Power Electronics

POWER SEMICONDUCTOR DEVICESTHYRISTORS

A Thyristors is one of the most important types of power semiconductor devices.They are used extensively in power electronics circuits. The word thyristor’ is originated from Greek word. The word “thy” means switch. The total word thyristor indicates that theswitch belongs to a transistor family. The Development of thyristor has revolutionized the use of semiconductors for power control. The thyristor was first invented by Bell Laboratory in U.S.A. in the year 1948 and it (100V,10A) was first introduced in the market by General Electric Company in the year 1957. However, commercially it started becoming available after 1960. Since this time there has been a rapid growth in the control equipment making use of thyristors.

Initially however for a period of 5-6 years the failure rate of the equipments hampered the growth of thyristorised equipment in industry. This however did not cause any break in the building up of know-how and technical literature has been piling up since, at a rate of about 200 reported articles per year on various aspects of this technology.It will not be wrong to say that the present state of thyristor technology has been a result of about ten years efforts in all directions including development in semiconductor and printed circuits technology. Today single thyristor is Capable of handling maximum of 25MW [5000V, 5000A].

TERMINAL CHARACTERISTICS OF THYRISTORSThe thyristor has four layers PNPN semiconductor switching device. It has three terminals: Anode, Cathode and gateWhen the anode voltage is made positive with respect to the cathode. The junction ‘J1’ and ‘J3’ are forward biased while junction ‘J2’is reverse biased. A very small leakage current will flow through the device. The thyristor is then said to be in the forward blocking state or off state condition. If the voltage at anode is continuously increased, the breakdown of reverse bias junction ‘J2’ occurs due to high voltage gradient and the device is switched on. The voltage drop would be due to ohmic drop in the four layers and it is small, typically 1v. In the on-state, the anode current is limited by an external impedance or a resistance,

,As shown in fig.1(a). The anode current must be more than ‘Latching Current’for on-state condition.If the anode current does not fall below the ‘Holding Current’the thyristor remains on.The latching current is usually double the holding current but both are low, even much less than 1 of the full load rated value. ‘Latching Current’is the minimum anode current required to maintain the thyristor in the on-state immediately after a thyristor has been turned on and gate signal has been removed

Figure-1 Thyristor Symbol and Three P-N Junctions.

Two Transistor Model of Thyristor.The regenrative or latching action due to positive feedback can be demonstrated by using a two transistor model of thyristor. A thyristor can be considered as two complementry transistors, one PNP transistor ‘Q1’ and other NPN-transistor ‘Q2’ as shown in fig. If a positive potential is applied to the gate electrode G when the anode A is positive with respect to cathode K, then transistor ‘Q2’ is turned on and starts to conduct. Since the collector current of ‘Q2’ is the base current of ‘Q1’, Q1 also starts to conduct. The collector current of ‘Q1’ is also the base current of ‘Q2’ Hence as long as the anode is positive, each transistor maintains the other in conducting state by regenrative action.The regenrative action of the thyristor operation can be explained mathematically

Figure-2 Two Transistor Model of Thyristor

When Q1 and Q2 have a very small forward bias the value 1 and 1 + 2 is small and hence IA is small. The sum of 1 + 2 can be made to approach momentarily by injecting a short duration positive current at the gate and thyristor will be on due to execessive anode current IA , which is clear from the above mathematical expression.The PN junction of the thyristor is capacitive because of the deplection layer during blocking. Whenever there is rapid rate of change the forward anode to cathode voltage (dv /dt), the charging current i = c dv / dt may, attain to sufficient, magnitude with the leakage current that there may be possibility of turning on of the thyristor. The dv / dt turn on of the thyristor is avoided due to the following reasons.Turning on by providing signal from gate takes negligible power to turn on the thyristor. The spreading of charged carriers over the entire area of thyristors take some time and as a result , localised over heating with in the crystal may reduce the life of the thyristor, of course,

there is no possibility of destruction of thyristor by the principle of voltage gradient firing of thyristor. It is known that in case of a transistor, Ic = IE +ILeakage Ic = IE +ICBO where ICBO is the leakage current of colector- base current and Ic / IE , For Q1 IE = IA and collector current is Ic1

