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Class C Chopper

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Circuit Diagram and Working

  • The type C chopper is parallel combination of type A and type B chopper.
  • The output voltage is always positive but output current either positive or negative in this type of chopper therefore it works in first as well as second quadrant.

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CH1 and CH2 OFF

  • The diode D1 and D2 does not forward biased because of CH1 and CH2 remain off at starting condition.
  • The load is completely isolated from the supply system.

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Diode D1 / Chopper CH2 Conducts

  • The output voltage becomes zero when diode D1 or CH2 conducts.
  • The output voltage becomes VDC When diode D2 or chopper CH1 conducts.

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Conduction

Output voltage

Diode D1 / chopper CH2

Zero

Diode D2 / chopper CH1

VDC

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  • The waveforms for gate signal G1, gate signal G2, output current and output voltage is shown in the figure C.

Chopper CH1 - ON

  • The chopper CH1 is switched on at point A and load current flows from supply to load side. The load current flows from Vdc - CH1 - L - Load - Vdc. The inductor current increases exponentially.

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Chopper CH1 - OFF

  • The chopper CH1 is switched off at point B and load current flows through path L – LOAD – D1 – L. When the value of L ( di/dt ) is equal to back emf Eb, the load current becomes zero ( Point C ).
  • The diode D1 conducts during point B to point C.

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Back emf Eb = L ( di/dt )

Load current becomes zero

Back emf Eb > L ( di/dt )

Load current flows in the negative direction

Back emf Eb < L ( di/dt )

Load current flows in the positive direction

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Chopper CH2 - ON

  • The chopper CH2 is switched on at point C when back emf of the motor is greater than the voltage across inductor.
  • The load current flows in the negative direction through path LOAD – L – CH2 – LOAD.
  • The negative current flows continue until chopper CH2 is switched off and chopper CH1 is switched on ( Point D ).

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Chopper CH2 - OFF

  • When the chopper CH2 is switched off, the stored energy of inductor dissipated its energy through path L – D2 – VDC – LOAD – L.
  • The chopper CH1 remains off in spite of it receives gate signal.
  • The chopper CH1 switched on at point E where the negative current becomes zero.

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SUMMARY

  • When chopper CH1 or diode D1 conducts, the load current becomes positive and load current becomes negative when CH2 or D2 conducts.
  • The chopper CH1 and CH2 never conducts simultaneously otherwise the DC supply source is short circuited.

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Thank You