SOLUTION

SCR is a unidirectional device and like a diode, it allows current to flow in one direction only. It can be used as a switch. The switching of SCR is controlled by the additional input called the gate and biasing conditions. It can be used as a rectifier to convert AC signals to DC signals.

Construction

The SCR is a four-layer p-n-p-n device, where p and n layers are alternately arranged. The outer layers are heavily doped. There are three p-n junctions called J₁, J₂, and J₃. The outer P-layer is called the anode and outer n-layer is called the cathode, middle P-layer is called the gate. The terminals are taken out from their layers. The anode must be positive with respect to the cathode so that SCR will be forward-biased. To turn on an SCR, a current is to be passed through the gate terminal. Hence it is a current operated device.

Two Transistor Model of SCR

The operation of an SCR can also be explained in a very simple way by considering it in terms of two transistors. This is known as the two transistor analogy of the SCR. The SCR can be considered as an NPN and a PNP transistor, where the collector of one transistor is attached to the base of the other and vice versa. This model is obtained by splitting the two middle layers of the SCR into two separate parts.

In the above figure, two middle layers of the SCR is split into two separate parts i.e PNP transistor T₁ and NPN transistor T₂. Collector current in the NPN transistor acts as the base current for the PNP, and analogously, the collector current of the PNP acts as the base current driving the NPN transistor.

I_A is the anode current

I_C is the cathode current

I_G is the gate current

I_E1 is the emitter current of the PNP transistor (T₁)

I_E2 is the emitter current of the NPN transistor (T₂)

I_C1 is the collector current of the PNP transistor (T₁)

I_C2 is the collector current of the NPN transistor (T₂)

I_B1 is the base current of the PNP transistor (T₁)

I_B2 is the base current of the PNP transistor (T₂)

α₁ is the current gain of the transistor T₁

α₂ is the current gain of the transistor T₂

The sum of two collector currents is equal to the external circuit current I_a, entering at anode terminal A.

∴ Ia = I_C1 + I_C2