PEX Rail on Pascal GPUs

This page talks about the PEX rail on Pascal series of GPUs, how it works, what it's used for, and common problems with it.

The Controller Circuit.
PEX is created by a uP1728Q by uPI semiconductor (datasheet) or similar IC by switching 3.3V to 1V Markings on the schematic and board could differ from GPU model to another but the circuit is almost always the same.

The uP1728Q uses 3.3V as Vin coming from the PCIe bus, 5V from 5V Rail at VDD, and gets enabled by the PGOOD signal from the Vcore controller.

The controller regulates the 3.3V on the input to 1V on the output. It regulates that voltage through a resistive divider on the FB pin (Figure 2)

Usage.
PEX rail goes directly to the core to power the pcie express logic on it.

No voltage out.
First thing to check is the EN signal which should be 2V+ followed by Vin which should be 3.3V (Figure 2) then VDD which should be 5V. If one of those is missing then check the schematic and follow the signal to find the culprit, it's usually a faulty resistor. Replace the controller if everything else is in order but still no voltage out.

Short on PEX rail.
Generally speaking, a short on the PEX rail is accompanied by a dead core well over 95% of the time. If you have a short on the PEX rail, your efforts should primarily be focused on proving that the core itself is the source of your short. Typically speaking for Nvidia cards, the "PEX" portion of the GPU is in the bottom right corner, and under a thermal camera a shorted PEX rail tends to be noticeably warmer in this location.