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PMP11254: Role of capacitor between PWRGND and GND (C25)

Ryo Akashi
Genius 9470 points
Part Number: PMP11254


Hello experts,

Could you tell me the role of capacitor connected with PWRGND and GND(C25)?
Is it helpful to reduce common mode noise which appear on secondary side due to primary side switching noise?
My customer is designing PoE design refer to this reference design and struggling to pass disturbance emission test(VCCI).
When customer remove C25, the performance became better in specific bandwidth.
Do you have any concern of removing C25?

Thanks and best regards,
Ryo Akashi
  • 0
    TI__Intellectual 2530 points

    Hi Ryo,

    Yes, C15 and C25 are used to help control EMI.  They provide a controlled path for the common mode noise.  Without them the common mode EMI will find its own path through parasitic capacitances to PWRGND or GND.

    In general, these capacitances work much better to reduce EMI with a common mode inductor on the input, which results in an LC filter.

    These capacitors can be removed, decreased in value or increased in value to improve EMI.  I would limit the value to 4700pF each without a common mode inductor since these also provide a path for electrical surges.

    Thanks,

    David