Practical CM Filter Design

There are two ways of going about this. Either way we are assuming that the mosfet heatsink is tied to the chassis. At one end of the parasitic capacitor Cp we are applying the trapezoid that best describes a typical drain waveform. This causes a CM noise current Icm to flow through the earth wire. We assume that the parasitic capacitances and/or X-caps cause this current to split up equally between the L and N wires. So we have Icm/2 in each of these two wires. 

We already know the Fourier components of the trapezoid, and so we can consider them individually and determine the injected current due to each harmonic. As an example, we take the forward converter. Here the peak to peak amplitude of the drain-to-source waveform ( Vmax or A ) is twice the supply rail (Vin). From Page 424, we have 

So assuming duty cycle is about 50%, the amplitude of the fundamental (first harmonic) is 

We realize that, as for the DM noise calculation, from the viewpoint of the noise envelope and its required attenuation, only the fundamental harmonic really counts. The current caused by this is 

The same situation occurs when the mosfet turns ON — only the directions are reversed. 

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