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Strange board behavior when switching power supplies (photos.app.goo.gl)

submitted 1 year ago by [email protected] to c/[email protected]

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This photo shows the power conversion side of two versions of the same board. Input is up to 60V DC, board converts to 5V and then down to 3.3V.

The top board works correctly all the time. But the bottom board has strange issues. Everything works perfectly when I connect it to my bench power supply at 53V. The board powers up with no problems. When I connect it to the daisy chain of other boards, it suddenly will no longer power on, despite ~54V being passed through correctly to downstream boards.

This is a real head-scratcher for me, and I'm having trouble making sense of it. I'm not so much looking for The Solution as to what steps I should take to troubleshoot this. There were some issues attaching the image, so in case it doesn't show up.

https://photos.app.goo.gl/MCx7Q4BDbKTv68EJ8

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[–] [email protected] 4 points 1 year ago* (last edited 1 year ago) (1 children)

Look for differences and bad joints

C4 looks not connected right, maybe c10 and c14
C16 missing
Solder joint messy up above C17
The two through hole pads by C19/20 arent soldered and the one above it looks to be a bad joint
jumpers on the right arent connected in the same spot
extra traces by the IC
components can die, especially ICs and capacitors from ESD and drops respectively
PCBs themselves can sometimes just be faulty from the factory and have damaged traces.

Another thing is that small ICs like that tend to be fairly difficult to stick down all the pads. Reworking that might be a good option if checking the above doesnt work

[–] [email protected] 1 points 1 year ago (1 children)

Thank you, I will reflow C4 and add C16, but C10 and C14 look fine from the sides.

The messy/missing joints near C17, C19, and C20 are optional components for the other side. (Unused surface mount USB port with through-hole mounting pins and an unused jumper.) Right-hand jumpers are all optional and unused from an earlier prototyping stage.

What do you mean by extra traces by the IC? And you're absolutely right about the small IC. It's the MAX17640, and at 2mm x 2mm it's a piece of work to land correctly.

[–] [email protected] 1 points 1 year ago* (last edited 1 year ago)

traces by the IC

I was referring to it more as a difference which might be the source of the issue. For example, one via is fairly close to the pad for C16 which could be shorted (probably isn't, but still a good idea to check). It could be a wiring issue but could also be a board manufacture issue.

[–] [email protected] 2 points 1 year ago (1 children)

You can try checking whether the 54v supply has any voltage spikes if you haven't already. 60-to-5 converter is most likely a switching converter and they draw high current instantaneously and can cause spikes if not filtered properly which may affect the functionality. This may not show up in DC voltage measurement, and daisy chained boards may still function if they are filtered well. One of the daisy-chained boards may be causing the disturbance too, and maybe top board is filtered well but bottom isn't for some reason, maybe a soldering error(e.g. filter capscitor ground connection, poor soldering may be conductive but with high inductance, hindering filter functionality). So it is good to check supply quality. Also what do you mean by not powering up? Is 3.3v not working? What about 5v?

[–] [email protected] 1 points 1 year ago

Correct, it's a switching converter, the MAX17640. 5V is not powering up (getting ~2V out) and then the 3.3V LDO is not powering up.

I put an older variant daisy-chained board before and after the newer board. Both of the older ones come up, but the newer one doesn't.

I will check soldering disturbances and put my scope on the input line to see if there's a lot of noise. Thank you!

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