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.
- 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.During the TOFF period, chopper is OFF condition, load current flows through the freewheeling diode FD.
As 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)
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.
 Varying TON and keeping the periodic time T constant. This is called constant frequency system.
 Variable frequency system. ie, Keeping either TON constant and varying TOFF or keeping TOFF constant and varying TON.
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