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Wiring and EMC

Discussion in 'Electronic Design' started by Raveninghorde, Nov 6, 2009.

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  1. I spent Wednesday on site looking at an EMC problem.

    We supply the boards and our customer builds them into a variety of
    units. Harmonics of an LM2675 simple switcher running at 260kHz were
    getting out onto the mains. 1.5MHz was about 0.2dB out of spec and
    several harmonics were within 6dB of the limit.

    The problem was the same as one I had identified about 4 years ago. On
    the controller I provide an EMC earth bond point for the I/O
    grounding. Instead of taking this direct to the chassis it was run
    back through about 2 foot of wire in the wiring harness.

    I removed the wire and used 1" of braid from the earth bond point to
    chassis and instant 6+dB improvement.

    I changed a pair of filter capacitors from 1n/500V to 470n/50V for
    another 6dB. That reflects a fault of mine. #when designing I still
    think of pth ceramic caps where higher values tended to be worse at
    HF.
     
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