Part Number: LM2596
We are using the LM2596S-ADJ to buck from around 28V to 20V drawing about 1.8A at the input to the circuit.
(The schematic is below.)
We have seen a number of instances where the catch diode D3 (SS34FA) fails. We suspect that the RMS Reverse Voltage of the SS34FA may be insufficient and have upgraded to the SS36FA but we have been unable to make either fail in the lab and would like to verify before re-deploying to the field.
The circuit is used in a vehicle application and is preceded by a 24V version of SNVA717 for protection which seems to work fine.
Any ideas what we may be doing wrong?
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In reply to Katelyn Wiggenhorn:
Thanks for the quick response!
I took some time to pull up the layout and review it. I do see a number of things that aren't ideal but I'll let you comment (wish I had caught them during review!)
We find the failures after a few days in the field so assuming it is during steady state during heavy load. (It does not fail during startup and the automotive protection circuit cleans up any transients.)
In reply to Kuenrg:
The max load is 1.8A and this lasts a couple of hours until the battery is charged up. Please clarify if you feel the current rating of the SS34FA or SS36FA (3A) is too low.
Unfortunately we do NOT use a heat sink on this device. This is a two layer board with 2oz copper pour. However the board is fairly compact so there is little room for heat dissipation.
Thank you for noticing R18, I will definitely have that updated.
C6 is an aluminum cap (UVK2A101MPD) from Nichicon, the ESR is not listed in the datasheet.
Yes, the routing comments you mention are well noted. I wish I had seen that earlier when I was reviewing the board.
In reply to Frank De Stasi:
I will redo the layout with those points in mind.
I'm working on getting the SW pin scoped - (Just out of office so have asked a co-worker to look into it)
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