Vcore Rail on Pascal GPUs

Vcore, how it works, how it's created, and problems that happens on it.

The Controller Circuit.
Nvidia uses the UP9511p by UPI Semi. The UP9511p is an 8-phase PWM controller on most Pascal high end cards, in some cases (Like the Hybrid version of EVGA cards) they use the NCP81274 instead.

Markings on the schematic and board could differ from GPU model to another but the circuit is almost always the same.

The controller gets powered up by the 5V rail through a 2.2Ohm resistor on pin 12.

Enabling it is the PGOOD signal from the 1.8V controller coupled with a protection circuit signal through an AND gate shown here:

The PGOOD signal from this controller will enable the Vmem controller.

Usage.
The PWM signals generated by the UP9511p controls the voltage going to power the GPU core. Switching 12V to 0.7v to 1v depending on the load on the GPU.

Depending on the model and manufacturer of the board and the components used, the PWM signals either go to the VRM directly, driver IC, or a doubler IC.

No Vcore voltage
In this case, check if the controller has 5V on its VCC, and 3V on EN. If one of them is missing trace their respective circuits and check for broken, knocked off, or defective components

If EN and VCC are present and still no output, check the VREF pin, it should be 2V if not, the controller itself is dead.

Very rarely the IMON resistor changes to a lower value which sets the output current limit to a lower value which could trip the Over Current Protection (OCP) of the controller.

Short on Vcore
By design, Vcore has low resistance to GND (less than 1Ohm) so using a low end multimeter won't yield you significant results. Often times, the short happens on one of the VRMs, how to find them is explained here Base Voltage Rail Short on Pascal GPUs.