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:




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

  • \$\begingroup\$ Is that from section 4.3 which says "Pick the higher value determined by the following two formulas" and gives an alternative formula? \$\endgroup\$ Feb 25, 2021 at 11:40
  • \$\begingroup\$ @AndrewMorton yes, I got one of them negative. \$\endgroup\$
    – Das D.
    Feb 25, 2021 at 11:44
  • \$\begingroup\$ And so which one is the higher value? Use that. (E.g. 5 is higher than -10.) \$\endgroup\$ Feb 25, 2021 at 11:45
  • \$\begingroup\$ -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 \$\endgroup\$
    – Das D.
    Feb 25, 2021 at 11:51
  • \$\begingroup\$ 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. \$\endgroup\$ Feb 25, 2021 at 12:27

1 Answer 1


Are you really sure you did the computation correctly? In equation (5) Cfb can be negative only if is negative either of

  • Lf : negative inductor? no, check
  • The thing after that? no, it's squared so it can't, check

Given that there are no complex numbers in that formula I don't get how you can get a negative value

  • \$\begingroup\$ I'm sorry, Cfa is negative. \$\endgroup\$
    – Das D.
    Feb 25, 2021 at 7:52
  • \$\begingroup\$ @DasD. Show how. \$\endgroup\$
    – winny
    Feb 25, 2021 at 10:46
  • \$\begingroup\$ @winny values are edited \$\endgroup\$
    – Das D.
    Feb 25, 2021 at 11:32

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