Electronics Forum / CAD / April 2005

Probably the current in the mains leads.

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

                                       ...Jim Thompson

           
I love to cook with wine.      Sometimes I even put it in the food.

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