Zener failure in a capacitive power supply

[panipat]

In capacitive power supply(transformerless supply) we are facing problem oz zener failure. It fails short.Is is 9v1 2W. We have done lots of analysis on surge and temperature but we found these are not affecting it
Does any one have idea about why its failing?

 

[ghydda]

Zener failure

Usually harmonics on the mains is the cause for the overload failure of the zener used in such power supplies.

Usually it is the 5th and 7th harmonic which carries the bulk of the energy among all the harmonics. But it depends on the environment your supply is in.

If you have access to a spice-tool, try simulate the circuit with and without harmonics and you will see the change in power which the zener must absorb. The extra energy provided by the harmonics can be quite large!


Cheers
/Ghydda