Class B Chopper

The commutating components L and C are connected in parallel with SCR T1 in the class B commutation. The class b commutation is also called as resonant commutation because L and C makes resonance circuit. It is also called as self-commutation because it turns off automatically and its time depends upon L and C parameter.

Power circuit diagram of class b commutation

Working of class b commutation

SCR T1 turned on

When SCR T1 is turned off, the charging of capacitor is done through path(+)V_dc – (+)C –C(-) – L – R_L–V_dc(-). When the SCR T1 is turned on, the load current flows through path (+)V_dc – SCR T1 –R_L– V_dc(-). As soon as SCR T1 is turned on, the discharging of capacitor is done through path (+)C – SCR T1 – L–C(-). Therefore the load current as well as capacitor discharging current flows through SCR T1.

SCR T1 turned off

Once capacitor discharges, it will again charge with opposite polarity ( upper plate negative and lower plate positive ). Therefore, the commutating current I_C flows in the opposite direction to that of load current I_L. When the capacitor current I_C is greater than the load current, the SCR T1 is turned off.

SCR T1 again turned on

As the SCR T1 is turned off, the charging of capacitor again done through path (+)V_dc – (+)C – C(-) – L – R_L– V_dc(-). When the voltage across capacitor becomes V_dc, the SCR T1 is again switched on. The SCR T1 turned on and turned off automatically in this way. The switching time of SCR T1 depends upon commutating components L and C.

CLASS B COMMUTATION DRAWING.PDF

Application of class B chopper

The application of class B chopper is in the DC chopper.
The SCR T1 is turned on during T_ON time and turned off during T_OFF time automatically.
The SCR automatically turned on and turned off in the class A and class B commutation therefore it is called as self commutation.