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  1. #1
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    The way to wound flyback transformer

    Hello!
    I am making a flyback transformer with input 33V output 12V using UC3842. So my flyback transformer have 3 winding: Primary, secondary, bias
    Volt bias = 18V (UC3842) + 1V (diode FR207) = 19V. I choose 20V
    I choose Te=0.5, inferred that Ns=Te * 12V = 6 Turns
    Np= Ns*Vin/(Vout +Vd) = 6*33/(12+1) = 15 Turns
    Nbias = Np*Vbias/Vin] = 15*20/33 = 11 Turns
    The order wound as follow
    Step 1: Primary winding: Start at pin 1, end at pin 4
    Step 2: Bias winding: Start at pin 3, end at pin 2
    Step 3: Secondary winding: Start at pin 5, end at pin 8
    All winding is wounded as clockwise


    Uploaded with ImageShack.us
    This is schematic:


    Uploaded with ImageShack.us
    I measured Bias voltage is over 40V, Vout 18V. The rectifier diode at secondary side overheat

    When I wound as order:
    The order wound as follow
    Step 1: Primary winding: Start at pin 1, end at pin 4
    Step 2: Bias winding: Start at pin 2, end at pin 3
    Step 3: Secondary winding: Start at pin 8, end at pin 5
    All winding is wounded as clockwise

    I measure bias voltage is 19.5V, Vout is 10.5V, the rectifier diode not overheat but mosfet and snubber resistor overheat.
    Please help me explain this, and which the right way to wound winding flyback transformer

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  2. #2
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    Re: The way to wound flyback transformer

    With the second scheme, you changed the flyback into a forward converter, not what you want, I guess.

    Twe too high output voltage in first case isn't a problem of transformer design, it means that the feedback circuit isn't working. Your schematic has a missing ground connection, but there may be other problems too. The TL431 circuit is rather unusual and doesn't provide stable regulation.



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  3. #3
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    Re: The way to wound flyback transformer

    Oh! In practice I fixed with ground connection. If the first scheme is right, how to reduce the voltage at bias and secondary winding meanwhile I calculated tunrs for each winding as equation. I wonder that if smps work at CCM mode. How to check a smps work at DCM or CCM mode? Please help me
    Last edited by vantusaonho; 26th December 2012 at 15:30.



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  4. #4
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    Re: The way to wound flyback transformer

    Either if you design the converter for CCM or DCM, it will need to use DCM at low loads. In a flyback converter, the duty cycle will also become rather low with low loads.

    You should be able to achieve the intended output voltage by making the feedback circuit work correctly.



  5. #5
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    Re: The way to wound flyback transformer

    The intended output voltage is 12V. When ouput voltage reach to 13V, through divider resistor circuit, TL431 will conduct, that cause feedback signal to UC3842. When ouput voltage below 13V, TL431 will not conduct and no signal to UC3842. However, output voltage raise to 18V



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