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EMC question

Discussion in 'Electronic Design' started by Tim Williams, Nov 30, 2009.

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  1. Tim Williams

    Tim Williams Guest

    Now, on a switching supply, you're supposed to have a Y-type cap between
    input and output commons, to shunt the RFI coupled across the transformer's
    interwinding capacitance (and maybe a common mode choke on input and/or
    output to improve that isolation with respect to the surroundings). But
    what should you do if the output side is also bouncing (different frequency,
    out of phase)?

    Specifically, imagine a high side gate drive circuit powered by a DC-DC
    converter. The driver's "common" is bouncing at, let's say 320Vp-p, so it's
    not really "common" in any useful sense. What then? Should there be CM
    chokes anywhere, or would those only make things worse (e.g., resonating
    with the parasitic capacitances)?

  2. Tim Williams

    Tim Williams Guest

    So it's no matter that the whole driver circuit is flying at some crazy
    voltage (and dV/dt, and potentially dI/dt across it)? Or is that something
    where you're just fucked and the only solution is to shield the lot of it?

    Now, you mention a common mode choke... but would that have to be sized so
    the resonant frequency is less than the driven fundamental? That'd be
    pretty huge. Otherwise, it'll resonate due to harmonics (or if damped, it
    will still be significantly conductive to HF), and that could be almost as
    bad (or maybe it's not, and it's the way to go?).

    Ultimately, the specific project I'm working on is probably going to throw
    off more RFI than I can do anything about (big DC motor PWM), but I'd like
    to get a handle on the subject while I'm at it. Right now, waving a
    "grounded scope probe" over my circuit shows about twice the noise of my
    laptop power supply, which is fairly remarkable given it's switching at
    least 10 times more voltage, and probably higher current, than my little
    DC-DC converter. There's definitely some room for improvement.

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