What's the physical base of 1nF capacitors (CY) between isolated grounds of DC/DC converters?
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\$\begingroup\$ In an addition to Andy's answer, here are two older questions similar to yours with why Cy wont break isolation : electronics.stackexchange.com/questions/151574/… electronics.stackexchange.com/questions/364271/… \$\endgroup\$– RahmanyCommented Aug 31, 2022 at 12:27
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2 Answers
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A couple of points to take on-board: -
- GND is a reliable "sink" node that can be used for connecting capacitors to for filtering any noise on the output of your DC-to-DC converter.
- Because the output of your converter is isolated, it doesn't mean that it contains zero noise with respect to GND.
- The output noise comes from the internal switching circuits. It's capacitively coupled via the isolation transformer at frequencies typically around 100 kHz (50 kHz to 200 kHz usually).
- That output noise does contain higher order harmonics up to hundreds of MHz.
So, the CY capacitor is present to reduce the high frequency noise on your isolated DC output. Call it an EMC compliance thing.
answered Aug 31, 2022 at 11:54
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The converter output is isolated and it is floating. Any signal coupling to it will affect the output isolated ground potential and it will have high frequency switching waveform capacitively coupled to it via the capacitance between switching mode transformer input and output windings.
The external Cy capacitor is much larger than the stray capacitances so the signal via small stray capacitance does not cause much voltage over Cy. Basically it acts as an AC or RF ground while still allowing DC isolation between input and output.
So the capacitor simply reduces output common mode voltage swings so the circuit passes EMI tests. So it does not cause problems to other circuits and other circuits do not cause problems to this circuit.
