What conditions could contribute to the failure of Zener diodes used as regulator in Capacitive AC-DC converter topology? Working in diagnostic job-nature, I have seen several products failed after 2 to 7 years (out of 10 years expected life-span) of operation due to the failure of zener diode in AC/DC converter circuit. The zener diode failure manifested as small resistance (0.20-100 Ohm).
The AC mains is subjected to voltage fluctuations due to daily load profile (up to >20%) and also subjected to seasonal intermittent supply.
What conditions could contribute to zener diode failure:
Could it be the AC input voltage fluctuation? Or high DC output ripply? Over voltage or over current in ESD-like events? Excess thermal dissipation?
How such problem can be reduced in replacing faulty diodes?