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 T_ON period, chopper is in ON condition and load voltage is equal to source voltage Vs.During the T_OFF 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 T_OFF period.
  In this manner, a chopped DC voltage is produced across the load terminals.

The average voltage is given as follows:

V_O = (T_ON/[T_ON+T_OFF] )V_S
      = (T_ON/T )V_S
      = α. V_S
Here α = Duty cycle = (T_ON/T)
where
T = T_ON +T_OFF
Chopper frequency f = 1/T
The above equation shows that, the load voltage is independent of load current.

From the T=T_ON +T_OFF 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 T_ON and T_OFF.
• Hence it is concluded that the average load voltage can be controlled by varying the value of T_ON and/of T_OFF in the following two ways.
  [1] Varying T_ON and keeping the periodic time T constant. This is called constant frequency system.
  [2] Variable frequency system. ie, Keeping either T_ON constant and varying T_OFF or keeping T_OFF constant and varying T_ON.

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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