thyristor

D.I.C.D.I.C.

Subject: - Power ElectronicsSubject: - Power Electronics

Code no: - IC - 657Code no: - IC - 657

Name: - Vasava Ashwin M.Name: - Vasava Ashwin M.

21406040052140604005

Kuntmal Rekha Kuntmal Rekha

21406042140604

Chapter: - ThyristorChapter: - Thyristor

Introduction of Thyristors:Introduction of Thyristors:• The invention of this high-power solid state switching The invention of this high-power solid state switching

device, known as thyristor, in the year device, known as thyristor, in the year 19571957 suggested a suggested a complete change in the concepts of control engineering. complete change in the concepts of control engineering.

• It was so named as its characteristics resembled those It was so named as its characteristics resembled those of its predecessor, i.e. gas-tube thyratron. of its predecessor, i.e. gas-tube thyratron.

• With the development of numerous similar devices of With the development of numerous similar devices of alike characteristics, the whole family of such high-power alike characteristics, the whole family of such high-power switching devices has come to be known as Thyristors. switching devices has come to be known as Thyristors.

• The whole thyristor family, therefore, includes the The whole thyristor family, therefore, includes the following devices:following devices:

• SCR (Silicon controlled rectifier, SCR (Silicon controlled rectifier, as the basic semiconductor as the basic semiconductor material is silicon)material is silicon)

• TriacTriac• SCS (Silicon controlled switch)SCS (Silicon controlled switch)• LASCR (Light activated silicon LASCR (Light activated silicon

controlled rectifier)controlled rectifier)• LASCS (Light activated silicon LASCS (Light activated silicon

controlled switch)controlled switch)• PUT (Programmable unijunction PUT (Programmable unijunction

transistor)transistor)• LAPUT (Light activated LAPUT (Light activated

programmable unijunction programmable unijunction transistor)transistor)

• DIACDIAC• SUS (Silicon unilateral switch) or SUS (Silicon unilateral switch) or

CSCR (Complementary SCR)CSCR (Complementary SCR)• SBS (Silicon bilateral switch)SBS (Silicon bilateral switch)

Application as a power controller Application as a power controller devices:devices:

• Speed controllers for ac and dc motorsSpeed controllers for ac and dc motors

• Temperature and illumination controllerTemperature and illumination controller

• AC and DC circuit breakersAC and DC circuit breakers

• Variable frequency dc-ac invertersVariable frequency dc-ac inverters

• Variable voltage DC-DC convertersVariable voltage DC-DC converters

• Variable frequency AC-AC convertersVariable frequency AC-AC converters

• Variable voltage AC-DC rectifiers,Variable voltage AC-DC rectifiers,

• HVDC transmission lines.HVDC transmission lines.

1.1 SCR, DIAC, TRIAC, UJT 1.1 SCR, DIAC, TRIAC, UJT CONSTRUCTION, OPERATIONS AND CONSTRUCTION, OPERATIONS AND

CHARACTERISTICS CHARACTERISTICS • SILICON CONTROLLED RECTIFIER (SCR):SILICON CONTROLLED RECTIFIER (SCR):• Why SCRs are used in control engineering?Why SCRs are used in control engineering?• Alternating current can be converted into direct Alternating current can be converted into direct

current using PN junction diodescurrent using PN junction diodes• In order to control the voltage/current series In order to control the voltage/current series

dropping resistor has to be used but there is, loss dropping resistor has to be used but there is, loss of power in the dropping resistor.of power in the dropping resistor.

• So it results in inefficient, uneconomical and So it results in inefficient, uneconomical and inconvenient operation. inconvenient operation.

• By using SCR the load control can be done with By using SCR the load control can be done with very small power control.very small power control.

•Fast switching action,Fast switching action,•Small size,Small size,•High current and high voltage ratings.High current and high voltage ratings.

Advantage of SCR:Advantage of SCR:• An advantage of the SCR over power An advantage of the SCR over power

transistors is the amount of drive necessary transistors is the amount of drive necessary for full conduction. In many silicon power for full conduction. In many silicon power transistors, it is necessary to inject up to half transistors, it is necessary to inject up to half an ampere of base current in order to conduct an ampere of base current in order to conduct five amperes from collector to emitter. In an five amperes from collector to emitter. In an SCR the amount of current conducted is SCR the amount of current conducted is dependent only on the external circuit once dependent only on the external circuit once the device has been triggered. Less than 100 the device has been triggered. Less than 100 mA is sufficient to trigger the SCR, which can mA is sufficient to trigger the SCR, which can conduct more then 100 amps.conduct more then 100 amps.

• In a transistor, the current at the base triggers In a transistor, the current at the base triggers it to conduction and the whole base region is it to conduction and the whole base region is affected by it. Therefore, when the base affected by it. Therefore, when the base current is removed the transistor returns to current is removed the transistor returns to the cut-off state. But in an SCR, the base the cut-off state. But in an SCR, the base electrode injects current only to a small electrode injects current only to a small fraction of the whole base region, P2 fraction of the whole base region, P2 ((Fig.1.1.1).Fig.1.1.1).

• Hence, once it is triggered to the Hence, once it is triggered to the conducting state, the gate electrode conducting state, the gate electrode loses control and remains in the loses control and remains in the conducting state, even when the gate conducting state, even when the gate current is removed.current is removed.

Description:Description:• A silicon controlled rectifier (SCR) is a A silicon controlled rectifier (SCR) is a

unidirectional power switch. unidirectional power switch.

• It allows current to flow in one direction only It allows current to flow in one direction only and, therefore, it is also called a and, therefore, it is also called a unidirectionalunidirectional, reverse blocking thyristor. , reverse blocking thyristor.

• It is a three terminal, four layer P-N-P-N It is a three terminal, four layer P-N-P-N semiconductor device. semiconductor device.

• It contains three electrodes—a cathode (C), It contains three electrodes—a cathode (C), an anode (A) and a control electrode called an anode (A) and a control electrode called the gate (G) as shown in the gate (G) as shown in Fig. 1.1.2 (a) & (b).Fig. 1.1.2 (a) & (b).

• The SCR can be switched from OFF to The SCR can be switched from OFF to ON state by a positive trigger pulse of ON state by a positive trigger pulse of right character applied to the gate. In right character applied to the gate. In the forward direction, a high resistance the forward direction, a high resistance is maintained until breakover occurs. is maintained until breakover occurs. The device exhibits the normal forward The device exhibits the normal forward characteristic of a silicon diode when characteristic of a silicon diode when breakover takes place. In the reverse breakover takes place. In the reverse direction, the device exhibits the direction, the device exhibits the reverse characteristic of a silicon reverse characteristic of a silicon rectifier.rectifier.

CONSTRUCTION: -CONSTRUCTION: -

• The cross-sectional views of The cross-sectional views of two typical SCRs are shown two typical SCRs are shown in in Figs. 1.1.3 (a) Figs. 1.1.3 (a) and and (b).(b).

• Basically, the SCR consists Basically, the SCR consists of a four-layer pellet of P-of a four-layer pellet of P-type and N-type type and N-type semiconductor materials. semiconductor materials.

• Silicon is used as the Silicon is used as the intrinsic semiconductor to intrinsic semiconductor to which the proper impurities which the proper impurities are added. are added.

• The junctions are either The junctions are either diffused or alloyed.diffused or alloyed.

• The planar construction in The planar construction in Fig. 1.1.3 (a) Fig. 1.1.3 (a) is used for is used for low-low-power SCRspower SCRs. .

• This technique is useful for making a numbers of units from This technique is useful for making a numbers of units from a single wafer. a single wafer.

• Here, all the junctions are diffused. Mesa construction Here, all the junctions are diffused. Mesa construction shown in shown in Fig. 1.1.3(b)Fig. 1.1.3(b) which is used for high-power SCRs. which is used for high-power SCRs.

• Here, the inner junction J2 is obtained by diffusion, and then Here, the inner junction J2 is obtained by diffusion, and then the outer two layers are alloyed to it. the outer two layers are alloyed to it.

• Because the P-N-P-N pellet is required to handle large Because the P-N-P-N pellet is required to handle large currents, it is properly braced with tungsten or currents, it is properly braced with tungsten or molybdenum plates to provide greater mechanical molybdenum plates to provide greater mechanical strength. strength.

• One of these plates is hard-soldered to a copper or an One of these plates is hard-soldered to a copper or an aluminum stud, which is threaded for attachment to a heat aluminum stud, which is threaded for attachment to a heat sink. sink.

• The heat sink provides an efficient thermal path for The heat sink provides an efficient thermal path for conducting the internal losses to the ambient .conducting the internal losses to the ambient .

CHARACTERISTICS OF AN SCRCHARACTERISTICS OF AN SCR

• The reverse region is typical The reverse region is typical of any silicon junction power of any silicon junction power rectifier cell, but the forward rectifier cell, but the forward characteristics are unique to characteristics are unique to the SCR.the SCR.

• A voltage V is applied A voltage V is applied between anode and cathode between anode and cathode with anode positive with with anode positive with respect to cathode as shown respect to cathode as shown in in Fig. 1.1.4.Fig. 1.1.4.

• If the voltage V is below the If the voltage V is below the breakdown potential of the breakdown potential of the device, only a small leakage device, only a small leakage current flows. Below the current flows. Below the breakdown potential, the cell breakdown potential, the cell forward characteristic is forward characteristic is actually the reverse actually the reverse characteristic of the center characteristic of the center pn junction, and thus pn junction, and thus provides a very high provides a very high resistance to current flow. resistance to current flow.

• Above the breakdown Above the breakdown potential, the device switches potential, the device switches from the blocking to the from the blocking to the conducting state. conducting state.

• Once the device is in the Once the device is in the conducting state the forward conducting state the forward characteristics is typical of characteristics is typical of any silicon junction rectifier any silicon junction rectifier exhibiting low dynamic exhibiting low dynamic resistance to current flow. resistance to current flow.

• Below the breakdown Below the breakdown potential, if the gate is made potential, if the gate is made positive with respect to positive with respect to cathode and gate current cathode and gate current flows, the device breaks down flows, the device breaks down and a heavy anode current and a heavy anode current flows, depending on the flows, depending on the voltage V and the resistance R voltage V and the resistance R of the anode circuit of of the anode circuit of Fig. Fig. 1.1.4.1.1.4. The V-I characteristic The V-I characteristic curve of the SCR is shown in curve of the SCR is shown in Fig.1.1.5.Fig.1.1.5.

Some terms regarding SCRSome terms regarding SCR• Some terms regarding SCR are as follows:Some terms regarding SCR are as follows:1. Forward break over voltage:1. Forward break over voltage:• The forward voltage at which the forward current starts The forward voltage at which the forward current starts

flowing suddenly at specific gate current is called the flowing suddenly at specific gate current is called the forward break over voltage.forward break over voltage.

2. Forward blocking voltage:2. Forward blocking voltage:• At zero gate current, the maximum forward voltage which At zero gate current, the maximum forward voltage which

can be applied to SCR without making the SCR ON is called can be applied to SCR without making the SCR ON is called the forward blocking voltage. It is shown by V DOM. i.e. the forward blocking voltage. It is shown by V DOM. i.e. 600V, 300V, 5000 V600V, 300V, 5000 V

3. Reverse break down voltage:3. Reverse break down voltage:• It is the reverse voltage at which the reverse current It is the reverse voltage at which the reverse current

suddenly rises to high value.suddenly rises to high value.4. Reverse blocking voltage:4. Reverse blocking voltage:• It is the maximum reverse voltage which can be applied to It is the maximum reverse voltage which can be applied to

SCR without making it to conduct. It is shown by VROM.SCR without making it to conduct. It is shown by VROM.5. Average forward current (IFAV):5. Average forward current (IFAV):• It is the maximum value of the average current which can It is the maximum value of the average current which can

be passed through SCR without damaging it. i.e. 16 A.be passed through SCR without damaging it. i.e. 16 A.

• RMS forward current (If):RMS forward current (If):• It is the RMS value of the average current which can be passed It is the RMS value of the average current which can be passed

through SCR without damaging it.through SCR without damaging it.• Holding current(IHO):Holding current(IHO):• It is the minimum vale of the forward current which should flow It is the minimum vale of the forward current which should flow

through the SCR to keep it in ON state at zero gate current.through the SCR to keep it in ON state at zero gate current.( associated with turning off process )( associated with turning off process )

• Latching current(IL):Latching current(IL):• It is the minimum value of the forward current necessary to bring It is the minimum value of the forward current necessary to bring

the SCR from OFF state to the ON state. Its value is about three the SCR from OFF state to the ON state. Its value is about three times the holding current. (associated with turning on process ).times the holding current. (associated with turning on process ).

• Forward voltage drop between anode and cathode (VT):Forward voltage drop between anode and cathode (VT):• It is the voltage drop across the anode and cathode when the It is the voltage drop across the anode and cathode when the

SCR is conducting.SCR is conducting.• Gate trigger voltage (VGT):Gate trigger voltage (VGT):• It is the gate voltage to make the SCR ON. i.e 3 VIt is the gate voltage to make the SCR ON. i.e 3 V• Gate trigger Current (IGT):Gate trigger Current (IGT):• It is the minimum value of the gate current which should pass It is the minimum value of the gate current which should pass

through the gate cathode circuit to make the SCR ON. i.e 20 mA through the gate cathode circuit to make the SCR ON. i.e 20 mA

DIACDIAC• A power device with four layers conducts A power device with four layers conducts

in both direction. in both direction. • A bidirectional device may be obtained A bidirectional device may be obtained

by connecting two of these back-to-back by connecting two of these back-to-back as shown in as shown in Fig.1.1.6 (a).Fig.1.1.6 (a).

• The fiveThe five layers,layers, n3,n3, p1,n1,p2,n2 can be p1,n1,p2,n2 can be combined into a single structure to form combined into a single structure to form a new device. a new device.

• This is shown in This is shown in Fig. 1.1.6(b).Fig. 1.1.6(b).• A Diac is a two-terminal, bi-directional A Diac is a two-terminal, bi-directional

conducting device that means it can conducting device that means it can conduct the current in both the conduct the current in both the directions like triac. directions like triac.

• But no gate current is required for its But no gate current is required for its operation. operation.

• This device can be switched from the This device can be switched from the OFF state to the ON state during either OFF state to the ON state during either the positive or the negative alteration of the positive or the negative alteration of an ac input wave. an ac input wave.

