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  1. #1
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    Common mode choke SRF should selected to be at the worst case EMC problem frequency?

    Hi,
    The following is an app note on use of common mode chokes to improve EMC….

    https://www.schaffner.com/fileadmin/...on_chockes.pdf

    Nowhere in this app note does it mention the self resonant frequency of the common mode choke.
    Do you know why this is?
    Surely THE big thing about use of common mode chokes in EMC , is that the SRF of the common mode choke should be placed at the worst case common mode EMC problem frequency?

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    Re: Common mode choke SRF should selected to be at the worst case EMC problem frequen

    It makes little sense to specify a common mode SRF because the actual in circuit resonance frequency is set by external capacitors and/or parasitic circuit capacitances. If you don't have common mode capacitance at the device side of the choke, no common mode disturbances need to be suppressed.


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    Re: Common mode choke SRF should selected to be at the worst case EMC problem frequen

    If you don't have common mode capacitance at the device side of the choke, no common mode disturbances need to be suppressed.
    Thanks, i take it you refer to "stray" capacitance here?....but isnt there always stray capacitance?

    It makes little sense to specify a common mode SRF because the actual in circuit resonance frequency is set by external capacitors and/or parasitic circuit capacitances.
    But surely the SRF of a common mode choke or an inductor is a major factor in the selection of inductors (CM or DM) in SMPS filters?

    At the common mode SRF, the impedance of a common mode choke peaks up very significantly, so surely you want this peak at your EMC problem frequency?



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    Re: Common mode choke SRF should selected to be at the worst case EMC problem frequen

    If the SRF matters in your application, you have usually curves of common mode and differential impedance, as in this data sheet https://katalog.we-online.de/pbs/dat...7446630047.pdf


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    Re: Common mode choke SRF should selected to be at the worst case EMC problem frequen

    SRF is of course important in minimising the size of any EMC filter, the BW can be extended by using plastic cups on each side of the toroid to reduce C to the core and various "figure 8" winding techniques to reduce ( cancel) pickup from nearby radiators of RF noise ...



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    Re: Common mode choke SRF should selected to be at the worst case EMC problem frequen

    Page 17 of AN15 (below) states that emissions from offline SMPS’s above 1MHz are usually common mode emissions….

    AN15: EMC app note:
    https://ac-dc.power.com/sites/defaul...pdf?download=1

    So then why is it that the SRF of most 250VAC rated, offtheshelf, common mode chokes, is below 1MHz?....eg, the wurth 7448640415 common mode choke has an SRF of 600kHz…

    Wurth 7448640415 common mode choke.
    https://katalog.we-online.com/pbs/da...7448640415.pdf

    Why so low?



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    Re: Common mode choke SRF should selected to be at the worst case EMC problem frequen

    Why so low?
    Because common mode L is high. 10 mH || 7 pF gives 600 kHz



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    Re: Common mode choke SRF should selected to be at the worst case EMC problem frequen

    Thanks, yes i agree, but then, surely it would be better to make L lower, so that SRF was higher?



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    Re: Common mode choke SRF should selected to be at the worst case EMC problem frequen

    I guess not, presumed you want also common mode filter action at frequencies below SRF



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    Re: Common mode choke SRF should selected to be at the worst case EMC problem frequen

    Thnaks but as the app note in the top post says, common mode emissions are usually above 1MHz.



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    Re: Common mode choke SRF should selected to be at the worst case EMC problem frequen

    this is why 2 stage LC CM filters are common ...



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