I have been designing a common mode choke for common mode noise filtering at the input of my SMPS. I have chosen the capacitance value to be 6.6nF on both Line to Ground and Neutral to Ground. I am following this equation to set the cut-off frequency of the filter:

Fc = 1 / (2pi * sqrt(L*2C)) = 39.025KHz

Using a T28 toroid core, I get an inductance of 1.26mH at 1KHz when I use an insulated wire with 18 turns per winding, with the windings placed seperately on both sides of the core (sectional winding). The choke is not placed on the PCB, but placed inside the case connecting power input connector to the SMPS board.

I am trying to get a much higher inductance from the choke by winding it such as the windings are cumulatively coupled (so that effective L = L1 + L2 + 2M). When I tried it out, as expected, I could get an inductane of (1.26mH + 1.26mH + (2 * 1.26mH) = 5.04mH) of inductance. I hope this would give me a much better filter performance from the same core as now the cut-off frequency comes down to 19.435KHz.

But, now since the winding is bifiliar, I wonder if there would be more parasitic C or L effects that would create problems. I know that leakage inductance would be lesser in bifiliar winding, but since I have a separate differential filtering stage onboard,that shouldnt be a problem.

Can some one share your experiences? I always see only sectional wound common mode chokes online and makes me surprised!

  • 1
    \$\begingroup\$ Are you able to use a simulator? If so then that is your best route for obtaining "experience". \$\endgroup\$
    – Andy aka
    Nov 29, 2018 at 8:17

1 Answer 1


Depends what frequencies you are trying to filter out. If you keep adding turns then the inductance increases but so does the inter-winding capacitance. At some frequency the choke will become parallel resonant. Above this frequency it will look capacitive and the impedance will decrease with frequency.


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