Ic1 = 1 IA +ICBO1, Similarly for transistor ‘Q2’Ic2 = 2 IK +ICBO2 IA = IC1+IC2 1 IA +ICBO1 + 2 IK +ICBO2 where IK = IA +IG IA = 1 IA +ICBO1 + 2 (IA +IG ) +ICBO2 1 IA +ICBO1 + 2 IA +2IG +ICBO2 1 IA +2 IA + ICBO1 +2IG +ICBO2 IA (1 +2 ) + ICBO1 +2IG +ICBO2 IA -IA (1 +2 ) = ICBO1 +2IG +ICBO2 IA[1 - (1 +2 ) ] = ICBO1 +2IG +ICBO2 IA = ICBO1 +2IG +ICBO2 / 1 - (1 +2 )

THYISTOR TURN-ON.A thyristor is turned on by increasing the anode current. This can be accomplished in one of the following ways.1. Temperature Triggering.If the temperature of a thyristor is high, there will be an increase in the number of electron hole pairs, which would increase the leakage current. This increase in currents would cause‘1’ and ‘2’ to increase. Due to regenrative action,(1 +2 ) may tend to be unity and the thyristor may be turned on. This type of turn-on may cause thermal runaway and is normally avoided. 2. Light Triggering.When light is thrown on the gate-cathode junction through a light window, the electron-hole pairs will increase ( free charge carriers electrons and holes are generated ). If the intensity of this light exceeds a certain value, the thyristor is turned on.Such a thyristor is known as light activated SCR (LASCR).

3. Forward Voltage Triggering.When the forward anode to cathode is greater than the forward breakdown voltage VBO.Sufficient leakage current will flow to initiate regenrative turn-on. This type of turn-on may be destructive and should be avoided.4. dv / dt Triggering.With forward voltage across the anode and cathode of a thyristor, the two junctions are forward biased but the inner junction J2 is reverse biased.This junction has the characteristics of a capacitive due to charges existing across the junction.If the entire anode to cathode forward voltage Va appears across junction J2 and the charge is denoted by ‘q’ than a charging current ‘I’ given by equation

Gate Triggering. The gate triggering is the most common method of turning on the SCRs, because this method lends itself accurately for turning on the SCRs at the desired instant of time

I = (dq / d t), d (Cj , Va )/ d t Cj dVa / d t + Va dCj / d tAs Cj, the capacitive of junction ‘J2’ is almost constant, the current is given by i = Cj dVa / d tIf the rate of rise of forward voltage ‘dVa / d t’ is high, the charging current plays the role of gate current and turns on the thyristor even when gate signal is zero. Such phenomena of turning on a thyristor, called ‘dVa / d t’ turn-on, must be avoided as it leads to false operation of the thyristor circuit. For controllable operation of the thyristor, the rate of rise of forward anode to cathode voltage ‘dVa / d t’ must be kept below the specified rated limit. Typically ‘dV / d t’ are 20-500v / sec. False turn-on of a thyristor can be prevented by using a snubber circuit in parallel with the device.

GATE CURRENT.If a thyristor is forward biased, the injection of gate current by applying positive gate voltage between the gate and cathode terminals would turn on the thyristor. As the gate current is increased, the forward blocking voltage is decreased.The following points should be considered in designing the gate control circuit:1. The gate signal should be removed after the thyristor turned on. A continuous gating signal would increase the power loss in the gate junction.2. While thyristor is reverse biased. There should be no gate signal; otherwise, the thyristor may fail due to an increased leakage current. 3. The width of gate pulse tG must be longer than the time required for the anode current to rise to the holding current value IH. In practice, the pulse width tG is normally made more than the turn on time ton of the thyristor

FIRING CIRCUITS FOR THYRISTORS

An SCR can be switched from off-state to on-state in several ways; these are forward voltage triggering, dv / dt triggering, temperature triggering, light triggering and gate triggering. The gate triggering is the most common common method of turning on the SCRs, because this method lends itself accurately for turning on the SCRs at the desired instant of time.

MAIN FEATURES OF FIRING CIRCUITS

The most common method for controlling the onset of conduction in an SCR is by means of gate voltage control. The gate control circuit is also called firing, or triggering circuit. These gating circuits are usually low power electronics circuits. A firing circuit should fulfil the following two functions.

