This is an ATX PSU rated for 550W powering 24/7 a computer load of about 200W, fed by a line-interactive 1500VA UPS. Mains is 230V.
I found both replacement ICs (CM6800 and TNY278PN) on ebay for ~5€ each, I may order them based on your recommendations.
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Images description:
20230124_185848.jpg and 20230124_185924.jpg show front / rear of the disassembled heat sink on the output side of the PCB.
- First issue: the PSU blew a mosfet on what I think is the active PFC boost converter, shorting its G D S pins together causing a short across the + and - lines produced by the rectifier on the 230V mains. The surge protector of the house tripped. When this happened I found that the UPS was unexpectedly disconnected from the wires of its battery. The UPS wasn't damaged, but I suspect this spontaneous detachment of a battery wire caused a condition that blew the power supply, because the UPS doesn't provide output voltage without its battery connected. The blown mosfet is a 16N50C3 in TO-220, replaced with an IRFP460 in a bulkier TO-247 which I had ready. The characteristics are mostly similar except particularly for the slower slew rate and higher gate capacitance of the IRFP460. The PFC PWM seems to be generated by a CM6800 which according to datasheet has a max frequency of 250ĸHz. The IRFP460 can switch up to about 3MHz.
After this replacement the short between + and - was gone.
Pictures shows two IRFP460 screwed to the input-side heat sink that replace two blown 16N50C3.
- Second issue: after I replaced the blown mosfet, I plugged the PSU to mains and tested the standby voltages on the bench. I had 5V VSB and the 5V power-on voltage from the green wire. All stable so far. I shorted green to black to turn on the PSU with a load of 3 x 12V fans (almost 1A total) plus the PSU's own fan. No load was on the 5V and 3.3V lines. For about a second it seemed to work fine and the fans powered on, then the other 16N50C3 on the same PFC circuit blew up, shorting + with - as before. The replacement IRFP460 physically next to it was undamaged.
- Current situation: I cleaned the PCB as I suspected that metal dust / fragments from cold solder joints may have caused a short in a control pin around the CM6800, then replaced the second 16N50C3 with another IRFP460 and tested again. I have a wall switch to quickly toggle mains power to the PSU on/off, to quickly cut power after repeated short tests of about 1 second each, so to prevent another blow up. Green remains shorted to black with a jumper wire. This time the dummy load is an old ~20 watt hard disk (1.2A on 12V + 0.75A on 5V). With the wall switch on, I measure ~12V between yellow and black and ~5V between red and black. The measures are approximate as the multimeter doesn't have time to stabilize before I turn the PSU off. The hard drive doesn't begin to spin up even after a second. The heat sink with the two IRFP460 becomes warm to the touch just after a second, and the 400V bulk capacitor of the boost converter remains discharged to 0V. It has a 1MΩ resistor in parallel so it would need several minutes to discharge.
With only a multimeter I tested all discrete components on the PCB for continuity. There are no shorts across components and all diodes' forward voltages are within expectation. BJTs tested fine for continuity, resistors are all within specs, the 3 optocouplers at the bottom center of annotated_2.jpg tested ~1.2V for the diode side, the transistor side isn't shorted. The integrated rectifier (visible in annotated.jpg, left side of heatsink) is fine. No capacitor is shorted, and I can observe the electrolyte ones charging up slowly under the multimeter's own voltage. The transformers don't short primary against secondary. The white glue has not become conductive due to deterioration.
I suspect what is broken is the PWM driver IC (CM6800) mounted on the vertical board close to the bulk capacitor. I tested its pins for shorts and there's none. One of its outputs connects to the base of both BJTs in front of the burnt/replaced mosfets, with one of these emitters connected to both mosfet gates thru a zener (unknown voltage) in parallel with a 22Ω resistor. The pair zener+resistor is replicated twice, once for each mosfet gate. (see annotated.jpg). It seems both mosfet gates are held open by the IC, conducting + straight to - via the boost converter coil (see annotated_2.jpg), preventing the capacitor from charging.
There's a second PWM IC, TNY278PN, which I'm unsure of its relation to the faulty circuit.
I found both replacement ICs (CM6800 and TNY278PN) on ebay for ~5€ each, I may order them based on your recommendations.
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Images description:
20230124_185848.jpg and 20230124_185924.jpg show front / rear of the disassembled heat sink on the output side of the PCB.
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