I need to filter out switching noise at the output of SMPS/DC-DC converter with an LC filter.

I have a bunch of CM chokes lying around.

Can I use a common-mode choke:

  1. with both CM windings connected in the same winding direction, to double the inductance?

  2. with one winding unused but left on core?

  3. with one winding only, with the second one unwound?

  • 2
    \$\begingroup\$ In (1) the inductance quadruples. Are you sure your noise isn't bad probing? Convince me. \$\endgroup\$
    – Andy aka
    Commented Nov 30, 2021 at 6:35
  • \$\begingroup\$ All three will saturate if you use them as you describe. Get yourself a DM choke instead. \$\endgroup\$
    – winny
    Commented Nov 30, 2021 at 7:20

1 Answer 1


Common mode chockes are not designed to carry their rated current unless the windings are connected anti-phase.

That's why CM chockes are way smaller than a similar rated inductor, I'm sorry but no free meals out there.

Trying use them like simple inductors will saturate the core, probably around one hundredths of nominal current.

  • 1
    \$\begingroup\$ As this answer covers the essential part already I'll add as a comment: It is hard to find very high inductances rated for really small currents. E.g. for signal currents a 1 mA current rating could well suffice in many situations but the smallest inductor ratings in the mH range are often >10 mA. Therefore using a series connected CMC can fill this role in a really tight space. Transformers would also work by the same logic but are often much larger. \$\endgroup\$
    – tobalt
    Commented Nov 30, 2021 at 7:50
  • \$\begingroup\$ Why are then some available cm chokes presented as for example 2mH, 10A ? Does that mean 10A of current in common mode configuration but much less in dm ? \$\endgroup\$
    – Platypus
    Commented Nov 30, 2021 at 20:16
  • \$\begingroup\$ @Platypus yes exactly. That's what they are made for and that's why they are comparatively so small \$\endgroup\$
    – carloc
    Commented Nov 30, 2021 at 20:27
  • \$\begingroup\$ Thanks everyone. I learned something new. I don't remember this differential vs common mode stuff ever covered in school. Well, i guess, it is because it is actually something useful in real life. \$\endgroup\$
    – Platypus
    Commented Dec 1, 2021 at 2:51

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