Question regarding MOSFET


I am having a bit of trouble with my inverter/UPS, which has worked great for two years by the way.

Lately, when switching from Wapda to Battery Mode, the inverter goes into Fault mode (continuous beep and no output with red LED) randomly. Sometimes it switches perfectly fine, sometimes it fails and upon restarting (turning it off and back on again) it starts to work fine. Sometimes it goes to Fault mode again immediately.

Almost a month ago, I faced the same problem, and it got worse to a point that it never worked fine and always went to Fault mode whenever it switched from Wapda to Battery. I took it to a look repair shop who deal with imported UPS repairing. After checking they said that FET was shorted and they have replaced the FET now and it should work fine.

It did work fine untill a few days back when the same problem appeared again. At this time, it is random (I have tried to relate it with load or continuous working time but none of this seems to be a factor. It looks purely random to me), although frequency of going into Fault mode is increasing with every passing day and my guess is that it will start constantly going into Fault mode any day now.

I don't know much about FETs (or MOSFETs). Just read a few bits here and there and they seem to be responsible for switching. My question is: is it really just the MOSFETs problem? If one (or more) of them are shorted then how come the switching works fine sometimes and fails sometimes. Shouldn't it always fail (or work)?

Secondly, any idea what else could be the problem? so that I can talk to the repair guy and convince him to look into that area as well?

BTW, the inverter is Lenoxx (Australian) 12V (Wattage specs are not available but it doesn't feel like to be more than 600-700W).

Any help will be appreciated. Thanks.


When a MOSFET burns (gets shorted) in a bank of few parallel MOSFETs, the surviving MOSFETs surely gets some internal damage and this will show up very soon.

The same happened in your case. Always change all the MOSFETs in that bank in which one or two got shorted.