Power electronics digital notes b. Tech III year

Class E Commutation 
This is also known as external pulse commutation. In this, an external pulse source is used to produce the 
reverse voltage across the SCR. The circuit below shows the class E commutation circuit which uses a 
pulse transformer to produce the commutating pulse and is designed with tight coupling between the 
primary and secondary with a small air gap. 
If the SCR need to be commutated, pulse duration equal to the turn OFF time of the SCR is applied. 
When the SCR is triggered, load current flows through the pulse transformer. If the pulse is applied to the 
primary of the pulse transformer, an emf or voltage is induced in the secondary of the pulse transformer. 
This induced voltage is applied across the SCR as a reverse polarity and hence the SCR is turned OFF. 
The capacitor offers a very low or zero impedance to the high frequency pulse. 
Figure: 1. 20. Class E Commutation circuit and waveforms 

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Natural Commutation 
In natural commutation, the source of commutation voltage is the supply source itself. If the SCR is 
connected to an AC supply, at every end of the positive half cycle the anode current goes through the 
natural current zero and also immediately a reverse voltage is applied across the SCR. These are the 
conditions to turn OFF the SCR. 
This method of commutation is also called as source commutation, or line commutation, or class F 
commutation. This commutation is possible with line commutated inverters, controlled rectifiers, cyclo 
converters and AC voltage regulators because the supply is the AC source in all these converters. 
Figure: 1. 21. Natural Commutation circuit and waveforms 
Dynamic Turn OFF Switching Characteristics 
The transition of an SCR from forward conduction state to forward blocking state is called as turn OFF or 
commutation of SCR. As we know that once the SCR starts conducting, the gate has no control over it to 
bring back to forward blocking or OFF state. 
To turn OFF the SCR, the current must be reduced to a level below the holding current of SCR. We have 
discussed various methods above to turn OFF the SCR in which SCR turn OFF is achieved by reducing 

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the forward current to zero. But if we apply the forward voltage immediately after the current zero of 
SCR, it starts conducting again even without gate triggering. 
This is due to the presence of charge carriers in the four layers. Therefore, it is necessary to apply the 
reverse voltage, over a finite time across the SCR to remove the charge carriers. 
Hence the turn OFF time is defined as the time between the instant the anode current becomes zero and 
the instant at which the SCR retains the forward blocking capability. The excess charge carriers from the 
four layers must be removed to bring back the SCR to forward conduction mode. 
This process takes place in two stages. In a first stage excess carriers from outer layers are removed and in 
second stage excess carriers in the inner two layers are to be recombined. Hence, the total turn OFF time 
t
q
is divided into two intervals; reverse recovery time t
rr
and gate recovery time t
gr
.