Two Transistor Model of a Thyristor | Power Electronics - Electrical Engineering (EE) PDF Download

Basic operating principle of SCR, can easily be understood by the two transistor model of SCR, as it is a combination of p and n layers.

This is a pnpn thyristor. If we bisect it through the dotted line then we will get two transistors i.e. one pnp transistor with J₁ and J₂ junctions and another is with J₂ and J₃ junctions as shown in figure below.

The relation between the collector current and emitter current is shown below

Here, I_C is collector current, I_E is emitter current, I_CBO is forward leakage current, α is common base forward current gain and relationship between I_C and I_B is
I_C = βI_B

Where, I_B is base current and β is common emitter forward current gain.

Let’s for transistor T₁ this relation holds

And that for transistor T₂

Now, by the analysis of two transistors model we can get anode current,

From equation (i) and (ii), we get,

If applied gate current is Ig then cathode current will be the summation of anode current and gate current i.e.

By substituting this valyue of Ik in (iii) we get,

From this relation we can assure that with increasing the value of
α₁ + α₂
towards unity, corresponding anode current will increase. Now the question is how
α₁ + α₂
increasing? Here is the explanation using two transistor model of SCR.
At the first stage when we apply a gate current I_g, it acts as base current of T₂ transistor i.e. I_B2 = I_g and emitter current of the T₂ transistor I_E2 = I_k. Hence establishment of the emitter current gives rise α₂ as

Presence of base current will generate collector current as

This IC₂ is nothing but base current IB₁ of transistor T₁, which will cause the flow of collector current,

IC₁ and IB₁ lead to increase IC₁ as
and hence, α₁ increases. Now, new base current of T₂ is,
 
which will lead to increase emitter current

and as a result α₂ also increases and this further increases.

As, α₁ again increases. This continuous positive feedback effect increases
(α₁ + α₂)
towards unity and anode current tends to flow at a very large value. The value current then can only be controlled by external resistance of the circuit.