In this article we will discuss about:- 1. Construction of SCR 2. SCR as a Switch 3. SCR in Normal Operation 4. Equivalent Circuit 5. Application.

Silicon-controlled rectifier (SCR) is a semiconductor device which acts as an electronic switch. A silicon-controlled rectifier can change an alternating current into direct one and also it can control the amount of power fed to the load. Thus in a sense it combines the features of both rectifier and transistor.

Contents:

  1. Construction of SCR
  2. SCR as a Switch
  3. SCR in Normal Operation
  4. Equivalent Circuit of SCR
  5. Application of SCR

1. Construction of SCR:

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If a P-N junction is added to a junction transistor, then the resulting P-N junction device is termed as a silicon-controlled rectifier. The construction of the SCR is shown in Fig. 10.1(a) while Fig. 10.1(b) shows its symbolic representation. It is a combination of a rectifier (P-N) and a junction transistor (N-P-N) in one unit to form a P-N-P-N device.

There are three terminals- one from the outer P-type material is called anode (A), the second from the outer N-type material is called the cathode (K) and the third from the base of transistor section is the gate (G). The anode is kept at high positive potential with respect to cathode while the gate is held at small positive potential with respect to cathode.

The SCR is a solid-state equivalent of thyratron. The anode, gate and cathode of SCR correspond to the plate, gird and cathode of thyratron and that is why SCR is also called as thyristor.

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2. SCR as a Switch:

The silicon-controlled rectifier has two states only:

(i) ON state, and

(ii) OFF state.

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If an appropriate value of the gate current is passed, the SCR begins to conduct heavily and remains in the position for an indefinite period even if the gate voltage is removed. This is the ON state of the SCR. But if the anode current is reduced to the holding current, the SCR is turned OFF. Thus it behaves as a switch, being an electronic device may be termed as an electronic switch.

There are a few advantages of using SCR as a switch over a mechanical or electromechanical switch:

i. Its switching speed is too high up to 109 operations/second.

ii. By a small gate current of few milliamperes, it permits control over large current (30 to 100 A).

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iii. Since it has no moving parts, it can provide noiseless operation.

iv. Its size is small and can give trouble-free service.

3. SCR in Normal Operation:

To operate a silicon-controlled rectifier in normal operation, some points are kept in view as stated below:

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(a) In general, the supply voltage is much less than the breakover voltage.

(b) If the gate current is increased above the required value, the SCR will close at a very reduced supply voltage.

(c) Bypassing an appropriate amount of current, a few µA say, the SCR is turned on and not by breakover voltage.

(d) To turn OFF the SCR from the ON state, the anode current should be reduced to holding current.

(e) If the SCR is operated by using an a.c. supply, then care should be taken so that the peak reverse voltage which arises during negative half-cycle does not exceed the reverse breakdown voltage.

Operation:

The load in an SCR is connected in series with the anode which is always kept at a positive potential with respect to the cathode.

The operation of the SCR can be explained by considering the following two cases:

i. When the Gate is Open:

The circuit diagram with gate open, i.e., when no voltage is applied to the gate is shown in Fig. 10.2. In this case the junction J2 is reverse biased but other two junctions J1 and J3 are forward biased. Thus the situation in J1 and J3 becomes similar to N-P-N transistor with base open.

As a result there will be a flow of current through the load RL and so the SCR is cut off. If the applied voltage is increased gradually, a stage is attained when the reverse biased junction J2 breaks down. As a matter of fact, the SCR begins to conduct heavily and is said to be in the ON state. The voltage applied at that instant due to which the SCR conducts heavily without gate voltage is known as breakdown voltage.

ii. When the Gate is Positive:

As shown in Fig. 10.3, the SCR can be made to conduct heavily by applying a small positive voltage to the gate. In this case, the junction J3 is forward biased but junction J2 is re-verse biased. From N-type material electrons move across junction J3 towards left while from P-type holes move towards the right. As a result, electrons from junction J3 are attracted across junction J2 and the gate current starts to flow.

With the flow of the gate current the anode current increases which in turn makes more electrons available at the junction J2. This process continues and within a very small time junction J2 breaks down and the SCR starts to conduct heavily. Once the SCR begins to conduct, the gate loses all control. The conduction stops only when the applied voltage is reduced to zero.

Considering the working of the SCR one may come to the following important conclusions:

i. An SCR either conducts heavily or it does not conduct. It has thus two states and there is no state in-between. Hence it behaves like a switch.

ii. There are two different ways to turn on the SCR. In the first method, the gate is kept open and the supply voltage is made equal to the breakover voltage. In the second method, the supply voltage is applied less than the breakover voltage and then it is turned on by a small voltage applied to the gate.

iii. It is the general way, to apply a small positive voltage to the gate for closing an SCR. This is because the breakover voltage is usually greater than the supply voltage.

iv. To make an SCR non-conducting, the supply voltage is reduced to zero.

Some Terms:

(a) Breakover Voltage:

Gate being open, it represents the minimum forward voltage at which an SCR starts to conduct heavily, i.e., turned on.

The breakover voltage of an SCR is 200 volts means it can block a forward voltage (i.e., the SCR remains open) so long the supply voltage is less than 200 volts. If the supply voltage is made greater than this value, then the SCR will be turned on. For practical purposes, the SCR is operated with a supply voltage less than the breakover voltage and it is then turned on by a low voltage applied to the gate. The breakover voltages of commercial SCRs are from about 50 volts to 500 volts.

(b) Holding Current:

Gate being open, it represents the maximum anode current at which the SCR is turned OFF from the ON conditions.

