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    PFC controller datasheets do not tell you that they need to damp LC filter ringing?

    Hi
    Why is it that PFC controller datasheets don’t tell you that they all have a facility to damp the LC filter ringing near the mains zero crossing?
    The PFC controllers all do this because PFC controllers don’t draw much current near the zero cross, and so when the mains suddenly forward biases the bridge diodes just after the zero cross, current then suddenly flows in to the converter, and the LC filter starts ringing which puts unwanted harmonics into the mains input current.
    Most PFC converters deal with this by switching the FET on for a long period at the zero cross. However, if the PFC’d converter is a PFC’d Buck, then this doesn’t work, and an external FET as in the attached example (M3) is needed, to provide the necessary damping.
    (LTspice simulation and pdf schematic attached)
    Why are there no off the shelf PFC’d Buck controllers which have a pin to switch on the said FET (M3)?

    The attached waveforms show the PFC'd Buck (mains input current) with and without the damping of the FET M3.

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    Re: PFC controller datasheets do not tell you that they need to damp LC filter ring

    how do you know the ringing isn't due to your input filters suddenly being unloaded when the buck ckt goes off - then on ...?

    sure M3 provides damping - "a load" - a sepic converter would give continuous input current ...


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  3. #3
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    Re: PFC controller datasheets do not tell you that they need to damp LC filter ring

    how do you know the ringing isn't due to your input filters suddenly being unloaded when the buck ckt goes off - then on ...?
    …thanks, it may well be that, but if it is, then that means it isn’t possible to do a PFC’d Buck converter without getting this ringing…because they will all have LC input filters. There are a number of PFC’d Buck converter controllers available off the shelf…with a low side FET…..and I am not sure how they can provide enough load near the zero crossing in order to damp away this LC ringing?
    I notice that even Boost converter PFC controllers actually have a feature whereby they deliberately draw overly high-ish current at the zero cross in order to damp away the LC ringing of the input filter.
    …eg the L6564 describes it on page 22 of its datasheet
    L6564 datasheet
    https://www.st.com/en/power-management/l6564.html



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