• The device essentially consists of two P-The device essentially consists of two P-N-P-N sections in antiparallel order as N-P-N sections in antiparallel order as shown in Fig. shown in Fig. 1.1.7(a) & (b).1.1.7(a) & (b).

• It has no gate terminal. The diac is also It has no gate terminal. The diac is also analogous to an inverse-parallel analogous to an inverse-parallel connection of two SCRs, but without the connection of two SCRs, but without the control gate. It is also called the trigger control gate. It is also called the trigger diode.diode.

CONSTRUCTION OF DIACCONSTRUCTION OF DIAC

• Diac is a three layer, two junctions, Diac is a three layer, two junctions, and two terminal semiconductor and two terminal semiconductor devices like transistor. devices like transistor.

• In this the doping concentrations of In this the doping concentrations of both the junction are kept equal (In both the junction are kept equal (In transistor, these are unequal). transistor, these are unequal).

• Due to this, similar characteristics Due to this, similar characteristics are available in both the directions. are available in both the directions.

• It is produced such that it can fire in It is produced such that it can fire in both directions with the breakover both directions with the breakover voltage of 29 to 35 V. voltage of 29 to 35 V.

• Only two leads are taken out. These Only two leads are taken out. These are denoted as MT. (Main terminals). are denoted as MT. (Main terminals).

• In In Fig. 1.1.8 (a)Fig. 1.1.8 (a) the schematic the schematic arrangement and in arrangement and in (b)(b) is various is various symbols are shown.symbols are shown.

Operation of & characteristics of Diac:Operation of & characteristics of Diac:• The two P-N-P-N sections are P1-N1-P2-N2 with junctions J2, The two P-N-P-N sections are P1-N1-P2-N2 with junctions J2,

J3, and J4 and P2-N1-P1-N3 with junctions J3, J2, and J1. J3, and J4 and P2-N1-P1-N3 with junctions J3, J2, and J1. • With a positive voltage at MT1 with respect to MT2, junctions With a positive voltage at MT1 with respect to MT2, junctions

J2 and J4 are forward biased, whereas J3 is reverse biased. J2 and J4 are forward biased, whereas J3 is reverse biased. The junction J1 is also reverse biased, so no current can flow The junction J1 is also reverse biased, so no current can flow through it.through it.

• For a small electrode voltage, the current flow through the For a small electrode voltage, the current flow through the device is extremely small as in an SCR. device is extremely small as in an SCR.

• When the electrode voltage is increased, J3 tends to When the electrode voltage is increased, J3 tends to avalanche and the current through the device increases. avalanche and the current through the device increases.

• The current flow through the device is, however, restricted to The current flow through the device is, however, restricted to the right-hand side of the device because it provides a smaller the right-hand side of the device because it provides a smaller resistance compared to the left-hand portion, since junction J1 resistance compared to the left-hand portion, since junction J1 opposes any current flow through it and the current has to opposes any current flow through it and the current has to overcome the lateral resistance of P1. overcome the lateral resistance of P1.

• As the current through the device increases, holes are As the current through the device increases, holes are injected from P1 to N1 and then, diffuse across N1 to be injected from P1 to N1 and then, diffuse across N1 to be collected by J3. collected by J3.

• The holes collected at the right-hand side of J3 are swept into The holes collected at the right-hand side of J3 are swept into region P2, and consequently it’s potential with respect to region P2, and consequently it’s potential with respect to region N2 and MT2 increases. region N2 and MT2 increases.

• But, at low current level, the hole current flows parallel to J4 But, at low current level, the hole current flows parallel to J4 to reach MT2 because the lateral resistance of region P2 is to reach MT2 because the lateral resistance of region P2 is smaller than the resistance of J4. smaller than the resistance of J4.

• However, the device current However, the device current increases further owing to an increases further owing to an increase in electrode voltage, increase in electrode voltage, the lateral voltage drop the lateral voltage drop becomes larger than that of the becomes larger than that of the forward-biased junction J4 and forward-biased junction J4 and the device is switched on from the device is switched on from the blocking state to the the blocking state to the conducting state as in an SCR.conducting state as in an SCR.

• When MT2 is made positive, a When MT2 is made positive, a similar action takes place and similar action takes place and current flows from MT2 to MT1 current flows from MT2 to MT1 through the left-hand portion of through the left-hand portion of the device as shown in the device as shown in Fig. Fig. 1.1.7 (a)1.1.7 (a). .

• The forward and reverse The forward and reverse characteristics of the device characteristics of the device are identical because its are identical because its construction is absolutely construction is absolutely symmetrical.symmetrical.

• The graph in The graph in Fig. 1.1.9 Fig. 1.1.9 shows shows that only a very low leakage that only a very low leakage current (due to minority current (due to minority carriers) +IBOcarriers) +IBO flows in the diac flows in the diac as MT1 is made more positive as MT1 is made more positive than the main terminal, MT2 than the main terminal, MT2 from 0 V to + VBO.from 0 V to + VBO.

• When the voltage across the diac has When the voltage across the diac has increased to + VBO,increased to + VBO, avalanche avalanche breakdown occurs at the reverse-biased breakdown occurs at the reverse-biased junction. junction.

• The current through the diac increases The current through the diac increases drastically at this point and is limited only drastically at this point and is limited only by external resistance. by external resistance.

• It is evident that the device exhibits a It is evident that the device exhibits a negative resistance characteristic in the negative resistance characteristic in the avalanche region, i.e. the current through avalanche region, i.e. the current through the device increases with the decreasing the device increases with the decreasing values of applied voltage. values of applied voltage.

• The voltage across the device then drops The voltage across the device then drops to break back voltage Vw.to break back voltage Vw.

• The value of breakover voltage for the The value of breakover voltage for the commonly used diac is about 32 V.commonly used diac is about 32 V.

• During the negative alteration of the During the negative alteration of the applied voltage when MT2 is made applied voltage when MT2 is made positive relative to MT1; the graph in positive relative to MT1; the graph in Fig.1.1.9Fig.1.1.9 shows a symmetrical but shows a symmetrical but opposite current in the diac. opposite current in the diac.

• When the applied voltage is less than + When the applied voltage is less than + VBOVBO (or – VBO), the diac blocks the flow of (or – VBO), the diac blocks the flow of current and effectively behaves as an current and effectively behaves as an open switch. open switch.

• It is possible to replace a diac by a It is possible to replace a diac by a suitable combination of SCRs, i.e. suitable combination of SCRs, i.e. antiparallel SCRs as shown in antiparallel SCRs as shown in Fig. 1.1.10. Fig. 1.1.10. The two gates of anti-parallel SCRs are The two gates of anti-parallel SCRs are not connected to the external circuit.not connected to the external circuit.

• Thus the characteristics of Diac are shown in Thus the characteristics of Diac are shown in Fig. 1.1.9.Fig. 1.1.9. Diac also conducts in quadrants 1st & 3rd like triac. Diac also conducts in quadrants 1st & 3rd like triac.

• When the voltage across the diac is less than the breakover When the voltage across the diac is less than the breakover voltage, it offers very high resistance so it does not voltage, it offers very high resistance so it does not conduct. conduct.

• But when the voltage exceeds the breakover voltage, diac But when the voltage exceeds the breakover voltage, diac offers very small resistance and it conducts. offers very small resistance and it conducts.

• Characteristic shows that the diac can conduct in either Characteristic shows that the diac can conduct in either direction.direction.

• Applications of the diac: -Applications of the diac: -• Diac are reliable and economical trigger devices for SCR Diac are reliable and economical trigger devices for SCR

control applications. control applications.

• They are extensively used as triggers for triac ac control They are extensively used as triggers for triac ac control circuits such as lamp dimmers, heat control of furnaces, circuits such as lamp dimmers, heat control of furnaces, motor speed control, and other similar applications.motor speed control, and other similar applications.

TRIACTRIAC

• A triac is a bilateral, bidirectional switching device with A triac is a bilateral, bidirectional switching device with three terminals. three terminals.

• It can be triggered either with positive or with negative It can be triggered either with positive or with negative pulses applied to the gate terminal, depending on the pulses applied to the gate terminal, depending on the potentials between the other two terminals called the main potentials between the other two terminals called the main terminals. terminals.

• Thus, the triac is an ac switch which can be made to Thus, the triac is an ac switch which can be made to conduct on both alternations (half-cycles) of an ac voltage. conduct on both alternations (half-cycles) of an ac voltage.

• The triac originates from two words, tri and ac. The triac originates from two words, tri and ac. • Tri indicates that there are three terminals, and acTri indicates that there are three terminals, and ac tells us tells us

that the device controls alternating current. that the device controls alternating current. • A triac is equivalent to an inverse-parallel connected pair of A triac is equivalent to an inverse-parallel connected pair of

SCRs. SCRs. • It is a low-power device of the thyristor family.It is a low-power device of the thyristor family.

CONSTRUCTION OF TRIACCONSTRUCTION OF TRIAC• The triac is a five-layer (N4-P1-N1-P2-The triac is a five-layer (N4-P1-N1-P2-

N2) device (if moved diagonally from N4 N2) device (if moved diagonally from N4 to N2) which may be considered to to N2) which may be considered to consist of an N2-P2-N1-P1 section in consist of an N2-P2-N1-P1 section in parallel with a P2-N1-P1-N4 section as parallel with a P2-N1-P1-N4 section as shown in shown in Fig. 1.1.11.Fig. 1.1.11.

• An additional lateral N3 region serves as An additional lateral N3 region serves as the control gate as it can be turned on the control gate as it can be turned on by applying a control voltage between by applying a control voltage between the gate and the main terminal.the gate and the main terminal.

• The gate terminal makes contact The gate terminal makes contact (ohmic) with both P2 and N3 type (ohmic) with both P2 and N3 type materials, thus allowing us to use either materials, thus allowing us to use either a positive or a negative pulse as trigger a positive or a negative pulse as trigger currents.currents.

• The characteristics of a triac The characteristics of a triac are similar to those of an SCR, are similar to those of an SCR, both in the blocking and both in the blocking and conducting states, except for conducting states, except for the fact that the SCR does not the fact that the SCR does not conduct in both directions. conduct in both directions.

• Due to this disparity, the terms Due to this disparity, the terms anode and cathode are not anode and cathode are not used for triac terminals. used for triac terminals.

• The two main terminals are The two main terminals are designated MT2 and MT1. designated MT2 and MT1. Fig. Fig. 1.1.12 (a) & (b) 1.1.12 (a) & (b) shows the shows the sectional view and symbol of a sectional view and symbol of a triac.triac.

Principle & characteristics of the triac:Principle & characteristics of the triac:• The triac is said to be positively The triac is said to be positively

biased when the main terminal 2 biased when the main terminal 2 (MT2) is positive with respect to (MT2) is positive with respect to the main terminal 1 (MT1). the main terminal 1 (MT1).

• This leads to operation in This leads to operation in quadrant l as shown in quadrant l as shown in Fig. Fig. 1.1.13.1.1.13.

• The triac operates in quadrant 3 The triac operates in quadrant 3 when MT2 is negative with when MT2 is negative with respect to MT1 and then it is said respect to MT1 and then it is said to be negatively biased. to be negatively biased.

• Like the SCR, the triac remains in Like the SCR, the triac remains in its OFF state until the break over its OFF state until the break over voltage is reached. voltage is reached.

• In its ON state, the voltage across In its ON state, the voltage across the triac drops to a low value and the triac drops to a low value and the current is limited by the the current is limited by the parameter of the external circuit. parameter of the external circuit.

• In the third quadrant when the In the third quadrant when the voltage polarity across MT2 and voltage polarity across MT2 and MT1 is reversed, current through MT1 is reversed, current through the triac is also reversed.the triac is also reversed.

Gate Triggering Modes of the Triac:Gate Triggering Modes of the Triac:

• Gate triggering of a triac in any of the four Gate triggering of a triac in any of the four operating modes is as follows:operating modes is as follows:

1. MT2 to MT1 voltage positive;1. MT2 to MT1 voltage positive; gate to MT1 gate to MT1 voltage positive—operation in quadrant 1voltage positive—operation in quadrant 1

2. MT2 to MT1 voltage positive;2. MT2 to MT1 voltage positive; gate to MT1 gate to MT1 voltage negative—operation in quadrant 1voltage negative—operation in quadrant 1

3. MT2 to MT1 voltage negative;3. MT2 to MT1 voltage negative; gate to MT1 gate to MT1 voltage negative—operation in quadrant 3voltage negative—operation in quadrant 3

4. MT2 to MT1 voltage negative;4. MT2 to MT1 voltage negative; gate to MT1 gate to MT1 voltage positive—operation in quadrant 3voltage positive—operation in quadrant 3

• When the terminal MT2 is positive and the terminal M1 When the terminal MT2 is positive and the terminal M1 negative, the triac can be turned on by applying a positive negative, the triac can be turned on by applying a positive voltage between the gate and MT1. voltage between the gate and MT1.

• This is the recommended method of triggering the device. In This is the recommended method of triggering the device. In this mode, the triac behaves as a conventional SCR, with four this mode, the triac behaves as a conventional SCR, with four layers P1-N1-P2-N2 and the MT1 terminal connected to layer layers P1-N1-P2-N2 and the MT1 terminal connected to layer N2 as shown in N2 as shown in Fig.Fig. 1.1.11.1.1.11.

• In this case, the device is switched on by a method called In this case, the device is switched on by a method called junction gate operation. junction gate operation.

• Initially, the left-hand portion of the triac comprising layers Initially, the left-hand portion of the triac comprising layers P1-N1-P2-N3 is turned on by the current flowing from P1-N1-P2-N3 is turned on by the current flowing from terminal MT1 to the gate through the junction P2-N3. terminal MT1 to the gate through the junction P2-N3.

• Terminal MT2 acts like the cathode gate. Terminal MT2 acts like the cathode gate. • When this left-hand portion conducts, the potential of the left When this left-hand portion conducts, the potential of the left

part of layer P2 in contact with N3 will go up, and because of part of layer P2 in contact with N3 will go up, and because of this potential gradient across layer P2,this potential gradient across layer P2, the current will flow the current will flow from left to right. from left to right.

• This is similar to the conventional gate current, and the right-This is similar to the conventional gate current, and the right-hand part of the triac comprising P1-N1-P2-N2 will turn on. hand part of the triac comprising P1-N1-P2-N2 will turn on.

