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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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    SRF of filter inductors in offline SMPS

    I have seen quite a few offline SMPS schematics with 100pF or so capacitors across the diff mode filter inductors. This is said to offer the opportunity to shift peaks in the EMC scan to different frequencies where they may be under the limit line of the conducted EMC test.

    I believe that this is not applicable for offline SMPS, and the only thing that should go across the diff mode inductors is damping resistors, would you agree?
    Also, would you agree that the switching frequency of an offline SMPS should never be at the SRF of any of the filter inductors in the AC filter section?...This only really applies to common mode chokes, because for diff mode inductors the SRF is usually well above the switching frequency anyway.

    So would you agree that one of the major considerations for a common mode choke selection in an offline SMPS , is where is the diff mode SRF?
    Last edited by FvM; 14th September 2019 at 16:18. Reason: Moved to previous thread with similar topic



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    Re: SRF of filter inductors in offline SMPS

    Part of your points have been already discussed in the previous thread.

    Also, would you agree that the switching frequency of an offline SMPS should never be at the SRF of any of the filter inductors in the AC filter section?
    Surely not. A choke has maximal impedance at it's SRF, respectively maximal isolation of the interfering frequency.

    100 pF is a huge capacitance compared to typical winding self capacitance. It will cancel the CM choke effect in the frequency range of interest. I can hardly think of a case where it would be helpful.



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    Re: SRF of filter inductors in offline SMPS

    Thanks, the 100pF caps were placed across diff mode inductors...i was assured by the engineer that it was for moving peaks on the scan to get the scan passing overall



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    Re: SRF of filter inductors in offline SMPS

    O.K., you can tune a diff mode filter to reject a particular frequency, at cost of wideband suppression. If it helped in a special case, it's surely no reasonable general suggestion.



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