If power circuit has more than one SCR, the firing circuit should produce gating pulses for each SCR at the desired instant for proper operation of the power circuit. These pulses must be periodic in nature and the sequence of firing must correspond with the type of thyristorised power controller. For example, in a single phase converter using two SCRs, the triggering circuit must produce one firing pulse in each half cycle ; in a 3-phase full converter using six SCRs, gating circuit must produce one trigger pulse after every 60 degree intervalThe control signal generated by a firing circuit may not be able to turn –on an SCR. It is therefore common to feed the voltage pulses to a driver circuit and then to gate-cathode circuit. A driver circuit consists of a pulse amplifier and a pulse transformer

A firing circuit scheme, in general consists of the components shown in above fig. . A regulated DC power supply is obtained from an alternating voltage source. Pulse generator, supplied from both AC and DC sources, gives out voltage pulses which are then fed to pulse amplifier for their amplification. Shielded cables transmit the amplified pulses to pulse transformers. The function of pulse transformer is to isolate the low voltage gate-cathode

circuit from the high voltage anode-cathode circuit

Types of Thyristor Firing Circuits1. Resistance Firing Circuit2. RC Firing Circuit3.UJT Firing Circuit4.Pulse Transformer Firing CircuitResistance Firing Circuit Resistance triggering circuit is the simplest and the most economical method.This however, suffer from a limited range of firing angle control (0 to 90 degree), great dependence on temperature and differnce in performance between individual SCRs R C FIRING CIRCUITS The limited range of firing angle control by resistance firing circuit can be overcome by RC firing circuit. The firing angle control range from 0 degree to 180

degree

Types of Thyristor Firing Circuits UJT triggering circuits.Resistance and RC triggering circuits give prolongedpulses. As a result, power dissipation in the gate circuit is large. This difficulty can be overcome by UJT triggering circuits.

RESISTANCE FIRING CIRCUITS Theory of operation As shown in the circuit, R2 is the variable resistance, R

is the stabilizing resistance. In case R2 is zero, gate current may flow from source, through load, R1, Diode D, and gate to cathode. This current should not exceed permissible gate current . This current can be limit with the value of R1

OPERATION OF RESISTANCE FIRING CIRCUITS It is thus seen that function of R1 is to limit the gate

current to a safe value as R2 is varied. Resistance R should have such a value that maximum

voltage drop across it does not exceed maximum possible gate voltage

R C FIRING CIRCUITS The limited range of firing angle control by

resistance firing circuit can be overcome by RC firing circuit.

Theory of operation of RC Firing Circuit

Fig illustrates RC triggering circuit. By varying the value of R, firing angle can be

controlled from 0 to 180 degree. In the negative half cycle, C charges through D2 .

This capacitor voltage remains constant at –Vm until supply voltage attains zero value.

When capacitor charges to positive voltage equal to gate trigger voltage Vgt, SCR is fired and after this, capacitor holds to a small positive voltage.

Diode D1 is used to prevent the breakdown of cathode to gate junction through D2 during the negative half cycle.

Unijunction Transistor (UJT). It is a three terminal device . The device input, is

called the emitter, has a resistance which rapidly decreases when the input voltage reaches a certain level. This is termed a “negative resistance characteristics’’.

three terminals called the Emitter (E), Base-one(B1) and Base-two(B2). It is made up of an N-type base to which P-type emitter is embedded. P-type emitter is heavily doped and N-type base is lightly doped

UJT Equivalent Circuit & Characteristics Curve

UJT Firing Circuit The unijunction transistor is a highly efficient switch ;

its switching time is in the range of nanoseconds. Since UJT exhibits negative resistance characteristics,

Fig. (a) shows a circuit diagram with UJT working in the oscillator mode. The external resistances R1 R2 are small in comparison with the internal resistances RB1, RB2 of UJT bases

Operation of UJT Firing Circuit In Fig. (a), when source voltage VBB is applied, capacitor C begins to

charge through R exponentially towards VBB, During this charging, emitter circuit of UJT is an open circuit. The capacitor voltage vC, equal to emitter voltage vE, is given by

