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SILICON CONTROLLED RECTIFIERS

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Question 1

Construction of SCR

A silicon controlled rectifier is a semiconductor device that acts as a true electronic switch.

It can change alternating current into direct current and at the same time can control the

amount of power fed to the load. Thus SCR combines the features of a rectifier and a

transistor.

When a PN junction is added to a junction transistor, the resulting three PN junction device

is called a silicon controlled rectifier as shown below.

Figure 1

It is clear that it is essentially an ordinary rectifier (PN) and a junction transistor (NPN)

combined in one unit to form PNPN device. Three terminals are taken, one from the outer

P-Type material called anode A, second from the outer N-Type material called cathode K

and the third from the base of the transistor section called gate G. In the normal operating

os SCR, anode is held at high positive potential with respect to cathode and gate at small

positive potential with respect to cathode.

Figure 2

The silicon controlled rectifier is a solid state equivalent of thyratron. The gate, anode and

cathode of SCR correspond to the grid, plate and cathode of thyratron. For this reason, SCR

is called thyristor.

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Working of SCR

In a silicon controlled rectifier, load is connected in series with anode. The anode is always kept at

positive potential with respect to cathode. The working of SCR can be studied under the following

two heads:

1. When the gate is open

Figure 3

Figure 3 shows the SCR circuit with gate open, i.e No voltage applied to the gate. Under this

condition J2 is reverse biased while J1 and J3 are forward biased. Hence, the situation in the

junctions J1 and J3 is just in npn transistor with base open . Consequently, no current flows through

the load RL and SCR is cut off. However, if the applied voltage is gradually increased a stage is

reached when reverse biased junction J2 breakdown. The SCR now conducts heavily and is said to be

ON state. The applied voltage at which SCR conducts heavily without gate voltage is called Breakover

Voltage.

2. When gate is positive with respect to cathode

Figure 4

The SCR can be made to conduct heavily at smaller applied voltage by applying small positive

potential to the gate as shown in Figure 4. Now junction J3 is forward biased and J2 is reverse

biased. The electron from n type material start moving across J3 towards left whereas holes from p-

type toward right. Consequently, the electrons from J3 are attracted across junction J2 and gate

current starts flowing. As soon as gate current flows anode current increases. The increased anode

current in turn makes more electrons available at J2. This process continues and in an extremely

small time, junction J2 breaks down and the SCR starts conducting heavily. Once SCR starts

conducting, the gate looses all control. Even if the gate voltage is removed anode current does not

decrease at all. The only way to stop conduction is to reduce the applied voltage to zero.

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SCR performance using two transistors Analogy

Figure 5

The two transistor equivalent circuit shows that the collector current of the NPN transistor TR2 feeds

directly into the base of the PNP transistor TR1, while the collector current of TR1 feeds into the

base of TR2. These two inter-connected transistors rely upon each other for conduction as each

transistor gets its base-emitter current from the others collector-emitter current. So until one of the

transistors is given some base current nothing can happen even if a Anode-to-Cathode voltage is

present.

When the thyristors Anode terminal is negative with respect to the Cathode, the centre N-P junction

is forward biased, but the two outer P-N junctions are reversed biased and it behaves very much like

an ordinary diode. Therefore a thyristor blocks the flow of reverse current until at some high voltage

level the breakdown voltage point of the two outer junctions is exceeded and the thyristor conducts

without the application of a Gate signal. This is an important negative characteristic of the thyristor,

as Thyristors can be unintentionally triggered into conduction by an overvoltage as well as high

temperature or a rapidly rising dv/dt voltage such as a spike.

If the Anode terminal is positive with respect to the Cathode, the two outer P-N junctions are

forward biased but the centre N-P junction is reverse biased. Therefore forward current is also

blocked. If now a positive current is injected into the base of the NPN transistor TR2, the resulting

collector current flows in the base of transistor

TR1. This in turn causes a collector current to flow in the PNP transistor, TR1 which increases thebase current of TR2 and so on.

each half cycle and as we now know, remains OFF until the application of a Gate trigger pulse.

Since an AC sinusoidal voltage continually reverses in polarity every half-cycle allowing the thyristor

to turn OFF, this effect is known as natural commutation and is a very important characteristic of

the silicon controlled rectifier.

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Derivation of SCR anode current in terms of current gain and leakage

current Ico

Figure 6

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Question 2

Explanation of SCR ratings

SCR Voltage Ratings

(i) Breakover Voltage.

The minimum forward voltage, when the gate is open, at which SCR starts conducting heavily (that is

turned-on) is called the breakover voltage. To specify this value, the gate is in the open-circuit

condition and the junction temperature is at its maximum permitted value, although VFB0 is still a

function of dv/dt. Though the SCR can be turned-on, with gate open, by making the supply voltage

equal to breakover voltage, but in practice, the SCR is operated with supply voltage less than

breakover voltage and it is turned-on by applying a small voltage (typically 1.5 V) to the gate.

