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TPS2373-4EVM-758: Transient voltage issue

T Kishi
Genius 4425 points
Part Number: TPS2373-4EVM-758
Other Parts Discussed in Thread: TPS2373

Hi

My Customer has question.

Earth the positive side of the DC ORing input and supply power as
48V.
When the48V line is turned off by SW, a transient voltage of about 33V is
generated at the POE port with respect to the Earth.
Is there a countermeasure for this transient voltage?

Please check the attached file for details.

TPS2373-4_transientV.pdf

Best Regard

T Kishi

  • 0
    TI__Mastermind 21320 points

    Hello T Kishi,

    Thank you for your question. The diode you are measuring across is rated for 100V, so a 33V spike is well within margin of the rating. 

    Since you are using 48V, you could put the adapter voltage on the front end of the PoE IC (at VDD and VSS) to allow the internal pass FET to limit the current inrush. But again, the diode is rated much higher than this spike. 

    Regards,

    Michael Pahl

  • 0 in reply to Michael Pahl
    Genius 4425 points

    Hi Michael

    Thank you for your reply.
    I basically understood that Diode won't break.
    Customers are concerned that it will not affect the ETH port. What should I think about this?
    And,This voltage is generated when SW = OFF. Can it still be restricted with TPS2373?
    If you can, please tell me how.

    Regard

    T Kishi

  • 0 in reply to T Kishi
    TI__Mastermind 21320 points

    Hello T Kishi,

    Since the IC is also rated for 100V, I think this spike should not affect the port. 

    The IC has current limiting and deglitched foldback that protects against 'faults' like these. The section titled, "7.4.9 PD Hotswap Operation" in the TPS2373 datasheet provides more information on this topic.

    Additionally, the following paper is a great resource for surge considerations of PoE PD's if you want to save it:

    http://www.ti.com/lit/an/slua736/slua736.pdf

    Regards,

    Michael Pahl