iMac 2017 PSU - shorted PFC mosfet

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  • mat2k
    Member
    • Mar 2021
    • 13
    • Argentina
    #1

    iMac 2017 PSU - shorted PFC mosfet

    I am trying to fix an iMac 2017 PSU, initially came with a blown fuse and shorted PFC MOSFET (6R199P). After visual inspection and some measurements I replaced the FET with a P20NM60 and tested with 100w series lamp, and it lasted 1 second before going into dead short. The PFC controller chip (NCP1605BG) and its passives appear fine (passives measure OK), but I do not know if there is a simple way of testing the NCP1605 as I wouldn't like to blind-replace it.
    There was some slight blackening of the solder on the PFC controller passives so I reflowed them all with some flux. I am waiting on new MOSFETs to try again.

    I have also measured the two drive transistors for the PFC MOSFET as I read these could be the culprit (PMBT4401 and 4403 pair) and they are okay.
    Any pointers as to what I should take a look at, before burning more new MOSFETs? Thanks
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    • Answer selected by mat2k at 03-26-2025, 10:52 PM.
      mat2k
      Member
      • Mar 2021
      • 13
      • Argentina
      Thanks to all who chimed in. Lol.
      Anyways, everything is working fine now. I replaced the 6R199P with a ST 24N60M2 and put a slightly overspecced diode in place of the bad one. replaced the HDD with an SSD and i'm installing Mac OS now.
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        • mat2k
          Member
          • Mar 2021
          • 13
          • Argentina
          #2
          Components arrived, I replaced the MOSFET and found a shorted diode (diode between PFC coil and MOSFET), replaced that too. The power supply worked fine for about 5 minutes, iMac turned on and worked, but the new diode (same model as original) went into short after that, and MOSFET may also be bad (did not remove and test yet, but it measured 250 ohms between drain and source in circuit). What could have caused this?

            Comment

            • mat2k
              Member
              • Mar 2021
              • 13
              • Argentina
              #3
              Thanks to all who chimed in. Lol.
              Anyways, everything is working fine now. I replaced the 6R199P with a ST 24N60M2 and put a slightly overspecced diode in place of the bad one. replaced the HDD with an SSD and i'm installing Mac OS now.
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