Why freewheeling diode is used?

1. Consider a simple half wave rectifier circuit consisting of an ac source a diode and a series R-L load.

2. now during the positve half cycle of the source current, the diode conducts but it stops conducting when the negative half cycle starts.

3. For an R-L load load current lags the source voltage and tries to flow in the previous direction even when the source voltage has reversed.

4. This action due to the inductance, builds a high voltage across the diode which may exceed its forward breakdown limit and permanently damage it.

5. Thus freewheeling diodes are used to short the load during negative half cycles and hence prevent the voltage breakdown of the semiconductor switches used in the control circuit.