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EMC question

T

Tim Williams

Jan 1, 1970
0
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)?

Tim
 
T

Tim Williams

Jan 1, 1970
0
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.

Tim
 
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