Hello!
I have a broken PC graphics card (EVGA GTX 970 SSC) with 4-phase GPU vcore power, one day the two low-side FETs from one of the buck converters just decided to short themselves and weld themselves to the copper on the PCB creating a permanent short circuit inside it. I was able to remove this short and make the card work with remaining 3 buck converters making power for the GPU ASIC. Now I need to restore the 4th phase, I recreated the destroyed traces on and inside the board, soldered down a new set of FETs (there are two low-side and one high-side), as well as a new driver for them (NCP81062). I did check all the connections based on the application circuits from driver and the buck controller datasheets (
https://www.onsemi.com/pub/Collateral/NCP81062-D.PDF and
https://www.onsemi.com/pub/Collateral/NCP81174-D.PDF), but now when I power on the card the controller seems to enable all the four converters for a couple milliseconds and then it disables them. Looking at the waveform of each phase (at the switch plane, before the output coil) I can see the probable reason for that, this is how it looks like on the 3 working phases:
(as expected, 300 kHz PWM signal)
And on the 4th phase:
And this doesn't look like it should do...
I don't know how to interpret that signal, maybe there will be someone that could tell me something more about what it could mean.
All of the NCP81174's PWM outputs look the same:
(300 kHz PWM signal)
At this point I don't know why it doesn't work, I already tried three different NCP81062 drivers from different batches, with the same result every time, I swapped the main buck controller (NCP81174), changed every FET thinking that they can be the issue, I even tried with a different output coil, but nothing seems to change anything in the way it behaves now.
Here is how the reconstructed buck converter and the whole power delivery section look like:
https://imgur.com/a/eHyJdEr