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enter image description here

Here's the data sheet if it helps. https://www.cui.com/product/resource/pdqe15-d.pdf

The DC\DC converter is an isolator unit so the left part of the circuit doesn't us a traditional GND, which leads me to be unsure of you -Vo would affect a polarized capacitor. I'm also worried CY2 would discharge into C3 and cause problems.

Also does the rating on C3 matter? the DC/DC converter outputs 24V but the data sheet doesn't mention a specific rating for C3. It does mention ratings for several other capacitors which leads to believe C3 doesn't have to be rated to a specific value.

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  • \$\begingroup\$ It does mention a rating. "C3 : Refer to Cout in table 2", and Table 2 says 10uF. All diagrams show that a dual output device should have two non-polarized capacitors, so two 10uF capacitors. I suppose C3 could be a polarized capacitor, but you are looking at an incorrectly drawn diagram, so there is no single C3. I don't see why you would even want to substitute those with a polarized capacitor or two, so maybe edit the question to refer to right picture and the motivation why do you ask if can be substituted? \$\endgroup\$
    – Justme
    Commented Apr 16, 2021 at 10:11

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Assuming +Vo is always positive in relation to -Vo, then there is nothing to prevent you using a polarised capacitor. This is indeed the case here.

It doesn't matter how -Vo or +Vo are related to Vin and GND as the converter is isolated, as the input voltages don't affect the rating of the output capacitor.

Your output capacitor should therefore be rated for the output voltage of the DC/DC. For example if you use a 24V output voltage device, then ideally a 30V or higher rated capacitor should be used.

The CY1 and CY2 capacitors are usually very low value (470pF to 2nF) to allow high-frequency noise to have a return path to reduce EMI. They will have no impact on the rating of your output capacitor C3. The voltage rating of the CY* capacitors does matter - it should be at least the 2kV value specified in the datasheet to avoid limiting the isolation voltage rating of the converter.

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C3 ought to low ESR so T=ESR *C =< 5us and rated for >=36V and rated for reliability at >=105’C

But you have shown dual output with no common ground for some reason. Dual outputs need independent polarized caps = 10uF as specified and implied is Vrating at least double the regulated voltage for reliability. Cy1 and Cy2 would be metal film and sometimes cross connection for reducing feedthru of PSRR for CM noise reduction. Test to make sure for EMI conducted and radiated.

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