EMI Filter Designer (LC)

Radio-frequency interference from electrical equipment became a regulatory issue in the 1930s as broadcast radio and emerging television services were disrupted by sparks from motor brushes, arc lamps, and early switched-mode power converters. The FCC codified the first conducted-emission limits in the 1940s, and CISPR (International Special Committee on Radio Interference) formed in 1934 to align regulations internationally.

The switched-mode power supply revolution of the 1970s made EMI filtering universal. A 100 kHz SMPS fundamental plus its harmonics radiates into the 150 kHz – 30 MHz CISPR 11/22 conducted EMI test band, requiring line filters on every product sold. The "π filter" (two capacitors bracketing an inductor) became standard for AC inputs, with common-mode chokes wound on ferrite toroids handling the common-mode noise that dominates at higher frequencies.

Modern low-voltage DC rail filters — downstream of a buck converter, upstream of a sensitive analog circuit — use simpler LC topology: a ferrite bead or inductor in series, a ceramic capacitor (typically 10 µF MLCC) to ground. Cutoff is placed a decade below the switcher frequency (e.g., 100 kHz switcher → 10 kHz cutoff) to achieve 40 dB of attenuation at the switcher fundamental.

Enter the target cutoff frequency (typically 1/10th the switcher frequency or at the low end of the noise band you need to attenuate) and an available inductor value. The tool solves for the capacitor: C = 1 / (L · (2π·f₀)²). Roll-off is 40 dB/decade above f₀ for any 2nd-order LC — doubling frequency above cutoff = 12 dB more attenuation.

Cautions: verify the inductor's saturation current exceeds your load current (otherwise L drops and the filter stops working at load); add a small series R or RC snubber across the L or across the C to damp the LC resonance peak (undamped LCs can actually amplify line disturbances at f₀); and check that the capacitor is rated for line-frequency continuous voltage. Everything runs client-side; no values leave your browser.