Can you connect isolated power supplies in series

Bare with me, I don't know much in electronics - but any advice would be appreciated.

Ive got these Eltek power supplies that are used in servers. They're 48V and can be turned up to 57VDC. Output capacitors are rated for 63V.

I tried checking for continuity between the output and input terminals and there was none, which I guess means that the power supplied are isolated.

Question is if they can be wired up in series to output ~105VDC. Is it safe to do it with the same ground connected between them. Are the capacitor rating of 63V sufficient if wired up in series?

If they are indeed isolated, you should be able to connect them in series, but I don't understand your comment about "the same ground connected between them", as one would be connected to ground and the other would be floating, since they are in series.

You should add a diode across each output (cathode to positive), so that if only one is turned on the other doesn't see excess reverse voltage across its output. The minimum diode current rating should equal the maximum output current of the supplies.

In theory, you can. But, in practice, there is a significant risk.

If they are connected to a load, and one of the power supplies is turned off, then the voltage of the other power supply appears, reversed, across the one that is off. In your case, the power supply that is off sees minus 48 V across its output terminals. That is not good.

The standard solution is to place a reverse diode across each power supply output able to handle the current that the load will take when powered by only one supply. That way, if a power supply is off, the other power supply powers the load at half voltage through the reverse diode across the other power supply.

Like this:



simulate this circuit – Schematic created using CircuitLab

There could also be an issue of isolation. In theory, you can add any number of power supplies in series and achieve any voltage you want. In practice, you must also consider the isolation between the line side and the output side of the power supply. It may be rated for only 1 kV. If the total voltage is too high, the power supplies at the ends may be exposed to a voltage between the input and the output that exceeds their isolation rating.

Ground = 0V by design of definition and may be floating.

This means usually V- = 0V OR the other V+=0V to get a -48V if floating and have 96V total but elsewhere protective earth(PE) = 0V but they are in different domains. They are not equal. It may be still used isolated or non-isolated sharing the PE=0V by choice and still have a 96V (or more) differential.

Yes, you can add them in a series but of course do not tie each V- together, as that's in parallel and would cause a short if also wired in series. The PE gnd's are already tied together at the AC input connected to non-isolate them to your 0V dc to reduce common mode floating output which was protected with ~3kV insulation when isolated (typically).

It is safe to run Caps up to their rated voltage, but there are MTBF acceleration factors on aging based on the % of Vmax margin for operation as the stress increases on the insulation as the margin reduces.

Just in case these Industrial Delta Power supplies do not power up in sync and have no reverse protection built-in (poor design, if so), use reverse power diodes on each PSU to shunt your load current from reversing the voltage on the last 1 of 2 series connected PSUs coming up in sequence.