Commercially available SCRs have Breakover voltages from about 50 V to 500 V.

(ii) Peak Forward Voltage.

This is the limiting positive anode voltage above which the SCR may get damaged. In general peak

forward voltage (PFV) is larger than forward breakover voltage VFB0 so that there is some inherent

protection for the device. However, if there is a voltage transient with an amplitude larger than the

transient rating of the SCR, although it is unlikely to damage the device, it may lead to

malfunctioning of the circuit if it causes the device to turn-on at the wrong instant. If the junction

temperature is low, it is possible that the PFV is lesser than VFBQ.

(iii) Maximum On-State Voltage.

It is the maximum value of the voltage appearing across the device during the conduction. For an

SCR with current load, the voltage across load is also included. Normally value of this voltage is of 1.5

V.

(iv) Peak Reverse Voltage.

The maximum reverse voltage (cathode positive with respect to anode) that can be applied to an

SCR without conduction in the reverse direction, is called the peak reverse voltage (PRV) or peak

inverse voltage (PIV).

Peak reverse voltage (PRV) is an important consideration while SCR is operating in an ac circuit.

During negative half cycle of ac supply reverse voltage acts across the SCR and if it exceeds beyond

PRV, there may be avalanche breakdown and SCR will get damaged if the current is not limited by

the external circuit. Commercially available SCRs have PRV ratings upto 2.5 kV.

(v) Critical Rate of Rise of Voltage.

Critical rate of rise of voltage, dv/dt, is the maximum rate at which the voltage in the forward

direction can rise without triggering the device. It is expressed in volts /microseconds.

It is found that sometimes an SCR unintentionally switches-on by itself during sudden variation of

the applied anode voltage at a time when there is no gate current applied and the SCR is supposed

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to be in the forward blocking state. This false triggering is because of the capacitance possessed by

the large area of junction J2. When the rate of rise of the applied voltage dV/dt is very large, the

capacitive charging current, CdV/dt may become large enough to trigger the device. This is not likely

to damage the device, but an unintended triggering may lead to either high di/dt through the SCR or

large shortest circuit current which may lead to the failure of the device.

(vi) Voltage Safety Factor.

To avoid puncture of SCR due to uncertain conditions, normal operating voltage is kept well below

PRV value of the device. The operating voltage and PRV are related by voltage safety factor Vf

defined as Vf = PRV/ 2x rms value of input voltage The normal value of V, lies between 2 and 2.5.

Current ratings of an SCR

The current carrying capability of an SCR is solely determined by the junction temperature. Except in

case of surge currents, in no other case the junction temperature is permitted to exceed the

permissible value

(i) Holding current

It is the maximum anode current, gate being open, at which SCR is turned off from ON conditions.

When SCR is in the conducting state, it cannot be turned OFF even if gate voltage is removed. The

only way to turn off or open the SCR is to reduce the supply voltage to almost zero at which point

the internal transistor comes out of saturation and opens the SCR. The anode current under this

condition is very small and nis called holding time. Thus SCR has a holding time of 5mA, I means that

if anode current is made less than 5mA, then SCR will be turned off.

(ii) Forward current rating.

It is the maximum anode current that an SCR is capable of passing without destruction. Every SCR

has a safe value of forward current which it can conduct. If the value of current exceeds this value,

the SCR may be destroyed due to intensive heating at the junctions. For example, if an SCR has

forward current rating of 40A, it means that the SCR can safely carry 40A. Any attempt to exceed this

value will result in the destruction of the SCR. Commercially available SCRs have forward ratings

from about 30A to 100A.

(iii) Circiut (I^2*t) rating.

It is the product of square of forward surge current and the time of duration of the surge, i.e Circuit

fusing rating = I^2*t. The circuit fusing rating indicates the maximum forward surge current

capability of SCR. For example, consider an SCR having circuit of 90. If this rating is exceeded in the

SCR circuit, the device will be destroyed by excessive power dissipation.

(iv) On-state Current.

When the device is in conduction, it carries a load current determined by the supply voltage and the

load. On-state current is defined in terms of average and rms values.

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ITav is the average value of maximum continuous sinusoidal on-state current (frequency 40-60 Hz,

conduction angle 180) which should not be exceeded even with intensive cooling. The temperature

at which the current is permissible has to be mentioned. It is this current which determines the

application of device.

ITrms is the rms value of maximum continuous sinusoidal on-state current (frequency 40-60 Hz,

conduction angle 180) which should not be exceeded even with intensive cooling.

Latching Current.

It is the minimum device current, which must be attained by the device, before the gate drive is

removed while turning-on, for maintaining it into conduction.

Holding Current.

It is the minimum on-state current required to keep the SCR in conducting state without any gate

drive. Its usual value is 5 m A.

(v) Critical Rate of Rise of Current.

The maximum rate of increase of current during on-state which the SCR can tolerate is called the

critical rate of rise of current for the device. This is specified at maximum junction temperature.

