DC - DC Chopper: Principle of Operation

DC to DC Chopper Circuit- Operating Principle:

A chopper is a high speed ON/OFF semiconductor switch.
It connects source to load and disconnects the load from source at a fast speed.
By doing this, a chopped load voltage is obtained from a constant DC supply of magnitude Vs.STEP_DOWN_CHOPPER

  • The principle of chopper operation can be explained with the following figures.
  • Here the SCR is used as a switch. 
  • The SCR can be turned ON and turned OFF with the help of triggering circuit and commutating circuit respectively.
  • It can be turned on or turned off as desired at very high frequency.
  • To avoid the complexity, the commutation circuits are not shown in the figure.
  • During the TON period, chopper is in ON condition and load voltage is equal to source voltage Vs.chopper_TonDuring the TOFF period, chopper is OFF condition, load current flows through the freewheeling diode FD.
    chopper_ToffAs a result, load terminals are short circuited by freewheeling diode FD.
    Thus the load voltage is zero during TOFF period.
    In this manner, a chopped DC voltage is produced across the load terminals.

The average voltage is given as follows:

VO = (TON/[TON+TOFF] )VS
      = (TON/T )VS
      = α. VS
Here α = Duty cycle = (TON/T)

where
T = TON +TOFF
Chopper frequency f = 1/T
The above equation shows that, the load voltage is independent of load current.

From the T=TON +TOFF equation, it is clear that the load voltage depends on two factors.
The supply voltage
The duty cycle of the chopper

  • Since the supply voltage is constant,load voltage is governed by the duty-cycle of the chopper. 
  • In other words, the load voltage is dependent on two factors TON and TOFF.
  • Hence it is concluded that the average load voltage can be controlled by varying the value of TON and/of TOFF in the following two ways.
    [1] Varying TON and keeping the periodic time T constant. This is called constant frequency system.
    [2] Variable frequency system. ie, Keeping either TON constant and varying TOFF or keeping TOFF constant and varying TON.

Read More:
How Class D Chopper Works?  
Class A Chopper Operation Tutorial  
Thyristor Turn OFF Methods in Chopper Circuits  
How Morgan Chopper does work?

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