Burnt a MOSFET, Let's Diagnose Why!

[DNAJAY]

Hi All,

Thank you in advance for your expertise!

I recently went to turn on my computer and it immediately turned off. Tried again and then I started to smell burnt electronics and saw a puff of white smoke. So I took out the GPU, One Stick of RAM, and unplugged everything that wasn't necessary. It powered on, then off again; more electronics smell. After that it wouldn't turn on at all :( . It's worth noting that my setup had been working fine for about 6 months and nothing has been changed since I built the machine. Went to turn it on one day and poof...


Upon inspection one of the MOSFETs in the VRM was charred, so the motherboard is on its way for a RMA from Gigabyte. So here are my questions:


1) I took the PSU to my local computer store and they tested the PSU and it's working properly and displaying the correct voltages. Can I effectively rule this out as the cause of the issue?

2) I am not quite sure how MOSFETs work. My question is on my MB, is there some inherent protection for my CPU following such an event? To put it another way, based on the location of the burn MOSFET is there a second line of defense before the CPU gets hit with full voltage? See Pic (Red Circle).




3) I think I know the answer to this, but would turning it on/off repeatedly in this state affect my other components? Just trying to gauge the damage potential and again what protections are in place on this MB.

My guess is the MB sacrificed itself to spare my other components, but until I get a working MB back from Gigabyte I won't know for sure. But I figured I would pose the above questions anyway just to try to narrow down why this happened, and become educated about the MOSFETs/VRM on this board.


Thanks again!


My Setup:

Gigabyte Z97N Wifi (Currently on RMA)
Intel 4790K (No OC)
TT Nic L31 CPU Cooler
MSI GTX 970 (Slight OC)
TT Tough Power 750w Gold
GSKill 16GB RAM (1600)
Samsung EVO 500GB
2 Seasonic 1 TB HDDs
TT Core V1 Case

Temps: NB ~50C; CPU~30C Idle; 60C Load.

[DNAJAY]

Re: Burnt a MOSFET, Let's Diagnose Why!

Here's a close up of the damage.

[Peakr]

Re: Burnt a MOSFET, Let's Diagnose Why!

If you smell smoke, best practice is to shut it down and not power it back up until you find whats burnt. When you get mobo back you can test individual components one at a time. I've burnt mosfets before and was lucky enough not to damage cpu, ram, cards but I can't speak for you. You can also check for other burnt spots on both sides of motherboard for clues. Other problems; molex connectors, loose wires, metal fragments in your case.

[profJim]

Re: Burnt a MOSFET, Let's Diagnose Why!

Sign up at Sin's Hardware site and post your questions and photos.

Sin's Hardware - Sin's Hardware

https://www.facebook.com/SinsHardwareReview

I haven't seen him in the forum for a while but he might reply if you send a private message to sin0822
Include a link to this thread in your PM.

[DNAJAY]

Re: Burnt a MOSFET, Let's Diagnose Why!

Thanks for the quick response all. I'll be sure to contact Sin to see his input!

[profJim]

Re: Burnt a MOSFET, Let's Diagnose Why!

You're welcome, let us know what you find out.

[DNAJAY]

Re: Burnt a MOSFET, Let's Diagnose Why!

You're welcome, let us know what you find out.

Will do. While we wait, is it likely that a few molex fans and LED lights (not connected directly to MB), could short out the MOSFET via the PSU? If so, wouldn't my PSU show some signs of damage? Isn't it more likely that a molex connector would burn? Again, my local computer store checked out my PSU. They tested the motherboard connections on the PSU, but didn't test the other modules.


I have a 750W GOlD TT Toughpower. It has one cable with 5 molex inputs. I don't put more than two items on each input, and I don't think I am even close to the max output. As so:

--X--------------------X------------------------X----------------------------------------X--------------------------X


Molex 1: 80mm Fan + 30mm LED
Molex 2: 80mm Fan + 30mm LED
Molex 3: 80mm Fan + 3 3-pin 50mm Fans (via a molex to 3 3pin splitter)
Molex 4: 2 12cmm LEDs


Yes, I know it's a lot of LEDs and Fans, but damn is it sexy. LEDs are bitfenix, Fans are Antec. I guess my question is, do molex power limits apply per connection, or per line of connections. Either way, was I pushing it?

