Class-A or Type-A chopper is defined as the chopper in which the direction of power flow is always from source to load. The operation of this chopper is always in first quadrant. This is the reason; it is also known as First Quadrant Chopper. In this type of chopper, the output voltage and load current are positive all the time. A step-down chopper is an example of Class-A or Type-A chopper.
Let us now understand the operation of a Class-A / Type-A chopper with the help of circuit diagram. Figure below shows the circuit diagram of a step-down chopper.
In the above circuit, when chopper CH is ON, the current flows from source to load and voltage across the load is equal to the source voltage. The direction of flow of current is shown using arrow. We assume the shown direction of voltage and current as positive. Thus, the output voltage Vo and Io both are positive when chopper is ON. You may refer Step-down chopper for output voltage and current waveform.
When chopper CH is switched OFF, the load gets disconnected from the supply. However, the flow of current to the load continues due to the presence of inductor L. The current through the inductor decreases during the OFF period and hence the polarity of induced emf across it is reversed from what shown in the circuit diagram. This makes free-wheeling diode forward biased and hence the load current continues to flow through free-wheeling diode (D), L and load. The direction of flow of load current is still the same as that was during the ON condition. This means, the Io is positive. The value of load voltage becomes ZERO during the OFF time of chopper.
Thus, we see that the average value of load voltage and load current is always positive from the step-down chopper. This is shown by the shaded area in the first quadrant of Vo-Io plane in the figure below.
From the above discussion, it is clear that the operation of Class-A or Type-A Chopper is always confined in first quadrant and hence power flow is always from source to load. Step-down chopper is a Class-A or Type-A Chopper.