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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
.
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