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TPS23754: Question from TPS23754's Application note (High Efficiency Class 4 PoE Powered Device)

Grady Lee
Genius 3945 points
Part Number: TPS23754

Hi, TI expert.

A customer is reviewing the TPS23754 and has a question in the Application note (High Efficiency Class 4 PoE Powered Device).

slua593.pdf

Looking at the schematic of the demo board shown in the application note (High Efficiency Class 4 PoE Powered Device), Capacitor, ceramic, 1000 pF, 2 kV, X7R, 10%, 1808 spec was applied to C5 and C4.

Is there a reason to use 2KV for internal voltage?
Will there be a problem if I apply a value lower than 2KV?
Are there any problems if the (Bob smith termination) value of C5 is changed?

Please check. Thank you.

  • 0
    TI__Mastermind 33241 points

    Hi Grady Lee,

    Thanks for reaching out to us.

    I will get back to you in 2-7 business days. 

    Best regards,

    Diang

  • +1 in reply to Diang Xing
    TI__Mastermind 33241 points

    Hi Grady,

    Thanks for your patience. 

    Is there a reason to use 2KV for internal voltage?

    - the 802.3 standard requires all PD & PSE provide isolation for all external conductors. Ethernet cable’s MDI leads should be included. 2 kV to meet the isolation voltage. 


    Will there be a problem if I apply a value lower than 2KV?

    - Please refer to the last picture. At least one of the 3 electrical strength tests needs to be meet.


    Are there any problems if the (Bob smith termination) value of C5 is changed?

    Bob-Smith is to prevent standing/reflected waves on the data cable in order to reduce CM EMI. If there is an Earth ground then the common BS connection can be connected to Earth through a HV capacitor. This also helps with transients. We did see different capacitance values on these cap. It may not have much effect on EMI or transients if the cap value is in the same level.

    Best regards,

    Diang