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  1. #1
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    Harmonic distortion of LED driver's mains input current waveform

    Hi,
    The attached is the LED current (green) and mains input current (red) of a 15W offline, non isolated Buck LED driver. There is deliberately no electrolytic input capacitor.
    Do you believe that this mains input current would violate THD regulations? I mean, it does have the ringing waveform at the mains voltage zero crossings, as you can see.
    (The pink waveform is the mains voltage)

    The basic schem is also attached...as you can see, its an extremely simple constant off time Buck

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    Re: Harmonic distortion of LED driver's mains input current waveform

    The question is a bit broad... but here are some thoughts.

    First, the THD/TDD regulations and standards applied to a device depend on the country of jurisdiction and the type of the device.

    The definitions of THD/TDD usually only include a certain number of harmonics... for example, the harmonics of order 2 ... 40. Very high frequencies are not included, so the ringing may not even be problem according to the regulations. Non-harmonic distortion is also often not included.

    This also applies to the limit values of individual harmonic components. Again, non-harmonic distortion may not be regulated at all. As far as I know, some standardization groups have recently taken a look at non-harmonic and high-frequency distortion.

    Is your simulator (LTSpice?) able to calculate the harmonic components and THD/TDD? I think LTSpice does have some frequency-domain capabilities.

    Is it a good idea to inject such distortion into the power network? Maybe not, but how much of a problem it would be? Have you taken power network impedances into account in your simulations? These will have an effect on the high-frequency components.

    I'm not sure if the waveform you posted has a DC component, but having a DC component in the mains current is bad.


    Just my 2cents, but I hope that this is useful. :P


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    Re: Harmonic distortion of LED driver's mains input current waveform

    More puzzling to me is where the ringing comes from. I suspect it is caused by the LC networks in the filter itself.
    As an experiment, try adding resistors in parallel with the filter inductors to reduce their Q and see if the ringing increases or reduces.
    Suggest 220R and work downwards in value.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.


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    Re: Harmonic distortion of LED driver's mains input current waveform

    Five years ago I did a video https://www.youtube.com/watch?v=veTVUhNiRC0 ( Pulsing Power Supply & AC Mains pollution )

    From the other hand at first post I am in denial to accept this graph as valid, this is not produced by respected electrical test and measurement equipment.


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    Re: Harmonic distortion of LED driver's mains input current waveform

    I am in denial to accept this graph as valid, this is not produced by respected electrical test and measurement equipment.
    No, it's a Ltspice simulation and probably not realistic in every detail.


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