Switching Characteristics During Turn off
The techniques used for turning off a thyristor is known as commutation techniques. A thyristor can be can be turn off by reducing the forward current to a level below the holding current IH.
The turn off time ( toff ) is time in which thyristor regain its forward blocking capability means thyristor changed its state form on state off state.
The turn off time t off is the sum of reverse recovery time Irr and recombination time trc. In t off time all the excess carriers must be removed from the four layers of SCR. After the forward current goes through the zero value, a reverse voltage appears across the thyrisotr as shown in figure1.12 this reverse voltage accelerants the turn off process by sweeping out the excess carriers from outer junctions J1 and J3,this is known as reverse recovery period and the time is reverse recovery time trr.
This reverse recovery time is shown in figure between time interval t1 and t3. The carriers around junction J2 and can be removed only by recombination because this junction is present between inner two layers. Actually after reverse recovery period junction J2 has trapped charges and these trapped charged cannot flow to the external circuit. Reverse voltage should be maintained across SCR for processing of recombination. Hence at the end of turn off a depletion layer develops across inner junction J2 and the thyristor recovers its ability to withstand forward voltages.
Figure Swithcing characteristics of SCR