• The junction gate operation involves high switching losses, The junction gate operation involves high switching losses, and therefore this form of gate drive is not normally used.and therefore this form of gate drive is not normally used.

• When the terminal MT2 is negative and the terminal MT1 When the terminal MT2 is negative and the terminal MT1 positive, the device can be turned on by applying a positive positive, the device can be turned on by applying a positive voltage between the gate and the terminal MT1. voltage between the gate and the terminal MT1.

• In this mode, the device is switched on by remote gate In this mode, the device is switched on by remote gate operation. The four layers used for this operation are P2-N1-operation. The four layers used for this operation are P2-N1-P1-N4. P1-N4.

• The reverse-biased junction is formed by layers N1-P1; it The reverse-biased junction is formed by layers N1-P1; it will be broken by increasing the carrier concentration in will be broken by increasing the carrier concentration in layer N1, assuming that the transistor is formed by layers layer N1, assuming that the transistor is formed by layers N2-P2-N1. N2-P2-N1.

• Since the gate is made positive with respect to terminal Since the gate is made positive with respect to terminal MT1, the transistor will be properly biased and a positive MT1, the transistor will be properly biased and a positive base current will flow into layer P2. base current will flow into layer P2.

• This will increase the emitter current and raise the carrier This will increase the emitter current and raise the carrier concentration in layer N1, and thus lead to the breakdown concentration in layer N1, and thus lead to the breakdown of the reverse-biased junction. of the reverse-biased junction.

• The device will then turn on.The device will then turn on.• If the gate is negative with MT2 negative, layers N3-P2-N1 If the gate is negative with MT2 negative, layers N3-P2-N1

will form the properly-biased transistor whose base drive is will form the properly-biased transistor whose base drive is provided by the positive voltage between MT1 and the provided by the positive voltage between MT1 and the gate. gate.

• The device will then turn on because of the increased The device will then turn on because of the increased current in layer N1. current in layer N1.

• When MT2 is negative and MT1 is positive, the When MT2 is negative and MT1 is positive, the recommended mode of triggering is by applying a negative recommended mode of triggering is by applying a negative voltage between the gate and the terminal MT1.voltage between the gate and the terminal MT1.

Applications of the triacApplications of the triac• Triac find extensive use in the control of ac power in ac Triac find extensive use in the control of ac power in ac

motors, heat control of furnaces, control of lamp dimmers, motors, heat control of furnaces, control of lamp dimmers, etc. Triac are also used in colour TV sets.etc. Triac are also used in colour TV sets.

• Advantages of the triac over SCR:Advantages of the triac over SCR:• The triac needs only one heat sink, though of a somewhat The triac needs only one heat sink, though of a somewhat

larger size.larger size.

• A triac needs only one fuse for its protection and this also A triac needs only one fuse for its protection and this also simplifies the construction,simplifies the construction,

• Triacs can be triggered with positive or negative polarity Triacs can be triggered with positive or negative polarity voltages.voltages.

• In some dc applications, the SCR is required to be fitted In some dc applications, the SCR is required to be fitted with a parallel diode to protect it against reverse voltage, with a parallel diode to protect it against reverse voltage, whereas a triac can work without a diode.whereas a triac can work without a diode.

Disadvantages of the triac over SCRDisadvantages of the triac over SCR

• Triacs have low dv/dtTriacs have low dv/dt rating compared with SCRs.rating compared with SCRs.

• Triacs are available in only smaller ratings.Triacs are available in only smaller ratings.

• The trigger circuit in triacs needs careful design as the triac The trigger circuit in triacs needs careful design as the triac can be triggered in either direction.can be triggered in either direction.

• The reliability of SCRs is more than that of triacs.The reliability of SCRs is more than that of triacs.

UNIJUNCTION TRANSISTOR (UJT)UNIJUNCTION TRANSISTOR (UJT)

• A unijunction transistor (UJT) or P-N transistor is a two-layer A unijunction transistor (UJT) or P-N transistor is a two-layer P-N device with three terminals. P-N device with three terminals.

• In transistor (BJT), there are two terminals but in UJT as its In transistor (BJT), there are two terminals but in UJT as its name suggests, there is only one junction. name suggests, there is only one junction.

• It is a unidirectional triggering element. Possessing only one It is a unidirectional triggering element. Possessing only one P-N junction, the UJT is a versatile semiconductor device P-N junction, the UJT is a versatile semiconductor device that exhibits negative resistance characteristics. that exhibits negative resistance characteristics.

• This means that an increasing emitter current results in a This means that an increasing emitter current results in a decreasing voltage between the emitter (E) and base-1 (B1) decreasing voltage between the emitter (E) and base-1 (B1) terminals. terminals.

• In other words, when the device is triggered, the emitter In other words, when the device is triggered, the emitter current increases regeneratively until it is limited by emitter current increases regeneratively until it is limited by emitter power supply.power supply.

Construction of the UJT:Construction of the UJT:

• The UJT is fabricated on an The UJT is fabricated on an N-type silicon bar with N-type silicon bar with ohmic contacts of gold film ohmic contacts of gold film for the two base terminals for the two base terminals as shown in as shown in Fig. 1.1.14.Fig. 1.1.14.

• The emitter section, a highly The emitter section, a highly doped P-type material, is doped P-type material, is deposited between the deposited between the base-1 (B1) and base-2 (B2) base-1 (B1) and base-2 (B2) regions. The P-type material regions. The P-type material is placed at a point closer to is placed at a point closer to B2 than B1. The N-type B2 than B1. The N-type silicon semiconductor region silicon semiconductor region comprising the base comprising the base material is lightly doped. material is lightly doped.

• The resulting small number of free The resulting small number of free electrons will support only a small electrons will support only a small current between the two base current between the two base terminals. terminals.

• Without the emitter, the dc resistance Without the emitter, the dc resistance across this N-type material from base-1 across this N-type material from base-1 to base-2 is approximately 4700 ohms to base-2 is approximately 4700 ohms to 9000 ohms for the 2N2160 type UJT. to 9000 ohms for the 2N2160 type UJT.

• The P-N junction acts as a silicon diode The P-N junction acts as a silicon diode connected to two base regions. connected to two base regions.

• The device is also called the double-The device is also called the double-based diode because the two terminals based diode because the two terminals are taken from the same section of the are taken from the same section of the diode.diode.

• Equivalent circuit of the UJT:Equivalent circuit of the UJT:• The interbase resistance RBB of the N-The interbase resistance RBB of the N-

type silicon bar appears as two resistors type silicon bar appears as two resistors RB1RB1 and RB2and RB2 where RBBwhere RBB = = RB1RB1 + RB2+ RB2 as shown in as shown in Fig. 1.1.15. Fig. 1.1.15.

• The relative values of RB1The relative values of RB1 and RB2 and RB2 depend on where the P-type emitter depend on where the P-type emitter material is located along the N-type material is located along the N-type bar. When the interbase voltage VBBbar. When the interbase voltage VBB is is applied between B2 and B1, a portion of applied between B2 and B1, a portion of the power supply voltage VBBthe power supply voltage VBB appears appears between the points C and B1 as shown between the points C and B1 as shown in in Fig. 1.1.15.Fig. 1.1.15.

• When there is no emitter current IE the voltage to the When there is no emitter current IE the voltage to the junction point of RB1 and RB2 with respect to B1 are given byjunction point of RB1 and RB2 with respect to B1 are given by

• VCB1VCB1 == VBBVBB··RB1/RB1+RB2RB1/RB1+RB2• == VBBVBB··RB1/RBBRB1/RBB (1.1)(1.1)• The ratio RB1/RBBThe ratio RB1/RBB == hh is called the intrinsic stand-off ratiois called the intrinsic stand-off ratio of of

a UJT. The value of h lies between 0.51 to 0.82. Therefore,a UJT. The value of h lies between 0.51 to 0.82. Therefore,• VCB1VCB1 == hVBBhVBB (1.2)(1.2)

• Principle operation of the UJT:Principle operation of the UJT:• If an external emitter voltage VE is applied to the emitter

terminal E with respect to B1, no current flows through the emitter as long as the emitter voltage is less than VCB1.

• If VE < hVBB, the emitter junction is reverse biased resulting in a small reverse leakage current.

• However, if VE > hVBB, the emitter junction gets forward biased, emitter current flows and holes get injected from the emitter into the high resistance N-type bar.

• These holes are attracted to base-1 (B1) and repelled by base-2 (B2). This results in an increase in the conduction of the region between the junction C and the base-1 (B1).

• This sudden increase in the number of available carriers quickly decreases the resistance of the RB1 portion of the UJT so that current, once started, flows easily between E and B1.

• Thus the conductivity of RB1Thus the conductivity of RB1 is is modulated or varied by the flow of modulated or varied by the flow of emitter current. emitter current.

• This phenomenon is known as This phenomenon is known as conductivity modulation.conductivity modulation.

• The increase in conductivity, in The increase in conductivity, in turn, decreases the voltage drop turn, decreases the voltage drop between the points C and B1.between the points C and B1.

• Consequently, this increases the Consequently, this increases the forward bias of the P-N junction. forward bias of the P-N junction. The emitter current IE,The emitter current IE, therefore, therefore, increases. increases.

• This process continues until the This process continues until the valley point valley point with the coordinates. with the coordinates. IvIv and Vvand Vv, , is reached as shown in is reached as shown in Fig. 1.1.16.Fig. 1.1.16.

• This current IvThis current Iv is so large that no is so large that no further increase in the conductivity further increase in the conductivity of the region between the junction of the region between the junction C and the base-1 (B1) is possible. C and the base-1 (B1) is possible.

• Hence beyond this valley point the UJT behaves as a Hence beyond this valley point the UJT behaves as a conventional forward biased junction diode. conventional forward biased junction diode.

• The emitter voltage at the peak point in the characteristic is The emitter voltage at the peak point in the characteristic is given as, VPgiven as, VP = hVBB+ VD= hVBB+ VD, , where VDwhere VD is the inherent base is the inherent base voltage drop.voltage drop.

• Between VPBetween VP and Vv,and Vv, increase in IEincrease in IE is accomplished by a is accomplished by a reduction in the emitter voltage VE.reduction in the emitter voltage VE.

• This is the negative resistanceThis is the negative resistance regionregion of the UJT. Beyond of the UJT. Beyond the valley point, increase in IEthe valley point, increase in IE is accomplished by an is accomplished by an increase in VE.increase in VE.

• This is known as the saturation region.This is known as the saturation region.

• At points to the left of VP,At points to the left of VP, the emitter-base 1 is reverse the emitter-base 1 is reverse biased, and there is no emitter current. biased, and there is no emitter current.

• This is called the cut-off region.This is called the cut-off region.

• Applications of UJT:Applications of UJT:• UJTs are extensively used in oscillator, pulse and wave UJTs are extensively used in oscillator, pulse and wave

sensing circuits as well as in delay timer circuits. This sensing circuits as well as in delay timer circuits. This device is also used to apply a sudden pulse of power to device is also used to apply a sudden pulse of power to energies a relay or to fire an SCR.energies a relay or to fire an SCR.

1.2 Two transistor analogy of SCR:1.2 Two transistor analogy of SCR:• In the structure of the SCR as In the structure of the SCR as

shown in shown in Fig. 1.2.1 Fig. 1.2.1 the inner N-the inner N-region is lightly doped. region is lightly doped.

• The outer P-region and the outer The outer P-region and the outer N-region are heavily doped. N-region are heavily doped.

• The doping of the inner P-region The doping of the inner P-region is intermediate between that of is intermediate between that of the inner N-region and the outer the inner N-region and the outer P-region (or the outer N-region).P-region (or the outer N-region).

• If a positive voltage VFIf a positive voltage VF is applied is applied to the anode with respect to the to the anode with respect to the cathode, junctions J1 and J3 are cathode, junctions J1 and J3 are forward biased but junction J2 in forward biased but junction J2 in the middle is reverse biased. the middle is reverse biased. Junction J2 passes a greater Junction J2 passes a greater value of leakage current than a value of leakage current than a simple P-N junction would pass simple P-N junction would pass because junctions J1 and J3 here because junctions J1 and J3 here act as sources of minority act as sources of minority carriers. carriers.

• Junctions J1-J2 and J2-J3 Junctions J1-J2 and J2-J3 can be considered to can be considered to constitute a P-N-P and N-constitute a P-N-P and N-P-N transistor, P-N transistor, respectively, as shown in respectively, as shown in Fig. 1.2.2, Fig. 1.2.2, where the where the junction J2 is the junction J2 is the collector-base junction collector-base junction common to both common to both component transistors.component transistors.

• In a transistor, the total In a transistor, the total collector current is, Ic = collector current is, Ic = aIE + IcoaIE + Ico where awhere a and Icoand Ico are the emitter-collector are the emitter-collector current gain and collector current gain and collector reverse saturation reverse saturation current, respectively. current, respectively.

• The collector currents of The collector currents of the two transistor Q1 and the two transistor Q1 and Q2, therefore, given byQ2, therefore, given by

• IC1IC1 == a1IA + ICO1a1IA + ICO1 andand (1.2.1)(1.2.1)

• IC2IC2 == a2IA + ICO2a2IA + ICO2(1.2.2)(1.2.2)

• The total current in the anode terminal of the SCR is then given The total current in the anode terminal of the SCR is then given byby

• IAIA == IC1 + IC2IC1 + IC2(1.2.3)(1.2.3)

• == (a1IA + ICO1) + (a2IA +ICO2)(a1IA + ICO1) + (a2IA +ICO2)(1.2.4)(1.2.4)

• Therefore, the forward current in the SCR is given byTherefore, the forward current in the SCR is given by

• IAIA == ICO1 + ICO2/1-(a1 + a2) (1.2.5)ICO1 + ICO2/1-(a1 + a2) (1.2.5)

• Normally, (a1 + a2) for silicon is less than unity. Normally, (a1 + a2) for silicon is less than unity.

• But a slight increase in as causes the denominator of Eq. But a slight increase in as causes the denominator of Eq. (1.2.5) to become extremely small and this produces a large (1.2.5) to become extremely small and this produces a large forward current IA. forward current IA.

• When the forward voltage VF is increased, the leakage current When the forward voltage VF is increased, the leakage current at junction J2 increases. This increases the values of a1 and a2, at junction J2 increases. This increases the values of a1 and a2, causing a reduction in the value of denominator of Eq. (1.2.5). causing a reduction in the value of denominator of Eq. (1.2.5).

