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I'm repairing switched power supply. It is custom inside device and cannot be changed with another power supply. It is not first time I've had to repair it. The problem is always the same: grilled capacitors as they are about 2mm from a heatsink (the device has no fan and all is in very tight spaces.) My guess is the power source is about 300W - no info on board.

This time I would like to improve the situation:

The capacitors' leads will be insulated only leaving last 3mm to mount on the PCB, this will allow 1-2cm of leads between capacitor and PCB. This 1-2cm will be bent (after soldering on the PCB) to get the capacitor as far from the heatsink as possible.

There is 'some' information about switched power supplies that states trace/wire length is important for design (power and noise.) How important is it really? What can I damage if my solution makes the path to capacitor 2cm longer?

EDIT: The original capacitors and my replacement capacitors are rated for 105 degree celcius, 125 degree capacitors are not available in the required capacity/voltage rating. When shopping for capacitors, always filter by capacitance, voltage, temperature and then order the one with highest ripple current.

Capacitor values (all rated to 105 degree celcius):

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    \$\begingroup\$ Perhaps the solution would be to use better capacitors with smaller ESR and higher temperature ratings? Also it depends what the capacitors are and what they do in the circuit. Output caps, input caps, some other caps? Sometimes they don't go bad due to heat but due to the ripple current they have to handle. \$\endgroup\$
    – Justme
    Commented Jun 14, 2023 at 8:45
  • \$\begingroup\$ my knowledge is not sufficient to find the real root cause, the heatsink was most obvious suspect, but you can be right. I always buy caps with highest ripple for repair, temperature rating of original and replacement caps is 105, there exist 125 celsius caps, but they are not available in required voltage+capacity rating. \$\endgroup\$
    – Peter Novotny
    Commented Jun 14, 2023 at 9:01
  • \$\begingroup\$ Value and voltage rating? \$\endgroup\$
    – Dirk Bruere
    Commented Jun 14, 2023 at 9:18
  • \$\begingroup\$ This are the capacitors lined 1-2mm from heatsink, all of them are more or less bulged on top, some on bottom also. Have added also ripple current from datasheets of the capacitors I'm using as replacement ones. 2pcs 1500uF 25V 2.6A, 2pcs 470uF 25V 1.33A, 470uF 10V 0.54A, 820uF 16V 1.96A, 2pcs 680uF 25V 1.76A \$\endgroup\$
    – Peter Novotny
    Commented Jun 14, 2023 at 10:53
  • \$\begingroup\$ After fixing the capacitors, can you force cool it with a fan? \$\endgroup\$
    – winny
    Commented Jun 14, 2023 at 16:45

1 Answer 1

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This might be due to AC (ripple) leakage into electrolytic caps. The remedy is to use ceramic caps, perhaps 0.1 μF, depending on frequency, across the leads of the electrolytic caps that fail. The ceramic caps are generally more heat tolerant and will bypass HF AC.

Place the ceramic caps as close as possible to the PCB -- if they can fit on the underside, solder right to the pads. The leads of the electrolytic caps can be left a bit longer, as you state, because the ceramic caps are right down to the PCB.

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  • \$\begingroup\$ Thanks for the hint, in general I liked your idea. In my case the power source became very loud after the modification, after removing the small ceramics it was silent and happy, I have shortened the leads little bit to not add that much distance, but still have bending space. Not all caps can be bend, but the most bulging ones were bend away from heatsink. Even when our solution not worked exactly as described, still I like it and for this reason marking as solution. \$\endgroup\$
    – Peter Novotny
    Commented Jun 14, 2023 at 20:50

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