The quadrant operation of chopper is basically the circuit configuration so adjusted to achieve the chopper voltage Vo and output current Io in all the four quadrants. A chopper can be operated in all four quadrants by employing required circuit configuration.
We know that, a chopper is a semiconductor power electronic device. Again, a semiconductor power device used in chopper circuit are unilateral devices. This means, the polarities of output voltage and direction of output current is restricted. If we want o change the polarity of either output voltage or current or both, we need to change the circuit configuration of chopper. Here comes the concept of four quadrant of operation of chopper.
To better understand the quadrant operation of chopper, let us have a look the figure below.
In the above diagram, positive current means current from source to load and negative current means current from load to source. Similarly, positive voltage means the voltage drop in the load in the positive current direction. Refer the circuit below.
In the figure, the current io and Vo are positive. If the direction of the io or vo changes from the shown direction, it will be assumed negative. The circuit diagram shown above correspond to step-down chopper. In step-down chopper the load voltage and load current is always in the direction shown in the figure. This means, io and Vo are always positive and hence the operation of chopper is in First Quadrant.
Similarly, in second quadrant operation, the Vo will remain positive but the load current io will be negative. A step-up chopper operates in second quadrant during OFF period. It may be noted that the circuit configuration for step-up and step-down choppers are different. In fact, we need to change the circuit to suit our requirement for a particular quadrant operation. However, there exists a circuit configuration which can be used to operate a chopper in any of the four quadrants. This configuration is called Class-E Chopper.
In third quadrant, both io and Vo will be negative and in fourth quadrant, the Io will remain positive but the Vo will be negative.
Thus, quadrant operation of chopper is nothing but to change the polarity of output voltage and current by means of suitable circuit configuration.