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Mohamed Tagelsir Mohammadat
09-26-2013 05:07 AM

Zener diodes in Capacitive AC-DC converter topology

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?
09-26-2013 07:24 AM
Top #2
Carole Sherrington
09-26-2013 07:24 AM
Hi Mohamed,
It seems to me that there are two main possibilities:
1.Over dissipation caused by incorrect choice of device.
2. Surge related failure due to impulse on the AC.
The device size issue is fairly straightforwards;
What is the maximum steady state AC voltage supply? What is the maximum frequency? What is the maximum value of the ballasting capacitor? These together will give the maximum supply current to be shunted by the zener. Consult the temperature rises for the packages, likely ambient temperatures etc and you'll come up with a zener rating for your particular conditions. Have you measured the temperature of the zener in normal use, without any load? Consider a device that you can heatsink.
The surge failures may be dealt with if your calculation of zener size comes up with something a lot bigger than you're actually using. However, surge failures may be prevented by feeding the ballasting caps into a large value electrolytic, then feeding the zener from that by a low value resistor. the zener then doesn't see the peak value of the surge, which could be thousands of amps depending on your circuit.
Hope this was helpful
09-26-2013 09:54 AM
Top #3
09-26-2013 09:54 AM
Hi Mohamed
According to my experience I also think, like Carol, that the Zener may be damaged by a surge voltage. Another root cause could be the inrush current at supply plug-in.
In case of surge, as Carol, said, increasing the DC capacitor could help. But it helps only for the reverse current (going from Cathode to Anode). For the forward current (from A to K), it will not help. But for sure, the reverse current is more stressful as you apply it through the whole breakdown voltage of the Zener.
Anyway, for both cases, I will also suggest to increase the inrush current limiting resistor (in series with the AC cap.). It will help to reduce the current applied to the Zener for both cases.
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