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This question already has an answer here:

Or if you can under some conditions, why is is a bad practice?

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marked as duplicate by RoyC, Finbarr, mkeith, laptop2d, W5VO Apr 26 at 4:51

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  • \$\begingroup\$ Because the way they add together (or cancel out) produces an equivalent single supply of a different voltage or current driving a short-circuit. \$\endgroup\$ – Toor Apr 23 at 18:28
  • \$\begingroup\$ Current sharing can be done only if they are identical source impedance and matched voltage. Even with this closed loop regulators can oscillate with no load. \$\endgroup\$ – Sunnyskyguy EE75 Apr 23 at 20:09
  • \$\begingroup\$ I put power supplies in parallel and series quite frequently. Sometimes I put them in parallel (same voltage) to get more amps, and sometimes I put them in series to get a higher voltage than either can supply alone. These are bench-type engineering supplies with programmable voltage and current limits. I am going to vote to close this question. \$\endgroup\$ – mkeith Apr 24 at 17:22
  • \$\begingroup\$ I'm voting to close this question as off-topic because it assumes something which is not true in the first place. \$\endgroup\$ – mkeith Apr 24 at 17:23
  • \$\begingroup\$ @mkeith You put two power supplies with different voltages in parallel and that works without problems? \$\endgroup\$ – DenR Apr 24 at 18:17
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Who says you can't or parallel power supplies?

It all depends the power supplies in question.

I used to design bench power supplies for a well known test and measurement company.

All our designs were capable of being connected in series as they had floating outputs you just had to ensure that they were never trying to sink current under any load they were asked to supply and that no terminal was more than 300V away from earth.

You could also parallel them. They would not share but were self protecting for an overload so if you drew more current that one could supply it would current limit and the other would start to conduct. For example you have two 20A supplies and ask for 15A it may all come from a single supply but if you ask for 30A one could be supplying 20A (current limit) and the the other 10A.

You need to check with the manufacturer however, not all power supplies are designed to work this way.

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For AC supplies like a floating transformer secondary winding, series connection is sometimes done, taking care to get the phasing correct.
For DC voltage supplies, series connections can be done, provided one is a floating supply. There are a few precautionary checks to be made first:

  • Can the floating supply withstand the common-mode voltage forced on it by the "bottom" supply?

  • Are both supplies able to accommodate reverse voltage applied to their outputs? Some supplies add a reverse-protection diode at their output terminals in an effort to prevent reverse voltage from destroying the regulating electronics. This diode must be rated to accept possible over-current resulting from the two-supply connection.

Should either DC supply not be protected from reverse-voltage, turning either/both on-to-off or off-to-on is problematic...it is unlikely that their output voltage will rise (or fall) simultaneously. The momentary surge of reverse current and/or voltage could easily destroy them.

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