SMPS problem

Discussion in 'General Electronics Chat' started by mikec, Nov 4, 2008.

  1. mikec

    Thread Starter Member

    Nov 4, 2008
    16
    0
    GS3

    I'm trouble shooting a smps power supply in my plasma at the moment. It currently has no 80V or 60V output which drives the screen sustain boards.

    I have traced it back to the flyback transformer which seems to have volts going in (using the ac setting on my DMM) but no volts coming out. Should I expect some sort of voltage on the output of this transformer. I dont have a scope so cant tell if the input volts are switching right.

    Mike C
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You replied to a year-old thread, which is not a good way to get noticed. We discourage starting a new topic on the tail of another thread.

    This is the original discussion - http://forum.allaboutcircuits.com/showthread.php?t=7738.

    Be extremely careful with switchers, as the input side has no gavanic isolation and lethal voltages present working or not.

    If the transformer has no output, the the driver FET may be bad.
     
  3. mikec

    Thread Starter Member

    Nov 4, 2008
    16
    0
    Cheers been there

    Thanks for moving the thread.

    I have an ac voltage across the primary of the switcher 160V. I cant verify the waveform as I don't have a scope but its not DC. I'm making an assumption that the AC is coming from the switching transistor.

    Mike C
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    A switcher runs well above the frequency a DVM's AC RMS circuit can respond to. You can check the rectifier diodes and see if you have any DC voltages on the filter caps.

    They are nasty things to troubleshoot - no schematics and prone to exploding capacitors.
     
  5. mikec

    Thread Starter Member

    Nov 4, 2008
    16
    0
    OK guys, I have made some progress on this now.:)

    First here is a hand drawn circuit made using gEDA (Open Source) schematic editor. I have had to trace this by eye so may have some of the components incorrect.

    http://farm4.static.flickr.com/3016/3010671611_147e8570eb_o.png

    I am getting 380V DC coming out of the Power Factor Corrector which is what I am expecting.

    There is 380V on the D pin of the switching transistor ( this is a 21N50C3) but no volts on the S pin, and zippo on the G pin. I assume that is because these are going at high frequency and I cant measure, or there is no pulse on the Gate and hence nada on the switching transformer.

    Interestingly enough the board is putting out all volts other than the 80V and the 60V that run directly off the switching transformer. Also Vcc 5V is showing as 0.28V so could be a problem as I'm sure some of the PWM ICs probably run of this. Looks like I'm going to have trace the circuit board some more.

    Any clue on how I can check the switching transistor is doing its thing without a scope?
    ( I have done a basic diode check on it, and it seems fine)

    Mike C
     
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