Differential Mode Filter Inductance

Formula

Description

Differential mode EMI filters use series inductors to block high-frequency noise currents while passing the desired DC or low-frequency signal. The required inductance is determined by the impedance needed at the noise frequency of interest. This formula rearranges the inductive reactance equation to find the inductance that presents the target impedance at the specified frequency. Higher inductance provides more attenuation but also introduces more DC resistance and potential saturation issues at high currents.

Variables

• L — Required filter inductance (H)
• Z — Target impedance at the noise frequency (Ω)
• f — Frequency of the noise to be filtered (Hz)

Practical Notes

For effective filtering, the inductor impedance should be at least 10× the source impedance at the noise frequency. Ferrite bead inductors are preferred over wire-wound inductors for EMI filtering because they are lossy at high frequencies, converting noise energy to heat rather than reflecting it back. Always verify the inductor performance with actual impedance vs. frequency curves from the datasheet.