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




  1. #1
    DNAJAY is offline Junior Member
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    Default Burnt a MOSFET, Let's Diagnose Why!

    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).


    Burnt a MOSFET, Let's Diagnose Why!-ga-z97n-wifi-itx-645x532-jpg

    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.

  2. #2
    DNAJAY is offline Junior Member
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    Default Re: Burnt a MOSFET, Let's Diagnose Why!

    Here's a close up of the damage.

    Burnt a MOSFET, Let's Diagnose Why!-photo-3-jpg

  3. #3
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    Default 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.
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  4. #4
    profJim's Avatar
    profJim is online now Chief Munchkin + moderator
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    Default 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.
    Q9650 @ 4.10GHz [9x456MHz]
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    Seasonic SS-660XP2 80 Plus Platinum psu (660w)
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  5. #5
    DNAJAY is offline Junior Member
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    Default 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!

  6. #6
    profJim's Avatar
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    Default Re: Burnt a MOSFET, Let's Diagnose Why!

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

  7. #7
    DNAJAY is offline Junior Member
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    Default Re: Burnt a MOSFET, Let's Diagnose Why!

    Quote Originally Posted by profJim View Post
    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?
    Last edited by DNAJAY; 10-29-2015 at 12:11 AM. Reason: formatting

  8. #8
    DNAJAY is offline Junior Member
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    Default Re: Burnt a MOSFET, Let's Diagnose Why!

    Quote Originally Posted by profJim View Post
    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.

  9. #9
    DNAJAY is offline Junior Member
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    Default 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!

  10. #10
    profJim's Avatar
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    Default 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:

    Q9650 @ 4.10GHz [9x456MHz]
    P35-DS4 [rev: 2.0] ~ Bios: F14
    4x2GB OCZ Reaper PC2-8500 1094MHz @5-5-5-15
    MSI N460GTX Hawk Talon Attack (1GB) video card <---- SLI ---->
    Seasonic SS-660XP2 80 Plus Platinum psu (660w)
    WD Caviar Black WD6401AALS 640GB (data)
    Samsung 840 Pro 256GB SSD (boot)
    SLI @ 16/4 works when running HyperSLI
    Cooler Master 120XL Seidon push/pull AIO cpu water cooling
    Cooler Master HAF XB computer case (RC-902XB-KKN1)
    Asus VH242H 24" monitor [1920x1080]
    MSI N460GTX Hawk (1GB) video card
    Logitech Z-5500 Digital 5.1 Speakers
    win7 x64 sp1 Home Premium
    HT|Omega Claro plus+ sound card
    CyberPower CP1500PFCLCD UPS
    E6300 (R0) @ 3.504GHz [8x438MHz] ~~ P35-DS3L [rev: 1.0] ~ Bios: F9 ~~ 4x2GB Kingston HyperX T1 PC2-8500, 876MHz @4-4-4-10
    Seasonic X650 80+ gold psu (650w) ~~ Xigmatek Balder HDT 1283 cpu cooler ~~ Cooler Master CM 690 case (RC-690-KKN1-GP)
    Samsung 830 128GB SSD MZ-7PC128B/WW (boot) ~~ WD Caviar Black WD6401AALS 640GB (data) ~~ ZM-MFC2 fan controller
    HT|Omega Striker 7.1 sound card ~~ Asus VH242H monitor [1920x1080] ~~ Logitech Z-5500 Digital 5.1 Speakers
    win7 x64 sp1 Home Premium ~~ CyberPower CP1500PFCLCD U.P.S
    .


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