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Regular chokes can certainly be used in parallel so that multiple smaller chokes with a lower current rating can be used. But does this also work for common mode chokes?

I would assume that differential currents are also affected by the common mode choke if the current goes in choke 1 and goes out choke 2. Can you assume that for regular differential output currents \$i_\mathrm{in1}\$ is always the same as \$i_\mathrm{gnd1}\$?

My intuition is that the common mode inductance will ensure that \$i_\mathrm{in1} = i_\mathrm{gnd1}\$ for differential output currents, at least at higher frequencies. Close to DC I'm not so sure anymore. For paralleling regular inductors I would not assume exactly equal current sharing.

Worst case I can imagine would be a DC differential output current leading to \$i_\mathrm{in1} \neq i_\mathrm{gnd1}\$ and saturating the choke, rendering it useless for actual common mode current. Is this a realistic scenario?

If you find any application note etc. regarding parallel CM-chokes, let me know. I couldn't find anything. But maybe this is a blatantly obvious topic.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Are you saturating your CM choke or are you at the thermal limit? \$\endgroup\$
    – winny
    Mar 7 at 9:18
  • \$\begingroup\$ @winny I'm asking about paralleling due to thermal limitations. In space constrained applications it might be easier to use two smaller chokes instead of one bulky choke. \$\endgroup\$ Mar 7 at 9:24
  • \$\begingroup\$ Regular chokes can certainly be used in parallel so that multiple smaller chokes with a lower current rating can be used. - this is a generally flawed idea so, maybe you have a particular use-case where they can? \$\endgroup\$
    – Andy aka
    Mar 7 at 9:50
  • \$\begingroup\$ @Andyaka The use-case in my mind for paralleling regular chokes/inductors would be something like an LC-filter at a power supply DC output. \$\endgroup\$ Mar 7 at 10:07
  • \$\begingroup\$ @LarsHankeln have you ever seen this done? Sure you can theorize that it might be done and sure, someone may have tried this but, you'll end up with more space taken up than if you used a proper sized core. \$\endgroup\$
    – Andy aka
    Mar 7 at 10:49

1 Answer 1

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Common-mode chokes can handle high currents because of the principle that the non-common-mode current to the load is equal and opposite to the return current to the source. If these two currents are out of balance, the choke quickly saturates and loses its effectiveness. In your drawing, if there is different line or contact resistance in your circuit, a portion of the current I_in1 could return via I_gnd2, unbalancing both of the chokes and rendering them ineffective. If you have space for two chokes, it would be better to have two common-mode chokes in series, each with one-half inductance value.

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