In this schematic a common mode choke is added between PWM output and PWM ground (return path.) There is no other return path to load (like through parasitic capacitance to chassis.) In this case, will it act as a common mode filter?

CMC is added between the output of PWM generator and LISN. DUT load for CE testing is on the right side of LISN:

CMC is added between the output of PWM generator and LISN. DUT load for CE testing is on the right side of LISN

This was suggested to suppress conducted emissions across supply line when tested across positive and negative lines.

  • 1
    \$\begingroup\$ Have you read: coilcraft.com/en-us/edu/series/… ? \$\endgroup\$ Feb 24, 2021 at 13:39
  • \$\begingroup\$ I don't see a CM choke added where you said it is added. \$\endgroup\$
    – Andy aka
    Feb 24, 2021 at 19:05
  • \$\begingroup\$ @Andyaka: Revised Diagram and description for better understanding. \$\endgroup\$
    – Akky
    Feb 25, 2021 at 5:27
  • \$\begingroup\$ LISNs have a ground connection as far as I'm aware. If not then circuit details of the LISN are required. \$\endgroup\$
    – Andy aka
    Feb 25, 2021 at 8:20
  • \$\begingroup\$ Yes, LISN is standard one with ground connection to test table. \$\endgroup\$
    – Akky
    Feb 25, 2021 at 11:06

1 Answer 1


It is exactly like that. Differential current (the load current, out of phase from the coil point of view) pass more or less unimpeded. Common mode noise from the drivers get attenuated (in phase from the coil point of view).

However there are subleties: the CM choke does not suppress (or, at least suppress much less) the PWM switching noise itself, since that's is differential mode. There are specially designed chokes that do also that or, simple, you put a simple inductor (called simply choke, in this context) to attenuate these. This often is magnetic field (for a number of reasons there's an electric field component too) and can be attenuated also simply twisting the wires.

Also your CM noise is actually returning thru the parasitic ground (there's always a ground path to earth, even is maybe only about 4pF) since it's traveling from both the wires and emitted from these. This is only an electric field thing due to capacitive coupling to earth.

In any case be wary of the current rating of the chokes since they not only do heat up but also have a magnetic limiting value due to core saturation.


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