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I am having trouble building a power supply for my car audio amplifier board.

The board needs a ±50 V, 250 W RMS PSU, which is why I bought two switch-mode power supplies, each 50 V, to connect them in series and use 50 V as common ground.

One problem is that I didn't pay attention to choosing a galvanically isolated SMPS, which I did not. Now I have two power supplies and don't know what to do.

Is there any possibility of inverting one of these two voltage outputs to -50V? Is there any option to galvanically isolate them from eachother so I can connect them in series?

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  • \$\begingroup\$ There isn't enough information, what specific power supplies did you buy? \$\endgroup\$ Nov 16, 2022 at 15:48
  • \$\begingroup\$ A boost converter is not that different from a Cuk converter. Basically just needs a single capacitor and inductor inserted. Whether this is possible depends on the board layout. \$\endgroup\$
    – tobalt
    Nov 16, 2022 at 16:03
  • \$\begingroup\$ If your input is 12v automobile supply, as the description implies, then use of a galvanically isolated supply for the negative supply would work. This could be a separate 12V battery, or a 12/12v supply or a (gasp) 12v to mains and mains to 12v supply with galvanic isolation in the chain. A separate 1#v battery could be charged from the main battery, with polarity inversion, when the amplifier was not in use. \$\endgroup\$
    – Russell McMahon
    Nov 16, 2022 at 23:34

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Yeah, no. The PSUs you bought share a common primary and secondary ground, so there isn’t a way to ‘float’ one of them to make the -50V rail. Try to return them if you can.

In theory, a buck supply topology can be converted to a negative output, but I’m surmising that this isn’t feasible with a pre-built power supply that didn’t come with a schematic and other detailed information. Boost? No, the switch is in the wrong place.

Could you dismantle / cannibalize another car amp? This would likely have the kind of pos/neg booster you need.

Another wild idea: use a DC/AC inverter, then rectify the AC to DC. Many of these actually swing +/- 100V square wave to neutral (GND). One diode and one filter cap on each rail and you’d have your raw supplies for the amp.

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Is there any possibility of inverting one of these two voltage outputs to -50V? Is there any option to galvanically isolate them from another so I can connect them in series?

There is one option called a virtual ground but you have to build a current sink that can sink all of the ground current and at that power level its not worth it. Send the supplies back and get isolated ones.

The other option would be to find an isolated DC DC that can run at 50V and at least 2.5A for the negative rail (technically only the neg rail need to be isolated, but it's better if you have matching supplies, matching will make things run better.)

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