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TPS23756: PoE PD TPS23756 detection issue.

Part Number: TPS23756
Other Parts Discussed in Thread: TPS23754EVM-383, TPS23754

Hi,

please help to review the attached schematic,the issues is as follows:

1- the POE board cannot detected by a POE switch as a POE device,connect this POE board to a POE switch, the POE switch has no voltage output.

2- plug in a 52V external adapter between VDD and VSS, the MOSFET Q10 is brokendown, some board Q7&Q9 are also brokendown,remove Q7&Q10, test the output is a 12V out.

Hope to recieve a reply ASAP.7217.TPS23756-POE.pdf

  • Hi Relynn,

    We are currently reviewing the schematic and our comments will be posted when available. Thanks!
  • Hi Relynn,

    For the bob smith termination, it should be 10nF and not 100nF. I have seen this effect detection in the past.
    I don't see a VDD and VDD1 connection. I see test points but not a net connection.

    For these power requirements and 52V input, you might consider copying the TPS23754EVM-383. I say this because the TPS23756 and the transformer selection is designed for wide input. For 12V output applications, and the adapter is 12V (for example), the adapter is typically OR'd at the output and not on the primary.
  • Hi Darwin,
    Thanks for your reply.
    1- In my schematic, VDD1 is not a test point, it is a power symbol, VDD and VDD1 are connected.
    2- Change C12-C15 to 0.01uF, there is no improving.
    3- My design exactly copy the TPS23754EVM-383, but it doesn't work. Is there any other difference between TPS23756 and TPS23754 except input?
    4- In your official schematic, there is a TVS between VSS and primary GND, is it necessary or could effect the function?(in my design no this TVS)
    Could you give me any other ideas about my design? looking foward to you reply.
    Thank you!
  • Hi Relynn,

    There may be leakage through the diode bridge rectifier. I noticed that it is different from the one used in our EVM.

    Could you show a VDD-VSS & Iss waveform showing what happens during startup (detection & classification)?

    - Tom
  • Hi Relynn,

    I will be closing this thread & marking it as resolved. Otherwise, if you have the additional waveforms or comments, you may reply to this thread & reopen it.

    Thanks,
    Tom