after CPU VRM burned, CPU still good?

[kaboom]

Re: after CPU VRM burned, CPU still good?

xbox 360 power supplies have a touchy overcurrent protection.
the input cap charging on my hf radio will trip it if you plug it in live. (1)
but it does not seem to mind 25a peaks demanded on voice peaks.
theres a 3300uf cap on the dc input on this yaesu ft-847.
and i too have seen vrm's totally burned up and the cpu survived.
its a matter of luck. (2)

1- Most hard drives are the same way. Hotplug anything in parallel with one, and it will reset.

2- Maybe they just adhere the old power supply adage of "protect the expen$ive load, no matter what."

I have an Intel DG33 that shorted it's upper MOSFET twice on me (after working for a few minutes each time). Both cases resulted in PSU buzz (i.e. short-circuit protection kicking in). After repairing those, I had a lower MOSFET short, and again the result was the same - PSU short-circuit protection kicking in. I though, man, that CPU sure must be toast now. So I decided to play with the board a little. I swapped the inductors with ones from a completely different motherboard (4-turn larger core in place of 3-turn small core), changed all high-side MOSFETs with different (but matching) ones. When I tried to turn on the motherboard, I got a nasty squeal. Then, I decided, screw it, let's see if this possibly fried CPU can damage another motherboard. So I took the CPU and tried it in another motherboard I didn't care much about. But what do you know? The CPU (Core 2 Duo E2160) was perfectly fine and functional, even after all of that BS.
As for my Intel mobo, I can't really say the same . All I know is, all gate drivers are fine. It just eats MOSFETs or squals or shorts out the PSU .

Some of those substitute devices may not be optimal. While there are dedicated gate drivers, you've got to watch gate charge, gate capacitance, and feedthrough capacitance. If any trash couples via D-G capacitance, the drive waveform could get distorted, depending on the layout and capabilities of the driver IC(s).

What were the existing ones, and what did you use to replace them with?

Do you think one of the low side MOSFETs was leaky, and taking out the high side ones? Just because I'm curious, was it also squealing w/o the CPU?

This is not my only experience with a chip that survived a shorted upper MOSFET. I also have an AGP GeForce 7600 GS that went through a bad MOSFET. Still works with my replacement 85 Amp MOSFET (vs. 13A for the original one ).

This is good to know! I've often wondered about video cards implementing "protect the load no matter what." Of course, all cards don't use the same PWM controllers, but at least for you, all turned out OK.

[kc8adu]

Re: after CPU VRM burned, CPU still good?

xbox 360 power supplies have a touchy overcurrent protection.
the input cap charging on my hf radio will trip it if you plug it in live.
but it does not seem to mind 25a peaks demanded on voice peaks.
theres a 3300uf cap on the dc input on this yaesu ft-847.
and i too have seen vrm's totally burned up and the cpu survived.
its a matter of luck.

I tried messing with some dead Xbox 360 boards (badly warped due to fly-by "repair" shops working on them). It doesn't matter if you short 12V to GPU V_core or V_core to ground - both will cause an over-current condition and trip the PSU.

[Stefan Payne]

Re: after CPU VRM burned, CPU still good?

Yeah, got my replacement CPU from intel, was replaced and took about a week (and no cost for me).

The LGA2011 sys runs fine now with no real OC (well, I upped the turbo multi a little )

[momaka]

Re: after CPU VRM burned, CPU still good?

Those CPU PWM ICs turn the low side on if/when the output exceeds the VID value.

Unless the low side completely opened up/burned clear and was unable to shunt the incoming supply, the CPU may actually be good.

+1

I have an Intel DG33 that shorted it's upper MOSFET twice on me (after working for a few minutes each time). Both cases resulted in PSU buzz (i.e. short-circuit protection kicking in). After repairing those, I had a lower MOSFET short, and again the result was the same - PSU short-circuit protection kicking in. I though, man, that CPU sure must be toast now. So I decided to play with the board a little. I swapped the inductors with ones from a completely different motherboard (4-turn larger core in place of 3-turn small core), changed all high-side MOSFETs with different (but matching) ones. When I tried to turn on the motherboard, I got a nasty squeal. Then, I decided, screw it, let's see if this possibly fried CPU can damage another motherboard. So I took the CPU and tried it in another motherboard I didn't care much about. But what do you know? The CPU (Core 2 Duo E2160) was perfectly fine and functional, even after all of that BS.
As for my Intel mobo, I can't really say the same . All I know is, all gate drivers are fine. It just eats MOSFETs or squals or shorts out the PSU .

