What to look for that blew UPS MOSFETs?

[berniedd]

Hello guys. The 3 yr old office UPS blew one night and I was asked to look into it. It is a Singapore made Giant unit Model 1200 VA. There are 2 banks of FB50N06 power MOSFETS (push-pull circuit?). Each bank has 3 MOSFETS. One bank had 2 of the 3 MOSFETS blistered and burned. The middle one even had its Source pin melted. The two 12 volt batteries were dead.

Nothing else looked cooked and the main AC fuse was intact. I would want to ask what parts to suspect and replace before I change out the burned semiconductors. Thanks in advance.

[stj]

Re: What to look for that blew UPS MOSFETs?

the controller may have turned both on at the same time,
or one could have just shorted.
was the gate pin shorted?
if yes then the control circuit is probably damaged

[Behemot]

Re: What to look for that blew UPS MOSFETs?

Post some pics…

It's likely the batteries died first overloading the switchers…

[RCA2000]

Re: What to look for that blew UPS MOSFETs?

On my own APC ES-750UPS..I was AFRAID...I might have a sort of SMPS issue...beacuse very recently...it got to where it would NOT hold the load of the computer even for a BRIEF SECOND..and would drop out...crashing the computer when it did. BUT..it WOULD run with SMALL load...like the Cable modem.. I was afraid MORE was wrong than just a dead battery...but I HOPED that was all that was wrong..that the battery was dying and not strong enough...it made sense that was the problem...not too mention it had been in there a LONG time...

In the end...it WAS a bad battery. Got a new one in it today...works FINE now. Also...the old one must have swelled a bit...it was NOT easy to get out of the housing. But i did so...now it holds all loads on it fine. I even tested it with a 70's model Zenith chromacolor II tv set--with VRT tranny power supply...it held it FINE.. Normally...this UPS will keep this computer, the monitor, (17" rtrinitron tube) , Fioptics modem, Altec speaker system and printer--ALL on...for near a hour...and if I power the monitor down..WELL over an hour.