Electronics > Repair

SMPS failure

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johnmx:
I tried to fix an SMPS power supply without success. The schematic of the power supply is practically identical to the one attached. One difference is, for example, that C11 does not exist.
The switch mode regulator (FD9020D) was burned out with an almost short circuit (~6 Ohm) between the power pins (1 and 8 ).
I checked all the other components and they were good. I particularly checked all diodes and resistors. I measured the capacitor C10 with an LCR meter and it had 9.8uF with 4.5 Ohm series resistance. A somewhat high value, but I don't have any of these capacitors to replace it. I even tested the secondary circuit and it seemed to be working fine.
I bought 10 units of the FD9020D on AliExpress, replaced it, turned on the power supply, and the 12V appeared at the output (correct value), but after about 10 seconds it burned out again.
The failure was the same, low resistance between pins 1 and 8.
I replaced the PWM controller again, turned on the power supply, and it burned out again after 10 seconds. But this time the 3 current sensing resistors (R4 and R9 from the attached circuit) also burned out, probably because the chip burned out and kept the output transistor on.

What do you think it could be?

wraper:
С1 would be a first suspect to me. Also R2/R3, C3, D5.

xavier60:
If C5 goes high ESR, flyback doesn't get clamped properly on the primary side. Also check D5.

johnmx:
I desoldered the main capacitors to measure them on the LCR meter and obtained the following results:

Original value |Meas. cap. |Meas. RsC122uF/50V19.6uF1.13ΩC5470uF/16V450uF0.07ΩC4470uF/16V435uF0.07ΩC1010uF/400V10.0uF3.45ΩC32.2nF/1200V2.3nF274Ω
The only capacitor that stands out is C3 with a very high equivalent series resistance.
R2, R3 and D5 are fine.

Note, L3 does not exist in the real board. The circuit is slightly different. All component designators mentioned here are based on the schematic, not on the actual board.

wasedadoc:
Does your LCR meter give a similar apparently high ESR reading for another capacitor in the 2.2nF ballpark?  Does not need to be a high voltage one for that check.

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