Class-B or Type-B chopper is defined as the chopper in which the direction of power flow is always from load to source. The operation of this chopper is confined in the second quadrant. This is the reason; it is also known as Second Quadrant Chopper. In second quadrant chopper, the output voltage is positive but the load current is negative all the time. Class-B or Type-B chopper is also known as Step-up chopper.
Operation of Class-B or Type-B Chopper:
Let us now understand the operation of a Class-B / Type-B chopper with the help of circuit diagram. Figure-1 below shows the circuit diagram of a Second Quadrant Chopper.
It should be noted that load must contain a DC source (E) in Class-B / Type-B chopper. When chopper CH is ON, the load voltage or output voltage vo becomes zero. But, load voltage E drives current through inductor L and CH. During this ON period of chopper, the inductor stores energy in it. It may be noted that diode D is reversed biased during ON time and hence the load is disconnected from the source.
When chopper CH is switched OFF, the load gets connected from the supply source Vs. However, due to the presence of inductor L, the current cannot be reversed suddenly. Rather, the current through inductor must die down before the current gets reversed. During this time when current is decaying through the inductor, the current io flows from load to source. In fact, the voltage across the load vo during this time is equal to (E + Ldi/dt) which is more than the source voltage. This is the reason; this type of chopper is also called step-up chopper. This voltage (E + Ldi/dt) makes the diode D forward biased and hence the current starts to flow from load to source. Hence, power flows from load to source. Now, if we switch ON the chopper CH before the current through inductor gets reversed then source will never get a chance to transfer power to the load. The chopping frequency is kept high so that load always feeds power to the source.
From the above discussion, it is clear that current io always flows out of the load irrespective of the fact that whether chopper is ON or OFF. Therefore, current io is always negative (A general convention is that current is positive if it flows from source to load). Since, output voltage Vo is always positive and current is negative, the load power is negative. Negative load power means that load feeds the source.
Thus, we see that the average value of load voltage is positive whereas load current is always negative for Class-B / Type-B chopper. This means, the operation of chopper is in second quadrant. This is shown by the hatched area in the second quadrant of Vo-Io plane in the Figure-2.