Class-B Chopper Tutorial

How Class-B Chopper Works? Class-B Chopper Tutorial

Type B Chopper - An Introduction:

In this post we are going to see how the class B chopper does work? Before proceeding further it is highly recommended to refresh about he basics of the chopper. Please click here to know more about the Basics of Choppers, Types of choppers and applications.

The Class-B chopper or Type-B Chopper is also called as Step Up chopper, because the power can be transferred from a lower voltage (V0) to higher voltage (VS) only. This reverse power flow from the load to the source will occur only if the load is active. (ie) capable of providing continuous power output.
A common example for reverse power flow is the regenerative braking in DC motors.
In that case, the kinetic energy stored in the DC motor is returned to the input DC source.
Here kinetic energy stored in the DC motor is returned to the input DC source.
Other example for transfer of energy from load to source is energy stored in large inductors.

As shown in the below figure( vO-iO place), the class-B chopper is a single quadrant chopper (operates in 2nd quadrant only).Class_B_Chopper_quadrantIn this chopper,

  • The output current IO is restricted to negative values by the SCR T1 and diode D1. The output voltage VO is restricted to be positive and lower than VS.
  • The load current is negative. It means, the load current flows out of the load.
    If the current flows from source to load it is considered as positive current.
    If the current flows from load to source it is considered as negative current.]

As the current flows from load to the source, the energy is transferred from load to the source.
It is known as inverting operation.
This kind of situation occurs during the braking of DC motor.
The energy associated with back emf (Eb) of the motor is fed back to the source.
Thus the current is known as negative current.
Since motor rotates in the same direction, it is called Forward Regenerative Braking.
Regeneration means energy is fed from load to the source during braking.

Type-B Chopper Operation:



  • The diode D1 allows the current flow only from load (E) to the source (VS).
  • As shown in the figure, the load is assumed as motor load.
  • The motor has internal inductance (L) and resistance (R). To avoid complexity, the R is ignored in all figure.
  • The Eb is the back emf of the motor load.This back EMF Eb is responsible for negative current flow.

Mode- I: SCR(T1) is ON


  • When the switch (T1) is turned ON, the output voltage Vo = 0 but the load voltage E drives current through L and T1.
  • The negative current flows from Eb, L and T1.
  • This current keeps on increasing.
  • The current IO flows through Eb, R, L and switch in negative direction.
  • The inductance stores energy (which is supplied by back emf Eb) during this period.

Mode- II: SCR(T1) is OFF


  • When the Switch (T1) is turned OFF, the output voltage is VO= (E+L .di/dt).
  • This VO value exceeds supply voltage VS.
  • Consequently the diode D1 is forward biased and begins conduction.
  • Thus negative output current flows through supply Vs. ie,the current is forced through supply voltage Vs.
  • Thus supply consumes power.
  • This power is transferred from load inductance and Eb.
  • The switch is turned on again and the cycle repeats.

Remember that the class B operation is never implemented independently. it takes place in two and four quadrant chopper having motor loads.

Read More:

SCR Parallel Operation 
Flyback Converter Working Principle 
Squirrel Cage Induction Motor vs Slip Ring Induction Motor 
Power Electronics Interview Questions and Answers: Set-8 

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