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  1. #1
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    Flyback core with no gap.

    Hello,

    I have tested an isolated flyback with:-

    F(switch) = 67KHz
    Vout = 16V
    Power out = 42W
    Vin = 400V

    I took apart the flyback transformer, and the core is two "E" shaped halves

    The dimension of the "upright" bit of each "E" is around 2cm.

    Its a small core, seemingly , for 42W.

    The strange thing is, that there is no gap in the core halves at-all.

    There was no spacers between the halves, and no gap in the centre rod of the E-core halves.

    The core is marked "PC40 7123 Z"

    I am wondering if it has a "distributed" gap......that is special ferrite mixture which has an "integrated" gap, so that it doesn't need a physical gap?

    The transformer had its secondary winding sandwiched between the two primary sections.

    The power dissipation in the RCD clamp resistor at max load/min line was just 170mW.

    The drain voltage peak is just 500V

    I am wondering how on earth this core has managed 42W output with seemingly no gap?

    •   Alt22nd April 2011, 15:38

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  2. #2
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    Re: Flyback core with no gap.

    Hello,

    I think you are right, it must be a material with low permeability, otherwise it can't store the energy. Is there any other mark on the ferrite do determine brand.

    I don't believe it is TDK PC40 material, as that has about 2300 initial relative permeability. The Ferroxcube iron powder materials start with 2P (for example 2P40 and they sell in toroids only).

    If you can't find the specs, your only option maybe putting some wire around it and measure the LF inductance. From that you can derive the initial permeability.

    With kind regards,

    Wim


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    •   Alt22nd April 2011, 16:15

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  3. #3
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    Re: Flyback core with no gap.

    Hi,
    Thanks, there is no other marking on it.
    It is an ETD24 core.

    It is used in Japanese Television.

    Maybe for TV they have special custom cores,

    but as you say, i must do that test you kindly descibed



    •   Alt23rd April 2011, 12:38

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  4. #4
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    Re: Flyback core with no gap.

    Hello,

    If you feel uncomfortable with your calculation, just mention the number of turns and the inductance you measured. I will do the calculation in that case also and you can compare my result with your result.

    If the effective relative permeability turns out to be very low, your result will depend on the separation (distance) between the turns of you winding (due to leakage effects).

    With kind regards,

    Wim



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