During initial period of turning-on, only a small area near the gate conducts the anode current. If the

current increases too fast, localised overheating may take place. This is called the hole storage

effect. Due to localised heating the device may get permanently damaged. To-day devices are

available which can withstand rate of rise of current upto 200-250 A/microsecond, however in

application this rate is hardly allowed to exceed beyond 5-10 A/micro second.

Protection against dI/dt is provided by series inductor.

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Question 3

Detailed explanation of the turn off mechanisms of an SCR

The SCR can be brought back to the forward blocking state from the conduction state only by

reducing the forward current to a level below that of the holding current.

This can be done by the following turn off mechanisms

1. Natural commutation: In this type of turn off, the supply voltage becomes zero or negative,Hence SCR is reverse biased. Therefore it is turned off.

2. Forced commutation: When the supply voltage is DC, then external commutationcomponent are used to turn off the SCR. The commutation components apply reverse bias

across the temporarily or pass impulse of negative current. Therefore SCR turns off.

The method of discharging a capacitor in parallel with an SCR to turn off the SC is called

forced commutation. As shown in figure 7, the forced commutation of SCR where capacitorC performs commutation. Assuming the SCRs are switches with SCR1 ON and SCR2 OFF,

current flows through the load and C. When SCR2 is triggered on, C is effectively parallel

across SCR1. The charge on C is then opposite to SCR2s forward voltage, SCR1 is thus turned

off and the current is transferred to R-SCR2 path.

Figure 7

3. Anode current interruption. When the anode current is reduced below a minimum valuecalled holding current, the SCR turns off. The simple way to turn off the SCR is to open the

line switch S as shown in figure 8

Figure 8

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Question 4

The operation of R-C firing circuit

Firing means the turning ON of an SCR.

RC HALF WAVE

Capacitor C in the circuit is connected to shift the phase of the gate voltage. D1 is used to prevent

negative voltage from reaching the gate cathode of SCR. In the negative half cycle, the capacitor

charges to the peak negative voltage of the supply (-Vm) through the diode D2 . The capacitor

maintains this voltage across it, till the supply voltage crosses zero. As the supply becomes positive,

the capacitor charges through resistor R from initial voltage of -Vm , to a positive value. When the

capacitor voltage is equal to the gate trigger voltage of the SCR, the SCR is fired and the capacitor

voltage is clamped to a small positive value.

Figure 9

RC half wave trigger circuit

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Case 1: R = Large: When the resistor R is large, the time taken for the capacitance to charge from -

Vm to V gt is large, resulting in larger firing angle and lower load voltage.

Case 2: R =Small: When R is set to a smaller value, the capacitor charges at a faster rate towards

Vgt resulting in early triggering of SCR and hence VL is more. When the SCR triggers, the voltage

drop across it falls to 1 1.5V. This in turn lowers, the voltage across R & C. Low voltage across the

SCR during conduction period keeps the capacitor discharge during the positive half cycle.

DESIGN EQUATION

From the circuit Vc =Vgt +Vd1 . Considering the source voltage and the gate circuit, we can write vs =

Igt*R +Vc . SCR fires when vs> Igt*R +Vc that is vs>Igt*R +Vc . Therefore R1.3(T/2).

RC FULL WAVE

A simple circuit giving full wave output is shown in figure below. In this circuit the initial voltage from

which the capacitor C charges is essentially zero. The capacitor C is reset to this voltage by the

clamping action of the thyristor gate. For this reason the charging time constant RC must be chosen

longer than for half wave RC circuit in order to delay the triggering. The RC value is empirically

chosen as 50 2 RC T . Also s gt gt v V R I - .

Figure 9: RC full wave trigger circuit

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References

1. http://www.electronics-tutorials.ws/blog/thyristor.html2. https://www.google.com.na/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&cad=rja&ved=0

CFMQFjAG&url=http%3A%2F%2Ftalkingelectronics.com%2FDownload%2520eBooks%2FPrin

ciples%2520of%2520electronics%2FCH-

20.pdf&ei=sqhYUYSxIcevPO3ZgdAG&usg=AFQjCNHLg2vJU8HiEWOD_9f0og9F_jihlA&bvm=bv

.44442042,d.d2k

3. https://www.google.com.na/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&ved=0CD8QFjAC&url=http%3A%2F%2Fwww.pkb.edu.my%2Felearning%2Ftopic%25201.pdf&ei=sq

hYUYSxIcevPO3ZgdAG&usg=AFQjCNFL6iR0IWy5HcUfbYslGnhsz5iNOQ&bvm=bv.44442042,d.

d2k

4. http://books.google.com.na/books?id=lGePnwccy3UC&pg=SA6-PA18&lpg=SA6-PA18&dq=Detailed+explanation+of+the+turn+off+mechanisms+of+an+SCR&source=bl&ots=

AbWTibmXbD&sig=U4OxMi64ITSBuOB3afxPRAhjLHw&hl=en&sa=X&ei=pbBYUeWYDMG2O8SZgbAC&ved=0CF8Q6AEwCQ