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TPS23754: PoE not powering with Ethernet/Network Switch(PSE) works with PoE injector

Karthik M
Prodigy 20 points
Part Number: TPS23754

Tool/software:

I am using the TPS23754PWPR PoE PD controller in my router designs.

  • In my Gigabit router design, everything works fine. The device powers successfully from both a PoE injector and a PoE-capable Ethernet switch (802.3at PSE).

  • In my Fast Ethernet (10/100 Mbps) router design, the exact same TPS23754-based PoE circuit is used. However, in this case the board only powers when using a PoE injector. When connecting to a PoE Ethernet switch (PoE PSE), the board does not power on.

So far:

  • PoE detection/classification appears normal with the injector.

  • No issues observed in Gigabit version.

  • Only the Fast Ethernet PoE switch → board case fails.

Questions:

  1. Are there known differences between PoE implementations in Fast Ethernet (10/100) magnetics versus Gigabit that could cause this behavior?

  2. Do I need to modify anything in the front-end design (RJ45 magnetics, center tap connections, Bob Smith termination, etc.) when using TPS23754 with a 10/100 Mbps transformer?

  3. Could the issue be related to Mode A/Mode B powering or diode bridge orientation in the Fast Ethernet layout?

Any guidance on debugging steps or required schematic/layout modifications would be greatly appreciated.

Schematic

PoE Injector Detect WaveForm

PoE-capable Ethernet switch Detect waveform

  • 0
    TI__Intellectual 920 points

    Hi Karthik,

    Thanks for reaching out.

    Questions:

    1. Are there known differences between PoE implementations in Fast Ethernet (10/100) magnetics versus Gigabit that could cause this behavior?

      A: For data transformers, please make sure both design has PoE compatibility.

    2. Do I need to modify anything in the front-end design (RJ45 magnetics, center tap connections, Bob Smith termination, etc.) when using TPS23754 with a 10/100 Mbps transformer?

      A: You have posted only one design in the thread, here is the issue I found from the schematic:   

    4,5-7,8 are shorted with two 75 Ohm resistor. When the PSE are doing detection, these 150 Ohm in total will cause detection issue. And please make sure all 4 pairs are terminated to the diode bridge correctly.

    Please refer to this reference design for the correct RJ45 connection.

    TPS23753AEVM-004: Evaluation Module for TPS23753A.. (Rev. F)

    Best regards,

    Zhining