• If the forward voltage is sufficiently large, carrier multiplication If the forward voltage is sufficiently large, carrier multiplication at J2 causes its breakdown and the leakage current increases at J2 causes its breakdown and the leakage current increases rapidly. rapidly.

• As a result, 1 - (a1+ a2) reduces to zero. The increase in As a result, 1 - (a1+ a2) reduces to zero. The increase in forward current IA at J2 is accelerated by an increase in forward current IA at J2 is accelerated by an increase in current gains a1 and a2, until avalanche breakdown occurs. current gains a1 and a2, until avalanche breakdown occurs.

• The current becomes so large that the component The current becomes so large that the component transistors are saturated. transistors are saturated.

• The voltage drop across the SCR falls nearly to 1.5 V and The voltage drop across the SCR falls nearly to 1.5 V and the forward current IA is limited by the external resistance the forward current IA is limited by the external resistance RL.RL.

• Once the forward breakdown occurs, the device remains in Once the forward breakdown occurs, the device remains in the ON state with a very low resistance until (a1 + a2) the ON state with a very low resistance until (a1 + a2) becomes less than unity.becomes less than unity.

• Thus, if a current is caused to flow across J3, the current Thus, if a current is caused to flow across J3, the current through the entire device is increased. through the entire device is increased.

• This enhancement is due to the increasing values of a1 and This enhancement is due to the increasing values of a1 and a2. If the current going to P-layer between J3 and J2, i.e. the a2. If the current going to P-layer between J3 and J2, i.e. the gate current, is made greater than zero, the breakover gate current, is made greater than zero, the breakover voltage VBO is lowered. voltage VBO is lowered.

• The effect of increasing gate current is to increase a2 only, The effect of increasing gate current is to increase a2 only, and a1 is determined by the overall forward current. and a1 is determined by the overall forward current.

• This means that 1 - (a1 + a2) is controlled both by the gate This means that 1 - (a1 + a2) is controlled both by the gate current and the total forward current. current and the total forward current.

• As the gate current increases in value, the forward As the gate current increases in value, the forward breakover voltage becomes smaller and equal to the breakover voltage becomes smaller and equal to the forward voltage drop of one P-N junction at some value of forward voltage drop of one P-N junction at some value of the gate current.the gate current.

• When the SCR is turned on, all the When the SCR is turned on, all the four layers are filled with carriers, four layers are filled with carriers, and even if the gate supply is and even if the gate supply is removed, the device will continue to removed, the device will continue to stay on because of internal stay on because of internal regeneration. regeneration.

• Therefore, a gate signal is required Therefore, a gate signal is required only for turning on the SCR. Once only for turning on the SCR. Once the SCR starts conducting, the gate the SCR starts conducting, the gate loses all control and even if the gate loses all control and even if the gate voltage is removed, the anode voltage is removed, the anode current does not decrease at all. current does not decrease at all.

• The only way to stop conduction is The only way to stop conduction is to reduce the applied voltage to zero to reduce the applied voltage to zero or bring the forward current below or bring the forward current below the value of holding current IH.the value of holding current IH.

• The SCR shown in The SCR shown in Fig. 1.2.1 Fig. 1.2.1 can be can be visualized as consisting of two visualized as consisting of two separate transistors as shown in separate transistors as shown in Fig. 1.2.2. Fig. 1.2.2.

• Thus, the equivalent circuit of the Thus, the equivalent circuit of the SCR is composed of P-N-P and N-P-N SCR is composed of P-N-P and N-P-N transistors connected as shown in transistors connected as shown in Fig. 1.2.3.Fig. 1.2.3.

• It is clear that the collector of each transistor is coupled to It is clear that the collector of each transistor is coupled to the base of the other, thereby making positive feedback the base of the other, thereby making positive feedback loops. loops.

• With the gate G open, there is no base current in the With the gate G open, there is no base current in the transistor Q2. Therefore, no current can flow in the collector transistor Q2. Therefore, no current can flow in the collector of Q2 and hence in the collector of Q1. of Q2 and hence in the collector of Q1.

• Under such a condition, the SCR is open.Under such a condition, the SCR is open.• If the switch S is closed, a small gate current IG (which is If the switch S is closed, a small gate current IG (which is

the base current of Q2) will flow through the base of Q2 the base current of Q2) will flow through the base of Q2 which means that its collector current IC2 will increase. which means that its collector current IC2 will increase.

• The collector current of Q2 is the base current of Q1, The collector current of Q2 is the base current of Q1, therefore, the collector current of Q1 increases. therefore, the collector current of Q1 increases.

• But the collector current of Q1 is the base current of Q2 But the collector current of Q1 is the base current of Q2 which combines with the gate current IG. which combines with the gate current IG.

• It thus results in the enhancement of the base current of It thus results in the enhancement of the base current of Q2. Q2.

• This phenomenon is cumulative since an increase in current This phenomenon is cumulative since an increase in current of one transistor causes an increase in current of the other of one transistor causes an increase in current of the other transistor. transistor.

• As a result of this action, both the transistors are driven into As a result of this action, both the transistors are driven into saturation causing a large value of current IA to flow saturation causing a large value of current IA to flow through the load RL, and in this condition the SCR is said to through the load RL, and in this condition the SCR is said to be turned on.be turned on.

1.31.3 Overview of following properties of Overview of following properties of SCR:SCR:

1.3.11.3.1Voltage & Current Rating:Voltage & Current Rating:

• Current Rating:Current Rating:• The rating of an SCR is based primarily on the amount of The rating of an SCR is based primarily on the amount of

heat generated within the device and the ability of the heat generated within the device and the ability of the device to transfer the internal heat to its external case. The device to transfer the internal heat to its external case. The power generated inside the junctions of the device depends power generated inside the junctions of the device depends onon

• turn-on switching,turn-on switching,

• forward conduction,forward conduction,

• turn-off or commutation,turn-off or commutation,

• reverse blocking, andreverse blocking, and

• Triggering.Triggering.

• The forward current rating of an SCR is a function The forward current rating of an SCR is a function of its maximum junction temperature, the device of its maximum junction temperature, the device thermal impedances, the total device losses and thermal impedances, the total device losses and the ambient temperature. the ambient temperature.

• Forward conduction is the main source of heat generation Forward conduction is the main source of heat generation for normal low-frequency operations. for normal low-frequency operations.

• However, for high-frequency applications or for large rates However, for high-frequency applications or for large rates of change of current, di/dtof change of current, di/dt, , the ratio of the peak to average the ratio of the peak to average current is high and turn-on switching losses become the current is high and turn-on switching losses become the predominant source of heat generation. predominant source of heat generation.

• The rating of the SCR is such that in no case the maximum The rating of the SCR is such that in no case the maximum junction temperature of the device is exceeded. junction temperature of the device is exceeded.

• The ON-state current rating of an SCR indicates the The ON-state current rating of an SCR indicates the maximum, average, rms and peak (surge) current that maximum, average, rms and peak (surge) current that could be allowed to flow through the device under the could be allowed to flow through the device under the stated ON condition. stated ON condition.

• For heat sink mounting, the rating depends on the For heat sink mounting, the rating depends on the temperature of its case.temperature of its case.

• Power Rating:Power Rating: • This is strictly applicable to current conduction and the This is strictly applicable to current conduction and the

forward voltage drop, therefore, the power loss is a better forward voltage drop, therefore, the power loss is a better consideration. The following are the power losses in an SCR consideration. The following are the power losses in an SCR which are usually specified by the manufacturers.which are usually specified by the manufacturers.

• Forward conduction lossForward conduction loss• Gate power lossGate power loss• Turn-on lossTurn-on loss• Turn-off lossTurn-off loss• Forward conduction loss: - Forward conduction loss: - The mean anode The mean anode

current multiplied by the forward voltage drop current multiplied by the forward voltage drop across the SCR (1.5 V) is the average power across the SCR (1.5 V) is the average power dissipated that is termed forward conduction loss in dissipated that is termed forward conduction loss in the SCR.the SCR.

• Gate power loss: - Gate power loss: - This is the product of the gate This is the product of the gate voltage and the gate current for continuous signals. voltage and the gate current for continuous signals. The loss is small if pulse signals are used to turn on The loss is small if pulse signals are used to turn on the SCR.the SCR.

• Turn-on loss: - Turn-on loss: - Due to finite time taken by the Due to finite time taken by the switching process, there is a relatively high voltage switching process, there is a relatively high voltage across the SCR while a current flows. Accordingly, across the SCR while a current flows. Accordingly, appreciable power is dissipated during this turn-on appreciable power is dissipated during this turn-on period. Hence the turn-on loss is the product of this period. Hence the turn-on loss is the product of this high voltage across the SCR and the turn-on current high voltage across the SCR and the turn-on current during the switching process.during the switching process.

• Turn-offTurn-off loss. loss. During a rapid turn-off of the SCR, it is possible During a rapid turn-off of the SCR, it is possible for the reverse current to rise to a value comparable to the for the reverse current to rise to a value comparable to the forward current. When the SCR impedance starts to increase, forward current. When the SCR impedance starts to increase, dissipation occurs as the current falls and the reverse voltage dissipation occurs as the current falls and the reverse voltage builds up. To limit the rate of change of current at turn-off, and, builds up. To limit the rate of change of current at turn-off, and, therefore, the energy to be dissipated, circuit inductance is therefore, the energy to be dissipated, circuit inductance is used. Hence the turn-off loss is the product of the reverse used. Hence the turn-off loss is the product of the reverse voltage that is building up and the reverse current that is falling voltage that is building up and the reverse current that is falling down during the turn-off period.down during the turn-off period.

• 1.3.21.3.2 Latching Current & Holding Current:Latching Current & Holding Current:• Latching Current:Latching Current:• The latching current of an SCR specifies a value of the anode The latching current of an SCR specifies a value of the anode

current slightly higher than the holding current, which is the current slightly higher than the holding current, which is the minimum value, required to sustain conduction immediately minimum value, required to sustain conduction immediately after the SCR is switched from the OFF state to the ON state and after the SCR is switched from the OFF state to the ON state and the gate signal removed as shown in the gate signal removed as shown in Fig. 1.1.5. Fig. 1.1.5. Once the Once the latching current is reached, the SCR remains in the ON state latching current is reached, the SCR remains in the ON state until the anode current is decreased below the holding current until the anode current is decreased below the holding current value.value.

• Thus, if latching current IL of an SCR is 7 mA, it means that if Thus, if latching current IL of an SCR is 7 mA, it means that if the anode current is made less than 7 mA, then the SCR the anode current is made less than 7 mA, then the SCR continues to stay ON. The latching current rating is an important continues to stay ON. The latching current rating is an important consideration when the device is used with an inductive load consideration when the device is used with an inductive load because the inductance limits the rate of rise of the anode because the inductance limits the rate of rise of the anode current. Precautions should be taken to ensure that under such current. Precautions should be taken to ensure that under such conditions the gate signal is present until the anode current conditions the gate signal is present until the anode current rises to the latching value so that the complete turn-on of the rises to the latching value so that the complete turn-on of the SCR is assured.SCR is assured.

• Holding Current:Holding Current:• It is the minimum value of the anode current, necessary in It is the minimum value of the anode current, necessary in

the anode circuit to keep the SCR conducting (with gate the anode circuit to keep the SCR conducting (with gate open) as shown in open) as shown in Fig. 1.1.5. Fig. 1.1.5. If the anode current is If the anode current is reduced below this critical holding current value, the SCR reduced below this critical holding current value, the SCR cannot maintain regeneration and reverts to the OFF state. cannot maintain regeneration and reverts to the OFF state. When the SCR is in the conducting state, it cannot be When the SCR is in the conducting state, it cannot be turned off even if the gate voltage is removed, and the turned off even if the gate voltage is removed, and the device will continue to stay ON as the depletion layer and device will continue to stay ON as the depletion layer and the reverse-biased junction J2 no longer exist because of the reverse-biased junction J2 no longer exist because of the free movement of carriers. When the forward current the free movement of carriers. When the forward current falls below holding current IH, the depletion region will falls below holding current IH, the depletion region will begin to develop again around the reverse-biased junction begin to develop again around the reverse-biased junction J2 and the SCR will go to the blocking state.J2 and the SCR will go to the blocking state.

• The holding current IH is usually lower than, but very close The holding current IH is usually lower than, but very close to, the latching current IL. The holding current is to, the latching current IL. The holding current is temperature sensitive and decreases with the increase in temperature sensitive and decreases with the increase in temperature. Thus, if the holding current IH of an SCR is 6 temperature. Thus, if the holding current IH of an SCR is 6 mA, it means that if the anode current is made less than mA, it means that if the anode current is made less than the 6 mA, then the SCR will be turned off. The holding the 6 mA, then the SCR will be turned off. The holding current rating is generally specified at room temperature current rating is generally specified at room temperature with gate open.with gate open.

1.3.31.3.3Turn-on Time, Turn-off Time, dv/dt, Turn-on Time, Turn-off Time, dv/dt, di/dt:di/dt:

• Turn-on Time:Turn-on Time:• When a triggering signal (a When a triggering signal (a

positive voltage between the positive voltage between the gate and cathode) at the gate of gate and cathode) at the gate of an initially-off SCR is applied, the an initially-off SCR is applied, the time period needed to turn the time period needed to turn the switch from the OFF state to the switch from the OFF state to the fully ON state is termed the turn-fully ON state is termed the turn-on time. on time.

• In other words, it is defined as In other words, it is defined as the time from the initiation of the time from the initiation of triggering (when the SCR offers triggering (when the SCR offers infinite impedance to the flow of infinite impedance to the flow of anode current) to the time when anode current) to the time when an equilibrium charge an equilibrium charge distribution is established distribution is established throughout the device together throughout the device together with a steady state forward with a steady state forward voltage drop. voltage drop.

• The turn-on time varies from 2 The turn-on time varies from 2 msms to 4 msto 4 ms for the commercially for the commercially available SCRs.available SCRs.

• Figure 1.3.3.1 Figure 1.3.3.1 indicates the indicates the form of current rise during the form of current rise during the transition from the non- transition from the non- conducting to the fully conducting to the fully conducting state. conducting state.

• The time period tdThe time period td is the delay time between the front of the is the delay time between the front of the gate pulse and the start of the rapid rate of increase in the gate pulse and the start of the rapid rate of increase in the anode current. anode current.

