help identify failure mode of SMPS?

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decibel08

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Attached is a picture of a failed PSU that was providing 24V at 5A on the red lead. PSU is rated for 140W so it should be able to handle this. There are obvious burn marks on the metal chassis above the connector, and also a strange white fuzz on nearby metal surfaces inside the case (might be hard to see). It looks/moves sort of like cotton lint.

Any ideas on what is happening here? I've established that the wire gauge (22) should be swapped for thicker (maybe 18). Does this look like a case of over heating at the contact point of the crimp and the cable, or maybe this dust/lint built up and was the cause somehow? Nothing else on the PSU looks damaged.

Many thanks for any input!


 

the second photo is of the cover?

first photo makes it look like a bad connector as you surmised

BTW I am pretty sure the wattage rating for a PSU is the overall rating. Check the label for the amperage for the 5V line.
 

kam1787 said:
the second photo is of the cover?

first photo makes it look like a bad connector as you surmised

BTW I am pretty sure the wattage rating for a PSU is the overall rating. Check the label for the amperage for the 5V line.
Click to expand...

Yes, the second photo is the cover. This PSU is a single output, so all 140W should be available to the 24V output. I'm thinking the main problem was using the max output (5A) but only one effective wire and a single crimp. Both the 22AWG and the crimp are iffy at 5A although technically barely in spec.

Any ideas what the white stuff is?
 

I'm betting the cause is just insufficient current capacity on the wire + connector. I mean looking at the location it's pretty obvious it's not going to receive any forced air cooling. Probably should have used more parallel contacts on that connector, with gold plating.

As for the fuzz, it's likely some sort of chemical precipitate from the burning plastic.
 

This appears to be caused by passing high frequency current through a loose contact. Instead of over-sizing the wire, solder it to the port.
 

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