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Wiring and EMC

R

Raveninghorde

Jan 1, 1970
0
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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