We know that if the SCR is in the conducting state, it cannot be turned off even if the voltage applied to gate is removed. To turn off the SCR the supply voltage is almost reduced to zero at which point the internal transistor comes out of saturation. Under this condition, the anode current becomes very small and is called the holding current.

The holding current of an SCR is 5 mA means that if the anode current is made less than 5 mA, then the SCR will be turned off.

(c) Forward Current Rating:

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

If the value of the current exceeds the forward current, then due to intensive heating at the junction, the SCR may be damaged. In fact, for every SCR there is a safe value of forward current at which it can conduct.

The forward current rating of an SCR is 40 A means that the SCR can safely carry only 40 A. The forward current ratings of commercial SCRs are from about 30 A to 100 A.

4. Equivalent Circuit of SCR:

The SCR in Fig. 10.4(a) shows its structure. It is a four-layer semiconductor device which can be separated into two transistors as shown in Fig. 10.4(b).

Thus the equivalent circuit of the SCR can be treated as constituted of two transistors, a P-N-P transistor and an N-P-N transistor connected as shown in Fig. 10.5. It is seen from the figure that the collector of each transistor is coupled to the base of the other and thus making a positive feedback loop.

The equivalent electrical circuit of an SCR is shown in Fig. 10.6. Let at first the gate terminal G be not connected to any external circuit and the gate current Ig be zero. Also a supply voltage Vss is connected with a series resistor RL between the anode A and cathode K. With this biasing arrangement the junctions J1 and J3 are forward biased while the junction J2 gets reverse biased. In this way, the three junctions’ J1, J2 and J3 are properly biased for the operations of the transistors T1 and T2.

The collector currents of the transistors T1, and T2 are then respectively given by-

SCR is so designed that its (α1, + α2) becomes slightly less than unity. In order to satisfy the condition that (α1 + α2) < 1, an SCR is made from silicon or germanium.

The current-voltage characteristics of an SCR is shown in Fig. 10.7 for different values of gate currents lg. Let us consider the curve for Ig = 0. In the low current region OA, (α1 + α2) < 1. But as the applied voltage increases, current increases slowly causing an increase of (α1 + α2) which further increases the current I. Thus in the region AB with the increase of voltage a rapid increase of current occurs. Such an increase of the current continues until (α1 + α2) becomes unity at B.

The corresponding voltage VBO is called the breakover voltage or triggering voltage. The current corresponding to the triggering voltage is marked by IH in the figure and is called the holding current. Once SCR gets fired and comes to ON condition, can be stopped only by reducing the anode voltage to reduce the anode current below IH. When the current comes down below IH, the conduction ceases and the operating point shifts from C to O.

To explain the effect of the gate current on the working to the SCR, we consider that the supply voltage Vss is less than that needed to fire the SCR. When Ig is applied α2 increases so that (α1 + α2) becomes unity and the SCR fires even with low supply voltage. It is also clear from the figure that if the value of Ig is greater, the applied voltage V becomes lower.

5. Application of SCR:

A very important application of an SCR is the controlled rectification. Half-wave and full-wave rectification using SCR is discussed below.

SCR Half-Wave Rectifier:

Fig. 10.8(a) reveals the circuit diagram of an SCR used as half-wave rectifier. In the figure, T is a transformer, RL the load resistance connected in series with the anode while Rh is the variable resistance inserted in the gate circuit for controlling the gate current.

i. Operation:

The a.c. supply is applied to the primary of the transformer. Let the peak inverse voltage (PIV) across the secondary is less than the reverse breakdown voltage of the SCR which ensures that the SCR will not break down during the negative half-cycle of the a.c. supply.

If suitable gate current is allowed to flow, the SCR will conduct during the positive half-cycle.

The greater the gate current, the smaller is the supply voltage at which the SCR is turned on. The gate current can be changed by means of the variable resistance Rh.

Let us now assume that the gate current is adjusted to such a value that SCR closes at positive voltage V1 which is smaller than the peak value of the voltage Vm. From Fig. 10.8(b) it is seen that the SCR will conduct when secondary a.c. voltage becomes V1 in the positive half-cycle. Beyond this SCR will continue to conduct till V becomes zero when it is turned OFF. It is also clear from the figure that the firing angle is α and the conduction angle is ф [= 180° – α].

Let v = Vm sin 0be the alternating voltage appears across the secondary of the transformer and α be the firing angle. During the positive half-cycle the rectifier will conduct from α to 180°.

It is thus seen that greater the firing angle α, the smaller is the average current.

SCR Full-Wave Rectifier:

The circuit of SCR full-wave rectifier is shown in Fig. 10.9(a). It is similar to an ordinary centre- tap circuit except that the two diodes are replaced by two SCRs. One SCR conducts during the positive half-cycle while the other during the negative half.

i. Operation:

By adjusting the gate currents the angle of conduction can be changed. Let the gate currents be so adjusted that SCRs conducting as the secondary voltage becomes V1. During the positive half-cycle of the a.c. input voltage the upper end of the secondary is positive and the lower end negative causing SCR1 to conduct? The conduction, however, begins when the voltage across the upper half of the secondary becomes [Fig. 10.9(b)].

In this manner only the shaded part of the positive half-cycle will pass through the load. During the negative half-cycle of the a.c. input voltage, the upper end of the secondary becomes negative and the lower end positive. When the voltage across the lower half of secondary becomes V1, the SCR2 starts to conduct.

This circuit has an added advantage over the ordinary full-wave rectifier circuit as by adjusting the gate currents, the conduction angle and the output voltage can be changed.

Let v = Vm sin θ be the alternating voltage that appears between centre-tap and either end of the secondary. If α be the firing angle, then-

Other Applications of SCR:

An SCR can also be used in switching and control applications. It can further be used for over light detection.

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