Electrical Revolution: Type D Chopper ( Class D Chopper )

Working

The two quadrant type B chopper or type D chopper is shown in the figure A.
This chopper works in first as well as fourth quadrant therefore the output current remain positive but output voltage may be either positive or negative.
The polarity of output voltage depends upon chopper ON time and chopper OFF time. When

T_ON > T_OFF – Output voltage positive

T_ON < T_OFF – Output current negative

T_ON = T_OFF – Output voltage zero

First quadrant

When chopper CH1 and CH2 both are switched on simultaneously, the current flows through path (+)V_dc – A – CH1 – B – LOAD – CH2 – V_dc( – ).
The voltage across load is equal to V_dc and current increases exponentially.
The chopper works in first quadrant because the output voltage and output current both are positive.

Fourth quadrant

When chopper CH1 and CH2 both are switched off simultaneously, the diode D1 and D2 forward biased and current flows through path B – LOAD – D2 – D – (+)V_dc– V_dc( – ) – C – D1 – B.
The chopper works in fourth quadrant because the output voltage negative and output current positive becomes positive.
The application of type D chopper is in the forward motoring and regenerative braking of motor. The output voltage and current waveform is shown in the figure B.