[DNAJAY]

Re: Burnt a MOSFET, Let's Diagnose Why!

You're welcome, let us know what you find out.

Found this on SinHardware: Sin's Hardware - VRM Guide


It's way outside my wheelhouse, but from what I gather there's a pretty good chance my CPU is fine. Here's why. There are usually two MOSFETs per powerphase (High-Side and Low-Side). The Low-Side turns on to absorb the remaining voltage once the High-Side turns off. I can't say for sure which MOSFET burnt out, but only one melted so it make me think the other MOSFET may have absorbed the extra voltage. Finally, Intel chips have their own voltage regulators as do PSUs. So between the three fail-safes I'm hopeful that my CPU survived.

As for what caused this, I have to go with a faulty MB. From my research MOSFETs blow mostly when people do OCing or their PSUs go out. Since the PSU checks out and my CPU is stock, that leaves the fault MB.

[DNAJAY]

Re: Burnt a MOSFET, Let's Diagnose Why!

Well bad news,

The new MB fried instantly with a new PSU. So that leaves the culprit as either the case or the CPU. Never heard of a CPU frying a MB, so it's got to be the case!

Keep in mind I only had the cpu, cooler, and one stick of RAM connected.

I even unplugged the 8 pin cpu connector and the MB still wouldn't power on for more than 1 second. Am I missing anything? Remember, I just went to turn on my PC one day and poof!

[profJim]

Re: Burnt a MOSFET, Let's Diagnose Why!

The 8-pin or 4-pin cpu connector is needed for your system to start up.

What do you mean by "fried instantly"? Was there smoke? Sparks? A loud pop?

Did you get a new motherboard and did it fail the first time you powered it on with a minimal hardware setup?

Have you tried powering on with your motherboard placed on a cardboard box outside of the computer case?


Examples of burnt cpu socket pins and cpu pads:






Bent cpu pins:

[DNAJAY]

Re: Burnt a MOSFET, Let's Diagnose Why!

[QUOTE=profJim;502180]The 8-pin or 4-pin cpu connector is needed for your system to start up.

What do you mean by "fried instantly"? Was there smoke? Sparks? A loud pop?

Did you get a new motherboard and did it fail the first time you powered it on with a minimal hardware setup?

Have you tried powering on with your motherboard placed on a cardboard box outside of the computer case?




Yup, it power went on, the CPU fan spun for about 1 second, then off. Tried again, then smoke. This was on the board I got back from gigabyte. So I bought a new board and tried again. New PSU, NEW MB, NEW PSU, and .... on, then immediately off.

As for the 8pin thing, the computer won't post without, but the MB should power on. I initially tried with the 8pin connect, then again without, then again with and poof!

I don't think it's bent pins because it has been working fine for five months, then one day..poof. The CPU itself looks immaculate actually. However, it's either the CPU or the RAM...so I am going to RMA both. I've never heard of a CPU or RAM dying, then killing the MB in this way...but that's all that's left.

I'll probably pick up a new case as well. I'm certain it's not the case's fault, but it's probably best to start from scratch here.

Thanks for all the help! I'll keep you all updated on the journey!

[profJim]

Re: Burnt a MOSFET, Let's Diagnose Why!

Closely inspect the motherboard and computer case for signs of damage, such as signs of a short circuit, including burn marks.

A high quality power supply will quickly shut down when it detects over-voltage, over-current and with several other conditions.

Double check that each of the nine motherboard stand-offs were securely mounted to the case and that each stand-off was aligned with the appropriate motherboard stand-off mounting holes. Make sure that there aren't any extra/unused stand-offs mounted underneath the motherboard.

Short circuits can sometimes occur where the motherboard's rear I/O panel contacts the computer case.

Are there any cables or wires that were located between the underside of the motherboard and the computer case?

Have you closely inspected all wires and cables for signs of chafing or damage? USB cables? Audio cables?