I want to design a low pass filter with common-mode choke that has a cutoff frequency of 100 Hz. This is going to be used in a bus 24V DC power system that will deliver 10A to the load. I want to use it to clear EMI, RF interference and noises. Since there is no AC line in a bus, 50 or 60 Hz noise will be no problem.

  1. Will Butterworth LC pi filter be a good choice?
  2. If so, I was given 470uH and 1.8mH common-mode chokes. How to calculate capacitor values?
  3. Will ouput impedance of the filter matter?
  • \$\begingroup\$ Is it AC power that you are providing? Might be helpful to provide a little bit more detail about what the power is. \$\endgroup\$
    – Kellenjb
    Oct 4, 2011 at 14:00
  • \$\begingroup\$ Ops sorry, it is DC. Let me edit. \$\endgroup\$ Oct 4, 2011 at 14:05
  • 2
    \$\begingroup\$ You could add also some specifics about the other parameters: it's more important a narrow transition band or a good linearity in the passing band? How complicated can it be? How much power (this is very important IMHO) could the suppressed band have? This affect the power that are you going to dissipate in the filter, so it's important. \$\endgroup\$
    – clabacchio
    Feb 23, 2012 at 12:48

1 Answer 1


Common-mode (CM) chokes on AC primary inputs serve in both directions to:

  1. prevent conducted noise from device or circuit pulse currents from coupling back into the AC source and in turn causing radiated noise via the AC power cable. (CE/FCC conducted emissions spec.)

  2. reduce switch-mode noise from other devices causing ingress noise interference to this device. In general, the range of frequencies of concern are 10KHz to 1MHz, NOT 100 Hz, as this would require huge passive values in the filter (read cost over-kill).

Generally this is handled by using high impedance, high permeability CM ferrite wire wound in a compact part with small high voltage caps that if connected to ground must not exceed leakage currents for Common mode AC test for ground fault leakage safety tests (CE/UL test usually 0.5mA max; leakage typically much less).


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