Peak inverse voltage:

The PIV rating of the diode is the instantaneous inverse, voltage that it can withstand without avalanche going on. A good power supply has diodes whose PIV ratings are significantly greater than the peak voltage of the alternating current at the input.

If PIV rating is not great enough, diode or diodes in a supply will conduct for part of reverse cycle. This will degrade efficiency of supply; the reverse current will buck the forward current.

Diodes can be connected in series to obtain a higher PIV capacity than the single diode alone. This scheme is seen in high voltage supplies, such as those required for tube type ham radio power amplifiers. High value resistors, of around 500 Ω for each peak inverse volt, are placed across each diode in set to distribute reverse bias equally among the diodes. Also, each diode is shunted by the capacitor of 0.005 µF or 0.1 µF.

 
Figure--Diodes in series should be shunted by resistors and capacitors.