[SI-LIST] Re: DC converter iEMI simulation with PSpice

Jean-Pierre :
I don't know the mechanics of Orcad that well. I use Icap/4 from Intusoft. I
have done exactly what you have just described. In my simulations I use a
LISN that measures whatever is on the 50 OHm resistor and don't separate the
CM from the DM. The way I separate the CM from the DM is that all returns *
except* for my filter, to reference, reference is the LISN return, go
through a voltage source that is zero volts. Icap allows a current
measurement through  the voltage source, that current is CM by definition.

With respect to simulation of such, the results can be very good or very
bad. So if your results are bad find out why and fix the model. Next time
the results will be better but still iterate improvements. The best thing
about the model you can do what if situations. Radiated fields will
certainly be much more challenging to render acceptably predictability.

Her is something that you need to know, before you can use the results,
typically the post filtering for the FFT uses a filtering scheme which is
relative to the mandated requirements. ICAP uses a Han filter algorithm
which is relative to CISPR 22. Though you can take the CSV output file and
do your own post filtering if you need to.

To cross check you results, you *can* predict the fundamental frequency to
within a few dB, pencil, paper and calculator, harmonics a bit more paper
and perhaps fresh batteries.
EA
On Fri, May 15, 2009 at 9:13 AM, jean-pierre bouthemy <
jean-pierre.bouthemy@xxxxxxxxxxxxxxxxxx> wrote:

> Hi,
> I am trying to simulate the EMI noise of a DC-DC Forward converter with
> Orcad Pspice FFT.
> I introduced LISN model between the power source and the converter input
> and measure the following voltages with Pspice probe :
>
> * Vlisn+ - Vlisn- : differential noise
> * (Vlisn+ + Vlisn-)/2 : common mode noise (due to 100pF stray
> capacitance between Mosfet drain and earth ground)
>
> Then I perform FFT on these voltages and change units to Log for both X
> and Y.
> I can thus look nice differential and common mode noise EMI spectrum,
> but I can not get dBµV on my FFT window.
>
> Does anyone of you know how to do so (if possible)?
> And is it correct to calculate as follows: Noise (dBµV) = 20*Log(V/1µV) ?
>
> And more genreral question : does it make sense to simulate EMI with a
> tool like Spice?
>
> Thank you.
>
> Regards.
> Jean-Pierre Bouthemy
>
>
>
>
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