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FLYBACK SMPS PROBLEM

Discussion in 'General Electronics Chat' started by ravilite, Oct 29, 2017.

  1. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    All answered in early posts.
     
  2. kubeek

    kubeek Well-Known Member

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    Call me blind, but I donĀ“t see the exact type of the fet anywhere, only voltage and current rating, and heatsink is not mentioned anywhere in the thread, only that the diode is a to220 package.
     
  3. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    You are right. My mistake.
    I thought it was on the schematic. Is not. All we know is voltage and current.
    The diode is on the schematic.

    We do not know much. The schematic looks good. We do not have scope pictures. Don't know if the transformer is at saturation. Are the diodes hot? Is the MOSFET hot. I really need to see the diode current and MOSFET current on a scope. MOSFET & Diode voltage on the scope.
     
  4. dave

    Dave New Member

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  5. ravilite

    ravilite New Member

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    The IC I am using have max Gate drive voltage of 20 Volts and Mosfet's gate voltage range is -20V to +20 V
     
  6. ravilite

    ravilite New Member

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    Yes diode is mounted on heat sink ( PI56-25MM size is H=20mm,L=16mm,W=12mm ).

    K12A80W mosfet is super junction mosfet DTMOS
     
  7. ravilite

    ravilite New Member

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    I am Using two separate Heat sink of Same size ( PI56-25MM size is H=20mm,L=16mm,W=12mm ) for mosfet and diode , the temperature of both the components is between 80 to 90 ( max operating temperature of diode and mosfet is 150 degrees ).

    I have some problem with my current probe , i am trying to get the waveforms from my friend's lab, i will upload shortly .

    There is one more thing I noticed today the value of the capacitor in RCD snubber used for mosfet is changed instead of 4.7nf it shows 6.8nf and the value was not stable it was continuously increasing. could this be the reason of my SMPS failure.

    Can anyone please tell me how saturated transformer damages the mosfet.
     
  8. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I understand the max voltage on those parts. The question is; did you measure the voltages? How did you measure? How do you know that there is never a voltage above 20V?

    If I was there ...... but I am not and can not see what is happening. I don't know what test equipment you have.
    I do not know how to get you to measure the voltage.
     
  9. tomizett

    tomizett Active Member

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    Remember that 150 degrees is the maximum junction temperature - not the external case temperature. If you know the heatsink temperature and thermal resistance (see the heatsink's datasheet) and the junction-to-case thermal resistance of the MOSFET (see the MOSFETs datasheet), you should be able to estimate the junction temperature. My inclination is that this sounds rather too hot, but I don't design SMPS... any thoughts folks?

    I also agree with the earlier comments that it's likely that the diode fails first - it's far more likely that a short diode kills the MOSFET than a short MOSFET kills the diode (I can't see how that could happen.
     
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  10. kinarfi

    kinarfi Well-Known Member

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    My guess is one of the diodes fail, shorted? and then you're done.
     
  11. ci139

    ci139 Active Member

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    TK12A80W_datasheet_en_20160205.pdf -- there is note on de rating pg.1 /// also the mosfet diode timings and dv/dt capabilities are not proportional , are more for mosfet & U16C40 - there are *** what i'm not sure what it exactly means
    the_characterization_of_dv_dt_capabilities_of_cree_sic_schottky_diodes_using_an_avalanche_transistor_pulser -- there is some pointouts at pg.1
    U16C30-U16C60.pdf --it says something RMS = 280V *** Fig.5 NUMBER OF CYCLES AT 60 Hz
    i'm not too very good at this if at all - - i remember seeing a pdf doc that strictly described the reverse protection diode parametric requirements with some calculation example . . . seems to be something in ' Consideration for Circuit Design from https://toshiba.semicon-storage.com/ap-en/product/mosfet/detail.TK14N65W5.html
     
    Last edited: Nov 7, 2017

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