Connecting two of these DC power supplies in series

Question

I have two identical power supplies which output 16.5V DC. They seem to be isolated by a transformer and an optocoupler, but there is also a 1kV 2.2nF capacitor between the primary and secondary side (or rather, between the negative pole of the output and negative pole of the input). Without this capacitor, there is no galvanic connection between the input and output. I want to connect these PSUs in series, but I suspect this could possibly create a short circuit via the capacitor (while it's still uncharged).

I tried connecting the positive terminal of one PSU to the negative of the other, first through a large resistance, then a lower resistance and finally directly. Nothing happened. I also measured current as I did this ‒ the multimeter showed some microamps here and there, of course only transiently until the cap charged up. I'm worried that this cap might cause some problems and I would like to just remove it. Does anyone have a clue what is its purpose, and what's the worst that could happen if it weren't there?

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Here are some photos. I can try drawing a schematic if necessary. The large vertically-mounted diode on the output side is a SR5100 (100V 5A Schottky); optocoupler is PS817c.

• Entire PCB:

  

• Primary-secondary separation (left = input; right = output):

  

Answer 1

The capacitor is there to 'short-circuit' the common mode high frequency noise that the converter generates between the input and the output.

As it is tiny, and rated for a high voltage, there is no danger that this capacitor will cause a DC problem when putting the supply outputs in series.

You could remove the capacitors, if your load or application is not susceptible to the noise. Be aware that the capacitor may be there to allow the PSU meet EM emissions standards, so operating without it might cause interference to other electronic products in your or your neighbours' houses.