Calling any SMPS experts... Help...

Discussion in 'General Electronics Chat' started by weeb, Jan 21, 2013.

  1. weeb

    Thread Starter New Member

    Jan 21, 2013
    2
    0
    Hello, I joined the forum because I am looking for a bit of help with a SMPS that needs repair.

    The board in question is a 715G2824-P03 which comes out of an Asus VE278 LED monitor. There are a few revisions of this type of PCB, (other revisions have inverter circuits included); this board omits the inverter section as it only needs a 6 pin connector for DC output to the LED backlight. In any case I believe the primary side circuits are basically the same or very similar throughout all revisions.

    The problem with my board appears to be on the primary side. I have a schematic for a similar PCB, different revision, which I have been using to troubleshoot my board.

    When I got the monitor, the primary MOSFET appeared to have blown up. I believe it was a STP10NK70. All I have done really is replace the MOSFET with a 2SK3797, as I did not have the ST part. Then I replaced the controller IC, LD7576 by Leadtrend. I also replaced a couple of resistors.

    If I connect the PSU to the mains I get a voltage on Vcc (pin 6) of the LD7576, which appears to fluctuate between approx 11v and 14v. This is within spec according to the datasheet? But I don't know if it should be fluctuating or steady.

    I checked the gate voltage on the MOSFET and most of the time it was zero, but it does sometimes go up to approx 2.5V then back to 0. I don't know what is happening, but for some reason it appears the LD7576 is not switching the MOSFET at all? In any case there appears to be 350V on the source but zero on the drain. So that is as far as the PSU wants to go; no output from primary side.

    I was wondering if anybody on here could shed a little light on how this LD7576 primary circuit works and whether the 2SK3797 is a valid replacement for the STP10NK70 ?

    Much appreciated.
    Included a schematic and a picture of both sides of the PSU.
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    First, when you have a cheap offline flyback that suffers a catastrophic failure, a lot of parts get damaged. When the FET blows, it briefly allows high voltage to be connected to the low voltage circuitry hanging on the gate line.

    Second, understand that often when a power supply fails it is because something on the load side failed and overloaded it. So, the first thing always to do is "substitute" other supplies (lab supplies which have overload protection) to try to run the load lines and see if the device works or has major problems. If you don't do that, you could just keep blowing up the power supply.
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,137
    1,786
    Even if you could "repair" the circuit it would be difficult to have confidence in it going forward since you have no way of knowing which components are now "stessed" or marginal. If I were you I would move on to more productive pursuits. I've built lots of SMPS circuits and never even attempted to resurrect one; it's just not worth the time and effort.
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    That symptom sounds like the PSU is "hiccupping" because there is not enough load connected.

    It is possible you have fixed the PSU but cannot test it properly without adding the right dummy load to its output.
     
  5. sheldons

    Active Member

    Oct 26, 2011
    616
    101
    i repair a few of these things week in week out ....the supply should quite happily run on its own.check the opto in the feedback circuit by replacement,on the primary side check all resistors/capacitors for oc,diodes for oc or sc,check on the secondary side of the supply for oc or sc diodes,same for electrolytics.....check zd902 for being sc....fault may have been due to hi esr electrolytics upsetting the regulation side of the supply causing the 16v supply to rise too high...also replace the KIA 431 .check C 939 and C915 for hi esr....
     
  6. weeb

    Thread Starter New Member

    Jan 21, 2013
    2
    0
    Thanks for the responses. This PSU was eventually repaired when I replaced the switching transistor for another. I found that the gate pin of the original transistor was not making contact with the pad, where it was too short to go through the board. That's the kind of thing that happens when you try to save money (and time) by pulling used parts off old PCBs and lash them in to complete a repair.

    I found a short troubleshooting guide for these power supplies on the net, included an attached image.
     
  7. sheldons

    Active Member

    Oct 26, 2011
    616
    101
    that is part of the service manual for that monitor which i have here....seem to be getting quite a few of these in of late with power supply failure or invertor failure after 2-3 years use
     
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