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I have added the Silvertel Ag9205S 5 V PoE module to my design in order to provide 5 V power to the system using Ethernet. I also have USB power that can provide 5 V as well. I have a diode on the USB's VBUS, but did not add one to the PoE module since it has one internally.

It works fine, but today I noticed that with just 5 V coming from the USB and no other source of power, a MOSFET on the PoE module heats up pretty good before what I suspect is it going into some sort of shutdown (the heat comes and goes).

In order to widdle down any other components causing problems, I removed everything except what is shown in the schematic below. It still happens in this case. As stated, everything works just fine, so my thought is it's operating as normal. I planned on adding a diode to the output even though it has one, but stumbled across this post that advised against it. I don't have a thermal camera, but my guess is it's reaching 50-60oC before shutting down.

Just seems odd in that the module should in theory be powered down at this point - with only 5 V coming externally reaching the module's output. Thoughts?

enter image description here

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I reached out to Silvertel and they responded with this:

What you are seeing is not the recommended behaviour of the module, the Ag9205 is a forward converter with synchronous rectification on the output. As a result, if an external voltage is provided on the output, then the output MOSFETs will oscillate and back feed through the module (generating high voltages on the input), this will cause power dissipation in the module and could lead to damage over time.

It is essential with any forward converter with synchronous rectification to have gating on the output to prevent this, the diode you mention from the block diagram was to indicate the rectification stage rather than an ORing diode, sorry for the confusion.

They provided this circuit that could work:

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