## 12/03/2017

### 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

### 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.