SMPS repair problem

[Electric flower]

I have been working on some SMPSes, when they came to repair shop me and one other guy were fixing them and on 70-80% of the times it were faulty capacitors (mostly bad capacitance and in every  occasion bad ESR), and we replaced those caps and they all worked, but  quite a lot of them blew up after working for more than minute, you hear a "pop" and there are burned SMD parts (resistors).

Sorry i did not write/remember ICs and models of power supplies .

What could it be? Anyone else here with same or similar experience?

[Tony.T]

The thing that crosses my mind is that it is to high voltage across the caps.   You should have seen if the polarity was wrong....  Could be something in the circuit that makes the voltage to rise to high and makes it to pop.  I guess that they are more...silently popped rather than a "bang"....

change the caps to higher voltage specifications....or just look why the circuit is giving to high voltage...

Bok..!!...

[HackedFridgeMagnet]

Did you check the semiconductors to make sure they weren't also blown short?

[Electric flower]

Capacitors that we used were installed correctly, in some cases we used even higher voltage rated caps. All semiconductors were fine before replacing caps.

Few minutes after it works we hear a loud pop, not bang. And after it we had greatzes, mosfets and ICs shorted. Quite few times circuit breaker tripped off.

My only guess was that problem could be caused by replacing the mains capacitor with one that has just bit more capacitance and that by doing so we have made resonance between that capacitor and primary of transformer, and that resonant frequency was same or similar to switching frequency and that it caused higher voltage that burned oscilator ICs that made mosfet go short and after that it's all gonne mad and greatz blew up after it.

Thanks

Bok...

[SeanB]

My guess is that running in startup with the switcher driving the power device with lower and lower drive ( before it shut down) overheated the power device junctions and degraded them, thus with a new capacitor the high voltage generated in normal operation finally broke it down, causing the destruction of all the parts. You night consider if you have a lot of those supplies that do that to simply change the power device at the same time as the capacitor, to improve reliability. The controllers typically rarely survive the power device going short circuit as this puts a massive current spike through the sense resistor blowing this open and then the chip. Diode bridge is the final failure trying to conduct mains through a short. Then the fuse blows to limit the fire....

[Shock]

Check the date codes on the caps as well.

[retiredcaps]

we replaced those caps and they all worked, but  quite a lot of them blew up after working for more than minute

What brand and series did you use for replacement and more importantly, where did you buy them from?

[Electric flower]

From Croatian supplier "Elmatis", higher capacitance value capacitors are mostly Jamicon, but lower capacitance ones were are mostly KSC and Huang. All of them were tested okay on ESR meter.