The Curious Case Of The Exploding Switchmode

[dexters_lab]

just bumping this old thread sorry!

Yesterday i was out on-site and working on a customers panel of these blind controllers, i was just testing everything prior to leaving and one of these Bias Power PSUs failed right there in front of me.

Thankfully i had put the metal cover on the panel but by god they go with a bang! 

Now interestingly the panel was on and had been for a minute or two, i activated a controller which began running a blind (shade!) down, about 3 seconds later BANG! and the MCB trips, but it turns out it wasn't the controller that was operating at the time that failed, it was a different one.

[Chipmunk]

I'd be tempted to have a look at the mains quality. Voltage might be running high anyway. If this client has a fairly good budget, a power analyzer could be rented to do a 24-48hr analysis of the mains supply to look for transients, and other weird stuff

[Rerouter]

on the later module that popped, i have seen some horror stories when it comes to smd soldering, but he is up in the top 20, i think that is half the issue, the giant blobs of solder reduce the gaps, something arcs and pop goes the supply,

[dexters_lab]

on the later module that popped, i have seen some horror stories when it comes to smd soldering, but he is up in the top 20, i think that is half the issue, the giant blobs of solder reduce the gaps, something arcs and pop goes the supply,

yea it's a bit blobby around the bridge rectifier that the back and that cap at the top, the rest looks ok. The potting compound makes it look worse in places.

[Pentium100]

Do they have active PFC? I have seen a couple of blown up computer power supplies with active PFC. The chain of events goes something like this:
Hot side reservoir cap fails -> voltage spikes rise to >900V -> a transistor or chip shorts out and blows up -> circuit breaker of the house trips.

So, now my advice for buying computer PSUs is this: buy high quality PSUs but if you really need a cheap one, get one that does not have active PFC.

[Psi]

Now interestingly the panel was on and had been for a minute or two, i activated a controller which began running a blind (shade!) down, about 3 seconds later BANG! and the MCB trips, but it turns out it wasn't the controller that was operating at the time that failed, it was a different one.

Yep, it definitely sounds like a mains spike is causing the PSU to fail shorted.