12 March 2017

Methods of Chopper Output Voltage Control

There are following methods to control output voltage of the chopper.

Constant frequency control ( Pulse width modulation control )

  • The chopping frequency is kept constant in this method therefore it is called as constant frequency control. 
  • The chopper ON time or chopper OFF time is adjusted in this method therefore it is called as pulse width modulation control. 
  • When the chopper ON time and OFF time is kept equal, the output voltage is equal to half the input voltage.

       VO = ( TON / TON + TOFF ) Vdc
             = ( TON / TON + TON) Vdc( TON = TOFF )
             =  0.5Vdc
  • If the chopper ON time is kept ¼ times chopper OFF time, the output voltage becomes 20% of the input voltage.

       VO = ( TON / TON + TOFF ) Vdc
             = ( TON / TON + 4TON) Vdc  ( TON = TOFF / 4)
             =  0.2Vdc
                     = 20% Vdc

constant frequency control of chopper

Variable frequency control ( Frequency modulation control )

  • The chopping frequency is kept variable in this method and chopper ON time or OFF time is kept constant therefore it is called as frequency modulation control. 
  • The chopper OFF time is kept constant and chopper frequency is kept variable as shown in the figure B. 
  • Similarly the chopper ON time is kept constant and chopper frequency is kept variable in the figure C.

Disadvantages of frequency modulation techniques over pulse width modulation techniques

  • The design of filter circuit is difficult because the range of chopping frequency is very large in order to control output voltage.
  • The load current becomes discontinuous if the chopper ON time is kept higher than the chopper OFF time.
  • There is communication interference with nearby telecommunication lines and other digital signal.

variable frequency control of chopper



Current limit control





current limit control of chopper


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