• Power dissipation in the SCR is the greatest during the period trPower dissipation in the SCR is the greatest during the period tr (rise time), because the current rises rapidly over a small area (rise time), because the current rises rapidly over a small area while the voltage drop is still appreciable. while the voltage drop is still appreciable.

• In other words, the turn-on time is the time period between the In other words, the turn-on time is the time period between the steep-fronted gate trigger pulse and the instant when the steep-fronted gate trigger pulse and the instant when the forward on-state voltage has fallen to 90% of its initial value. forward on-state voltage has fallen to 90% of its initial value.

• The turn-on time depends onThe turn-on time depends on• anode circuit parameters,anode circuit parameters,• the gate signal amplitude, andthe gate signal amplitude, and• the rise time.the rise time.• Turn-off Time:Turn-off Time:• Turn-off condition means that all forward conduction has ceased Turn-off condition means that all forward conduction has ceased

and the reapplication of positive voltage to the anode will not and the reapplication of positive voltage to the anode will not cause current to flow without a gate signal. cause current to flow without a gate signal.

• The turn-off time is defined as the time interval, starting from The turn-off time is defined as the time interval, starting from the dropping of the anode voltage, needed to complete the the dropping of the anode voltage, needed to complete the switching-off process. switching-off process.

• In other words, it is the maximum amount of time that an SCR In other words, it is the maximum amount of time that an SCR will require to achieve gate control for the specified circuit will require to achieve gate control for the specified circuit conditions. conditions.

• A typical value of turn-off time is 50 ms for converter grade A typical value of turn-off time is 50 ms for converter grade SCRs and it can be as low as 6 ms for faster devices normally SCRs and it can be as low as 6 ms for faster devices normally called inverter grade SCRs. called inverter grade SCRs.

• For any duration less than this, For any duration less than this, the device will start the device will start reconducting with reapplication reconducting with reapplication of the forward voltage. of the forward voltage.

• The turn-off time is temperature The turn-off time is temperature sensitive. sensitive.

• For natural commutation, the For natural commutation, the turn-off time is between 10 ms turn-off time is between 10 ms and 100 msand 100 ms while for forced while for forced commutation it can be between commutation it can be between 7ms7ms and 20 ms.and 20 ms. More More specifically, the turn-off time for specifically, the turn-off time for a low-voltage, low-current SCR is a low-voltage, low-current SCR is 10 ms. 10 ms.

• The turn-off time is divided into The turn-off time is divided into two parts:two parts:

i. Reverse recovery time trr for i. Reverse recovery time trr for which reverse anode current which reverse anode current flows after the application of a flows after the application of a reverse voltage as shown in reverse voltage as shown in Fig. Fig. 1.3.3.2.1.3.3.2.

ii. Gate recovery time tgrii. Gate recovery time tgr required for the recombination required for the recombination of all excess carriers in the two inner regions around the of all excess carriers in the two inner regions around the middle junction J2 of the device.middle junction J2 of the device.

• The turn-off timeThe turn-off time

• increases with junction temperature because increases with junction temperature because recombination takes longer at higher recombination takes longer at higher temperatures,temperatures,

• decreases because of a reverse current as decreases because of a reverse current as junctions J1 and J3 become reverse biased in a junctions J1 and J3 become reverse biased in a shorter time, andshorter time, and

• increases with the increase in the magnitude of increases with the increase in the magnitude of the anode current.the anode current.

SCR dv/dt Calculation:SCR dv/dt Calculation:

• In practice a dc voltage is switched In practice a dc voltage is switched across an SCR, and the rate of across an SCR, and the rate of change of voltage across the device change of voltage across the device must be below the dv/dt rating of must be below the dv/dt rating of the device, otherwise the device the device, otherwise the device would start conducting even without would start conducting even without the application of a gate trigger the application of a gate trigger pulse. The circuit is shown in pulse. The circuit is shown in Fig.1.3.3.3.Fig.1.3.3.3.

• The rate of change of voltage across The rate of change of voltage across the SCR may be calculated in the the SCR may be calculated in the following two ways as explained following two ways as explained below:below:

1.1. When the switch is closed, the When the switch is closed, the voltage equation across the voltage equation across the SCR SCR (assuming load resistance R is (assuming load resistance R is negligible) is given bynegligible) is given by

• VAKVAK == VB(1-e-1\t) VB(1-e-1\t) ··VV(1.3.3.1)(1.3.3.1)

• Where tWhere t == L/RsL/Rs(1.3.3.2)(1.3.3.2)

• Therefore, dVAK/dt = Rs/LTherefore, dVAK/dt = Rs/L··VBe-t/tVBe-t/tvolts/secvolts/sec

• The maximum rate of change of voltage is when t=0,The maximum rate of change of voltage is when t=0,

• or,or, dVAK/dt maxdVAK/dt max == Rs/LRs/L·· VB VBvolts/secvolts/sec

• IfIf RsRs == √(L/Cs)√(L/Cs)

• ThenThen dVAK/dt maxdVAK/dt max == √VB/(LCs)√VB/(LCs)(1.3.3.3)(1.3.3.3)

2.2. When the switch is closed, the voltage oscillation across the When the switch is closed, the voltage oscillation across the SCR is SCR is given bygiven by

• VAKVAK == Vsin wtVsin wt (1.3.3.4)(1.3.3.4)

• Where wWhere w == √1/(LCs)√1/(LCs)

• Therefore,Therefore, dVAK/dtdVAK/dt == wVcos wtwVcos wt

• == √V/(LCs) cos wt√V/(LCs) cos wt

• The maximum rate of change of voltage occurs at t=0,The maximum rate of change of voltage occurs at t=0,

• dVAK/dt maxdVAK/dt max == √V/(LCs)√V/(LCs) (1.3.3.5)(1.3.3.5)

• Hence, by proper choice of L, Cs, and Rs in the circuit, the Hence, by proper choice of L, Cs, and Rs in the circuit, the dV/dt across the SCR could be limited to an acceptable dV/dt across the SCR could be limited to an acceptable value. Rs also limit the discharge current through the SCR value. Rs also limit the discharge current through the SCR when the SCR is gated to turn it on.when the SCR is gated to turn it on.

SCR di/dt Calculation:SCR di/dt Calculation:• The maximum permissible rate of The maximum permissible rate of

change of current through a change of current through a device is dependent on the type of device is dependent on the type of component being used. component being used.

• The manufacturer’s data sheet The manufacturer’s data sheet specifies the maximum di/dt specifies the maximum di/dt capability. capability.

• The price of the device increases The price of the device increases with di/dt capability. Through with di/dt capability. Through proper circuit design techniques, proper circuit design techniques, the rate of change of current the rate of change of current through the SCR must be kept to a through the SCR must be kept to a minimum. minimum.

• This section calculates the di/dt This section calculates the di/dt through an SCR as a function of through an SCR as a function of other circuit components and the other circuit components and the applied voltage. applied voltage.

• The circuit is shown in The circuit is shown in Fig. Fig. 1.3.3.4.1.3.3.4.

• The circuit voltage equation is given byThe circuit voltage equation is given by• vv == Ri + L di/dtRi + L di/dt

(1.3.3.6)(1.3.3.6)• Solving for current,Solving for current,• ii == v/R(1-e-t/t)Av/R(1-e-t/t)A• WhereWhere tt == L/RL/R• Therefore, di/dtTherefore, di/dt == v/Rv/R··1/t1/t··e-t/te-t/t

(1.3.3.7)(1.3.3.7)• or,or, di/dtdi/dt == v/Lv/L··e-t/te-t/t

(1.3.3.8)(1.3.3.8)• The maximum di/dt is at t=0,The maximum di/dt is at t=0,• or,or, di/dtmaxdi/dtmax == v/Lv/L• == Vm/LVm/L A/secA/sec

(1.3.3.9)(1.3.3.9)• It is seen from Eq. 1.3.3.9 that the rate of change of current It is seen from Eq. 1.3.3.9 that the rate of change of current

through the SCR is dependent on the supply voltage and through the SCR is dependent on the supply voltage and inductance in the circuit. The di/dt could be reduced by inductance in the circuit. The di/dt could be reduced by proper selection of L (inductance) in series with the SCR.proper selection of L (inductance) in series with the SCR.

1.3.41.3.4 Forced & Natural Forced & Natural Commutation:Commutation:

• Commutation methods:Commutation methods:

• What meaning of commutation?What meaning of commutation?• The process of opening or turning off of a The process of opening or turning off of a

conducting Thyristor is called Commutation. A conducting Thyristor is called Commutation. A conducting thyristor may be turned off by conducting thyristor may be turned off by reducing its anode current below holding current reducing its anode current below holding current value & then applying a reverse volt across the value & then applying a reverse volt across the device to enable it to regain its forward blocking device to enable it to regain its forward blocking capability.capability.

• Two types of Commutation:Two types of Commutation:

• 1.1. Natural or Line or Phase Commutation.Natural or Line or Phase Commutation.

• 2.2. Forced Commutation.Forced Commutation.

Natural Commutation:Natural Commutation:

• In ac circuits as the current through an SCR goes through In ac circuits as the current through an SCR goes through its natural zero, a reverse voltage automatically appears its natural zero, a reverse voltage automatically appears across the SCR and causes it to turn off quickly. across the SCR and causes it to turn off quickly.

• As the voltage in an ac circuit reverses every half-cycle, an As the voltage in an ac circuit reverses every half-cycle, an SCR in the line would be reversing biased every negative SCR in the line would be reversing biased every negative cycle and thus get turned off. cycle and thus get turned off.

• This is known as natural or phase or line commutation. This is known as natural or phase or line commutation. Line-commutated converters and inverters are the Line-commutated converters and inverters are the examples of natural commutation. examples of natural commutation.

• This type of commutation is applied in ac volt controllers, This type of commutation is applied in ac volt controllers, phase controlled rectifier, cyclo converters.phase controlled rectifier, cyclo converters.

Forced Commutation:Forced Commutation:• In dc circuits where there is no natural zero value of In dc circuits where there is no natural zero value of

current, forward anode current can be reduced either by current, forward anode current can be reduced either by shunting the SCR by another device (commutating element) shunting the SCR by another device (commutating element) or by applying a reverse voltage across its anode-cathode or by applying a reverse voltage across its anode-cathode terminal in order to force the anode current to a zero value. terminal in order to force the anode current to a zero value.

• This is called forced commutation.This is called forced commutation. In the case of dc circuits, In the case of dc circuits, there are six common ways (Class A to Class F) by which there are six common ways (Class A to Class F) by which the forward current through the conducting SCR can be the forward current through the conducting SCR can be forced to zero (commutation). forced to zero (commutation).

• The rectifier circuits with large inductive loads, choppers The rectifier circuits with large inductive loads, choppers and inverters are examples of forced commutation. and inverters are examples of forced commutation.

• The reverse bias turn-off method belongs to the category of The reverse bias turn-off method belongs to the category of forced commutation.forced commutation.

• Reverse bias turn-off:Reverse bias turn-off:• Reverse anode-to-cathode voltage (the cathode is positive Reverse anode-to-cathode voltage (the cathode is positive

with respect to the anode) will sweep out the holes from the with respect to the anode) will sweep out the holes from the outer P-layer and electrons from the outer N-layer, resulting outer P-layer and electrons from the outer N-layer, resulting in a reverse recovery current. in a reverse recovery current.

• Thus, the SCR will continue to conduct the reverse anode Thus, the SCR will continue to conduct the reverse anode current but with a small positive voltage drop (0.7 V) current but with a small positive voltage drop (0.7 V) because of the presence of trapped charges in the outer because of the presence of trapped charges in the outer junctions J1 and J3. junctions J1 and J3.

• After a small period of time, the reverse anode current After a small period of time, the reverse anode current suddenly falls to a low value. suddenly falls to a low value.

• At this stage, the recovery of the device to its original state At this stage, the recovery of the device to its original state is not fully complete since a high concentration of carriers is not fully complete since a high concentration of carriers still exists around the middle junction. still exists around the middle junction.

• This concentration of excess carriers decreases by the This concentration of excess carriers decreases by the process of recombination. process of recombination.

• After the hole and electron concentration at the middle After the hole and electron concentration at the middle junction J2 has decreased to a low value, the reverse anode junction J2 has decreased to a low value, the reverse anode current suddenly drops to zero and the SCR regains its current suddenly drops to zero and the SCR regains its blocking states. blocking states.

• This method belongs to the category of forced This method belongs to the category of forced commutation.commutation.

• Different methods of Forced Commutation:Different methods of Forced Commutation:• In dc circuits, the forward current has to be forced to zero In dc circuits, the forward current has to be forced to zero

by an external circuit in order to turn off the SCR. This is by an external circuit in order to turn off the SCR. This is known as forced commutation.known as forced commutation. The dc input is required for The dc input is required for SCR controlled circuits used for SCR controlled circuits used for dc to dc converters dc to dc converters (choppers) (choppers) and for and for dc to ac converters (inverters). dc to ac converters (inverters). The The basic principle of forced commutation is to decrease the basic principle of forced commutation is to decrease the SCR current below the holding current IH of the device.SCR current below the holding current IH of the device.

• In series inverters, the commutating components (inductors In series inverters, the commutating components (inductors and capacitors) are connected in series (parallel) with (to) and capacitors) are connected in series (parallel) with (to) the load, thus forming an under damped circuit. the load, thus forming an under damped circuit.

• This method of turn-off is known as resonant turn-off (since This method of turn-off is known as resonant turn-off (since the forward current is made zero by a resonant circuit) and the forward current is made zero by a resonant circuit) and is also termed current commutation.is also termed current commutation.

(a) (a) Current Commutation:Current Commutation:• In this scheme an external pulse of current greater than the In this scheme an external pulse of current greater than the

load current is passed in reversed direction through the load current is passed in reversed direction through the conducting SCR. When the current pulse attains a value conducting SCR. When the current pulse attains a value equal to the load current, next pulse current through equal to the load current, next pulse current through thyristor becomes zero & the device is turned off.thyristor becomes zero & the device is turned off.

• There are many ways by which the forward current in the There are many ways by which the forward current in the SCR can be forced to zero:SCR can be forced to zero:

• Shunting the conducting SCR by a low resistance element & Shunting the conducting SCR by a low resistance element & thereby reducing the forward current to a level below IH in thereby reducing the forward current to a level below IH in order to turn-off SCR requires large off times since no order to turn-off SCR requires large off times since no reverse volt is applied across SCR. therefore, this method is reverse volt is applied across SCR. therefore, this method is not convenient when periodic switching off of the SCR is not convenient when periodic switching off of the SCR is required.required.

