DC Chopper Voltage Control

DC Chopper Voltage Control Methods

There are following methods for DC Chopper Voltage Controls.

• Constant frequency control
• Variable frequency control
• Current limit control

DC Chopper Voltage Control: Constant Frequency Control

DC Chopper Voltage Control: 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.

When Chopper ON time is equal to chopper OFF time

       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

DC Chopper Voltage Control: 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 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.

DC Chopper Voltage Control: Current limit control

The maximum current and minimum current pass-through chopper is determined in this method 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.

Disadvantages of current limit control 

The semiconductor switching losses are higher due to high chopping frequency.

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