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Simulating dBuV for FCC compliance

M

Mike

Jan 1, 1970
0
I'm trying to design a switch mode power supply of about 1200W. And I'd like
to simulate the noise I should expect to get from this circuit. So I want to
use OrCad Probe to plot out the spectrum in dBuV and compare it with the
requirements of FCC.

I know how to do the FFT. But I'm not sure what signal to FFT. Do I need to
simulate a LISN that is done in actual FCC compliance testing? Or do I
simply do an FFT on the line voltage assuming a small resistance in the
line?

Also, I'd like to plot on the same graph the maximum acceptable FCC noise in
dBuV, one for commercial, one for residential, for comparison purposes. How
do I do that on a plot of dBxx verses frequency?

Thanks.
 
J

Jim Thompson

Jan 1, 1970
0
I'm trying to design a switch mode power supply of about 1200W. And I'd like
to simulate the noise I should expect to get from this circuit. So I want to
use OrCad Probe to plot out the spectrum in dBuV and compare it with the
requirements of FCC.

I know how to do the FFT. But I'm not sure what signal to FFT. Do I need to
simulate a LISN that is done in actual FCC compliance testing? Or do I
simply do an FFT on the line voltage assuming a small resistance in the
line?

Also, I'd like to plot on the same graph the maximum acceptable FCC noise in
dBuV, one for commercial, one for residential, for comparison purposes. How
do I do that on a plot of dBxx verses frequency?

Thanks.

Probably the current in the mains leads.

VdB(...) is a valid entry in Probe.

...Jim Thompson
 
B

blazeinferno

Jan 1, 1970
0
Hey Mike,

I'm not an EMI guru but I recently went through a few
EMI tests. The first question I would ask is: How good is your power supply
model? Is it very representative of what you see in practice?
You will probably have to go through conducted common mode, conducted
differential mode tests and radiation. We will neglect the immunity part.

At best your simulation model will give you an idea for differential mode
only (my opinion). For the common mode part, no component model (spice
based) includes the parasitic capacitance between the component and chassis,
transformer windings capacitance etc do you follow? You would probably have
to come up with a common mode simulation model based on approximations.

Although, the best tip I can give you is look for possible EMI sources:

1) Ringing due to reverse recovery of
diodes
2) Ringing due to power switches
(mosfets IGBTs etc)

I know fundamental switching frequency is a source but
what can we do about it?hahahah
Parts you might want to consider:

Add EMI common and
differential mode filters to your input and output usually it's a PI filter.
If you are using RC snubbers,
try using non inductive resistors (regular wirewound resistors usually ring
about 5MHz-10MHz range).
Be sure to be using a good
chasis connection

I hope these tips help you and let me know how your simulation model
goes.....It would be pretty cool if your method works.

Ciaooooooo
Fern
 
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