Class E - External pulse source for commutation & Class F, AC line commutated
Class E – External Pulse Source for Commutation:
Fig: Class E external pulse source for commutation
- The transformer is designed with sufficient iron and air gap so as not to saturate. It is capable of carrying the load current with a small voltage drop compared with the supply voltage.
- When SCR1 is triggered, current flows through the load and pulse transformer. To turn SCR1 off a positive pulse is applied to the cathode of the SCR from an external pulse generator via the pulse transformer.
- The capacitor C is only charged to about 1 volt and for the duration of the turn-off pulse it can be considered to have zero impedance.
- Thus the pulse from the transformer reverses the voltage across the SCR, and it supplies the reverse recovery current and holds the voltage negative for the required turn-off time.
Class F, AC line commutated:
Fig: Class F natural commutation by supply voltage
- If the supply is an alternating voltage, load current will flow during the positive half cycle. With a highly inductive load, the current may remain continuous for some time till the energy trapped in the load inductance is dissipated.
- During the negative half cycle, therefore, the SCR will turn off when the load current becomes zero 'naturally'. The negative polarity of the voltage appearing across the outgoing SCR turns it off if the voltage persists for the rated turn-off period of the device.
- The duration of the half cycle must be definitely longer than the turn-off time of the SCR. energy trapped in the load inductance is dissipated. During the negative half cycle, therefore, the SCR will turn off when the load current becomes zero 'naturally'.
- The negative polarity of the voltage appearing across the outgoing SCR turns it off if the voltage persists for the rated turn-off period of the device. The duration of the half cycle must be definitely longer than the turn-off time of the SCR.
, where α the triggering angle and Isc and Vs
- This expression is obtained with the simplifying assumption that the input inductance contains no resistances. When the current rises in the incoming SCRs, which in the outgoing ones fall such that the total current remains constant at the load current level.
- When the current in the incoming ones reach load current level, the turn-off process of the outgoing ones is initiated. The reverse biasing voltage of these SCRs must continue till they reach their forward blocking state. As is evident from the above expression, the overlap period is a function of the triggering angle. It is lowest when α ~ 900. These SCRs being 'Converter grade', they have a larger turn-off time requirement of about 30-50 μsecs.
- The period when both the devices conduct is known as the 'overlap period'. Since all SCRs are in conduction, the output voltage for this period is zero.
- If the 'fully-controlled' converter is used as an inverter with triggering angles > 900, the converter triggering can be delayed till the 'margin angle' which includes the overlap angle and the turn-off time of the SCR - both dependent on the supply voltages.