Broken SMPS

Hello,

I have a SMPS for a kitchen appliance (bread maker) that has failed. It's 240v in with four DC outputs: ground, 0v, 5v and 12v. Unfortunately I'm getting nothing out of any of these when plugged into 240 v. Tracing the voltage through the board, everything seems to be ok (I think) up to the transformer. On the high voltage side I have 400v DC, but there is nothing on the low voltage side. Of course this appears to be a failed transformer, but I just wanted to check if I'm reading this right.

If it is a failed transformer does anyone have any advice on replacing it? Can't seem to find an exact replacement anywhere online, and I presume it needs a specific number of windings for this board, though I may be wrong - ?

Any help greatly appreciated!

Ringtail said:

Hello,

I have a SMPS for a kitchen appliance (bread maker) that has failed. It's 240v in with four DC outputs: ground, 0v, 5v and 12v. Unfortunately I'm getting nothing out of any of these when plugged into 240 v. Tracing the voltage through the board, everything seems to be ok (I think) up to the transformer. On the high voltage side I have 400v DC, but there is nothing on the low voltage side. Of course this appears to be a failed transformer, but I just wanted to check if I'm reading this right.

If it is a failed transformer does anyone have any advice on replacing it? Can't seem to find an exact replacement anywhere online, and I presume it needs a specific number of windings for this board, though I may be wrong - ?

Click to expand...

Almost certainly NOT the transformer, these very rarely fail, and are custom made for each SMPSU, so you can't buy replacements anyway, other than from the manufacturer of the SMPSU (which is highly unlikely to be possible).

Most SMPSU faults are fairly catastrophic, and require replacement of all faulty parts, or it just blows again.

R2 and R7 look to be burnt up, but without a schematic it's impossible to know that they do. Drawing out the schematic, and posting it here, would be a help - as would posting the number off the IC, and it's datasheet if available.

Many thanks for the reply.

Yes, those resistors did get a little fried unfortunately when a multimeter probe slipped, but I don't think they're the issue (well not the main issue). I wasn't getting anything out of the transformer before the resistor incident.

The IC is an AP3706P-G1, which I've already tried replacing with a brand new one. I've also taken out the voltage regulator and triode and both seem to test fine.

As for the schematic, I haven't been able to find one anywhere online, and I'm not really great at plotting them out (though maybe I should try).

Shorting things out can (and almost always does) cause serious further damage, you have to really careful not to damage SMPSU's.

Here's an example diagram from the datasheet:




Interestingly the chip is for battery chargers.

R1 and R6 are the startup resistors, these often go high or O/C, D1 and D2 are diodes that commonly fail, Z1 could fail, obviously the chip can fail, Q1 as well, C1, C2 and C6 can go high ESR. R8 is likely to be duff if Q1 has blown. This is just general advice, not specific to this supply.

Your new chip could well have been killed, as you changed it without replacing everything that is faulty.

That is great, thank you so much Nigel for the comprehensive reply. I'll go through and test out the individual components again to make sure.

I do have another spare chip up my sleeve but I'll only replace that once I confirm everything else is ok/find the other fault(s).

Bear in mind the component numbers may well be different on your board.

I suggest that you buy a 12 V and a 5 V supply and fit both. There is likely to be enough space inside the bread maker to fit a second power supply.

Another approach would be 12 V power supply and then a 5 V step down switch-mode regulator.

As Nigel points out, fixing switch-mode supplies is difficult.

Something like this might work for you.

Many thanks everyone for the replies. I had thought I might persist with this PS and learn a few things along the way. Well I've certainly learned a lot, but my persistence is still not paying off. So I've ordered a replacement board. I will continue with this PS though to see if I can isolate the fault. My next step will be replacing the burnt out (but seemingly still working) resistors, as well as all the caps. Once I'm fairly confident all these components are working ok I'll do the voltage regulator and triode, and then the IC last.

All a lot of work, but the learning experience has been really great and has given me a lot more confidence working on small electronics.

I've repaired thousands of SMPSU's, to the extent that for the most common types I created repair kits - I then simply changed all the parts in the kit, and if that didn't cure it (in the VERY small number of cases it didn't) then I'd delve further.

From the diagram in post #4 C1 (the reservoir capacitor) is something that does occasionally fail (goes O/C), but not enough to be added to any of my kits. Depending on the exact PSU this can be catastrophic, with the PSU blowing up, yet some PSU's with almost identical circuits carry on working with no damage, and are still mostly usable. On the supplies where reservoir failure kills the PSU I'd check the capacitor, after I'd removed all the kit parts.

Even less common is transformer failure, and I used to stock specific transformers for two or three PSU's where they failed (others never did - presumably the transformers weren't very well made?) - failure modes were either shorted turns, O/C windings, and even shorts between windings. But as I said, it was pretty rare, and I'd fit the kit first and see what happened when powered up.