First Quadrant or Class A chopper
A chopper two mode operation of class A chopper can be explained as follow.
Figure class A chopper circuit V0- I0 characteristic
When the chopper switch is on V0= Vs and current flows in the direction in this mode V0 and I0 will be positive.
When the chopper SW is OFFF V0 = 0 (Since diode act like short Circuit). Now the inductor present in the load releases its energy and maintain the current in the same direction through the freewheeling diode FD. Again the V0 and I0 will be positive. This operation is shown in by the hatched area in figur.