EMI Filter Calculator — InnovChip

EMI Filter Calculator

Design an LC input filter for power supplies: compute cutoff frequency, characteristic impedance and attenuation at a chosen noise frequency.

Calculator

About This Calculator

An LC low-pass filter in front of a switching converter attenuates conducted EMI. The cutoff frequency fc = 1/(2π√(LC)) must sit far below the switching harmonics you want to remove, while the characteristic impedance √(L/C) should roughly match the system impedance to avoid resonances. Above cutoff, a second-order LC filter rolls off at 40 dB/decade.

How to Use

  1. Enter filter inductance and capacitance (µH, µF).
  2. Read the cutoff frequency and characteristic impedance.
  3. Enter the noise frequency to attenuate (typically the switching frequency or its strongest harmonic).
  4. Compare Z0 with the load/source impedance — large mismatch can create ringing.

Formulas

Cutoff frequencyfc = 1 / (2π √(L·C))
Characteristic impedanceZ0 = √(L / C)
Roll-offAtten ≈ 40·log10(f / fc) dB (2nd order)

Design Notes

Damping: an undamped LC network rings at light load. Add a small series damping resistor (often R = Z0·√(L/C)) or a damped output capacitor.

Include the parasitic ESR/ESL of real components — they limit attenuation above ~10 MHz.