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  1. #21
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    Re: Flyback SMPS power MOSFET getting hot? uc3843 pwm converter

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    the power mosfet don't have any RC snubber circuit. is it necessary in a flyback smps with power mosfet?

    the xformer have an RCD snubber with R = 47K/2W, C = 103pf & fast recovery diode FR107.



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  2. #22
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    Re: Flyback SMPS power MOSFET getting hot? uc3843 pwm converter

    RCD snubber is the appropriate method to absorb primary overvoltage caused by transformer leakage inductance. You have to check the actual primary voltage and current waveforms, an additional RC snubber will unlikely improve anything.

    The transformer air gap should be sufficient to avoid transformer saturation. If Bmax is actually 260 mT, transformer core will run very hot, but not cause problems for the transistor.

    Are you sure that the converter is operating regularly with constant duty cycle? Hard to determine without oscilloscope measurement.



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  3. #23
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    Re: Flyback SMPS power MOSFET getting hot? uc3843 pwm converter

    rcd snubber usually across transformer primary ...


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  4. #24
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    Re: Flyback SMPS power MOSFET getting hot? uc3843 pwm converter

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    this protective turn off RC snubber? is it necessary with power FET?

    the xformer have RCD snubber with R = 47k/2w, C = 103pf/1kv & fast recovery diode FR107.

    - - - Updated - - -

    surprised with the chinese smps PQ3230 xformer of this circuit. where the Nprim = 40T (swg25 x 3). and using the same air gap of about 0.9mm.

    how its possible? i tried my own xformer also with external 12v-1amp power supply but still mosfet heating.

    - - - Updated - - -

    if the cause of this problem is the core being going into saturation how can i check this?



  5. #25
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    Re: Flyback SMPS power MOSFET getting hot? uc3843 pwm converter

    you need to learn about leakage inductance - also as we cannot see you gate drive - this may be a problem too ...



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  6. #26
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    Re: Flyback SMPS power MOSFET getting hot? uc3843 pwm converter

    for the time being the problem of power FET heating solved by adding a turn off RC snubber across FET.

    the power FET now heat up normally with load.



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