VC = VE = VBB( 1 – e-t/RC)

The time constant of the charge circuit is 1 = RC

When this emitter voltage vE (or vC) reaches the peak-point voltage VP (= VBB + VD), the unijunction between E – B1 breaks down. As a result, UJT turns on and capacitor C rapidly discharges through low resistance R1 with a time constant t2 = R1C. Here t2 is much smaller than t1. When the emitter voltage decays to the valley-point voltage VV, UJT turns off

Pulse Transformer Firing CircuitSometimes pulse transformers are used in firing circuits for thyristors and GTOs, for isolation between the gate circuit and the load circuit. The main reason for this is that the load may use a high voltage ac supply, and the firing circuit may use a low voltage. The transformer generally used arc either l:l two-winding, or l'l:l three-winding types. These have transformers have a low winding resistance, and a low leakage resistance. The pulse transformer provides electrical isolation as it transfers a pulse from the primary 1o the secondary coil. The secondary coil of the pulse transformer is connected directly between the gate and the cathode, or may have series resistor, or a series diode to prevent reverse gate current..

There are various ways of connecting the pulse transformer to trigger the thyristor. Figure shows the basic pulse transformer coupling to drive a single thyristor

A pulse at the output of the pulse generator is given to the primary of the pulse transformer, this is transmitted faithfully at its secondary terminal through the resistor R to the gate of the thyristor. Figure 3.19 shows another way of using a pulse transformer to drive an anti-parallel pair of thyristors.

Here a three-winding transformer provides complete isolation and the pulse generator must supply enough energy to trigger both thyristors. Note the black dots on the primary and secondary windings. These dots are used to indicate the polarity of the windings. Transformer polarity is defined as the relative direction of the induced voltages in the primary and secondary windings with respect to the winding terminals. The dot is used to indicate which windings have the same instantaneous polarity

Pulse TransformersPulse transformers are used quite often in firing circuits for ,SCRs and GTOs. This transformer has usually two secondaries. The turns ratio from primary to the two secondaries is 2:1:1 or 1:1:1. These transformers are designed to have low winding resistance, low leakage reactar~ce and Iow interwinding capacitance. The advantages of using pulse transformers in triggering semiconductor devices are:(a) They provide isolation of low voltage firing circuit from high voltage anode-cathode power circuit and (a) The trigger pulse can be coupled to one or more devices from the same trigger source by means of pulse transformer.A square pulse at the primary terminals of a pulse transformer may be transmitted at its secondary terminals faithfully as a square wave or it may be

transmitted as a derivative of the input waveform.A general layout of the trigger circuit using a pulse transformer is shown in Fig. 2 Here, R1 limits the current in the primary circuit of pulse transformer. In practice, trigger pulses are preferred due to the following reasons:

(a) This pulse waveform is suitable for injecting a large charge in the gate circuit for reliable turn on.

(b) The duration of this pulse is small, and therefore, no significant heating of the gate circuit is observed.

(c) The fact stated (b) as mentioned permits Va to be raised to a suitable high value so that a hard drive of SCR is obtained. A device with a hard drive can withstand high di/dt at the anode circuit, which is desirable.

SNUBBER CIRCUIT The circuit used to prevent unwanted dv /dt triggering

of SCR is called Snubber circuit

For di/dt protection inductor is connected in series

THYRISTOR COMMUTATION TECHNIQUESThe gate has no control over thyristor once its turns on . It

can be turn off by reducing its forward anode current to a level below the holding current.There are mainly two types of commutation

Natural Commutation Forced Commutation In Phase controlled Rectifiers Thyristor turned off

automatically due to natural behaviour of input supply after half cycle.

In Choppers, and Inverters input supply voltages are DC. In these circuits Thyristor will turned off by applying following forced commutation techniques

Voltage Commutation Current CommutationThese techniques will be discussed in Chopper and

Inverters

ChoppersIntroduction To produce quality goods in any industry, the

processes necessarily require the use of variable speed drives.

Variable speed d.c. and a.c. drives are being in creasingly used in all industries. These drives and processes take power from d.c. voltage sources.

