I've been looking at board mount isolated DC/DC converters to use in an application. I found this rather reasonably priced for my needs. Here is what I'm confused about, what is C103 doing here? Isn't it breaking the isolation?

Or, is it because it's DC it won't break the isolation?

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1 Answer 1


It will not unduly compromise the galvanic isolation or safety because it is is an XY rated safety capacitor. It's also low capacitance (less than 1 nF) so the impedance at mains frequency is rather high and the resulting current will be low, even if L and N were accidentally reversed and a path was present from -DC OUT to earth.

Although the working voltage rating is only 250VAC it will withstand 4kV to meet the safety agency requirements. Do not substitute another type of capacitor without doing a proper engineering (safety) analysis.

The purpose of C103 is to conduct noise caused by the internal switching back to the input. If you leave C103 out, the output will have high frequency noise on it relative to the input (and thus relative to earth).

  • 6
    \$\begingroup\$ Last paragraph taught me hours worth of study, thx+ \$\endgroup\$
    – Dzarda
    Jan 29, 2015 at 21:13
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    \$\begingroup\$ L and N are routinely reversed with European (Schuko) unpolarized mains sockets and plugs. \$\endgroup\$
    – ntoskrnl
    Jan 29, 2015 at 23:35
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    \$\begingroup\$ Assuming D101 is a bridge rectifier, L and N are both symmetrically connected to -DC_IN. \$\endgroup\$
    – rioraxe
    Jan 30, 2015 at 5:02
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    \$\begingroup\$ In some particularly noisy devices, this is sometimes done using stacked ground planes in a multilayer board providing the necessary capacitance, esp in cases where the noise is radiated from the devices body. \$\endgroup\$ Feb 3, 2015 at 4:14

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