**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.

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