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.
last diagram is not right.
ReplyDeletewhat is mistake?
ReplyDeleteThe last diagram lacks specification regarding to BSS 17:1896
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