[Moved] MOSFET Burning Problem in APC UPS, please somebody help me...

[xman111]

Hi,
I`m new here and my english is not good but I`ll try to describe all details related to my problem at best possible way and I hope that somebody will help me in this problem...

I have old APC BACK-UPS PRO 420, and for some reason on this ups stop to work, after checking what is problem I see that main transformer is burned and one mosfet (q34 - schematic -> http://www.upsclub.org/schem/APC/218/218F5S5.pdf ), because I have one more same ups with some other problems I replaced this parts and when I start this ups to work on battery (unplugged from main power source) all working fine, without any problem, no heating no bad sounds, nothing, all fine...I leave this ups to work few minutes on battery and all was good...after this I plug it to 200v ups switch normal from battery to main power source and continue to work normal, no heating, no bad sounds...but after ~10 seconds ups stop to work, stop with beeps, fet q34 and q33 start to heat abnormal, from transformer I have some very bad sound like overloaded and I must unplug it from all power sources (now again don`t wont to work on battery, same sounds like on main power and after few seconds trying it switch to overload and stop to work)...

After checking mosfets again I see that Q34 is again burned (not q33), I replaced it again and again same test but again on same way same mosfet is burned!!!

Do you know why this mosfet burning after ~10seconds on main power source??? Without any load, without any heating of this mosfet (until it burned)...

All schematics you can see here: Index of /schem/APC/218

P.S. Mosfets q33 and q34 = IRFZ24N

 

[Tipu606]

Re: MOSFET Burning Problem in APC UPS, please somebody help me...

seems like the ups is overloaded
try a higher rated mosfet like the IRFZ44N(55V,49A) or better IRF3205(55V,110A)

 

[xman111]

Re: MOSFET Burning Problem in APC UPS, please somebody help me...

No no, this problem I have without any load! For me look like after 10s on main power ups also try to work on battery and than I have q34 burned but maybe that is not problem, maybe something else problem but I can`t find what is problem because that 10s all working fine, without any bad sound, overheating etc...q34 is cold that 10s but after that 10s burning in fire...

 

[radtech]

Re: MOSFET Burning Problem in APC UPS, please somebody help me...

Just replacing burnt components does not fix things.
You need to find the reason.
I would not repair a UPS without replacing at least some capacitors and checking a few waveforms.
If the FET is blown there are often a few parts around it that are there to protect it, if so then they may have clearly failed.
Also when the FET burns up it stresses parts around it.
I would not practice the "it could be this or that" approach, you need to check it by taking some measurements.

 

[xman111]

But what to check?
Here you can see q34 fet: http://www.upsclub.org/schem/APC/218/218F5S5.pdf
And all other parts: Index of /schem/APC/218