Tool/software:
I am using the TPS23754PWPR PoE PD controller in my router designs.
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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).
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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:
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PoE detection/classification appears normal with the injector.
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No issues observed in Gigabit version.
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Only the Fast Ethernet PoE switch → board case fails.
Questions:
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Are there known differences between PoE implementations in Fast Ethernet (10/100) magnetics versus Gigabit that could cause this behavior?
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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?
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Could the issue be related to Mode A/Mode B powering or diode bridge orientation in the Fast Ethernet layout?
Below attached the Schematic & Waveform for reference.
Schematic
PoE Injector Detect WaveForm
PoE-capable Ethernet switch Detect waveform
