Power electronics digital notes b. Tech III year

t
q
 = t
rr
 + t
gr
The figure below shows the switching characteristics of SCR during turn ON and OFF. The time t
1
to t
3
is 
called as reverse recovery time; at the instant t
1
the anode current is zero and builds up in the reverse 
direction which is called as reverse recovery current. This current removes the excess charge carriers from 
outer layers during the time t
1
to t
3
. 
At instant t
3
, junctions J
1
and J
3
are able to block the reverse voltage but, the SCR is not yet able to block 
the forward voltage due to the presence of excess charge carriers in junction J
2
. These carriers can be 
disappeared only by the way of recombination and this could be achieved by maintaining a reverse 
voltage across the SCR. 

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Figure: 1. 22. Dynamic characteristics of SCR 
Hence , during the time t
3
to t
4
, the recombination of charges takes place and at the instant t
4
, junction 
J
2
completely recovers. This time is called gate recovery time t
gr
. 

From the figure the turn OFF time is the time interval between the t
4
and t
1
. Generally, this time 
varies from 10 to 100 microseconds. This turn OFF time t
q
is applicable to the individual SCR. 

The time required by the commutation circuit to apply the reverse voltage to commutate the SCR 
is called the circuit turn OFF time (t
c
). For a safety margin or reliable commutation, this t
c
must 
be greater than the t
q
otherwise commutation failure occurs. 

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
The SCRs which have slow turn OFF time as in between 50 to 100 microseconds are called as 
converter grade SCRs. These are used in phase controlled rectifiers, cyclo converters, AC voltage 
regulators, etc. 

The SCRs which have fast turn OFF time as in between 3 to 50 microseconds are inverter grade 
SCRs. These are costlier compared to converter grade and are used in choppers, force 
commutated converters and inverters.