Switching Mode Power Supply - How to adjust cut off?

Discussion in 'The Projects Forum' started by Eddiie, Jun 21, 2015.

  1. Eddiie

    Thread Starter New Member

    Oct 12, 2014
    16
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    Hello,

    I have a 100-240ac to 24v ~5a power supply that is around 78mm square. Evidently this is hard to do because I cannot find a replacement.
    Trying to fix the one I have, wound up replacing every part on the board except some small resistors and 2 MBR20200's. The replacements I got are MBR20200CT.. Different, right?

    Anyway, it works now, which is cool, but it does not put out the power it once did, or it cuts off at about 3.5 amps.

    Is this something with the power sense circuit? Can I change a part to allow more load, closer to the 5 amps?

    I do have a couple working one's that I am using to set the 'bar', this is why I feel it is not performing as it should.

    Also need 3 additional one's, and being through this board several times, I think I could draw it in a program.
    Although there are values stamped on the board, the actual parts are different.

    I will upload some pics, ah they are too big.
    I posted them on my server -
    http://www.usbjunk.com/24v5aps1.jpg
    http://www.usbjunk.com/24v5aps2.jpg
    http://www.usbjunk.com/24v5aps3.jpg
    http://www.usbjunk.com/24v5aps4.jpg
    http://www.usbjunk.com/24v5aps5.jpg
    http://www.usbjunk.com/24v5aps6.jpg
    http://www.usbjunk.com/24v5aps7.jpg
    http://www.usbjunk.com/24v5aps8.jpg
    http://www.usbjunk.com/24v5aps9.jpg

    Common / key Parts used:
    MBR20200
    OB2269AP
    FQPF5N60C
    Transformer - G65-2S12V12 ??

    One more note: Even though there is a 3A slo blow fuse on the board (AC feed), there was an in-line fuse at the power feed that was 2AMPs. After working on this power supply I had to upgrade the 2A fuse to 3A. Strange? It will blow a 2 amp fuse.

    How to fix? Make it perform like the others?

    Thanks
    Eddiie
     
  2. #12

    Expert

    Nov 30, 2010
    16,298
    6,809
    The SMPS's that I know about are voltage regulated. You can usually hack that with the part labeled, "201". But low current output and had to raise a fuse on the input side? I think you have a low/compromised quality part in there or a shorted turn in the transformer. More measurements are in order.
     
  3. Eddiie

    Thread Starter New Member

    Oct 12, 2014
    16
    0
    Part 201 is the pot. It adjusts the voltage out, which is right on the money at 24v. I need it to put out another 500ma - 1amp before it goes in to this short-circuit mode... It starts going on/off/on/off/on/off at about 3 amps.

    I've run it for an hour or so at 3amps, it works fine, no smoke. I suspect a resistor needs to be tweaked somewhere?

    Big Electrolytic Capacitors have been replaced. Smaller one's tested OK.

    Parts NOT replaced -
    Ceramic capacitors
    Some transistor or a part that looks like a transistor.. TL431
    The blue capacitors (isolation caps?)
     
  4. #12

    Expert

    Nov 30, 2010
    16,298
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    The TL431 is a voltage reference chip. It can be used to set a voltage limit or a current limit.
    The on/off behavior suggests a protection circuit, possibly protecting against what it sees as the wrong output current. That's as far as I can go. If I had a schematic and some measurements, I might track this down. I don't have those things.
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    the chip OB2269 has a current sense pin6, on the mosfet source lead resistor,you can lower its value , or short pin 6 to ground (NOT RECOMMENDED) to override the current sense.


    http://dalincom.ru/datasheet/OB2269.pdf
     
    Last edited: Jun 22, 2015
    #12 likes this.
  6. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,035
    1,660
    Your current sensing part of the circuit has a resistor thats resistance has drifted a bit high creating the false high current shutdown.
     
  7. Eddiie

    Thread Starter New Member

    Oct 12, 2014
    16
    0
    ah, thank you. I will check this tonight.
     
  8. Eddiie

    Thread Starter New Member

    Oct 12, 2014
    16
    0
    Actually, I check this now.

    New picture.

    Pin 6 goes to the resistor labelled 470R. (I'll confirm the actual colors later; in a rush now).
    From 470R, it goes to the FQPF and the big resistor. The little ceramic cap is in there, too.

    Which resistor are you referring to? The big one or little one?
    The big one looks 'worked'.

    I also have what I thought was a replacement for the big resistor, it says R15 5% 3W. But did not put it in for some reason. Is it a suitable replacement for the big resistor? It also has a green coating on the outside, about the same size though.

    closingin.jpg
     
  9. Eddiie

    Thread Starter New Member

    Oct 12, 2014
    16
    0
    The color codes on the "470R" resistor are 56 Ohms 5%
     
  10. #12

    Expert

    Nov 30, 2010
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    The fat, gray resistor is 0.15 ohms. Likely candidate for drifted value.
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    BIG resistor is 0.15 ohms , it has probably gone higher, replace it..
    (the 56ohms and capacitor are for filtering,)
     
  12. Eddiie

    Thread Starter New Member

    Oct 12, 2014
    16
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    R15 is same as 0.15 ohm??
     
  13. #12

    Expert

    Nov 30, 2010
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    If we could see an R15 label, we would have said so. How many fat, gray resistors do you see on that board in post #8?
     
  14. Eddiie

    Thread Starter New Member

    Oct 12, 2014
    16
    0
    How many watts is the big gray resistor?
     
  15. #12

    Expert

    Nov 30, 2010
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    I'd guess, in the range of 3W to 5W
     
  16. Eddiie

    Thread Starter New Member

    Oct 12, 2014
    16
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    I think it is 3 watt, when you get to 5 watts they put it in a different package don't they?
    I screwed up by ordering a R15 resistor, not 0R15 that was my question, R15 is different than 0.15 ohm..
    The one I ordered just has R15 3W stamped on it. I'll double check when I get home but am prepared to order one that says 0.15 @ 3W
     
  17. #12

    Expert

    Nov 30, 2010
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  18. Eddiie

    Thread Starter New Member

    Oct 12, 2014
    16
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    here are a couple more pictures.. The replacement (labelled R15, purchased from Mouser) shows the same value as the one on board, except the replacement is 5%.
    Nonetheless, we know it should be .15 I will order it. Using your link above, there is current sense and wire wound. Probably the same thing, but should I select the current sense one (third down).

    I'd like to get one that is a bit lower, too.. Would that be .20?? heh The .15 seems to be a common value for power sense, so maybe just leave it there.

    Thank you!



    r15-onmeter.JPG r15-onmeter.JPG resistoronboard.JPG r15-onmeter.JPG resistoronboard.JPG
     
  19. JWHassler

    Member

    Sep 25, 2013
    201
    33
    Be sure to measure the probes' resistance.
    And .2 is not lower than .15
     
  20. Eddiie

    Thread Starter New Member

    Oct 12, 2014
    16
    0
    oh that's right, probe resistance, good point. fraq.

    they are 1.7, too. hehe guess I can't go that .. low?

    .2 not lower than .15, got it, going other direction.. .10
     
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