Classification of forced commutation methods
Turn-off dynamics of the SCR:
Fig: Turn-off dynamics of the SCR
- A thyristor can be turned ON by applying a positive voltage of about a volt or a current of a few tens of milliamps at the gate-cathode terminals.
- However, the amplifying gain of this regenerative device being in the order of the 108, the SCR cannot be turned OFF via the gate terminal. It will turn-off only after the anode current is annulled either naturally or using forced commutation techniques.
- These methods of turn-off do not refer to those cases where the anode current is gradually reduced below Holding Current level manually or through a slow process. Once the SCR is turned ON, it remains ON even after removal of the gate signal, as long as a minimum current, the Holding Current, Ih, is maintained in the main or rectifier circuit.
- A negative current flows through the device. This current returns to zero only after the reverse recovery time trr, when the SCR is said to have regained its reverse blocking capability. The device can block a forward voltage only after a further tfr, the forward recovery time has elapsed.
- Consequently, the SCR must continue to be reverse-biased for a minimum of tfr trr = tq, the rated turn-off time of the device. The external circuit must therefore reverse bias the SCR for a time toff > tq. Subsequently, the reapplied forward biasing voltage must rise at a dv/dt < dv/dt (reapplied) rated. This dv/dt is less than the static counterpart. General Electric has suggested six classification methods for the turn-off techniques generally adopted for the SCR. Others have chosen different classification rules.
- SCRs have turn-off times rated between 8 - 50 μsecs. The faster ones are popularly known as 'Inverter grade' and the slower ones as 'Converter grade' SCRs. The latter are available at higher current levels while the faster ones are expectedly costlier.
Classification of forced commutation methods:
The six distinct classes by which the SCR can be turned off are:
a) Class A Self commutated by a resonating load
b) Class B Self commutated by an L-C circuit
c) Class C C or L-C switched by another load carrying SCR
d) Class D C or L-C switched by an auxiliary SCR
e) Class E An external pulse source for commutation
f) Class F AC line commutation