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[FAQ] PoE PD Controllers: What is the purpose of the isolation capacitor (Cio) between RTN and isolated output ground?

Earl Smith97
Prodigy 60 points
Part Number: TPS23751
Other Parts Discussed in Thread: TPS23755EVM-894

What is the purpose of Cio in figure 32 of the TPS23751 datasheet?

I have a noise problem at a specific frequency and it appears to be related to my layout of the POE circuit. What is the purpose of this capacitor purpose and can it be removed without introducing a vulnerability to the overall design?  Looking at the design, I can only guess it is used to provide a return path for leakage due to the primary - secondary capacitance of the flyback transformer which would suggest removing it might make noise emissions worse.

  • 0
    TI__Genius 9230 points

    Hi Earl,

    Cio in figure 32. of the TPS23751 datasheet is an isolation capacitor. As noted on page 6 of SLUA469, "this capacitor suppresses EMI by providing a return path for noise that is coupled across the flyback transformer."

    Additionally, you can add another isolation cap from Vdd to secondary ground for the high side of the transformer to create a Y-network for common mode noise. An example of this can be seen on the schematic of the TPS23755EVM-894 User's Guide. (SLVUBG8)

    Thanks,
    Tom Amlee
    PoE PD Apps Engineer