Corshair RM750 - doesn't allow for board to turn on

[kotel studios]

Hi,

While back I got this Corshair RM750 PSU from a friend which said it was broken. Plugged it into my bulb protection and turned it on. Was greeted with two 60w bulbs turning on (barely at max with PSU generating the voltages. I thought it was the APFC needing more current so I removed the bulb protection and plugged into my wall socket.
All the rails were fine, all generating the correct voltages when turned on.
Now, I have plugged in an known good board (intel i7 0th gen with iGPU, nothing else) and pressed the power button. Nothing happened besides the 5vsb LED onboard going dim for a split second. No sounds, beeps, fan spin and other LEDs. Measured the PS_ON and 5vsb rails when pressing the power button and there weren't any issues I could spot (5vsb is 5.08v all the time, PS_ON dips from 5v to 4.8v and then back to 5v). No obvious signs of burns/damage on the PCB.

This is my first time seeing such an issue. I have no idea what's the cause for it.

Any ideas?

[stj]

over current protection circuit probably false-tripping.
look for a big current-shunt and work back from there.

[kotel studios]

Okay, I have done reverse engineering on the parts which were exposed and I couldn't find any big resistors which represented current shunts.
Here are the pictures.

Any ideas?

[kotel studios]

Nobody is gonna belive this, but when I was going outside to take better pics of it, the corner of the PCB (near the primary MOSFETs) had shorted against the case and now I have 10ohms between positive DC and negative DC on the diode bridge rectifier in circuit, 31 ohms between Drain and Source on all of primary MOSFETs. There is also an burning smell inside.

[CapLeaker]

I guess you have to fix the shorted out components in this PSU now. What you should have done first place was checking the state of the supervisor IC in the secondary of the PSU when connected to a motherboard and didn’t work, vs not connected and forced on.

[CapLeaker]

Drain and source of the mosfet short could be because you turned the gate on. Do a proper test of the mosfet with your DMM in diode mode and try turning the gate off an on. There are you tube vids on how to do that,

[kotel studios]

Quick update:
The resistance dropped to 5ohms on both the positive and negative outputs of rectifier and on gate and drain of primary MOSFETs. My idea is to remove the rectifier and if that doesn't fix it remove those MOSFETs (that's where the burnt smell is). Hopefully only one went bad and not anything else...

[kotel studios]

Order of solder wicks finally came and I found the culprit for the short on primary DC side.
One of the primary MOSFETs went short. Luckily it was only one so the damage should be minimal, but just for sanity I'll need to replace all 3 60R190E6 MOSFETs.

Now here's my question, what are the equivalents to 60R190E6 inifeon MOSFETs? I can't find any online.

[stj]

that part number covers 3 parts - which is it?

[kotel studios]

that part number covers 3 parts - which is it?

I believe the IPP60R190E6 (TO-220 case).

[stj]

it may have been replaced by the 60R190P6, TME sells it.
send a mail to infineon asking about P6 vs E6

[kotel studios]

Yup, the P6 series is an more efficient drop-in replacement for the E6. Now before I order the MOSFETs from TME I need to troubleshoot the current sensing part.

EDIT: Supervisor IC is on the fan controller board.
Useful link: https://www.techpowerup.com/review/corsair-rm750/4.html

[momaka]

Just to shed some light on the original issue of the PSU here...
If you have a PSU that turns On and appears to output the proper voltages on *all* of the output rails, but the connected motherboard does NOT turn On, then the issue is likely with the PG signal - either it's not there or not coming on at the correct time.

But now with the shorted MOSFET(s), that's a completely different issue and probably unrelated to what was going on originally. Just putting this here for anyone else who reads this thread so they can keep track of what's going on.

[kotel studios]

Just to shed some light on the original issue of the PSU here...
If you have a PSU that turns On and appears to output the proper voltages on *all* of the output rails, but the connected motherboard does NOT turn On, then the issue is likely with the PG signal - either it's not there or not coming on at the correct time.

Hmm... That's an theory I haven't thought of. Any suggestions on what could cause it?

[kotel studios]

MOSFETs had been ordered. They should arrive tomorrow. Hopefully it's gonna get fixed, otherwise I'll need to check the entire supervisor IC daughter board.

[kotel studios]

FETs had arrived. Ordered the TO-220 FP case to not bother with the thermal pads. Going to replace them and hope for the best.

[stj]

FP? if they are resin coated tabs you wont get the same wattage from them
34w instead of 150!
so 80% lower output

[kotel studios]

60zł down the drain....

At least after replacing I know that the PSU doesn't blow up, but I forgot to plug in the rest of the connectors on the vertical output PCB, so I didn't have 5VSB and PS_ON. So that's something...

But let's try to be on the bright side. How much wattage could I get if there are 3 FETs in series? I could try to keep the power draw bellow that so I could test it properly before correct replacements will arrive.

[stj]

maybe you can sand or file the back of the fets to expose the metal tab.

[kotel studios]

Couldn't find the metal tab which is present on the TO-220. Maybe I could sand down just enough for the metal plate under the switching part to show? Maybe, but then I'd be risking the entire switching part to fall out/break.

And now, after connecting everything and using an 500W toaster (120W bulbs weren't enough for the APFC to work and there wasn't any rail voltage generation (besides 5VSB and PS_ON)) the whole PSU got screwed again. I bet it's the FETs again since I don't get any PS_ON or 5VSB and the power LED on toaster is lit. Albeit if the PSU was dead short then I'd get the coils of the toaster to glow, which I don't 😕

What's weirder, there wasn't any load on it. Just the PSU so I have no idea what the hell is happening with it.