Cisco 2960-CX Not providing POE

[fuxxy]

First off, I'm sorry if this is the wrong subforum - None of the others seemed to fit the description.

I've got this Cisco 2960-CX. It works flawlessly otherwise, but won't provide POE to the ports.

On initial inspection, U4701 (An SI3458) has a physical hole in the die. I replaced with another chip out of a dead 2960, and it immediately popped a new hole.

All I've been able to pull up are datasheets for mosfets, and I'm not quite convinced this is a simple FET.

Popping the die implies an overcurrent downstream, correct?

I have 3 more of these chips until I'm SOL. I've also attached the POE board from a 24-port switch for reference.

[fuxxy]

Re: Cisco 2960-CX Not providing POE

Reference board from a Cisco WS-C2960X

[stj]

Re: Cisco 2960-CX Not providing POE

the mosfets are the 8pin parts on the left,
thats the mosfet driver.
one or more mosfets is shorted to the gate and is backfeeding high voltage to the driver chip

[fuxxy]

Re: Cisco 2960-CX Not providing POE

sweet, I can work with that. Let me do some testing...

[fuxxy]

Re: Cisco 2960-CX Not providing POE

Progress!

Took my multimeter in basic continuity (beep mode), U4701 was the only FET that read short across source and drain. I'm a novice and can't find datasheets, so I'm interpreting source and drain as the two of the three larger sinked pads. These are 8-pin SMD FETs, with 4 pins bridged together, as well as 3 other pins bridged together. The last pin I'm assuming is the gate.

I replaced U4701 (again) and Q4801 with similar components from my donor board. They're not quite exact, but appear to have the same footprint and family.

SUCCESS! (Sort of...)

When I initially received the switch, it would power on, boot, and respond to console commands. It just did not provide any POE, on any port.

Now after this repair I'm getting POE power applied to ports 1, 6, 7 8. No POE PD device is being detected on ports 2,3,4,5.

There's a small possibility that my donor U4701 is faulty. It makes sense to swap it out again for another chip, but this is quite the nerve-wracking experience. I'm afraid I'll overheat the board, POE controller, or both.

I also noticed an error in my initial attempt to replace U4701 before I started this thread. The chip was installed in the wrong orientation - for others attempting this type of repair, the silkscreened dot on the IC die should line up with the beveled line on the PCB outline.

[stj]

Re: Cisco 2960-CX Not providing POE

work out which fet is for each port,
then compare readings - maybe several are damaged

[fuxxy]

Re: Cisco 2960-CX Not providing POE

work out which fet is for each port,
then compare readings - maybe several are damaged

oh that sounds logical.
I was thinking they used some weird setup like one fet for each side of the power pairs or something.

[stj]

Re: Cisco 2960-CX Not providing POE

i have no idea how it works
just applying logic - i see lots of ports and lots of fets and a driver that can control them individually

[fuxxy]

Re: Cisco 2960-CX Not providing POE

i have no idea how it works
just applying logic - i see lots of ports and lots of fets and a driver that can control them individually

As far as the source for each FET, all 8 of them have roughly -33v DC being supplied to each FET.

The drains, on the other hand, seems hit or miss. Q4803 & Q4804 are not "on" - fluctuating around 3v output voltage. This is with port 1 powering a POE device.

When I remove the device, nothing changes.

I'll replace Q4803 & Q4804 and see what happens.