Odd MOSFET only on some ASUS x99/x299/z370 motherboards..

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  • Timp74
    Member
    • Apr 2018
    • 36
    • Spain
    #1

    Odd MOSFET only on some ASUS x99/x299/z370 motherboards..

    Hi All,
    Could anyone comment on the purpose of the MOSFET circled in red in the attached photo of an ASUS ROG X299 Extreme VRM?

    I'm trying to fix an ASUS motherboard(WS x299 SAGE) with a similar VRM where this MOSFET has burnt out along with one of the IR3555m driver chips. From looking at photos of motherboard VRMs on the internet, it seems only some ASUS motherboards have a MOSFET like this. Although I've seen some photos of ASUS z370 boards where it's not populated. The MOSFET is usually a NTMFS4C06N. It has its drain connected to the output of the Vcore VRM, and on my defective SAGE motherboard the source is connected to GND although it is possible this is a short in the PCB. The gate is connected to a quad op-amp and a bunch of discrete FETs. So I'm curious what this MOSFET does and to know if the source pin should be connected to GND or not. It doesn't make much sense that it is, but obviously if the PCB is faulty that would explain why the MOSFET and the driver chip burnt out.
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    • Timp74
      Member
      • Apr 2018
      • 36
      • Spain
      #2
      Re: Odd MOSFET only on some ASUS x99/x299/z370 motherboards..

      So to answer my own questions, the MOSFET is almost definitely for Vccin/Vcore Over Voltage Protection... I found a boardview file for an Asus X99 Deluxe board in another thread: https://www.badcaps.net/forum/showthread.php?t=94184

      It's clear from that the MOSFET's source pin is connected to GND and the gate is connected to a op-amp with network names like *_OVP_* Hopefully replacing the burnt components will fix the board..

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