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 Ic1 , transistor T2 the base current T2 is
Ic1 = Ib2 and Ib1 = Ic2
Also IK = Ia + Ig (1.1)
The relationship of transistor analogy is
Ib1 = Ie1 – Ic1 (1.2)
Where Ic1 = α Ie1+ Ico1 (1.3)
Reverse leakage current Ico1 of the J2 at reverse biased . J1 and J3 will be not present.
Now substituting equ (3) in (1) get
Ib1 = Ie1– α Ie1 – Ico1 (1.4)
Ib1 = (I- α) Ie1 – Ico1 (1.5)
The anode current of the device becomes emitter current of transistor T1 is
Ia = Ie1 (1.6)
Ib1 = (I- α) Ie1 – Ico1 (1.7)
And,
Ic2 = α Ie2+ Ico2 (1.8)
The cathode current of the device becomes emitter current of transistor T2 is
IK = Ie2 (1.9)
Ic2 = α Ik+ Ico2 (2.0)
Ib1 = Ic2 (2.1)
Sub equ (1.7) & (2.0) in equ (2.1)
(1- α1) Ia– Ico1= α2( Ia+ Ig) + Ico2 (2.2)
(1- α1– α2) Ia = α2Ig + Ico2 + Ico1 (2.3)
[1 – (α1+ α2)] Ia = α2Ig + Ico2 + Ico1 (2.4)
Ia = α2Ig + Ico2 + Ico1 / [1 – (α1+ α2)] (2.5)
The leakage current Ico2 & Ico1 is small and its negligible
Ia = α2Ig / 1 – (α1+ α2)] (2.6)
Equ (2.6) we have studied(α1+ α2) = 1 , the anode current (Ig) becomes infinite, instantaneously increased anode current is very high, oncoming infinity. In on other words the attain latching current (IL) 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 Mp and electrons multiplication Factor Mn is large. junction J2 close to break down voltage the multiplication factors (Mp & M n) is high. Increasing VBO rapidly increased the junction J2 near the breakdown voltage.