I’ve explained what a buck converter does here.
A common problem with the CPU vcore circuit is that one of the FETs in the circuit shorts and is always working, even without input to tell it to work. So it sends PPBUS_G3H to the CPU… all the time. This can burn out the CPU, and also bring down the PPBUS_G3H rail. We fix this problem under our Macbook logic board component level repair service.
So people will replace the affected FET and think they’re done… right? Wrong. The buck converter controller IC receives that very same output on a pin, and when it receives that constant 12.6v because the FET has shorted it dies. The FET is supposed to open and close rapidly, which makes those pulses of 12.6v average out to 1v. However when it dies it is putting out a constant 12.6v, which kills the controller.
Now the controller is dead, so it will tell the FET to open all the time. The FET will overheat from being opened all the time, so it will die. So you replace the controller, but now the controller killed the FET, and you have this vicious loop.
When you have an issue with a shorted FET in a buck converter
1) Replace EVERY FET!!!
2) Replace the controller
3) Now turn it on.
I talk more about this issue in the video below.