Post-mortem of a small power supply? - Page 1
Forum section: electronics / beginners

author: quicknick
author_rank: Contributor
timestamp: October 11, 2021, 09:36:22 pm
content: Got this 12V/3A power brick, and well, it was dead as a brick. Opened it up to see what's going on inside, main filter cap still charged to about 300V. No casualties here, I discharged it usingthe tip of my screwdrivera power resistor.Immediately I noticed a blown 3-leg IC, probably the TL431 on the secondary side (red arrow), and on the primary there's one blown trace going to the optocoupler (purple arrow), so the phototransistor inside must've been subjected to a quite hefty current. I never try to repair such badly blown supplies, but I'm curious to know what was the sequence of events that led to this failure.Cheers,Nick

author: Jwillis
author_rank: Super Contributor
timestamp: October 12, 2021, 02:11:58 am
content: I would say that the transistor or regulator shorted keeping the LED in the optocoupler ON . This would cause the supply to run away . That would certainly heat the traces on the high side until the phototransistor in the optocouple fails .The cause of the shorting could be caused by an overload or voltage spike . And that just happened to be the weakest link.

author: quicknick
author_rank: Contributor
timestamp: October 14, 2021, 05:18:59 pm
content: Fiddled a bit with the board today and couldn't find any other failed parts. I know I should throw it away, but tinyon my shoulder tells me to replace the TL431 and repair the blown trace and give it a test, which I might do in a few days. I'll be back with an update!

author: Gyro
author_rank: Super Contributor
timestamp: October 14, 2021, 06:20:10 pm
content: To me, that looks like a PSU that might have suffered a high voltage primary - secondary voltage spike (eg. nearby lightning) - other than the lack of evaporated copper. I can't really think of any situation that would simultaneously blow up the TL431andburn a track on the phototransistor side of the opto. I would have expected the opto to explode under those circumstances too. I doubt that even an opto going s/c between the LED and phototransistor would pass enough current to do that. It is a peculiar one!It would be fun to do as you say to as an exercise to see if anything else has failed, but I wouldn't necessarily trust the insulation afterwards though.P.S. It's a bit of a shame - that actually looks like a decently designed SMPS.EDIT: One other thought - a flashover to the opto track at the C6 footprint might be a more feasible explanation - the clearances are tight considering that the footprintidis on a track from the switching transistor. At the same time, it looks like the lower potential source side of the transistor, I can see what looks like the current sense resistor (R4).

author: magic
author_rank: Super Contributor
timestamp: October 14, 2021, 07:05:59 pm
content: This is TL432. Learn the difference before it's too lateIt's surprising that his chip would burn, there are series resistors at all its pins except ground. Maybe there is something to that secondary-primary breakdown theory.The opto may be toast too, I would test it before powering up.

author: quicknick
author_rank: Contributor
timestamp: October 14, 2021, 10:34:15 pm
content: @Gyro - I think you are spot on regarding the flashover from C6: under a powerful magnifier, the solder on the upper pad has some pitting suggesting a discharge took place from there (pity I don't have the gear to take a proper picture). The track is vaporized for about 10mm, until it passes well clear of R7, after which it goes to pin 1 of the (smps?/pwm?) controller, which is a unknown 6-pin chip marked "362J17 745Ca".@magic - dunno how you figured this out based only on my not-so-good pic, but I think you're right. Together with the above^^, this makes me not so sure I want to further experiment with this, as I don't like witnessing things blow up. However, these PSUs are available at a local flea market for exactly 1 eur apiece, so if you guys consider the unit to be well designed I'm tempted to get a few more, hoping that this one was indeed the unfortunate subject of a lightning strike or something akin.

author: magic
author_rank: Super Contributor
timestamp: October 15, 2021, 06:09:06 am
content: From pinout of course; one pin goes to the feedback divider (even resistor values are visible) and one goes to the optocoupler.OTOH, I can't see any of that PCB damage you people are talking about

author: Gyro
author_rank: Super Contributor
timestamp: October 15, 2021, 08:56:55 am
content: Quote from: magic on October 15, 2021, 06:09:06 amIt's at the Purple arrowhead (C6 footprint).Quote from: quicknick on October 14, 2021, 10:34:15 pmAlthough it looks well designed from the outside, primary-secondary PCB clearances, transformer leadouts clearly PTFE insulated, decent input filtering,realfuse, general build etc. They clearly screwed up the clearances on that track to the primary side of the opto. You would want to rip off the track and replace it with a properly insulated wire to be confident that this problem wouldn't happen again. All designs can have weaknesses, they obviously didn't spot that one.

author: quicknick
author_rank: Contributor
timestamp: October 15, 2021, 01:11:49 pm
content: They aren't easy to open, my usual techniques failed so in the end I had to use a hot knife. Not worth making a repair IMO, if you think this is a design fault.Here's a more detailed view of the area. The track is damaged from left of C6 footprint to the right of R7:

author: Gyro
author_rank: Super Contributor
timestamp: October 15, 2021, 04:14:01 pm
content: Wow, I didn't notice it being all the way back past R7, I thought it was just the discolouration at C6 footprint. I would have expected more copper residue around it - it almost looks as if the track was cut... other than there would then be no connection to the opto! I suppose the track and pads above the missing track are all connected to that same line from the switching transistor (top of C6, D3, R7), so the arc may have propagated, but I would still have expected lots of copper debris. Still confusing.Well, at least you don't have to worry about removing the track if you continue with the experiment, it's already ready to bypass! As Magic says, check function and isolation of the opto, and preferably replace it too.P.S. I'm not giving up on the lightning theory just yet, if you've never actually seen it operating. I'm as intrigued as you are!

author: magic
author_rank: Super Contributor
timestamp: October 15, 2021, 04:15:30 pm
content: So that whole segment is completely burnt out?Wouldn't be surprised if the controller is also gone, the current had to gosomewhere.I took a look at the photos. It seems that the big solder blob above D3 is the transformer-drain connection which is where high voltage is. That C6,D3,R7 circuit appears to be coming from the AUX winding, providing power supply to the controller and some sort of transformer sensing.There isn't normally much voltage in those things

author: quicknick
author_rank: Contributor
timestamp: October 15, 2021, 07:45:45 pm
content: Yes I've never seen it operate, I got it already blown. There was some residue/debris on the PCB but I cleaned it twice to be able to better see what's happenedAnyway, here's some better pictures:

author: Gyro
author_rank: Super Contributor
timestamp: October 15, 2021, 09:21:13 pm
content: Quote from: quicknick on October 15, 2021, 07:45:45 pmNowthat'swhat I was expecting to see - significant energy and metal evaporation!That track loss looks almost surgical. I bet there's some of it plated on the underside of D3 and R7 [Edit: actually, maybe not, there's glue under them]. I still can't see the energy path though (apart from possible breakdown at C6 footprint. Insulation breakdown in the opto would be a possible explanation but there isn't enough collateral damage between it and the TL432 (and the controller IC package still looks intact).