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Computer SMPS MOSFET replacement

Discussion in 'Troubleshooting and Repair' started by zombiehawk, Jun 11, 2013.

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  1. zombiehawk


    Jun 18, 2011
    Hi everyone:)
    I recently had a power surge which damaged my computer power supply. The fuse was blown and one of the rectifier diodes was shorted. I've promptly replaced them but one of the switching transistors on a second board had cracked wide open and I could not identify the part no. completely.:(
    It is from a company called ST microelectronics and upon investigation I found that there are four MOSFET switching transistors that match the description but they have different current ratings. I've uploaded the spec sheets for your consideration.Any idea which I should choose?
    BTW the SMPS is rated 235watt max(its an old computer), and the fuse is rated at 5 amps.

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  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    You never know what else is defect or on the brink of failing.
    You don't know whether the insulation still is in good and safe working order.
    Unless you are experienced in diagnosing and repairing power supplies (which, taking into account your question, I don't think you are) I recommend you don't mess with this supply and better get a new one.

    Safety first!
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I'd go for the highest current one (is it 9A?).

    Are you fixing this because you can't get a replacement?

    As Harald has mentioned, there may be other faults.
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Most likely the main switching transistor(s) or MOSFET(s) will also be damaged. These are on a big heatsink (usually the largest one). The damage may be visible, as with the small MOSFET you've already identified, or it may only show up if you measure resistance between the centre pin and the right hand pin (drain and source, or collector and emitter) and get a reading of zero ohms, i.e. short-circuited.

    There are other components that can get damaged by a power surge. Any semiconductor will fail if subjected to excessive reverse voltage or excessive current. The control IC may be damaged, and there are many diodes which may be destroyed internally (in which case they will read short-circuit in both directions) or compromised (meaning they may overheat during normal operation, and/or fail after a period of time).

    So as Harald and Steve said, you're probably better off buying a new one. Or if it's from an old computer, you can probably salvage one from a computer recycler or a friend.
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