In many cases, conversion of the d.c. source voltage to different levels is required. For example, subway cars, trolley buses, or battery operated vehicles require power from a fixed voltage d.c. source. However, their speed control requires conversion of fixed voltage d.c. source to a variable-volt age d.c. source for the armature of the d.c. motor.

Generally Following Methods are available for obtaining variable DC from fixed DC voltagesResistance control In this method, a variable resistance is inserted between the load and the source. This method is highly wasteful of energy. Also, for a given output voltage, different values of resistances are needed for different values of load current. This method is still used for older traction installations Motor-generator set Separate generator excitation gives a voltage whichcan be varied from zero to rated value with either polarity. The set-up is bulky, costly, slower in response, and less efficient because of the generator field time-constant A.C. link chopper (inverter-rectifier)In this method, the d.c. is first converted to a.c, by an inverter (d.c. to a.c. converter) . The obtained a.c: is then stepped up or down by a transformer and then rectified back to d.c. by a rectifier.  

D.C. chopper (d.c. to d.c. power converters)  A d.c. chopper is a static device (switch) used to obtain

variable d.c. voltage from a source of constant d.c. voltage. Therefore, chopper may be thought of as d.c. equivalent of an a.c. transformer . The d.c. chopper offers following advantages as compared to previous methods.

Greater efficiency, Faster Response, Lower Maintenance, Small Size, Smooth Control, Solid-state choppers due to various advantages are

widely used in trolley cars, battery-operated vehicles, traction-motor control, control of a large number of d.c. motors from a common d.c. bus with a considerable improvement of power factor,

Types of DC Chopper

According to the output voltages DC Chopper are classified as follows

Step Down Chopper

Step Up Chopper

Step Up-Down Chopper

Principle of Step Down Chopper The output of step down chopper is less than the input voltage.Figure . illustrates the principle of a chopper. The chopper is represented by an SCR inside a dotted square. It is triggered periodically and is kept conducting for a period TON and is blocked for a period TOFF. The chopped load voltage waveform is shown. The output can be controlled either by current limit control or time ratio control

CONTROL STRATEGIES OF CHOPPERSThe average value of output voltage. Vo can be

controlled by periodic opening and closing of the switches. The two types of control strategies for operating the switches are employed in d.c. choppers. They are: 1. Time-ratio Control (TRC). and 2. Current Limit Control.

Time-Ratio Control (TRC)In the time-ratio control, the value of TON /T is varied. This is effected in two ways.They are variable frequency operation and constant frequency operation. 

Constant Frequency System In this scheme, the on-time Ton is varied but chopping

frequency f (or chopping period T) is kept constant. Variation of Ton means adjustment of pulse width, as such this scheme is also called pulse-width-modulation scheme..

Fig. illustrates the principle of pulse-width modulation. Here chopping period T is constant. In Fig.(a), Ton = 1/4 T so that = 25%. In Fig.(b), Ton = 3/4T so that = 75%. The output voltage V0 can be varied between zero and source voltage VS

Variable Frequency System In this scheme, the chopping frequency f (or chopping

period T ) is varied and either (i) on-time Ton is kept constant or (ii) off-time TOFF is kept constant. This method of controlling ‘’ is also called frequency-modulation scheme

Principles of Step up Chopper In this chopper average output voltage VO is more than

the input voltage VO VS . Large inductor L in series with source voltage VS is

essential When the chopper is on, inductor stores energy . When the chopper CH is off, as the inductor current

cannot die down instantaneously, this current is forced to flow through the diode and load .

As a result, voltage across the load, given by VO = VS + L (di/dt), exceeds the source voltage VS. In this manner, the circuit of Fig. (a) acts as a step-up

chopper and the energy stored in L is released to the load.

Circuit Diagram and Waveform of Step UP Chopper

CHOPPER COMMUTATION CIRCUITS

There are three types of Chopper

Commutations

Voltage Commutated Chopper

Current Commutated Chopper

Load Commutated Chopper

Voltage Commutated ChopperAs shown Fig. The chopper feeds a constant current load (highly inductive load). To start with the capacitor is pre-charged with lower plate positive by closing the switch shown in the Figure. The operation can be explained using 4 modes.

Designing of commutation circuitThe following formulas are used to calculate the values of Capacitor and Inductors

where tC = Device turn off time,

IO = Output current and E = Supply voltage.

