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.

CLASS E COMMUTATION DRAWING.PDF

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