Power supply 20A overheating power mosfet

Discussion in 'Power Electronics' started by Blisk, Jun 6, 2016.

  1. Blisk

    Thread Starter New Member

    Jun 6, 2016
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    I hope I get some help here about power supply 20A 12V.
    When I get it, it worked ok than suddenly something blows up but it was not overloaded because it was less than 1A load on it.
    When I dissasembled it I see it was one leg of power mosfet badly soldered and I think this cause blow up.
    Below in picture is in red circle what was damaged and I changed it. Also power mosfet STW12NK90Z.
    Problem now is because mosfet is hot even when nothing is connected to 12V.
    If I connect some load on it it starts to overheat badly.
    What can be wrong, some advice is really appreciated,
     
  2. bushrat

    Member

    Nov 29, 2014
    97
    22
    If it is overheating, then it's most likely drawing a lot of current. Try to trace the output of that power mosfet, and see if something is shorted..
     
  3. Roderick Young

    Member

    Feb 22, 2015
    408
    168
    If the supply was not from a reputable brand name, it might be a cheap design that used marginally-rated components.
     
  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,937
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    That's the trouble with switch mode supplies - when they blow up they can take out many components and if you don't find them all you could be in for a repeat performance.
    Check all components that you can with a multimeter. All diodes, transistors and resistors, including the output rectifiers. There will be a snubber network (capacitor(s), resistor(s), diode) across the transformer primary and you should check all those components. You will need to trace tracks to find them.
    Do you still get 12V output now?
     
  5. ISB123

    Well-Known Member

    May 21, 2014
    1,239
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    What is the voltage at MOSFETs gate?
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,006
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    You likely need an oscilloscope to troubleshoot your switching supply.
    Do you have a schematic?
     
  7. Blisk

    Thread Starter New Member

    Jun 6, 2016
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    Yes I still get 12V output!
    I also have checked all didodes, transistors and resistors, but didn't checked capacitors.
    I don't have shematic I havy buyed this on ebay.
    I have osciloscope and also have almost the same power supply which is ok and works but it is 30A and have one more mosfet.
    Maybe there are gates too much opened.
     
  8. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Be very careful using an oscilloscope on this type of supply. The mains feeds a bridge rectifier into that large electrolytic capacitor. Everything to the left (on your photo) of the transformer is live. You cannot connect the ground of the oscilloscope probe anywhere in this circuit. Anything you connect the probe to will have the rectified mains waveform which makes it very difficult to spot the actual waveform you are interested in.
    That capacitor may also remain charged for quite a while after being switched off and will hurt a misplaced finger given half a chance.

    I spent many years repairing such supplies and it is exceedingly difficult without a schematic, and without experience of fault finding on the particular model.

    Good luck and take care.
     
  9. Blisk

    Thread Starter New Member

    Jun 6, 2016
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    Thanks.
    I have used osciloscope on some parts on left but problem is because signal is too high and can not see anything like I should.
    Frequency on mosfet was about 90KHz
    Voltage on Gate of mosfet is 110V. Voltage in my country it is 240V for all devices.
    Do you have any advice what can be wrong?
     
  10. Techno Tronix

    Member

    Jan 10, 2015
    140
    10
    Did you tried to cut out some output or tried to short it?
    Did you try to trace the output of mosfet?
     
  11. Blisk

    Thread Starter New Member

    Jun 6, 2016
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    Yes I have connected some load like 3A to output but mosfet heats too much.
    Don't know what you mean to trace output of mosfet?
     
  12. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,009
    1,530
    If that is the gate voltage, that is your problem. Most non-logic level mosfets only need around 10V on the gate, with a maximum of 20V or 25V.
     
  13. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    We really need to know the gate-source voltage to determine if it's being overdriven.
    If it had that amount of gate-source voltage it would likely blow, not overheat.
     
    shortbus likes this.
  14. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Remember everything left of the transformer has that kind of voltage relative to ground (which is all you can measure with a scope in the usual way).
    If you have a dual trace oscilloscope with an invert on at least one channel, and the ability to display the sum of both channels, then you can display the difference between the channels. Connect one probe to Mosfet source and the other to the gate and the 'scope will display the difference (do not connect the probe grounds to anything - indeed make VERY sure they cannot accidentally touch anything in the power supply). BUT you have to be careful with the attenuator settings as the high voltage will overload the channel input amplifiers if there is too little attenuation. So the gate source voltage may only be seen as a very small waveform. Some 'scopes cope better with this than others.
     
  15. Blisk

    Thread Starter New Member

    Jun 6, 2016
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    Ok I have measured now with osciloscope it have math function and I put A-B so I get 1V from gate to source.

    No actually it is somewhere 4.5V
     
    Last edited by a moderator: Jun 8, 2016
  16. Roderick Young

    Member

    Feb 22, 2015
    408
    168
    If the gate drive is going from 0 to 4.5 volts, that's too weak for this part. You need about a 10-volt drive. Check whatever circuit is driving the gate, there should be a supply of about 10 volts. Might be 8, might be 15, but not 4.5 volts.
     
  17. Blisk

    Thread Starter New Member

    Jun 6, 2016
    14
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    I have another power supply and will measure there and compare it
     
  18. Blisk

    Thread Starter New Member

    Jun 6, 2016
    14
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    Ok I found one resistor which should be 100 ohm it was 30Kohm so I changed it and put in 100 ohm. Circles shows 100ohm but when I measure it unsoldered it shows 30 Kohm.
    After I powered it up, fuse blows up and W12NK90Z also dies. Need to check other transistors for more.
     
  19. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Yes, I'm afraid they do that sort of thing. It is vital to find all the faulty components (again!).
    Can you get a schematic for this thing?
     
  20. Blisk

    Thread Starter New Member

    Jun 6, 2016
    14
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    No sorry already tried and search for schematic.
    I also found out that big resistor which is 0.22 ohm also died and two small transistors, now I am searching for more dead parts.
    Can it be some bad part also on right side where is low voltage???
    Or I can just check left side?
     
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