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I need a 48V power supply with 20-25A so I decided to use old ATX power supplies I have lying around.

I know already that the series of ATX PSU can be used to charge lithium packs for RC planes, it works by insulating ground from negative in all but one unit.

However, is it correct that only the unit at the highest potential will supply all the current?

Can I use differently specced PSUs with only the "top" one rated for the current I need? In this case, I could use even a simple step-up converter as the low PSU, powered by an independent DC supply.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ I think you may have misunderstood the meaning of Vcc and Vdd, usually they're both positive. Better stick to +, Gnd and -. \$\endgroup\$ – Oskar Skog Apr 24 '17 at 10:32
  • \$\begingroup\$ "I know already that the series of ATX PSU can be used to charge lithium packs for RC planes" -- uhhh, what? That is not how you charge lipos. \$\endgroup\$ – Bryan Boettcher Apr 24 '17 at 15:10
  • \$\begingroup\$ @insta Or so I heard from a friend who built one for that purpose. Maybe he had also a current limiter. \$\endgroup\$ – FarO Apr 24 '17 at 15:17
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The maximum current that you're going to be able to draw for that configuration is lowest PSU rated current in the series connected supplies, which means 3A in your case. If your try to draw more, current protection/limitation will kick in.

I should confirm that the PSU outputs have to be isolated for the input, to be able to connected their outputs like that.

In reponse to your question: The PSU with the highest output voltage will source the highest current (if not all, it depends on the voltage difference and output stages) in a parallel connection configuration, not a series one.

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ADDITION TO HatminB's answer...

Ensure the PSU case grounds are isolated from the Negative output pins, and ground both of them back to the Line-Ground. Whether you connect that ground to your internal ground is optional but recommended.

schematic

simulate this circuit – Schematic created using CircuitLab

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