# EMI Filter for Buck Converter

I'm trying to design an EMI filter for a buck converter and I'm following steps on: AN-2162

Here is the question:

At page:6, for the Cf capacitor; two different capacitor values are calculated and highest value is taken(Cfa, Cfb). In my situation, calculations for Cfa results a negative cap. value.

Since there is no negative capacitance, should I neglect this result or take it as a positive value?

Here is the calculation:

Cin=2uF(0.000002F)

L=1mH(0.001H)

fs=30kHz

for Cfa:

0.000002 / ((0.000002) x (0.001) x (((2 x 3.14 x 30000)/10)^2)-1)

0.000002 / ((0.000002) x (0.001) x (355000000))-1

0.000002 / (0.71-1)

0.000002 / -0.29

= -6.89uF

• Is that from section 4.3 which says "Pick the higher value determined by the following two formulas" and gives an alternative formula? Feb 25, 2021 at 11:40
• @AndrewMorton yes, I got one of them negative. Feb 25, 2021 at 11:44
• And so which one is the higher value? Use that. (E.g. 5 is higher than -10.) Feb 25, 2021 at 11:45
• -10 is not a valid value for capacitance, that's why I ask for it, I thought maybe minus sign has no importance here and it may become: 10 is higher than 5 Feb 25, 2021 at 11:51
• Please check the APEC seminar I taught in 2017 in which I show in slide 80 how to design an EMI filter for a buck and how to optimally damp it. Feb 25, 2021 at 12:27