Type D Chopper ( Class D Chopper )

 Introduction of type D chopper

The type D chopper is two quadrant chopper as it operates in the first and fourth quadrant. When the load voltage and current becomes positive, it operates in the first quadrant. Similarly, when load voltage becomes negative but load current is positive, it operates in the fourth quadrant.  The two quadrant type B chopper or type D chopper is shown in the figure A. 

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

Power circuit diagram of type D chopper

Working of type D chopper

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.

Application of type D chopper

The application of type D chopper is in the forward motoring and regenerative braking of motor.

Waveform of type D chopper

The output voltage and current waveform is shown in the figure B.

The output voltage remains negative when chopper off time less than the chopper on time. The load current remains positive during that period.

T_ON < T_OFF 

The output voltage remains positive when chopper on time is more than that of chopper off time. The load current remains positive during that period.

T_ON > T_OFF