CURRENT COMMUTATED CHOPPER (CCC) The circuit and modes of CCC are shown in Fig.. In current commutated chopper circuit, an inductor is connected in series with the capacitor Tm is the main SCR and TA is the auxiliary SCR. To start with the capacitor is pre charged with top positive and bottom negative. The details of commutation can be explained with the following modes

CURRENT COMMUTATED CHOPPER (CCC)

Designing of commutation circuitThe following formulas are used to calculate the values of Capacitor and Inductors

where tC = Circuit turn off time,

ICP = Peak values of capacitor current and VS = Supply voltage.

LOAD COMMUTATED CHOPPER The load commutated chopper (LCC) uses four SCRs. They are triggered in pairs. Initially the capacitor is charged with a — and b +. The working of LCC can be explained with the following modes. The circuit and modes of LCC are shown in Fig.

SWITCHING- MODE REGULATORS DC chopper can be used as switching-mode regulators to convert a DC voltage, normally unregulated DC output voltage. The regulation is normally achieved by PWD at a fixed frequency and the switching device is normally a power BJT, or MOSFET

Topologies of switching regulators

(1)              Buck Regulators(2)              Boost Regulators

(3) Buck-Boost Regulators

Buck Regulators. In a buck regulator, the average output voltage VO, is less than the input voltage ‘VS’

BOOST REGULATORS In a boost regulator, the output voltage is greater than the input voltage . A boost regulator using a power ‘MOSFET’ as shown in fig. .The circuit operation can be divided into two modes

Buck-Boost Regulators A buck-boost regulator provides an output voltage that may be less than or greater than the input voltage—hence the name "buck-boost"; the output voltage polarity is opposite to that of the input voltage. This regulator is also known as an inverting regulator

CYCLOCONVERTERS A device which converts input power at one frequency to

output power at a different frequency with one-stage conversion is called a cycloconverter.

A cycloconverter is thus a one-stage frequency changer. Basically, cycloconverters are of two types, namely :

(i) Step down cycloconverters (ii) Step up cycloconverters

In step-down cycloconverters, the output frequency f0 is lower than the supply frequency fS fo < fS .

In step-up cycloconverters, fQ >fs.At present, the applications of cycloconverters include the following :(i) Speed control of high-power ac drives (ii)      Induction heating (iii) For converting variable-speed alternator voltage to constant frequency output volt age for use as power supply in aircraft or shipboards  

Single-phase to Single-phase Step-up Cyclocbnverter For understanding the operating principle of step-up device, the load is assumed to be resistive for simplicity. It should be noted that a step-up cycloconverter requires forced commutation. The basic principle of step-up device is described here first for mid-point and then for bridge-type cycloconverters

. Waveforms for Step-up Cyclocbnverter

Single-phase to Single-phase Step-down Cycloconverter A step-down cycloconverter does not require forced commutation. It requires phase-controlled converters connected as shown in Fig. .1. These converters need only line, or natural, commutation which is provided by ac supply. Both mid-point and bridge-type cycloconverters are described in what follows

INVERTERS Introduction

It is DC to AC converter An Inverter enables one to convert a supply of dc input

voltage to a symmetrical ac output voltage of a desired magnitude and frequency

This output voltage may be fixed or variable, at a fixed or variable frequency.

The variable output voltage can be obtained either by varying the input dc voltage, keeping the inverter at a constant gain or by varying the gain of the inverter

The gain of the inverter is defined as the ratio of the ac output voltage to the dc input voltage

Although the output voltage waveforms of an ideal inverter should be sinusoidal, in practice, they are non-sinusoidal and contain certain harm For low- and moderate-power applications these harmonics may be acceptable, but for high-power applications, low distorted sinusoidal waveforms are required These harmonic contents of the output voltage can be reduced or minimized by using high-speed switching power semiconductor devices Applications of inverters are: variable speed ac motor control, induction heating, stand by/uninterrupted power supplies, etc

Classification of InvertersThere are many ways to classify inverters, but they are broadly divided in to two types, viz., single-phase, and three-phase; these are further classified according to the taxonomy of Figure .1.

