cisco poe switch PSU issue

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  • madan1
    Badcaps Veteran
    • Nov 2016
    • 670
    • Bulgaria
    #1

    cisco poe switch PSU issue

    Hello everyone.
    I'm troubleshooting a cisco switch, which seems to have an issue with the PSU.
    The psu has the following power rails - +12v, -53.5V and +3.3V
    When connected to power, the +12 and -53.5 seem to work fine, but the +3.3 starts at ~3v and then goes down to ~0 (I guess this is the cap discharging). This happens both when the cable is connected and not to the switch board.
    Any ideas what might be causing this behavior?
    Click image for larger version Name: 20240423_222005-1.jpg Views: 153 Size: 3.48 MB ID: 3258494 Click image for larger version Name: 20240423_222107-1.jpg Views: 486 Size: 2.67 MB ID: 3258491 Click image for larger version Name: 20240423_222111-1.jpg Views: 143 Size: 2.91 MB ID: 3258492 Click image for larger version Name: 20240423_222043-1.jpg Views: 146 Size: 3.40 MB ID: 3258493
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    • madan1
      Badcaps Veteran
      • Nov 2016
      • 670
      • Bulgaria
      #2
      So I started taking apart the psu, the caps measured ~290uF for the 330uF, ~410uF for the 470 and 45 for the 47, which I think is OKish.. at least for now.
      Traced the 3.3V rail to the 33063 regulator. It gets 12v on Vcc (between pin4 and pin6), so I guess it has failed.
      Click image for larger version Name: 20240424_014013.jpg Views: 168 Size: 1.78 MB ID: 3258635

        Comment

        • madan1
          Badcaps Veteran
          • Nov 2016
          • 670
          • Bulgaria
          #3
          Well, I changed the IC and still have the same issue - on power on it outputs ~3v and immediately shuts down. On power off I see another surge of ~3v, so I guess something else in that circuit is wrong.

            Comment

            • stj
              Great Sage 齊天大聖
              • Dec 2009
              • 30937
              • Albion
              #4
              33063 is a buck convertor - it needs an inductor and a fast diode followed by a low esr cap.
              check the example and then check your board.
              https://www.ti.com/lit/ds/symlink/mc33063a.pdf

                Comment

                • CapLeaker
                  Leaking Member
                  • Dec 2014
                  • 8000
                  • Canada
                  #5
                  Sounds to me like a feedback or protection like OVP problem.

                    Comment

                    • harp
                      Badcaps Veteran
                      • Jun 2022
                      • 592
                      • Planet Earth
                      #6
                      What is resistance on 3.3v rail? If he managed to hold 3.0v it is relative high... after you disconect psu from mainboard, check all diodes and major transistors, you may want to inject some current into 3v3 rail to see where is problem.

                      Also, advice to clean pcb, check and resolder all THT pads, especialy over main transformer and power semiconductors, looks crack or cold joined.
                      Last edited by harp; 04-26-2024, 09:14 AM.

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