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  1. #1
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    Boost PFC controller must contain anti-distortion feature for zero crossings

    Pages 22 and 23 of the L6564 datasheet show that Boost PFC controllers need to sense the instantaneous mains voltage, and then increase the FET current at the zero crossings. This is to reduce crossover distortion and thus reduce THD.
    Why does the LT1248 PFC controller not mention this feature? Does it not have this feature? How does it avoid the LC filter ringing that otherwise occurs near the zero crossing if this feature is not present?

    L6564 datasheet
    https://www.st.com/resource/en/datasheet/l6564.pdf
    LT1248 datasheet
    https://www.analog.com/media/en/tech...ets/1248fd.pdf

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    Re: Boost PFC controller must contain anti-distortion feature for zero crossings

    LT1248 is from about 1999
    see what looks like a date code at the bottom of the last page
    1248fd LT/GP 0799 2K REV D • PRINTED IN USA
    i think 0799 is a date, july 1999
    package is a 16 pin plastic DIP - an "old" package
    see page 6, the section on Pin 6, IAC, at the bottom

    L6564 is from about 2013
    note copyright on the last page
    and date at the bottom of page 1
    package is a SSOP10 - a "new" package

    I think the difference is that the ST part is newer and has the feature added
    to help correct an issue with the older version


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    Re: Boost PFC controller must contain anti-distortion feature for zero crossings

    The assumption implied by the title is wrong. A boost PFC must not necessarily contain this feature to comply with power quality regulations.


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    Re: Boost PFC controller must contain anti-distortion feature for zero crossings

    average current mode PFC chips ( like the original UC3854 ) automatically raise the PWM around the zero xing to keep the average input current tracking the mains voltage waveshape - controllers like the 6564 are not average mode and so need extra bits to force them to go to higher PWM around the zero xing to help improve the power factor ...


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    Re: Boost PFC controller must contain anti-distortion feature for zero crossings

    average current mode PFC chips ( like the original UC3854 ) automatically raise the PWM around the zero xing to keep the average input current tracking the mains voltage waveshape
    Thanks… the attached (including LTspice sim) is a basic PWM controller that I’ve hacked to give average current mode power factor correction for a boost converter. I haven’t implemented the feature whereby the current is increased at the zero crossings. As you can see, there are unwanted blips in the mains input current waveform….these cannot be gotten rid of by adjusting the filter values etc…..i assume that the only way to get rid of this will be to somehow make the FET ON time higher at the zero crossings, which should damp out the sudden blip that occurs just after the zero crossing, where the mains bridge suddenly begins flowing current into the post bridge capacitance, which unfortunately stays a little charged up and so causes this blip.
    So what do you reckon?…I should put in a zero crossing detector and just artificially increase the reference voltage near the zero crossings so as to discharge the post bridge capacitance at the zero crossing?...that will get rid of the blipping?

    Or should i just forget it...it surely wont give a failure of THD?



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    Re: Boost PFC controller must contain anti-distortion feature for zero crossings

    most design engineers would not worry about such a small artifact, it does indeed loo to be caused by bridge rect noise - and would be filtered by the EMC filters ...


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    Re: Boost PFC controller must contain anti-distortion feature for zero crossings

    most design engineers would not worry about such a small artifact, it does indeed loo to be caused by bridge rect noise - and would be filtered by the EMC filters ...
    Thanks but that is in the mains input current , albeit the simulator....but it didnt get filtered out, and i find even increasding the fitration doesnt solve it......but as you say, it looks like it can be ignored.



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    Re: Boost PFC controller must contain anti-distortion feature for zero crossings

    it looks to be more an artifact of the diode bridge in the sim - this is quite common to see - try putting a snubber across each diode in the sim to remove ...


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