This is not my only experience with a chip that survived a shorted upper MOSFET. I also have an AGP GeForce 7600 GS that went through a bad MOSFET. Still works with my replacement 85 Amp MOSFET (vs. 13A for the original one ).

In other news, I have a Shuttle PC with (likely) bad NB. But that one was due to faulty OST caps. NB had only two caps on its VRM low side (large OST RLX and small OST RLG). Both bulged and failed, thus causing the upper MOSFET in the buck converter to die. With no (or very little) output capacitance from those dead caps, NOW you have recipe for disaster.

It works much faster than that... Besides, the output caps have to follow the increasing voltage, so the change is not instantaneous. It might seem like that to us, but there's plenty of time for the error amp to back down the duty cycle, then ultimately keep the low side devices on.

Yup.
I tried messing with some dead Xbox 360 boards (badly warped due to fly-by "repair" shops working on them). It doesn't matter if you short 12V to GPU V_core or V_core to ground - both will cause an over-current condition and trip the PSU.

So most likely the OP's CPU is fine. But I guess that is a moot point now.

In any case, CPUs are some tough cookies.

[Stefan Payne]

Re: after CPU VRM burned, CPU still good?

Well, contacted Intel and they say, they're able to exchange my CPU. So it doesn't matter if the CPU is still good or not...

[tom66]

Re: after CPU VRM burned, CPU still good?

As I said, this is a best case scenario. You've still got a large amount of bulk capacitance (PSU, probably at least 2200uF, plus motherboard and others) that will dump a lot of energy into the processor through the upper FET. And, you're assuming that the IRF devices (controller included) implement this function. As I said many controllers don't implement it because it complicates the control loop during transient response.

[kaboom]

Re: after CPU VRM burned, CPU still good?

Output Crowbar
As part of the protection for the load and output components of
the supply, the PWM outputs will be driven low (turning on the
low side MOSFETs) when the output voltage exceeds the upper
Power Good threshold. This crowbar action will stop once the
output voltage has fallen below the release threshold of approxi-
mately 450 mV.
Turning on the low side MOSFETs pulls down the output as the
reverse current builds up in the inductors. If the output overvolt-
age is due to a short of the high side MOSFET, this action will
current limit the input supply or blow its fuse, protecting the
microprocessor from destruction.

...

[tom66]

Re: after CPU VRM burned, CPU still good?

If the top FET is shorted, there is nothing the error loop can do. You will basically form a resistive divider with about 10mohm on each ladder. Output voltage will be briefly determined by the ratio of resistances. Most controller ICs however will rely on the body diode to commute an over-voltage event. Very few of them switch on the bottom FET during transients because it makes the control loop trickier to design.

A big "it depends", really....

[kaboom]

Re: after CPU VRM burned, CPU still good?

Those CPU PWM ICs turn the low side on if/when the output exceeds the VID value.

Unless the low side completely opened up/burned clear and was unable to shunt the incoming supply, the CPU may actually be good.

If on the high side, marginal. CPU would have been subject to +12V for at least a millisecond or so which would probably destroy it. If on the low side it might survive.

What error amp, especially one for a CPU regulator, is so slow and useless?

It works much faster than that... Besides, the output caps have to follow the increasing voltage, so the change is not instantaneous. It might seem like that to us, but there's plenty of time for the error amp to back down the duty cycle, then ultimately keep the low side devices on.

(The PWM IC drives the MOSFETs via dedicated hi/low gate drivers, but the effect is the same.)

[Stefan Payne]

Re: after CPU VRM burned, CPU still good?

Well, it looks like that was the high side...

The connector on the upper right corner is the EPS12V CPU connector, followed by a coil.
So that burned one looks like it was on the high side...

[tom66]

Re: after CPU VRM burned, CPU still good?

If on the high side, marginal. CPU would have been subject to +12V for at least a millisecond or so which would probably destroy it. If on the low side it might survive.

[Stefan Payne]

I've got a Gigabyte X79-UD5 with burned VRMs, looks like this.

How probable is it that the CPU survived (or not)??