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.

       V_O = ( T_ON / T_ON + T_OFF ) V_dc

             = ( T_ON / T_ON + T_ON) V_dc( T_ON = T_OFF )

             =  0.5V_dc

• If the chopper ON time is kept ¼ times chopper OFF time, the output voltage becomes 20% of the input voltage.

       V_O = ( T_ON / T_ON + T_OFF ) V_dc

             = ( T_ON / T_ON + 4T_ON) V_dc  ( T_ON = T_OFF / 4)

             =  0.2V_dc

                     = 20% V_dc

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.

Current limit control

• The maximum current and minimum current pass through chopper is determined in this method in order to turn on and turn off it. 
• When the current becomes maximum, the chopper becomes OFF. Similarly when the current becomes minimum, the chopper becomes ON. 
• The chopper ON – OFF control is shown in the figure D.
• If there is small difference between maximum and minimum current, the ripple is produced at the output waveform.
• The semiconductor switching losses are higher due to high chopping frequency.  

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