• For efficient forced commutation, reverse volt must appear For efficient forced commutation, reverse volt must appear across SCR. This reverse volt can be obtained from a across SCR. This reverse volt can be obtained from a charging circuit consisting of an inductor and a capacitor charging circuit consisting of an inductor and a capacitor which are called the commutating components. The volt which are called the commutating components. The volt across the capacitor is used for obtaining forced turn-off of across the capacitor is used for obtaining forced turn-off of SCR.SCR.

(b)(b) Volt Commutation:Volt Commutation:• In this scheme a conducting thyristor is commutated by the In this scheme a conducting thyristor is commutated by the

application of a pulse of large reverse volt which reduces application of a pulse of large reverse volt which reduces the anode current of conducting thyristor to zero rapidly. the anode current of conducting thyristor to zero rapidly.

• Then the presence of reverse volt across SCR aids in the Then the presence of reverse volt across SCR aids in the completion of its turn-off process. (In regaining forward completion of its turn-off process. (In regaining forward blocking capability of SCR).blocking capability of SCR).

• When the current is zero, the capacitor will get charged to a When the current is zero, the capacitor will get charged to a voltage higher than the supply voltage, and reverse voltage voltage higher than the supply voltage, and reverse voltage will appear across the SCR after it is turned off. will appear across the SCR after it is turned off.

• Therefore, when the SCR is fired (triggered), it Therefore, when the SCR is fired (triggered), it automatically gets turned off after approximately one-half automatically gets turned off after approximately one-half period of the resonant circuit. period of the resonant circuit.

• No other SCR need be fired to commutate the conducting No other SCR need be fired to commutate the conducting SCR as is done in parallel or bridge inverters. SCR as is done in parallel or bridge inverters.

• This method of turn-off is also referred to as self This method of turn-off is also referred to as self commutation.commutation.

• There are six distinct classes of forced commutation.There are six distinct classes of forced commutation.• Class A (series resonant turn-off)—self-commutated by Class A (series resonant turn-off)—self-commutated by

resonating the load.resonating the load.• Class B (parallel resonant turn-off)—self-commutated by an Class B (parallel resonant turn-off)—self-commutated by an

LC LC circuit.circuit.• Class C—Class C—C C or or LC LC switched by another load carrying SCR.switched by another load carrying SCR.• Class D—Class D—C C or or LC LC switched by an auxiliary SCR.switched by an auxiliary SCR.• Class E—an external pulse source is used for commutation.Class E—an external pulse source is used for commutation.• Class F—ac line commutation.Class F—ac line commutation.

• Class A Commutation (Series Resonant Class A Commutation (Series Resonant Commutation by an LCCommutation by an LC Circuit):Circuit):

• The commutating components The commutating components L L and C are connected to and C are connected to load load RL RL as shown in as shown in Fig. 1.3.4.1(a), Fig. 1.3.4.1(a), so that the overall so that the overall circuit becomes under damped with R2 < 4L/C.circuit becomes under damped with R2 < 4L/C. When an When an under damped circuit is excited by applying a fixed dc under damped circuit is excited by applying a fixed dc voltage, the waveform of the oscillatory current voltage, the waveform of the oscillatory current i i will be as will be as shown in shown in Fig. 1.3.4.1(b). Fig. 1.3.4.1(b). Application of the Kirchhoff's Application of the Kirchhoff's voltage law to the circuit as shown in voltage law to the circuit as shown in Fig. 1.3.4.1(a)Fig. 1.3.4.1(a) results in equationresults in equation

• V =V = Ri + LRi + L··di/dt + 1/C ∫ i dtdi/dt + 1/C ∫ i dt 1.3.4.11.3.4.1

• Differentiating Eq. (1.3.4.1),Differentiating Eq. (1.3.4.1),• R R · · di/dt + Ldi/dt + L··

d2i/dt2 + i/C = 0d2i/dt2 + i/C = 0• This is a second order differential This is a second order differential

equation. The response of the circuit equation. The response of the circuit will be oscillatory if R2 < 4L/C.will be oscillatory if R2 < 4L/C.

• When the SCR is turned on by a gate When the SCR is turned on by a gate pulse, an oscillatory current i flows pulse, an oscillatory current i flows in the circuit and charges up the in the circuit and charges up the capacitor C. capacitor C.

• Capacitor C is charged up to the Capacitor C is charged up to the supply voltage Vsupply voltage V as the oscillatory as the oscillatory current reaches its peak value. current reaches its peak value.

• As and when the capacitor charges As and when the capacitor charges up to the point wt = p/2,up to the point wt = p/2, the the induced voltage in inductor Linduced voltage in inductor L opposes the oscillatory current [see opposes the oscillatory current [see the polarity of the induced e.m.f in the polarity of the induced e.m.f in the inductor in the inductor in Fig. 1.3.4.1(a)Fig. 1.3.4.1(a)].].

• Beyond wt = p/2,Beyond wt = p/2, the oscillatory the oscillatory current decreases from the peak current decreases from the peak value, the induced voltage in the value, the induced voltage in the inductor changes its sign according inductor changes its sign according to Lenz's law (polarity of the induced to Lenz's law (polarity of the induced e.m.f is shown by encircled + and -). e.m.f is shown by encircled + and -).

• Again the capacitor charges from the supply voltage VAgain the capacitor charges from the supply voltage V to a to a higher voltage which is 2 V. When the capacitor is higher voltage which is 2 V. When the capacitor is completely charged up to 2 V,completely charged up to 2 V, a differentiated voltage of (2 a differentiated voltage of (2 V - V) = V across the capacitor reflects at the cathode of V - V) = V across the capacitor reflects at the cathode of the SCR and turns it off.the SCR and turns it off.

• Series inverters function on this principle. Series inverters function on this principle.

• Since the load current also flows through the commutating Since the load current also flows through the commutating components, obviously this method cannot be a practical components, obviously this method cannot be a practical proposition for inverters of large capacity. proposition for inverters of large capacity.

• However, this method may serve the purpose of high-However, this method may serve the purpose of high-frequency low-power inverters.frequency low-power inverters.

Class B Commutation (Parallel Resonant Class B Commutation (Parallel Resonant Commutation by an Commutation by an LC LC Circuit):Circuit):

• Figure 1.3.4.2(a)Figure 1.3.4.2(a) shows the shows the circuit for Class B commutation. circuit for Class B commutation. The commutating components LThe commutating components L and C are connected across the and C are connected across the SCR. Thus, they need not form a SCR. Thus, they need not form a resonant circuit with the load. resonant circuit with the load.

• Besides, the commutating Besides, the commutating components do not carry the load components do not carry the load current.current.

• Initially, the capacitor CInitially, the capacitor C remains remains charged to the dc voltage Vcharged to the dc voltage V with with its upper plate positive as shown in its upper plate positive as shown in Fig. 1.3.4.2(a).Fig. 1.3.4.2(a). When the SCR (T) When the SCR (T) is triggered, load current iR flows. is triggered, load current iR flows.

• Due to triggering on of T, the Due to triggering on of T, the stored charge on the capacitor stored charge on the capacitor drives an oscillatory current drives an oscillatory current through the loop formed by through the loop formed by capacitor C, SCR (T) and inductor capacitor C, SCR (T) and inductor L. L.

• The overall SCR current iTThe overall SCR current iT will be the algebraic will be the algebraic sum of load current iRsum of load current iR and oscillatory current and oscillatory current ic.ic. Beyond point 'a' as shown in Beyond point 'a' as shown in Fig. Fig. 1.3.4.2(b),1.3.4.2(b), the electromagnetic energy of L the electromagnetic energy of L is is converted to electrostatic energy and converted to electrostatic energy and capacitor C recharges in the reverse direction. capacitor C recharges in the reverse direction.

• Thus when oscillatory current icThus when oscillatory current ic reverses its reverses its direction (i.e. when capacitor C discharges direction (i.e. when capacitor C discharges from - Vfrom - V) ) and tends to exceed the load current and tends to exceed the load current iR,iR, the SCR current iTthe SCR current iT becomes zero. becomes zero.

• As a result, the SCR (T) gets turned off at point As a result, the SCR (T) gets turned off at point ‘b’ and the circuit looks like that shown in ‘b’ and the circuit looks like that shown in Fig. Fig. 1.3.4.2(c).1.3.4.2(c).

• Referring to the waveform [Fig. Referring to the waveform [Fig. 1.3.4.2(b)], it is seen that at the instant 1.3.4.2(b)], it is seen that at the instant of commutation (i.e. iT = 0), the of commutation (i.e. iT = 0), the remaining voltage of the capacitor (V’)remaining voltage of the capacitor (V’) is is additive to the dc supply voltage V;additive to the dc supply voltage V; the the load current iR flowing through the load current iR flowing through the branch C, L,branch C, L, and RLand RL continues to increase continues to increase corresponding to point 'd' as shown in corresponding to point 'd' as shown in the wave diagram (even after the wave diagram (even after commutation) but soon becomes zero commutation) but soon becomes zero (point 'e') when the capacitor C is (point 'e') when the capacitor C is charged to dc voltage Vcharged to dc voltage V with its upper with its upper plate positive. In the case of an inductive plate positive. In the case of an inductive load, a freewheeling diode is used across load, a freewheeling diode is used across the load.the load.

Class C Commutation (Complementary Class C Commutation (Complementary Commutation or Parallel Capacitor Turn-off):Commutation or Parallel Capacitor Turn-off):

• In this method, the commutating In this method, the commutating element C (or LC) is switched across element C (or LC) is switched across the conducting SCR by another load the conducting SCR by another load carrying SCR. carrying SCR.

• Triggering of one SCR commutates Triggering of one SCR commutates the other SCR. the other SCR.

• Both SCRs carry the load current.Both SCRs carry the load current.

• When T1 is turned on, the capacitor When T1 is turned on, the capacitor C is charged to the supply voltage VC is charged to the supply voltage V through R2,through R2, CC and T1 with polarity and T1 with polarity as shown in as shown in Fig. 1.3.4.3(a).Fig. 1.3.4.3(a).

• When T2 is triggered, the charged When T2 is triggered, the charged capacitor C gets connected across capacitor C gets connected across T1 and discharges through it. T1 and discharges through it.

• The discharging current of the The discharging current of the capacitor opposes the load current capacitor opposes the load current in T1, and the SCR T1 commutates. in T1, and the SCR T1 commutates.

• Now, SCR T2Now, SCR T2 conducts the load current. conducts the load current. The capacitor C will eventually be The capacitor C will eventually be charged in the opposite direction charged in the opposite direction through RL, C, and T2. When T2 is through RL, C, and T2. When T2 is triggered, a voltage (2V) which is twice triggered, a voltage (2V) which is twice the dc voltage is applied to the RL-C the dc voltage is applied to the RL-C series circuit. series circuit.

• If T1 conducts, the same capacitor If T1 conducts, the same capacitor reverse biases T2 and turns it off.reverse biases T2 and turns it off.

• Referring to Referring to Fig. 1.3.4.3 (b) Fig. 1.3.4.3 (b) it is seen it is seen that as long as T1 is conducting, the that as long as T1 is conducting, the current iR1 through RL-T1 will be a current iR1 through RL-T1 will be a steady dc quantity. steady dc quantity.

• But when T2 is triggered, the capacitor But when T2 is triggered, the capacitor voltage adds to the supply voltage V voltage adds to the supply voltage V and causes a decaying current through and causes a decaying current through RL, C, and T2. RL, C, and T2.

• Thus, though T1 gets OFF, the current Thus, though T1 gets OFF, the current suddenly jumps to a higher value and suddenly jumps to a higher value and then dies away with a time constant then dies away with a time constant RLC. The current in through T2 will be RLC. The current in through T2 will be the sum of this decaying dc component the sum of this decaying dc component and the steady dc component (through and the steady dc component (through R2-T2 branch). R2-T2 branch).

• Parallel inverters, McMurray-Bedford Parallel inverters, McMurray-Bedford inverters, etc. work on this method of inverters, etc. work on this method of commutation.commutation.

Class D commutation (Auxiliary Class D commutation (Auxiliary Commutation):Commutation):

• In the circuit shown in In the circuit shown in Fig. Fig. 1.3.4.4 (a), 1.3.4.4 (a), an auxiliary SCR an auxiliary SCR (T2) is turned on to commutate (T2) is turned on to commutate the main load carrying SCR (T1). the main load carrying SCR (T1).

• The SCR (T2) is turned on first to The SCR (T2) is turned on first to charge capacitor C to supply charge capacitor C to supply voltage V, or alternatively, a voltage V, or alternatively, a charging switch SW may be charging switch SW may be used for starting. used for starting.

• If triggering of T2 is made use of If triggering of T2 is made use of to charge capacitor C, it will be to charge capacitor C, it will be automatically turned off as the automatically turned off as the charging current falls below the charging current falls below the holding current level of T2. holding current level of T2.

• Besides, when SCR (T1) is Besides, when SCR (T1) is triggered, the charge on triggered, the charge on capacitor C reverse biases T2 capacitor C reverse biases T2 and turns it off.and turns it off.

• The triggering of T1 causes currents in The triggering of T1 causes currents in two paths. In one path there will be a two paths. In one path there will be a steady dc load current iR through the steady dc load current iR through the load RL. load RL.

• In the other path there will be sinusoidal In the other path there will be sinusoidal capacitor current iC through the loop C, capacitor current iC through the loop C, T1, L, and D. Because of the oscillatory T1, L, and D. Because of the oscillatory sinusoidal current iC, the charge on C is sinusoidal current iC, the charge on C is reversed and then held in this condition. reversed and then held in this condition.

• Since diode D prevents any reverse Since diode D prevents any reverse current through this loop, iC will be only a current through this loop, iC will be only a sinusoid of positive half-cycle. sinusoid of positive half-cycle.

• At any desired time, SCR (T2) may be At any desired time, SCR (T2) may be triggered which then places capacitor C triggered which then places capacitor C across T1, reverse biases T1 and turns it across T1, reverse biases T1 and turns it off.off.

