Ethernet – IC to Switch from Ethernet Port A to Port B

Don't do it! Here's some reasons why you shouldn't:

• The isolation transformer is not likely to be able to handle the current that PoE will push through it.
• You can't support the full PoE discovery protocol without modifying the firmware of the Ethernet switch. I am referring to LLDP.
• Not supporting the PoE discovery protocol could allow you to power a device that isn't able to handle it-- causing it to damage the device.
• Careful current regulation is required to prevent creating a fire hazzard.

I have to admit that I am making a couple of assumptions about your ability to design this thing properly-- assumptions that might be wrong. I am assuming that you don't know how to design it properly because you don't apparently know how the Ethernet signaling, isolation transformers, and PoE discovery works.

If you knew how PoE and Ethernet actually works then this project might be barely possible-- but hardly cost effective since there are other PoE switches and injectors that are not super expensive. Even as a learning experience this has limited value, since there are better ways to learn PoE.

This is not a complete answer, but it's a good first step.

I recommend checking the PHY's mode switches. It's probably set to auto-negotiate, meaning the PHY will fall back on a slower data rate if the faster one proves marginal. If you can force the faster data rate, it'll be much easier to debug the problem.