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I have a custom board which is powered by Ethernet and I am using the LT4275B from Linear using the recommended schematic.

When I am using a cable with only the data pairs [1-2]&[3-6] ([4-5]&[7-8] are disconnected), I can plug as many of my boards I want and I have no problem.

But now, if I use a complete cable with the four pairs connected I have this problem : If I plug a board I can see in the web page of the PoE switch the Class of my board and the consumption. Now I plug a second board (which is the same hardware) BUT I can't see my board, neither the Class nor the consumption. The switch consider the consumption of the second as if it was the one from the first board.

So for example I have two boards with 2W of power consumption, if a plug one board I have 2W on the first board. Then I plug the second board and I have 4W for the first one and 0W for the second one. And so on (6W for three, etc.).

I have this problem on all the PoE switch I had in my hand (Sisco, HP, and more).

Do you have any ideas ? Is it possible that the problem come from my RMII chip (DM9161A) ?

Thank you for your help !

EDIT :
When we cut the spar pairs it works properly as well when we use Ethernet Crossover cables... Strange...

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Find the solution :

After a LOT of tests it turns out the problem was the Earth connection of the shield of the Ethernet cable.
In our design the shield of the Ethernet connector was connected to the ground through a ferrite bead of 220 Ohm at 100MHz, so we replaced it with a capacitor of 2.2nF/1kV (like the one for decoupling fly-back converters).

We also had an "Earth loop" problem, sometimes we need to have an Ethernet connection AND a power supply, but you can't have two different Earth on a system. You can found more information in this document (in French but schematics are comprehensible).

Hope this helps !

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  • \$\begingroup\$ If your device has any non-isolated external connections then you need to use an isolated power converter for the PoE. \$\endgroup\$ – Peter Green Sep 13 '17 at 13:04
  • \$\begingroup\$ Yes, it is an isolated power converter for the PoE. \$\endgroup\$ – Tagadac Sep 13 '17 at 13:07

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