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I'm hoping to get a +/-48V supply out 2 of these PDME1-S5-D24-S +/-24V split power supplies. From what I've read about connecting power supplies in series, it's necessary to have diodes across each of the series outputs to protect them in the case of short circuit or if one starts up slower than the other. In this case where I'm trying to use split supplies, is it necessary to have diodes between each rail (i.e. between +24 and 0 & 0 and -24) or just the +24 to -24 of each supply?

This schematic is what I was thinking about be okay, though the diodes shown would actually be Schottky diodes.

schematic

simulate this circuit – Schematic created using CircuitLab

Also, if there would be any issues running these both off the same 5V, please let me know. These supplies do have isolated outputs so I think it's okay, but I'm not super confident.

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    \$\begingroup\$ Should work, but aren’t there any single supply 48 V ones instead? The devil could be in the details for that split in each supply. \$\endgroup\$
    – winny
    Mar 8, 2022 at 14:24
  • \$\begingroup\$ @winny Thanks for your help. I couldn't find any other suitable supplies that could run on 5V and weren't ridiculously expensive. Would doing 4 24V supplies in series be better than doing the 2 +/-24V supplies? What are the potential issues with the split in the supplies? \$\endgroup\$ Mar 8, 2022 at 14:41
  • \$\begingroup\$ Perhaps. Worst case you would need a bypass diode across each rail, so 4 in total. I'm thinking of the edge case where each 24 V rail is regulated individually and one triggers OCP but not the other. It depends on the implementation of it and manufacturers seldom specify it. \$\endgroup\$
    – winny
    Mar 8, 2022 at 14:44
  • \$\begingroup\$ @winny Thanks for the explanation, feel free to put that as an answer so the question can be resolved. \$\endgroup\$ Mar 8, 2022 at 14:50

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Assuming your power supplies are isolated, the edge case you need to protect against is at high currents where one OCP (over current protection) will kick in before the others. You already have bypass diodes across them, but the same problem may or may not exist inside each power supply depending on how it's implemented. If it's one OCP on the primary and two secondary windings, both would cut out and you are fine. If not, and one kicks in before the other, you will have soon have up to -36 V applied across it which it won't survive. Synchronous rectification with could clamp to ground in this condition would make it quite energetic or fiery.

If you go with +-24 V supplies, I would recommend four Shottky diodes in total for bypassing each of them in case of OCP or startup shenanigans.

A bit safer option would be two single rail 48 V supplies which would be an easier guess how they behave and only two diodes needed.

Best scenario is of course a +-48 V supply.

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