SMPS Blown When Connected to Mains

[mr_push_pull]

this is an audio SMPS I have been using for a few years now in an amplifier. it was connected to mains with one of those extenders that include overvoltage protection and after that one longish (6-7 meters) extender to an earthed outlet. when I plugged the long extender back in the SMPS blew.
inside the amplifier there is also a 5W mains transformer that powers a control circuit which, among other things, controls an enable input of the SMPS. the microcontroller that does that job is now partially melted.

the fuse on the SMPS blew but it's still dead after replacing it.

all other equipment connected to the same extender survived.

what might have caused this? someone said that extenders that include overvoltage protection have coils that sometimes can cause trouble.

[eem2am]

Generally , the most common time for an smps to blow is at switch on, whether or not you have "extenders" or "surge limiters" etc.

Have you checked to see if a component has gone short.?

It may have been some resonance at switch on with the coils that you speak of.

What about the MOV....is that ok?....did that blow.....?

Very often, especially if you switch on exactly at the mains peak, you get a high resonance with all the L's and C's at the smps input...and this can , over the years, weaken the mov, and eventually it just goes

has the mov gone short,? or is it an open circuit now? even when exposed to greater than its break over voltage.?

Maybe the el caps were dried out and this caused some problem.

[mr_push_pull]

hi and thanks for answering

MOV are visually OK, I would guess that thermal failure would show. there are many of them at different stages of the input filter. I hope that the fuse blew because of the MOVs. if they managed to protect the input section, I'm guessing the reason it's not working now is that overvoltage entered it at the enable pin (remember the burnt MCU that used to control the enable).

a rather superficial cold testing did not reveal any shorted diode. mains rectifier is ok. the specialized ICs I don't know how to test. no visually damaged parts on the SMPS.

I'm leaning towards resonance too.

[eem2am]

sorry i forgot the mcu...........

Maybe that little 5W transformer suffered insulation failure?...

or some other problem with that 5w circuit.

was the mcu supply regulated to 5V?.....or did sometimes it drift above this.?

[mr_push_pull]

looks like the SMPS is not blown after all.

the enable input doesn't work like it used to. I was connecting it to the 5V rail through a 180R resistor but now I have to short it to work. I checked and the enable goes directly to the input of an optocoupler though a 1K resistor. looks like for some reason the 5V rail isn't providing enough current after the failure. will need to investigate further.

[sbrads]

Optos get weaker over the years and to switch need a bit more current than when new, so if the original design was marginal it may not work. Some also get leaky after a few years on their output side when warm.