The Class B Chopper is typically used in applications which require transfer of power from the load to the source. An example would be regenerative braking in trains, where the power from the driving motor is sent to the power mains. This is also known as inverting operation. 
In the Class B chopper, the output voltage is positive while the output current is negative.
In a class B chopper, a diode, in reverse bias, blocks power from the source to the load. The chopper is connected parallel to the load and the source. The load voltage is the back-emf of the winding of a DC motor. When the chopper is in the ON condition, the current due to the back-emf flows through the inductance and the resistance through the chopper. The diode does not conduct as the voltage across the chopper is zero as the chopper is 'ON'. No Current flows into the source.
When the chopper is switched OFF, the voltage across the chopper increases and this biases the diode in the forward direction. The diode conducts and the power reaches the source. The source may be a battery or any other power source.