Electrical Revolution: Class E Commutation ( External Pulse or Type E commutation )

The class E commutation is also called as external pulse commutation because the SCR is turned off by external source.
The SCR is turned off by applying reverse voltage across it.
Figure A shows the external pulse commutation by using transistor.
The SCR T1 is turned off by applying reverse voltage through transistor Q and voltage source V.
When the SCR T1 is turned off , the load current flows through path (+)V_dc– SCR T1 – R_L–V_dc(-).
When a base signal is applied to transistor Q, it turns on and reverse voltage applies to SCR T1 by DC voltage source V.
The base signal must be greater than the maximum load current.
The time for reverse voltage across SCR T1 must be greater than the turn off time of the SCR T1.

Figure B shows the circuit for commutation of the SCR T1 by using pulse transformer.
The secondary of the pulse transformer is connected in series with the load.
When the SCR T1 is turned on by gate pulse, the load current is flowing through secondary of pulse transformer and load.
When a pulse is given to primary of the pulse transformer, the voltage across secondary of pulse transformer -V_dc provides reverse voltage across SCR T1 resulting SCR T1 is turned off.
The voltage drop across capacitor is very low because frequency of the pulse transformer is very high.
The voltage drop across capacitor is hardly 1.0 Volt.
The following points to be noted while designing pulse transformer. The efficiency of external pulse commutation is very high due to low energy requires.

Electrical Revolution