Power Electronics: Turning off SCR [Commutation]

Commutation

To turn ON a SCR, a low voltage, short duration pulse is applied to the gate (typically 4V, 100µs).
Once the SCR is turned-on, the gate loses control and the SCR will only turn off when the load current falls virtually to zero, or the SCR is reverse biased.
The SCR will turn off naturally with a.c. supplies as the voltage reverses (which is called as Natural Commutation), but no such reversal occurs with d.c. supplies and it is necessary to force a voltage reversal if turn-off is to occur. This process is called Forced Commutation.
The process of turning OFF SCR is defined as "Commutation".
In all commutation techniques, a reverse voltage is applied across the SCR during the turn OFF process.
By turning OFF a SCR we bring it from forward conducting to the forward blocking mode.

The conditions to be satisfied in order to turn OFF an SCR are:

IA < IH ( Anode current must be less than holding current).
A reverse voltage is applied to SCR for sufficient time enabling it to recover its blocking state.

      There are two methods by which a SCR can be turned OFF.
                I. Natural Commutation
               II. Forced Commutation

In AC circuit, the current always passes through zero for every half cycle.
As the current passes through natural zero, a reverse Voltage will simultaneously appear across the device. This will turn OFF the device immediately.
This process is called as natural commutation, since no external circuit is required for this purpose. It is also known as line commutation.
AC voltage controllers or phase voltage controllers are the example for the natural commutation.

To turn OFF a SCR, the forward anode current should be brought to zero for sufficient time to allow the removal of charged carriers.
In case of DC circuits, the forward current should be forced to zero by means of some external circuits. This process is called as forced commutation.