TPS23861: Port Always Detected as Short

Hi Ryan,

Can you confirm for me when you say "disconnected the center taps on the magnetics’ secondary for one of the ports ", did you remove F6-F9 on page 1 to 4 and it still shows short circuit? Can you ask their engineer to check if there's any resistor between V48V_PSE and PSE_N1(2,3,4) ? 

In addition, TPS23861 requires power on sequence which is described in this app note:http://www.ti.com/lit/an/slva723/slva723.pdf. Please ask your customer to double check this. Thanks.

Best regards,

Penny

Hi Penny,
1. When we say "disconnected the center taps on the magnetics’ secondary from one of the ports", we mean we disconnected the center taps on the Phy side of the transformer (opposite to the side that has power injected). Our design has all four center taps connected together at the transformer pins (pins 1,4, 7, and 10), then connected to logic ground through capacitors. We noticed that your reference design leaves the center taps open on the phy side, so we lifted the pins on one port. This made no difference to our short circuit problem.
2. We have not removed the fuses F6-F9. We can in order to verify the short circuit goes away.
3. There is not a resistor directly between V48V_PSE and PSE_N1(2,3,4). After the fuse there is the diode and 0.1uF cap in parallel. The other side of the fuse connects through the 47 ohm resistor to the TPS23861 Drain pin and directly to the Drain of the FET.
4. We'll look at the reset circuit again, although I don't believe this is a problem since the reset is held active until long after both the 48V and 3.3V supplies are up and stable.
Please let us know if you have any other ideas.
Thanks,
Mike Plummer
Vadatech

Hi Mike,

Thanks for the update. Please help to verify if completely disconnecting the PSE port from the transformer resolves the short circuit issue. I want to isolate the problem. Thanks.

Best regards,
Penny

Hi Penny,
We removed the fuse from one of the ports as you suggested and the port now reads as an "Open Circuit" in the PI Commander GUI, while the other three with the fuses installed still read as "Short Circuit".
Please advise,
Thanks,
Mike

Hi Penny,

Thank you, for your help, we have determined the problem:

We noticed that our design was missing the 22nF caps in series with the 75ohm resistors on the line side center taps. The missing capacitor resulted in a measured 150 ohms between each center tap. Once we added the 22nf caps in series, the PoE started to work.

Thanks again for the assistance!

-Mike