Question

Explain the operation of thyristor when gate terminal is open.

Answer 1

I_co = I_co1 and I_co2
Where I_co1 and I_co2 are the reverse saturation currents of CB junction of Q₁ and Q₂ transistros. When V_AK is increased upto avalanche breakdown voltage of junction J₂, I_co1 increases, both α₁ and α₂ values increase and α₁ + α₂ approaches unity. Under this condition large anode current starts flowing which is controlled only by external load resistor, The voltage drop in the external resistance causes a collapse of voltage across thyristor. The CB junctions of both Q₁ and Q₂ become forward biased and the total voltage drop across the device becomes equal to voltage drop across the diode. The thyristor is then said to be in ON state. Soon after turn ON, if i_A is larger than a specified current called latching current, a, and a₂ remain high enough to keep thyristor in ON state. The only way thyristor can be turned OFF is bringing I below a specified current called holding current I_H where α₁ and α₂ values start decreasing. The thyristor can region the forward blocking capacity once excess stored charge at J₂ is removed by the application of a reverse voltage across A and K.