Classification of InvertersIn amplifier-type inverters, transistors are used as amplifiers and due to high power dissipation in the device itself, they generally give a low efficiency In saturated-type inverters, devices are used as a switch; therefore, they show high efficiency Transistors, and IGBTs are generally used in low- and medium-power requirements Thyristors, and GTOs are used in high-power applications. These inverters can also be classified into two groups: voltage-driven inverters and current-driven inverters

VOLTAGE CONTROL IN SINGLE-PHASE INVERTERS AC loads may require constant or adjustable voltage at their input terminals. When such loads are fed by inverters, it is essential that output voltage of the inverters is so controlled as to fulfill the requirement of AC loads The various methods for the control of output voltage of inverters are as under External control of ac output voltage External control of dc input voltage Internal control of inverter. The first two methods require the use of peripheral components whereas the third method requires no peripheral components. These methods are now briefly discussed

External Control of dc Input Voltage In case the available voltage source is ac, then dc voltage input to the inverter is controlled through fully-controlled rectifier, Through an uncontrolled rectifier and a chopper, Through an ac voltage controller & an uncontrolled rectifierIf available voltage is dc, then dc voltage input to the inverter is controlled by means of a chopper

External Control of ac Output Voltage There are two possible methods of external control of ac output voltage obtained from inverter output terminals. AC voltage control Series-inverter control

AC voltage control In this method, an ac voltage controller is inserted between the output terminals of inverter and the load terminals as shown in Fig. 1. The voltage input to the ac load is regulated through the firing angle control of ac voltage controller

Series-inverter control This method of voltage control involves the use of two or more inverters in series. Fig. (a) illustrates how the output voltage of two inverters can be summed up with the help of transformers to obtain an adjustable output voltage.

Internal Control of Inverter Output voltage from an inverter can also be adjusted by exercising a control within the inverter itself. The most efficient method of doing this is by pulse-width modulation control used within an inverter. This is discussed briefly in what follows Pulse width modulation control.

In this method, a fixed dc input voltage is given to the inverter and a controlled ac output voltage is obtained by adjusting the on and off periods of the inverter components. This is the most popular method of controlling the output voltage and this method is termed as pulse-width modulation (PWM) control. The output voltage control with this method can be obtained without any additional components With this method, lower order harmonic can be eliminated or minimised along with its output voltage control. As higher order harmonics can be filtered easily, the filtering requirements are minimised.

FORCE-COMMUTATED THYRISTOR INVERTERS For low-and medium-power applications, inverters using transistors, GTOs and IGBTs are becoming increasingly popular. However, for high-voltage and high-current applications, thyristors are more suitable.In voltage fed inverters, thyristors remain forward biased by the dc supply voltage. This entails the use of forced commutation for inverter circuits using thyristors

Modified McMurray Half-bridge Inverter It consists of main thyristors Tl, T2 and main diodes Dl, D2. The commutation circuit consists of auxiliary thyristors TA1, TA2, auxiliary diodes DA1, DA2 ;

damping resistor Rd, inductor L and capacitor C

Principle of Phase Controlled Converters The simplest form of controlled Rectifier circuit is

consist of single thyristor feeding DC power to a resistive load R, as shown in fig 1(a).

During positive half cycle of input voltage , the thyristor anode is positive with respect of its cathode and the thyristor is said to be forward biased. When T1 is fired at t = , T1 conducts and input voltage appears across the load.

When the input voltage starts to be negative at, t = , the thyristor anode is negative with respect to cathode and T1 is said to be reverse biased and it is turned off.

Circuit Diagram and Waveforms with Resistive Load

Single Phase Half Wave Circuit with RL Load

As shown in fig. At t = , thyristor is turned on by gating signal. The load voltage Vo at once brcomes equal to source voltage Vs as shown. But the inductance L forces the load or output current Io to rise gradually. After some time, Io reaches maximum value and then begins to decrease. At t = , Vo is zero but Io is not zero because of the load inductance L. After t = , SCR is subjected to reverse anode voltage but it will not be turned off as load current Io is not less than the holding current.At some angle , Io reduces to zero and SCR is turned off as it is already reverse biased. After t = , Vo = 0 and Io = 0.

Single Phase Half Wave Circuit with RL Load