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TPS2375-1: 300ms interrupt PoE hold-up

Olan Way
Prodigy 190 points
Part Number: TPS2375-1

I need to hold up the incoming IEEE 802.3af PD power bus during 300ms power interruptions, avoiding a PoE re-handshake. 

Theoretically, if I have a large cap in the PD that slowly charges up to the incoming bus potential (48VDC), then when the PSE interrupts the PoE, the large cap in the PD keeps the TPS2375-1 operating until the PSE comes back up.  Do you think this would actually work without a PoE re-handshake, as long as the TPS2375-1 input never drops below 36V? 

(Note:  the large cap charging rate would be purposely limited, but the large cap discharging into the TPS2375-1 input would be as quick as needed through a Schottky diode.)

  • 0
    TI__Mastermind 21320 points

    Hello,

    Yes this would work. I would suggest using resistors to force a charge current and a diode for force current direction like below: 

    The circuit above also puts it on the output, but either input or output is fine. 

    The other thing to note is that such a large cap, on input or output, is going to take time to charge. The PoE standard has a limited in-rush phase that limits the input current that is supposed to charge the input bulk cap. If the caps are not charged by the end of the time limit, the PSE will shut off power. However, the hold up circuit will then slowly discharge, and the next PoE handshake should have some charge left over. It make take a few tries, and technically it will not be complaint, but it will work. So you will probably have a slower initial turn on from the end user perspective. 

    Additionally, you could consider doing dual inputs with hitless failover. This essentially uses two PoE ports to ensure that if one fails the other seemlessly takes over. We have a blog,  training video and working reference design if you're interested. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments 

  • 0
    Prodigy 190 points

    Hi Michael,

    Thanks for your message.

    How much time does it take to perform the 802.3af typical handshake, between PSE and PD (without considering the hold-up discussion)?

  • +1 in reply to Olan Way
    TI__Mastermind 21320 points

    Hello, 

    That depends on the PSE since it controls the negotiation. It would probably be safer to do more like 500ms. 

    If this post answers your question, please indicate so by marking this thread as resolved. Thank you.

     

    Regards, 

     

    Michael P.

    Applications Engineer

    Texas Instruments