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    Frequency of common mode noise in offline SMPS?

    The following article (on page 4) from mag-inc.com gives a design example for a common mode choke for an offline SMPS. It specifies that the common mode choke will have an impedance of 1000 ohms at 10khz.
    Why does it bother to specify impedance at such low frequency? Common mode noise is noise that is typically above several MHz…because it needs to be that high in frequency to allow it to couple out through stray capacitances etc.

    “Common Mode inductors for EMI filters require careful attention to core material selection.”
    http://www.mag-inc.com/File%20Librar...ture/fc-s5.pdf

    Common mode noise is noise that has come in to the offline SMPS through the live line, then gone in to the SMPS, but instead of coming back out through the neutral, it couples out of the power supply and flows back to earth ground. -As you can tell, signals at 10khz simply cannot do this, at least not with significant amplitude....so why is their example at 10khz?

    Related thread:
    https://www.edaboard.com/thread231845.html
    Last edited by FvM; 18th November 2015 at 19:10. Reason: Link fixed

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    Re: Frequency of common mode noise in offline SMPS?

    Also, we all know that a common mode choke at the input to an offline SMPS can saturate due to the leakage flux. –Especially with small common mode chokes used with multi kilowatt power supplies.

    Now, this saturation is most likely to happen with “ferrite” torroids when used to make the common mode choke. Because of this, when the SMPS is high power, is it not worth doing one common mode choke on a ferrite torroid, but also having another common mode choke wound on say a powder core torroid so that at least it doesn’t saturate?

    I mean, I know it will have a lower inductance, because of the lower permeability of powder core torroids in comparison to ferrite torroids, but we surely agree that common mode noise is really high frequency, so you don’t need a really high inductance to staunch it. …Or rather, a significant amount of common mode noise in offline SMPS is at such a high frequency that the powder core common mode choke will staunch a good deal of the common mode noise problem?

    So, are you up for a powder core torroid common mode choke?

    Eg wound on APH46P90 (amogreentech) torroid
    http://52ebad10ee97eea25d5e-d7d40819...H46P90_1DS.pdf

    ..page 18 for APH46P90 torroid core



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    Re: Frequency of common mode noise in offline SMPS?

    Quote Originally Posted by treez View Post
    The following article (on page 4) from mag-inc.com gives a design example for a common mode choke for an offline SMPS. It specifies that the common mode choke will have an impedance of 1000 ohms at 10khz.
    Why does it bother to specify impedance at such low frequency? Common mode noise is noise that is typically above several MHz…because it needs to be that high in frequency to allow it to couple out through stray capacitances etc.
    I don't think they're suggesting this is a standard requirement, it's just an example spec they came up with from nothing.

    Common mode noise is noise that has come in to the offline SMPS through the live line, then gone in to the SMPS, but instead of coming back out through the neutral, it couples out of the power supply and flows back to earth ground. -As you can tell, signals at 10khz simply cannot do this, at least not with significant amplitude....so why is their example at 10khz?
    You should rethink this. Usually we are not concerned so much with common mode transients coming from the mains as we are with preventing the smps from injecting common mode interference back into the mains. This is done by bypassing the common mode interference back to earth ground through Y caps and blocking the mains with the CM choke.


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    Re: Frequency of common mode noise in offline SMPS?

    Usually we are not concerned so much with common mode transients coming from the mains as we are with preventing the smps from injecting common mode interference back into the mains. This is done by bypassing the common mode interference back to earth ground through Y caps and blocking the mains with the CM choke
    The thing is, common mode noise is very high frequency, and the app note of the first post goes on about making the inductance several millihenries......surely the inductance can be less than that for a common mode choke?

    I thought the point of the common mode choke was that we don’t want the noise being drawn in through the live and then letting it go out of the earth wire…….so the common mode choke makes this path high Z. The noise then chooses to come back out of the earthed enclosure, and instead go up the y cap, and then go down the neutral, thus preventing it being common mode noise, because it came in via live, and exited via neutral so it is made into differential noise.
    Last edited by treez; 19th November 2015 at 07:19.



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    Re: Frequency of common mode noise in offline SMPS?

    Also, we all know that a common mode choke at the input to an offline SMPS can saturate due to the leakage flux. –Especially with small common mode chokes used with multi kilowatt power supplies.
    Seriously, I'm not even sure to which load case you are referring. The term "leakage flux" suggests that you are talking about possible saturation of a common mode choke by differential mode current. But shouldn't we expect that keeping the (differential) current rating avoids core saturation and respective loss of choke function by design?

    Low frequent common mode currents can easily saturate the choke, but they don't involve leakage flux.

    Quite obviously, the lowest common mode frequency component generated by a SMPS is the switch frequency fundamental. As stated correctly, common mode current can only return through capacitances, e.g. inter-winding capacitance and dedicated Y capacitors. The common mode voltage is however high, so you need a considerable amount of common mode inductance to cut the common mode current path. That's, I think, the background of the Magnetics article, including the misleading 10 kHz specification.


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    Re: Frequency of common mode noise in offline SMPS?

    Seriously, I'm not even sure to which load case you are referring. The term "leakage flux" suggests that you are talking about possible saturation of a common mode choke by differential mode current.
    The article in the first post refers to that very saturation. Page 7, near the bottom.



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    Re: Frequency of common mode noise in offline SMPS?

    I see, they discuss possible saturation with high crest factors of non-PFC power supply. Well, the article is quite old (2001).


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    Re: Frequency of common mode noise in offline SMPS?

    Sso do you think this common mode choke can handle mains 32a rms and 45a pk

    http://www.farnell.com/datasheets/1736546.pdf

    ..without saturating...it says 35a max but not say of this is rms or peak



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    Re: Frequency of common mode noise in offline SMPS?

    Rated current of a common mode choke is DC or RMS, peak won't matter as long as you don't expect partial core saturation. But I guess, the Wuerth guys didn't check for it specifically.


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