• It should be noted that at the instant of It should be noted that at the instant of triggering T2, the capacitor voltage aids triggering T2, the capacitor voltage aids the supply voltage V and the combination the supply voltage V and the combination drives a much higher current through the drives a much higher current through the branch C, T2, and RL. branch C, T2, and RL.

• Therefore, iR will suddenly jump to a Therefore, iR will suddenly jump to a higher value [point 'a' in higher value [point 'a' in Fig. 1.3.4.4 Fig. 1.3.4.4 (b)(b)] from the steady dc level but then ] from the steady dc level but then ultimately decays to zero with a time ultimately decays to zero with a time constant RLC. constant RLC.

• The corresponding capacitor The corresponding capacitor current also decays, but its current also decays, but its direction is obviously opposite to direction is obviously opposite to that of the positive half-cycle that of the positive half-cycle sinusoid (during oscillatory sinusoid (during oscillatory current through C, T1, L, and D). current through C, T1, L, and D).

• The current iT1 will be the sum The current iT1 will be the sum of the sinusoidal parts of the iC of the sinusoidal parts of the iC and the steady dc part of iR. and the steady dc part of iR. Jone's chopper is an example of Jone's chopper is an example of this class.this class.

• Class E Commutation (External Class E Commutation (External Pulse Commutation):Pulse Commutation):

• This type of commutation This type of commutation depends on the commutation depends on the commutation energy being supplied from an energy being supplied from an external source. The peak external source. The peak amplitude of the current pulse amplitude of the current pulse must be greater than that of the must be greater than that of the load current through the SCR, load current through the SCR, and the duration of the reverse and the duration of the reverse voltage applied after turn-off of voltage applied after turn-off of the SCR must be longer than the the SCR must be longer than the SCR turn-off time. The SCR T is SCR turn-off time. The SCR T is turned off by means of an turned off by means of an auxiliary transistor switch Q as auxiliary transistor switch Q as shown in shown in Fig. 1.3.4.5 (a).Fig. 1.3.4.5 (a).

• The SCR is assumed to be initially ON when turn-off is The SCR is assumed to be initially ON when turn-off is desired. A signal applied to the base of Q turns it on and desired. A signal applied to the base of Q turns it on and reverse biases the SCR. The SCR is now turned off.reverse biases the SCR. The SCR is now turned off.

• The drive signal to the base of Q must be of sufficient The drive signal to the base of Q must be of sufficient duration to ensure SCR’s turn-off and of sufficient amplitude duration to ensure SCR’s turn-off and of sufficient amplitude to place Q in saturation. to place Q in saturation.

• If Q comes out of saturation before the SCR turn-off is If Q comes out of saturation before the SCR turn-off is complete, it results in communication failure. complete, it results in communication failure.

• The waveforms are shown in The waveforms are shown in Fig. 1.3.4.5 (b).Fig. 1.3.4.5 (b).

• Class F Commutation (AC Line Commutation):Class F Commutation (AC Line Commutation):

• AC line commutation is also recognized as a method of AC line commutation is also recognized as a method of forced commutation, as commutation is forced by the forced commutation, as commutation is forced by the opposite half-cycle of the ac supply. Actually, the negative opposite half-cycle of the ac supply. Actually, the negative ac voltage of the line reverse biases the SCR and turns it off. ac voltage of the line reverse biases the SCR and turns it off.

• Since external commutating elements are not required to Since external commutating elements are not required to commutate the conducting SCR, this type of commutation is commutate the conducting SCR, this type of commutation is not generally referred to as forced commutation in most not generally referred to as forced commutation in most literature. This method is commonly known as AC line literature. This method is commonly known as AC line commutation or natural commutation. It is extensively used commutation or natural commutation. It is extensively used in line-commuted rectifiers, inverters, and cycloconverter.in line-commuted rectifiers, inverters, and cycloconverter.

• Fig. 1.3.4.6 (a) Fig. 1.3.4.6 (a) and and (b) (b) show the circuit diagram and show the circuit diagram and waveforms. waveforms.

1.41.4 Turn on Methods:Turn on Methods:

• An SCR can be triggered by using any of the following An SCR can be triggered by using any of the following ways:ways:

• Radiation Triggering or Light Turn-on:Radiation Triggering or Light Turn-on:• A beam of light bombarded on the gate-cathode junction J3 A beam of light bombarded on the gate-cathode junction J3

can produce sufficient energy to break electron-hole pairs can produce sufficient energy to break electron-hole pairs in the semiconductor, resulting in an increase in blocking in the semiconductor, resulting in an increase in blocking current and the SCR thus gets fired. Such SCRs are often current and the SCR thus gets fired. Such SCRs are often referred to as LASCRs. The LASCRs are widely employed in referred to as LASCRs. The LASCRs are widely employed in HVDC transmission lines.HVDC transmission lines.

• Voltage Triggering or Breakover Voltage Turn-on:Voltage Triggering or Breakover Voltage Turn-on:• An increase in the anode-cathode forward voltage increases An increase in the anode-cathode forward voltage increases

the width of the depletion layer at the junction J2 and also the width of the depletion layer at the junction J2 and also increases the external accelerating voltage for minority increases the external accelerating voltage for minority carriers across the same junction. These carriers collide carriers across the same junction. These carriers collide with the fixed atoms and dislodge more minority carriers with the fixed atoms and dislodge more minority carriers until there is an avalanche breakdown of the junction. This until there is an avalanche breakdown of the junction. This makes the junction J2 forward biased. The anode current in makes the junction J2 forward biased. The anode current in this case would be limited only by the external load this case would be limited only by the external load impedance. At this forward breakover voltage + VBO, the impedance. At this forward breakover voltage + VBO, the SCR changes its characteristic from a high voltage across it SCR changes its characteristic from a high voltage across it (with a low leakage current) to a low voltage across it (with (with a low leakage current) to a low voltage across it (with a large forward current).a large forward current).

dv/dt Turn-on:dv/dt Turn-on:

• A rapid rate of increase in forward anode-cathode voltage can A rapid rate of increase in forward anode-cathode voltage can produce a transient gate current, which is caused by anode-gate produce a transient gate current, which is caused by anode-gate and gate-cathode capacitances. and gate-cathode capacitances.

• This can then turn on the SCR. Practically; the dv/dt for switching This can then turn on the SCR. Practically; the dv/dt for switching is increased by using a low external gate-cathode resistor.is increased by using a low external gate-cathode resistor.

• Gate Turn-on or Gate Triggering:Gate Turn-on or Gate Triggering:

• Additional minority carriers can be injected into the gate Additional minority carriers can be injected into the gate region through the gate electrode to switch on the SCR. If region through the gate electrode to switch on the SCR. If the gate current is large enough, the SCR will switch on as the gate current is large enough, the SCR will switch on as soon as the anode becomes positive with respect to the soon as the anode becomes positive with respect to the cathode. This method of turning on is most commonly used.cathode. This method of turning on is most commonly used.

• The best shape of a gate signal is one with a sharp leading The best shape of a gate signal is one with a sharp leading edge, and the shorter the signal duration the greater must edge, and the shorter the signal duration the greater must be its magnitude for a reliable turn-on. Once the SCR is ON, be its magnitude for a reliable turn-on. Once the SCR is ON, the gate current is no longer required to flow for the device the gate current is no longer required to flow for the device to remain conducting, so a gate pulse is enough. The to remain conducting, so a gate pulse is enough. The following points must be noted while designing the gate following points must be noted while designing the gate control circuit.control circuit.

• Appropriate value of the gate-cathode voltage must be applied Appropriate value of the gate-cathode voltage must be applied for turn-on when the SCR is forward biased.for turn-on when the SCR is forward biased.

• The gate signal must be removed after the device is turned on.The gate signal must be removed after the device is turned on.• No gate signal should be applied when the device is reverse No gate signal should be applied when the device is reverse

biased.biased.• There are three ways of triggering the SCR by gate control.There are three ways of triggering the SCR by gate control.

• Triggering by a dc gate signal:Triggering by a dc gate signal:• A dc voltage of proper polarity and magnitude is applied A dc voltage of proper polarity and magnitude is applied

between the gate and the cathode when the SCR is to be between the gate and the cathode when the SCR is to be turned on. The SCR is a current-operated device and it is that turned on. The SCR is a current-operated device and it is that value of the gate current which can turn the SCR on. The value of the gate current which can turn the SCR on. The drawbacks of this method are as follows:drawbacks of this method are as follows:

• The internal power loss is more as the gate drive is continuous.The internal power loss is more as the gate drive is continuous.• There is no isolation of the gate control circuit from the main There is no isolation of the gate control circuit from the main

power circuit.power circuit.• Triggering by an ac gate signal:Triggering by an ac gate signal:

• The gate-to-cathode voltage is obtained from a phase-shifted The gate-to-cathode voltage is obtained from a phase-shifted ac voltage derived from the main supply. The firing angle ac voltage derived from the main supply. The firing angle control is obtained very conveniently by changing the phase control is obtained very conveniently by changing the phase angle of the control signal. However, the gate drive is angle of the control signal. However, the gate drive is maintained for one half-cycle after the device is turned on, and maintained for one half-cycle after the device is turned on, and a reverse voltage is applied between the gate and the cathode a reverse voltage is applied between the gate and the cathode during the negative half-cycle. The advantage of this scheme during the negative half-cycle. The advantage of this scheme is that proper isolation of power and control circuits can be is that proper isolation of power and control circuits can be provided.provided.

Triggering by a pulsed-gate signal:Triggering by a pulsed-gate signal:• The gate drive consists of a single pulse appearing The gate drive consists of a single pulse appearing

periodically, or a sequence of high frequency pulses. periodically, or a sequence of high frequency pulses. • This is known as carrier frequency gating. This is known as carrier frequency gating. • A pulse transformer is used for isolation. A pulse transformer is used for isolation. • The advantage of this scheme is that the gate loss is very The advantage of this scheme is that the gate loss is very

low since the drive is discontinuous. low since the drive is discontinuous. • For power control in ac circuits, the instant of firing the SCR For power control in ac circuits, the instant of firing the SCR

is controlled by applying at the proper time a train of high-is controlled by applying at the proper time a train of high-frequency pulses generated by a logic circuit.frequency pulses generated by a logic circuit.

1.51.5 Firing of SCR using UJT:Firing of SCR using UJT:• A firing of SCR using UJT is shown in A firing of SCR using UJT is shown in Fig. Fig.

1.5(a). 1.5(a). There is a circuit using UJT to There is a circuit using UJT to obtain the pulse control of SCR is shown in obtain the pulse control of SCR is shown in Fig. (a).Fig. (a).

• R1 is the variable resistor through which R1 is the variable resistor through which the capacitor C charges towards the supply the capacitor C charges towards the supply voltage. voltage.

• When the voltage across the capacitor is When the voltage across the capacitor is less than the voltage necessary to fire the less than the voltage necessary to fire the UJT, the UJT offers very high resistance. UJT, the UJT offers very high resistance.

• The capacitor continues to charge. The capacitor continues to charge. • When the voltage across the capacitor When the voltage across the capacitor

reaches the value equal to ηVBB, the UJT reaches the value equal to ηVBB, the UJT offers very low resistance and the capacitor offers very low resistance and the capacitor discharges through UJT. discharges through UJT.

• Part of the current flows through the gate Part of the current flows through the gate cathode circuit of the SCR. cathode circuit of the SCR.

• Hence the SCR triggers. Hence the SCR triggers. • Load current flows. Load current flows. • When the capacitor discharges, voltage When the capacitor discharges, voltage

across it reduces & the UJT now offers high across it reduces & the UJT now offers high resistance and the capacitor starts resistance and the capacitor starts charging again.charging again.

• When resistance R1 is increased, the When resistance R1 is increased, the charging rate is decrease, the charging rate is decrease, the capacitor takes more time to the UJT capacitor takes more time to the UJT triggering voltage, and the firing angle triggering voltage, and the firing angle is increased, load current reduces. is increased, load current reduces.

• The waveforms are shown in The waveforms are shown in Fig. (b).Fig. (b).

• Since there is no synchronism Since there is no synchronism between the d.c. supply of the UJT and between the d.c. supply of the UJT and that of a.c. supply, the firing angle in that of a.c. supply, the firing angle in each half cycle does not remain same each half cycle does not remain same so this is not desirable.so this is not desirable.

1.61.6 Series & Parallel operation of Series & Parallel operation of SCRs:SCRs:

• Why there is necessary to operates SCRs in Series & Parallel?Why there is necessary to operates SCRs in Series & Parallel?• The SCRs are required to be connected in series when a The SCRs are required to be connected in series when a

single SCR of the voltage rating consistent with the circuit is single SCR of the voltage rating consistent with the circuit is not available, or when in some high reliability circuits, the not available, or when in some high reliability circuits, the failure of an SCR is not expected to disturb the circuit. failure of an SCR is not expected to disturb the circuit.

• A parallel connection becomes necessary when a single SCR A parallel connection becomes necessary when a single SCR is not capable of supplying the required current. is not capable of supplying the required current.

• SCRs are now available with voltage ratings up to 10 kV and SCRs are now available with voltage ratings up to 10 kV and current ratings up to 1200 A. current ratings up to 1200 A.

• In many power control applications, the required voltage and In many power control applications, the required voltage and current ratings are lower than these maximum values. current ratings are lower than these maximum values.

• Therefore, even though it may be possible to obtain a single Therefore, even though it may be possible to obtain a single SCR of proper voltage and current ratings, on many SCR of proper voltage and current ratings, on many occasions the designer is forced to use lower-rated SCRs for occasions the designer is forced to use lower-rated SCRs for reasons of economy and availability. reasons of economy and availability.

• In such a situation, the lower-rated SCRs have to be In such a situation, the lower-rated SCRs have to be connected in series and parallel combinations to suit the connected in series and parallel combinations to suit the voltage and current requirements of the circuit for a voltage and current requirements of the circuit for a particular application. particular application.

• Series and parallel combinations are also often used when it Series and parallel combinations are also often used when it is required to control power in low-voltage high-current is required to control power in low-voltage high-current circuits or high-voltage low-current circuits because an SCR circuits or high-voltage low-current circuits because an SCR of suitable voltage and current ratings may not be available.of suitable voltage and current ratings may not be available.

Series Connected SCRs:Series Connected SCRs:• Semiconductor devices may be connected in series to Semiconductor devices may be connected in series to

enhance their peak inverse voltage beyond what is enhance their peak inverse voltage beyond what is available in a single device. available in a single device.

