In a commutation circuit employed to turn off an SCR, satisfactory turn off is obtained when

In a commutation circuit employed to turn off an SCR, satisfactory turn off is obtained when Correct Answer circuit turn off time > device turn off time

Concept:

Thyristor commutation:

• Commutation is defined as the process of turning off a thyristor.
• Thyristor commutation is a necessary mechanism for obtaining the controlled output in many of the thyristor circuits
• A thyristor is a semi-controlled device, It cannot be turned off directly by the gate signal.
•  So by the use of external commutation circuits, we can turn off the SCR.

Thyristor turnoff time (t_q):

• It is the time between the instant anode current becomes zero and the instant SCR regains forward blocking capability.
• During this time (t_q) all the excess carriers from four layers of SCR are removed.
• The turn off time is divided into two intervals of reverse recovery time (t_rr) and gate recovery time (t_gr)

t_q = t_rr  + t_gr

Circuit turnoff time (t_c):

It is defined as the time between the instant anode current becomes zero and the instant reverse voltage due to practical circuit reaches zero.

Note:

The circuit turn off time should be more than the device turnoff time for reliable turn off, otherwise, the device may turn on at an undesired instant, a process called Commutation failure.