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I am designing a flyback converter that works at an operational frequency of 100kHz. Now, I need 2 levels of filtering at the input phase viz - common mode filter and differential mode filter. The common mode filtering is done using a common mode choke while the differential mode filtering is done using a pi filter composed of ferrite bead and capacitors. The current requirement is about 3Amps.

Now, my doubt pertains to the selection of these filters. For the common mode choke I went through the datasheets of different chokes that had decent impedance at the frequency of 100kHz. Likewise for ferrite beads.

But most of the said filters show impedance curves at frequencies greater than 1MHz. So, how can I select one for 100kHz ? Or is my thought process wrong ?

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But most of the said filters show impedance curves at frequencies greater than 1MHz. So, how can I select one for 100kHz ? Or is my thought process wrong ?

The flyback converter works with square waves when switching and it produces an array of harmonics that can still be significant past 1 GHz in some cases. This means that to prevent conducted emissions to the AC power (or DC feed to the power supply), a common mode choke may need to be "useful" above 1 GHz.

Generally, in my experience (and broadly speaking) it's the 100 MHz range of frequencies that tend to be the normal problem area so, your common mode choke does need to be specified across a vast range of frequencies of which 100 kHz is the lowest.

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  • \$\begingroup\$ So, as you say, I must select a common mode choke that has filtering in the 100kHz to almost 1GHz. The higher frequencies for the harmonics. But, then my question still stands. Are common mode chokes available for such a wide range ? \$\endgroup\$
    – Board-Man
    Mar 12, 2016 at 11:17
  • \$\begingroup\$ It'll be a compromise because at the high end and the low end of the spectrum it won't be as good as the middle range but, like I implied it's the mid range that tends to be where all the problems are. You now appear to be asking a question about availability of such devices and implying that was your main question. Are you aware that this site doesn't tend to explicitly recommend devices because that information will become outdated. Also a device cannot be generally recommended without intimate circuit knowledge. \$\endgroup\$
    – Andy aka
    Mar 12, 2016 at 11:23
  • \$\begingroup\$ Unfortunately, I am not a native english speaker. So apologies if my question was detailed in a wrong manner and confusing for you. \$\endgroup\$
    – Board-Man
    Mar 12, 2016 at 11:25
  • \$\begingroup\$ I wasn't confused - you changed your question. \$\endgroup\$
    – Andy aka
    Mar 14, 2016 at 15:35
  • \$\begingroup\$ "Now, my doubt pertains to the selection of these filters." That was my precise question. How was it wrong ? This site does not recommend components is a different aspect. That is my mistake. Probably I should not have asked it in this forum. But, I have not changed my question. \$\endgroup\$
    – Board-Man
    Mar 14, 2016 at 15:43

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