• Like any other electronic device, characteristic/ properties Like any other electronic device, characteristic/ properties of two SCRs of the same make and ratings are never the of two SCRs of the same make and ratings are never the same. same.

• For either positive or negative voltage applied to the SCRs, For either positive or negative voltage applied to the SCRs, the leakage current at a given temperature and at that the leakage current at a given temperature and at that voltage is somewhat different. voltage is somewhat different.

• Naturally, with a voltage applied to a series combination of Naturally, with a voltage applied to a series combination of the SCRs either to the reverse or to the OFF-state direction, the SCRs either to the reverse or to the OFF-state direction, unequal distribution of voltage occurs across the SCRs unequal distribution of voltage occurs across the SCRs owing to unequal leakage current. owing to unequal leakage current.

• The SCR with minimum leakage current has the maximum The SCR with minimum leakage current has the maximum voltage across it. voltage across it.

• Although the voltage-current characteristics of these Although the voltage-current characteristics of these devices are nonlinear, they would, at any given condition, devices are nonlinear, they would, at any given condition, divide the total applied voltage in inverse proportion of divide the total applied voltage in inverse proportion of their leakage currents. their leakage currents.

• Therefore, some sort of forced equalization method must be Therefore, some sort of forced equalization method must be applied to distribute the voltage equally across each SCR.applied to distribute the voltage equally across each SCR.

Voltage equalization:Voltage equalization:• If SCRs are connected in series as shown in If SCRs are connected in series as shown in Fig. 1.6.1 (a),Fig. 1.6.1 (a),

voltage across T1 will be higher than that across T2 as the voltage across T1 will be higher than that across T2 as the leakage current of T1 for the same voltage is smaller than leakage current of T1 for the same voltage is smaller than that of T2. that of T2.

• The V-I characteristics of these two SCRs are shown in The V-I characteristics of these two SCRs are shown in Fig.1.6.1 (b).Fig.1.6.1 (b).

• The first method of equalization is to vary the value of the The first method of equalization is to vary the value of the parallel resistances in accordance with the value of the parallel resistances in accordance with the value of the leakage currents of SCRs, and thus there will be unequal leakage currents of SCRs, and thus there will be unequal parallel resistances down the string. parallel resistances down the string.

• This method is rather cumbersome and generally not This method is rather cumbersome and generally not followed in practice.followed in practice.

• The second method is to allow different but fixed voltages to The second method is to allow different but fixed voltages to appear across different SCRs, within permissible limits, but appear across different SCRs, within permissible limits, but with the same value of the parallel resistances. Supposewith the same value of the parallel resistances. Suppose

• V1 + V2+ ....+ Vn = VT = total applied voltageV1 + V2+ ....+ Vn = VT = total applied voltage• andand• dV = V1 – V2 = maximum allowable difference in voltagedV = V1 – V2 = maximum allowable difference in voltage• wherewhere• V1 is the maximum voltage allowed to be daveloped across V1 is the maximum voltage allowed to be daveloped across

T1T1• V2 is the minimum voltage allowed to be developed across T2V2 is the minimum voltage allowed to be developed across T2• Naturally, the leakage current I1 through T1 is minimum, and Naturally, the leakage current I1 through T1 is minimum, and

the leakage current I2 through T2 is maximum. the leakage current I2 through T2 is maximum. • Therefore, the maximum difference in leakage current at the Therefore, the maximum difference in leakage current at the

maximum allowable operating junction temperature is DI = maximum allowable operating junction temperature is DI = I2- I1. I2- I1.

• Since the sum of the SCR leakage current and the current Since the sum of the SCR leakage current and the current through the resistance is constant throughout the string, through the resistance is constant throughout the string, therefore,therefore,

• V1/R + I1 = V2/R + I2 = Vn/R + InV1/R + I1 = V2/R + I2 = Vn/R + In• Hence, the equalizing resistance is given byHence, the equalizing resistance is given by• R R == V1 –V2/I1 – I2V1 –V2/I1 – I2• == dV/DidV/Di• The power dissipated in the equalizing resistor R is minimum The power dissipated in the equalizing resistor R is minimum

when the resistance is large. when the resistance is large.

• The transient sharing of the voltage is The transient sharing of the voltage is accomplished by connecting a capacitor accomplished by connecting a capacitor across each SCR. across each SCR.

• The resistor RT in series with the capacitor C The resistor RT in series with the capacitor C is used to prevent a flow of large discharge is used to prevent a flow of large discharge current through the SCR during turn-on. current through the SCR during turn-on.

• The voltage sharing can also be achieved by The voltage sharing can also be achieved by connecting metal-oxide varistors across connecting metal-oxide varistors across each SCR in a string.each SCR in a string.

• Parallel Connections of SCRs:Parallel Connections of SCRs:• If it is required to deliver a load current that If it is required to deliver a load current that

is much larger than that which can be is much larger than that which can be controlled by the largest available SCR used controlled by the largest available SCR used singly, then a parallel connection of more singly, then a parallel connection of more than one SCR becomes necessary as shown than one SCR becomes necessary as shown in in Fig. 1.6.2. Fig. 1.6.2.

• Because of unequal dynamic resistances of Because of unequal dynamic resistances of the SCRs, sharing of current among them the SCRs, sharing of current among them will not be equal. will not be equal.

• If one of the SCRs having lower dynamic If one of the SCRs having lower dynamic impedance in a parallel unit carries more impedance in a parallel unit carries more current than the other SCRs, its internal current than the other SCRs, its internal power dissipation will be more, thereby power dissipation will be more, thereby raising the junction temperature and raising the junction temperature and decreasing the dynamic resistance of the decreasing the dynamic resistance of the SCR. SCR.

• This, in turn, will further increase the current This, in turn, will further increase the current shared by this SCR and the process shared by this SCR and the process becomes cumulative, resulting in burn-out becomes cumulative, resulting in burn-out of the SCR. of the SCR.

• This is followed by the burning out of the other SCRs too, This is followed by the burning out of the other SCRs too, one by one.one by one.

• The non-uniformity of the forward characteristic of the SCRs The non-uniformity of the forward characteristic of the SCRs is the major factor responsible for current imbalance. is the major factor responsible for current imbalance.

• Problems which directly or indirectly affect the circuit Problems which directly or indirectly affect the circuit operation are the differences in turn-on time, delay time, operation are the differences in turn-on time, delay time, finger voltage and the loop inductance. finger voltage and the loop inductance.

• The designing of parallel connections in SCRs demands not The designing of parallel connections in SCRs demands not only the knowledge of the turn-on characteristic, delay only the knowledge of the turn-on characteristic, delay time, finger voltage, and loop inductance, but also the time, finger voltage, and loop inductance, but also the knowledge of latching current, holding current, on-state knowledge of latching current, holding current, on-state voltage drop, and thermal resistance.voltage drop, and thermal resistance.

• Fig. 1.6.3 Fig. 1.6.3 represents the forward represents the forward characteristics of SCRs showing characteristics of SCRs showing current division between them. current division between them.

• When two or more SCRs are When two or more SCRs are connected to an ac source and fired connected to an ac source and fired at an angle a quite close to zero, all at an angle a quite close to zero, all the SCRs would not turn on the SCRs would not turn on simultaneously owing to a difference simultaneously owing to a difference in their finger voltage. in their finger voltage.

• As a result, there would be a severe As a result, there would be a severe imbalance of current for a brief period imbalance of current for a brief period of time.of time.

CAUSES OF DAMAGE TO SCRs:CAUSES OF DAMAGE TO SCRs:• The SCRs can get damaged on account of voltage and The SCRs can get damaged on account of voltage and

current ratings being exceeded. current ratings being exceeded. • Under transient conditions, the derivatives of voltage Under transient conditions, the derivatives of voltage

(dv/dt) and current (di/dt) can also damage the SCR. (dv/dt) and current (di/dt) can also damage the SCR. • The behaviour of an SCR is, therefore, specified in terms of The behaviour of an SCR is, therefore, specified in terms of

average, repetitive and non-repetitive voltage and current average, repetitive and non-repetitive voltage and current ratings for anode and gate circuits.ratings for anode and gate circuits.

• The SCRs are robust devices within their rated capabilities. The SCRs are robust devices within their rated capabilities. However, they get easily damaged by excessive voltages. However, they get easily damaged by excessive voltages.

• An SCR will be switched to its ON state if the forward An SCR will be switched to its ON state if the forward voltage exceeds the forward breakover voltage VBO or if voltage exceeds the forward breakover voltage VBO or if the forward voltage is supplied at a rate that exceeds the the forward voltage is supplied at a rate that exceeds the dv/dt (max) for the device. dv/dt (max) for the device.

• Most of the SCRs get damaged by the application of the Most of the SCRs get damaged by the application of the reverse voltage in excess of the specified reverse voltage.reverse voltage in excess of the specified reverse voltage.

• Such destruction of SCRs results in tracking across the Such destruction of SCRs results in tracking across the surface or melting of silicon near the junction because of surface or melting of silicon near the junction because of excessive leakage currents. excessive leakage currents.

• Repeated low energy transients cause local heating of Repeated low energy transients cause local heating of silicon, resulting in increased reverse leakage currents. silicon, resulting in increased reverse leakage currents.

• Excessive reverse power dissipation destroys the device. Excessive reverse power dissipation destroys the device.

• Over voltage in the forward direction causes the breakdown of the Over voltage in the forward direction causes the breakdown of the SCR into the conduction stage.SCR into the conduction stage.

• Semiconductor devices cannot withstand an over voltage greater Semiconductor devices cannot withstand an over voltage greater than the rated non-repetitive peak reverse voltage even for a very than the rated non-repetitive peak reverse voltage even for a very small period of time. small period of time.

• On the ac input side, voltage surges can be caused by remote On the ac input side, voltage surges can be caused by remote switching operations on other parts of the main supply, by the switching operations on other parts of the main supply, by the switching off of loads on the same distribution line, or by local switching off of loads on the same distribution line, or by local switching inside the area.switching inside the area.

• Transient voltages are also produced by the make and break of the Transient voltages are also produced by the make and break of the ac input contactors, and by the blowing out of fuses. ac input contactors, and by the blowing out of fuses.

• Sometimes, severe surges may occur on the overhead transmission Sometimes, severe surges may occur on the overhead transmission lines because of lightning. lines because of lightning.

• The surge because of lightning may occur either on the input side, The surge because of lightning may occur either on the input side, or on the dc output side of a rectifier used for distribution purposes.or on the dc output side of a rectifier used for distribution purposes.

• On the dc side of the equipment, severe surges can also occur On the dc side of the equipment, severe surges can also occur owing to the blowing out of fuses or tripping off of switches or circuit owing to the blowing out of fuses or tripping off of switches or circuit breakers.breakers.

• The rate of rise of the surge voltage may be as high as 1000 V/ms, The rate of rise of the surge voltage may be as high as 1000 V/ms, owing to faulty operation of contactors. owing to faulty operation of contactors.

• Generally, the transient voltage appears as spikes on the supply Generally, the transient voltage appears as spikes on the supply voltage waveform. voltage waveform.

• Surge voltage is also produced by the turn-on or turn-off of the SCRs Surge voltage is also produced by the turn-on or turn-off of the SCRs with a highly inductive load. with a highly inductive load.

• Again, because of very rapid drop of reverse current (hole storage Again, because of very rapid drop of reverse current (hole storage current), harmful voltage surges are induced across the inductances current), harmful voltage surges are induced across the inductances in the circuit.in the circuit.

1.71.7 Importance of Snubber circuits:Importance of Snubber circuits:• Preventing Damage to SCRs:Preventing Damage to SCRs:

• The SCRs can be protected by the following methods:The SCRs can be protected by the following methods:• Snubber circuit or dv/dt protection:Snubber circuit or dv/dt protection:• Because of relatively sudden interruption of reverse anode current Because of relatively sudden interruption of reverse anode current

towards the end of the commutation, it causes a large e.m.f. to be towards the end of the commutation, it causes a large e.m.f. to be induced in the anode circuit inductance. induced in the anode circuit inductance.

• This high transient voltage (called commutation transient) may cause This high transient voltage (called commutation transient) may cause damage to the SCR. damage to the SCR.

• To avoid such damage, a Snubber circuit (consisting of a resistance R To avoid such damage, a Snubber circuit (consisting of a resistance R in series with a capacitance C) is connected in parallel with the SCR in series with a capacitance C) is connected in parallel with the SCR such that the energy trapped in the anode circuit inductance during such that the energy trapped in the anode circuit inductance during commutation transient finds a bypass path through this R-C circuit as commutation transient finds a bypass path through this R-C circuit as shown in shown in Fig. 1.7 (a)Fig. 1.7 (a) and thus no damage to the SCR takes place. and thus no damage to the SCR takes place.

• The R-C network controls the rate of change of voltage across the SCR The R-C network controls the rate of change of voltage across the SCR during its blocking state. Snub means to repress here the large e.m.f.during its blocking state. Snub means to repress here the large e.m.f.

• Resistance R helps to prevent damage caused by minority Resistance R helps to prevent damage caused by minority carrier storage during commutation oscillations. carrier storage during commutation oscillations.

• Capacitor C provides a path for reverse current when the Capacitor C provides a path for reverse current when the SCR suddenly blocks at the end of the minority carrier SCR suddenly blocks at the end of the minority carrier storage time, but charges with opposite polarity during storage time, but charges with opposite polarity during forward blocking and then discharges rapidly through the forward blocking and then discharges rapidly through the SCR at turn-on. SCR at turn-on.

• The resistance R limits the initial forward current when the The resistance R limits the initial forward current when the SCR turns on and damps out oscillations caused by carrier SCR turns on and damps out oscillations caused by carrier storage effects when the SCR is reverse blocking.storage effects when the SCR is reverse blocking.

• To limit di/dt, when circuit conditions are such that there is To limit di/dt, when circuit conditions are such that there is a danger of exceeding the specification, an inductance L a danger of exceeding the specification, an inductance L may be added to the snubber circuit as shown in may be added to the snubber circuit as shown in Fig. 1.7.Fig. 1.7.(b). (b).

• This is usually a coil of a few turns, capable of carrying the This is usually a coil of a few turns, capable of carrying the full SCR current. In some designs, this coil is wound on a full SCR current. In some designs, this coil is wound on a small ferrite ring. This reduces the size of the coil for the small ferrite ring. This reduces the size of the coil for the same inductance value. same inductance value.