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    Reducing radiated emissions in an offline flyback SMPS transformer

    If I have a offline 30w flyback transformer with a split primary sandwiching a one layer secondary ……then can I reduce radiated emissions by doing the following?...

    1…Put on a fourth coil (outside of all others) which is just terminated at one end to the primary DC bus ground…this will shield the other coils and reduce radiated emissions?

    2….Ensure that the Switching node end of the primary coil is right at the bottom of the bobbin (the most inner coil on the bobbin), so that the switching node is more shielded by the other coils?

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    Re: Reducing radiated emissions in an offline flyback SMPS transformer

    I doubt that the RF is radiated from the transformer. More likely capacitive coupling between the windings transmits the interference which is radiated by the chassis and cables.


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    Re: Reducing radiated emissions in an offline flyback SMPS transformer

    I agree with FvM


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    Re: Reducing radiated emissions in an offline flyback SMPS transformer

    Thanks, but do you suspect that the grounded outer coil stops that?...like a flux band

    Like pg20 of this
    https://ac-dc.power.com/sites/defaul...pdf?download=1
    Last edited by treez; 2nd September 2019 at 06:11.



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    Re: Reducing radiated emissions in an offline flyback SMPS transformer

    The flux band is no "grounded coil", it's a shorted turn to reduce parasitic magnetic fields.


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    Re: Reducing radiated emissions in an offline flyback SMPS transformer

    The best way to reduce emissions is to go to a resonant flyback ...


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    Re: Reducing radiated emissions in an offline flyback SMPS transformer

    Quote Originally Posted by FvM View Post
    I doubt that the RF is radiated from the transformer. More likely capacitive coupling between the windings transmits the interference which is radiated by the chassis and cables.
    Agreed, it's most likely a conducted EMC (probably common mode) which is being radiated via long interconnects. So it needs to be treated like a conducted EMC issue. Increase common mode filtering, increase the Y cap, or decrease the source of the EMC (use a soft switching topology, or better layout).



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