SOLUTION

Turn on time:- The turn-on time of the SCR is defined as the time from initiation of gate drive to the time when anode current reaches its full value.

The turn-on time of the SCRs is about 1 to 3 microseconds. The turn-on time can be effectively reduced by applying higher values of gate currents. Because of high gate currents, more electron-holes are injected near junction j₂. Hence avalanche break-down of J₂ takes place fast. Therefore anode current rises fast. Thus effective turn-on time is reduced. To turn on the SCR, the gate pulse is thus sufficient.

Turn off time:- The turn-off time of the SCR is the time required to achieve forward blocking capability after commutation is initiated.

The turn-off time of the SCR varies from 5 to 200 microseconds. The turn-off time of the commutation circuit is called circuit turn-off time (t_c). And hence circuit turn-off time must be greater than the turn-off time of the SCR (t_c > t_q).

Hence the turn-on time is greater than the turn off time in an SCR

SCR Turn-off

We know that SCR can be turned off when its forward current falls below the holding current. The can be done by two methods:

1. Natural commutation
2. Forced commutation

Natural Commutation: In this type of turn-off, the supply voltage becomes zero or negative, Hence SCR is reverse biased. Therefore it is turned off.

Forced commutation: When the supply voltage is DC, then external commutation components are used to turn off the SCR. The commutation components apply a reverse bias across the SCR temporarily or pass impulse of negative current. Therefore SCR turns off.

Since the commutation transient lasts for a short period, the turn-off time in forced commutation is much smaller than in the Natural commutation