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WT7527 supervisor shutting down a good PSU

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    #1

    WT7527 supervisor shutting down a good PSU

    I have a dead ATX power supply. It is a Coolermaster RS-500-PCAP-A3 and the PCB is labelled PC7013-000G. The 5v standby is present and the troublesome output supervisor IC is a WT7527.

    When you pull pin 4 (Power On) low the WT7527 starts the primary side by pulling Pin 3 (Fault Protection Latch) low. It goes to startup but then immediately releases Pin 3 and shuts the primary down again. I checked all the caps and only 1 2200u on the output side was down at 1400u but changing it makes no difference. The 431 is tested and is fine. The optocouplers also tested fine.

    It was my old bench test PSU until it failed. I wasn't going to spend too much time on an old test PSU so I grounded Pin 3 to force it to stay running and see what the problem was. To my surprise, the 3.3V, 5V and 12V were all OK on Pins 8, 12, 13 and 14 and the current sense pins 5, 7, 10 and 11 were all just a few millivolts less so they are good as well. PGI has a threshold of 1.2V and it is at 2.8V. Everything seems OK but the WT7527 is seeing a fault somewhere and pulls Pin 16 (PGO) low.

    The datasheet says that to reset the PGO latch you release Pin 4 and bring it low again. I do this with it running (it won't turn off with Pin 3 held low) and PGO goes high immediately. So, at start up the WT7527 is unhappy with something and shuts the PSU down but if you force it to start everything is just fine.

    The pins I am not sure of from the datasheet are Pin 6 (RI) which is 1.2V and Pin 9 (VX) is at 0.49V. Any ideas what I am missing? This PSU seems just fine but the WT7527 is not having it…
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    #2
    Re: WT7527 supervisor shutting down a good PSU

    I have a similar issue, i am thinking of changing the WT7527.

    Comment

      #3
      Re: WT7527 supervisor shutting down a good PSU

      Read this one:
      https://www.badcaps.net/forum/showthread.php?t=106011
      Had a similar thing happening.
      Last edited by CapLeaker; 11-05-2023, 05:53 PM.

      Comment

        #4
        Re: WT7527 supervisor shutting down a good PSU

        What brand was the bad 2200 uF cap? Did you check all of the other caps from this brand. Usually I find that when one brand goes bad in a PSU, the rest from the same brand are usually not too far behind. That said, I think the issue is more likely to be one of the small electrolitic caps near the WT7527. Pull all of those small caps out and check them. Good chance one of them could be bad.

        Also, you tried forcing the PSU on, but did you try forcing it on with a load? If not, try putting about 1-2 Amps on each rail and see what it does and how the rails look like.

        Lastly, check all small resistors that go back from the output rails to the WT7527. IIRC, regulation is monitored separately from SCP/OVP/UVP. So PSU can still regulate but think one of its rails has a problem.

        *EDIT*
        CapLeaker mentioned a good point too - if your PSU has a single 400/420/450V cap, it more than likely has an APFC circuit, and that too can cause all sorts of random behavior if the main cap is dry / going bad.
        Last edited by momaka; 11-05-2023, 07:18 PM.

        Comment

          #5
          Re: WT7527 supervisor shutting down a good PSU

          Thanks for the comments all.

          The capacitor was a Ltec brand and Googling tells me it is not well thought of so I will swap the others anyway when I win with it. I have pulled every cap on the PSU and only the one 2200u was down.

          I have 2 capacitors in series after the bridge in my PSU so no APFC.

          I have plugged it into an old motherboard to act as a load and it still shuts down but it runs for longer so maybe it is a regulation issue at 1st startup. If I use my Hamtek scope it logs the max voltage and at startup the 12V goes to 13.8V, the 5V to 6.1V and the 3.3V to 3.8V.

          If the Hantek can trusted then the WT7527 supervisor is doing its job and shutting down and over voltage PSU. I have never used that function before so I cannot say if the scope is reliable in that way. The scope is a DSO2D10 and I know it can log the waveform and I can replay it and measure it on screen later but that will require a bit of manual reading for me.

          Thanks again.

          Comment

            #6
            Re: WT7527 supervisor shutting down a good PSU

            Put FPOB pin 3 of the WT7527 to ground on the secondary and fire the PSU up. The fault protection would be disabled like that and it should keep running.

            Comment

              #7
              Re: WT7527 supervisor shutting down a good PSU

              Originally posted by CapLeaker View Post
              Put FPOB pin 3 of the WT7527 to ground on the secondary and fire the PSU up. The fault protection would be disabled like that and it should keep running.
              I does keep running and all the rails are fine but the PG is held low. If you toggle PSON while it is running the WT7527 resets and PG goes high.

              The issue is that at startup the WT7527 sees something it is not happy with but I can't figure out what or why.

              The V pins on the WT7527 are all OK. The I pins are all a few millivolts lower. Resistance between the V and I pins on 3.3 and 5 are 470 Ohm and both the 12's are 1K. PGI has a threshold on 1.2v and sits at 2.8v. It all looks just fine but the WT7527 says no it is not???

              Comment

                #8
                Re: WT7527 supervisor shutting down a good PSU

                Obviously you modified the PSU for bench usage. So why using that supervisor anyway?
                A: you could have a bad WT7527. Supervisor IC have gone bad before.
                B: it doesn’t like something at the startup. Bad capacitance.
                I go with B, since your PGO is going good with by quickly turning off and the PSU back on.
                Get a hair dryer and warm the PSU up, then turn it on. See what happens.

                Comment

                  #9
                  Re: WT7527 supervisor shutting down a good PSU

                  Originally posted by CapLeaker View Post
                  Obviously you modified the PSU for bench usage. So why using that supervisor anyway?
                  A: you could have a bad WT7527. Supervisor IC have gone bad before.
                  B: it doesn't like something at the startup. Bad capacitance.
                  I go with B, since your PGO is going good with by quickly turning off and the PSU back on.
                  Get a hair dryer and warm the PSU up, then turn it on. See what happens.
                  When I say it is my bench PSU I mean for testing, I haven't modified it. I have warmed up the whole board but it makes no difference. I have ordered a few WT7527 IC's and this will have to wait until they land. Thanks.

                  Comment

                    #10
                    Re: WT7527 supervisor shutting down a good PSU

                    Aah, ok! I don’t think the supervisor is bad in this case. Overshoot in voltage can be bad capacitors. The PowerMan IP in that link did the same thing. So heating up that power supply didn’t do anything… can we see some clear straight shot, high resolution pictures of that PSU?

                    Comment

                      #11
                      Re: WT7527 supervisor shutting down a good PSU

                      Originally posted by CapLeaker View Post
                      Aah, ok! I don't think the supervisor is bad in this case. Overshoot in voltage can be bad capacitors. The PowerMan IP in that link did the same thing. So heating up that power supply didn't do anything… can we see some clear straight shot, high resolution pictures of that PSU?
                      Thanks for the replies CapLeaker. I am away dealing with a family issue but I will update with pics when I get back. Thanks again.

                      Comment

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