Correct Answer - Option 3 : commutation
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 (tq):
- It is the time between the instant anode current becomes zero and the instant SCR regains forward blocking capability.
- During this time (tq) all the excess carriers from four layers of SCR are removed.
- The turn off time is divided into two intervals of reverse recovery time (trr) and gate recovery time (tgr)
tq = trr + tgr
Circuit turnoff time (tc):
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.
