Class-C or Type-C Chopper Explanation

by

Class-C or Type-C Chopper is a category of chopper which can operate in first as well as second quadrant. This basically means that, the power can either flow from source to load or load to source in this chopper. This kind of chopper is also known as Two Quadrant Class-A chopper. In this article, we will discuss Class-C or Type-C chopper, its working principle with the help of circuit diagram and its application.

Operation / Working of Class-C or Type-C Chopper:

As we know that, a Class-A and Class-B chopper operates in first and second quadrant respectively. Therefore, if we connect both these types of chopper in parallel then it is possible to have chopper operation in first as well as second quadrant.

In fact, Class-C or Type-C chopper is obtained by the parallel connection of Class-A and Class-B chopper. Figure below shows the circuit diagram of this type of chopper.

circuit-diagram-of-class-c-or-type-c-chopper

Carefully observe the circuit diagram. You will notice that chopper CH1, free-wheeling diode (FD) and load are forming Class-A Chopper whereas chopper CH2, D2 and Load are forming Class-B chopper. Both these choppers are connected in parallel. To obtain first quadrant operation we should switch ON chopper CH1 and for getting second quadrant operation we should switch ON chopper CH2. Let us discuss this in detail.

Case-1: When CH1 is switched ON / OFF

When chopper CH1 is switched ON, source Vs directly gets connected to the load and hence, load voltage Vo is equal to source voltage. The direction of load current is from source to load as shown in the circuit diagram which is assumed positive.

When CH1 is switched OFF, the free-wheeling diode FD comes into the circuit as it gets forward biased and hence shorts the load. Therefore, the output voltage Vo becomes zero. However, the io continues to die down through the FD and L in the same direction as shown in circuit diagram. Thus, the average output voltage Vo and current Io are positive and hence operation of chopper is in first quadrant. In fact, this is the Class-B mode of operation.

Case-2: When CH2 is switched ON / OFF

When chopper CH2 is switched ON, load DC source E drives current through CH2 and load. The direction of this current io will be opposite to that shown in circuit diagram and hence is assumed negative. Output voltage Vo is zero during this time. When CH2 is made OFF, diode D2 gets forward biased and hence the current into the source from the load. The output voltage is Vs in this time as the load is connected to the source through D2 during OFF time of chopper CH2. Thus, the load current is always negative i.e. operation of chopper is within second quadrant. In fact, this is the Class-B mode of operation.

From the above two cases, we can conclude the following points:

  • The output voltage Vo is zero when chopper CH2 is ON or free-wheeling diode FD conducts.
  • The output voltage Vo is equal to source voltage Vs when chopper CH1 is ON or diode D2 conducts.
  • The load current flows in the direction shown in circuit diagram i.e.io is positive when CH1 is ON or FD conducts.
  • The load current flows opposite to the direction shown in circuit diagram i.e.io is negative when CH2 is ON or D2 conducts.

The average load voltage is always positive but the average load current may be positive or negative. Therefore, power flow may be from source to load (first quadrant operation) or load to source (second quadrant operation). The operating region of Class-C or Type-C chopper is shown below by hatched area.

quadrant operation of class-c or type-c chopper

Caution:

Choppers CH1 and CH2 should not be ON simultaneously as this would lead to direct short circuit on the supply lines.

Application:

Class-C or Type-C choppers are used for motoring and regenerative breaking of DC Motors.

Leave a Comment