When using multiple common mode inductors in series, how can we avoid the overlap of resonance points?

The resonance of common-mode inductance is determined by its inductance (L) and parasitic capacitance (C, such as inter-winding capacitance), with the resonance frequency f0​=1/(2πLC​). When multiple stages are connected in series, if the resonance points are close, it can lead to a sharp decrease or even amplification of interference suppression in a certain frequency band.

To avoid the overlap of resonance points, differentiated design methods are employed: by adjusting the parameters of each inductance (such as core material, number of turns, winding structure), the resonance points of each stage are staggered. For example, the front stage uses a high permeability core (such as manganese-zinc ferrite) to increase inductance and reduce resonance frequency; the rear stage uses a low permeability core (such as nickel-zinc ferrite) to decrease inductance and increase resonance frequency, ensuring that the resonance point spacing is ≥2 octaves.

Introducing damping: connecting small resistors (such as 10-100Ω) between stages to consume resonance energy and suppress resonance peaks, without significantly affecting the common-mode impedance.

Parasitic capacitance control: the rear stage inductance adopts layered winding or adds a shielding layer to reduce parasitic capacitance, shifting the resonance frequency to a higher frequency band and complementing the front stage.​