Mosfet heating without any load

Discussion in 'The Projects Forum' started by suzane, Jun 17, 2016.

  1. suzane

    Thread Starter New Member

    Jun 17, 2016
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    I have been trying to find the mosfet heating issue in some other threads posted by someone. Unfortunately, could not find something vital to resolve my issue.
    In a half bridge smps the mosfet is heating up appreciably high. A similar near identical board which i have working perfectly. To find out the issue i disconnected all the connection at the secondary side of the Transformer.
    The oscilloscope probe is showing appreciably square wave of 88.65khz. What could be the issue to resolve
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Not the easiest schematic to read. I see only two MOFETs and they are in parallel, so I don't see a half bridge. What are the output voltage and current, and where is the output on the schematic?

    k
     
  3. suzane

    Thread Starter New Member

    Jun 17, 2016
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    +V on the left hand side is the output. The output voltage is 12volt and 20 amps
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,809
    1,105
    At what node, what voltage and with reference to what?
     
  5. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Agree with AnalogKid about the schematic. It appears to show two windings on the transformer which are short-circuited!

    Before you disconnected the secondaries, what output voltage was it producing?
    Do both MOSFETs heat up similarly?
     
  6. suzane

    Thread Starter New Member

    Jun 17, 2016
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    Sorry in a hurry the transformer diagram i could not modify on TIME - its not short - one end of the wire has three pin viz. 7,8,9 and the other end of this winding is 10, 11, 12

    The problem started in this way : After powering on, the fuse started blowing - Investigating further, I found Diode D15 -1amps/1kv schottky diode connected to drain shorted. I replaced the diode with fresh one but the diode keeps on blowing.

    I presumed the diode D15 rating should be increased so I replaced with a 2 amps rating - NOW, after power on the diode saved and mosfet blown

    To carry our further test i have removed the secondary connection from the board - the mosfet is not blowing but its heating appreciably high and it increases gradually.

    You have a good point - do both mosfet heat up similarly - My answer is NO - while touching the mosfet body i found the mosfet on board which is near to the pwm IC tl3845 is heating more than the other one
     
  7. suzane

    Thread Starter New Member

    Jun 17, 2016
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    0
    Probably, my above statement of both mosfet heating is incorrect. The temperature difference between the two fets is marginal.
    Further, i removed one mosfet and did the same with the other mosfet - the temperature increases.
     
  8. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,968
    387
    You need to check the components around D15 - R8, C8, C9. That includes checking the value of the capacitors. If you can't check the values then it would be best to assume they are faulty and replace them. If you find nothing wrong with any of those then, again, it would be wise to replace all those components as you can't test them at 1-2kV and that may be when the problem shows up.
     
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