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 I_H.

The turn off  time ( t_off ) 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 I_rr and recombination time t_rc. 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 t_rr.

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