Electronics > Repair
Seasonic FOCUS SPX 750w blows fuse when started. 5Vsb works. No shorts found
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acedogblast:
Hello everyone. I have a Seasonic FOCUS SPX 750w SFX (smaller ATX) PC power supply that blows its main fuse (12A) on startup (Green PS_ON signal is sent). After replacing fuse and one shorted diode (PFC bypass rated at 3A) I plugged in to find that 5V stand by is working fine but right when PS_ON signal from motherboard/PSU tester is sent a spark was seen and the fuse blows again. Replacing the fuse again and using a current limiting dim bulb tester, 5Vsb works but when starting up the dim bulb tester is rapidly flashing on and off. PSU tester does not turn on.
Probing around the PSU again on all main components gives me no shorts. Checked PFC mosfet and diode, 2 main switching mosfets, and large bulk filter cap shows no shorts. I can't even find any burn marks from the spark. I suspect that the 2 main switching mosfets in a half-bridge configuration is somehow driven to be activated at the same time (this should never happen normally). No short between the gates of these 2 mosfets though. So now I suspect either faulty controller or gate driver ICs.
Do you all have any other possibilities that I should check?
See tear-down and component pictures here: https://www.techpowerup.com/review/seasonic-focus-spx-750/3.html
acedogblast:
Found where the spark was coming from. Used a larger 200w bulb instead of a 40w. This was enough to repeatedly make a spark from the main PFC diode to the heatsink it was attached too. Almost certain the insulation between them is compromised. I did not find this before with multimeter so it must be the thermal expansion of the diode.
MathWizard:
Yeah that's something I'd probably forget about, or not consider till the end. This winter I was working on an old stereo, and when trying to fit smaller power transistors, with mica insulators and plastic collars, I was still shorting the screws to the HS. Actually there was no drilled out section for the collar, so I had to drill the holes. Then it all fit secure.
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