The Reverse Break down (VBD) voltage of a diode is defined as the reverse bias voltage across the diode at which it starts conducting currents heavily. At voltages greater than Reverse Break down voltage the diode currents increase for even a small change in reverse bias voltage. Sufficient heat sink has to be provided to remove the heat dissipated at the diode junction for voltages greater than Reverse Break down voltage; otherwise it will lead to permanent damage of diode