Two Transistor Analogy of SCR, Working, Formula, Circuit Diagram

Two Transistor Analogy of SCR (Two Transistor Analogy):

The two transistor namely PNP and NPN are connected one of the collector connected to another one of the base of and other terminal anode and cathode terminals. known as “ two transistor analogy” . This type is determined by separate the middle two regions of the SCR into two individual parts.

The transistor T₁ the collector current I_c1 , transistor T₂ the base current T₂ is

I_c1 = I_b2 and I_b1 = I_c2

Also I_K = I_a + I_g (1.1)

The relationship of transistor analogy is

I_b1 = I_e1 – I_c1 (1.2)

Where I_c1 = α I_e1+ I_co1 (1.3)

Reverse leakage current I_co1 of the J₂ at reverse biased . J₁ and J₃ will be not present.

Now substituting equ (3) in (1) get

I_b1 = I_e1– α I_e1 – I_co1 (1.4)

I_b1 = (I- α) I_e1 – I_co1 (1.5)

The anode current of the device becomes emitter current of transistor T­₁ is

I_a = I_e1 (1.6)

I_b1 = (I- α) I_e1 – I_co1 (1.7)

And,

I_c2 = α I_e2+ I_co2 (1.8)

The cathode current of the device becomes emitter current of transistor T­₂ is

I_K = I_e2 (1.9)

I_c2 = α I_k+ I_co2 (2.0)

I_b1 = I_c2 (2.1)

Sub equ (1.7) & (2.0) in equ (2.1)

(1- α₁) I_a– I_co1= α₂( I_a+ I_g) + I_co2 (2.2)

(1- α₁– α₂) I_a = α₂I_g + I_co2 + I_co1 (2.3)

[1 – (α₁+ α₂)] I_a = α₂I_g + I_co2 + I_co1 (2.4)

I_a = α₂I_g + I_co2 + I_co1 / [1 – (α₁+ α₂)] (2.5)

The leakage current I_co2 & I_co1 is small and its negligible

I_a = α₂I_g / 1 – (α₁+ α₂)] (2.6)

Equ (2.6) we have studied(α₁+ α₂) = 1 , the anode current (I_g) becomes infinite, instantaneously increased anode current is very high, oncoming infinity. In on other words the attain latching current (I­_L) the SCR conduct ON stage to Holds the it OFF stage. this called as “regenerative action” . the gate current (α₁+ α₂) = 0 (unity). The SCR is ON state.

(α₁+ α₂) 1 (at turn on condition)

(a) device temperature is very high, increase the leakage current.

(b) the current is small , for break over condition alphas very small its satisfied the multiplication factor is M_p and electrons multiplication Factor M_n is large. junction J₂ close to break down voltage the multiplication factors (M_p & M _n) is high. Increasing V_BO rapidly increased the junction J₂ near the breakdown voltage.