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LC EMI Filter Design Calculator

Design LC EMI filters for conducted emissions compliance. Calculate inductance, capacitance, cutoff frequency, and attenuation for CISPR 22/32 and FCC Part 15. Free, instant results.

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Formula

n=⌈AdB20⌉n = \left\lceil\frac{A_{dB}}{20}\right\rceil

Reference: Williams & Taylor, "Electronic Filter Design Handbook" 4th ed.

Z0— Characteristic impedance (Ω)
ωc— Angular cutoff frequency (rad/s)
n— Filter order

How It Works

The LC EMI Filter Calculator designs low-pass filters for conducted emissions compliance — essential for CISPR 32/22 (IT equipment), CISPR 11 (industrial), and FCC Part 15 certification. EMC engineers use this to achieve 20-60 dB attenuation at 150 kHz (CISPR lower limit) while maintaining stable power delivery and avoiding filter resonance issues.

Per Henry Ott's 'EMC Engineering,' a second-order LC low-pass filter provides 40 dB/decade attenuation above cutoff frequency f0 = 1/(2 x pi x sqrt(L x C)). For 40 dB attenuation at 150 kHz, cutoff must be f0 = 150 kHz / 10^(40/40) = 15 kHz. The characteristic impedance Z0 = sqrt(L/C) should match source/load impedance (50 ohm for LISN measurements) to prevent resonant peaking.

CISPR 32 Class B limits conducted emissions at 66 dBuV (150-500 kHz), 56 dBuV (500 kHz-5 MHz), and 60 dBuV (5-30 MHz) using a LISN (Line Impedance Stabilization Network). Pre-compliance measurements typically show SMPS emissions 70-90 dBuV — requiring 20-30 dB filter attenuation plus 6-10 dB margin for measurement uncertainty and production variation.

Filter topology matters: Pi-filter (C-L-C) provides 60 dB/decade; T-filter (L-C-L) provides same rolloff but better common-mode rejection. Per Ott, mains filters combine common-mode choke (addresses CM noise) with X-capacitors (DM across line-neutral) and Y-capacitors (CM to earth). Complete EMI filter modules integrate these in safety-certified packages.

Worked Example

Problem: Design LC filter for 200W SMPS showing 82 dBuV at 150 kHz. CISPR 32 Class B limit is 66 dBuV. 50-ohm LISN reference.

Solution per Ott:

  1. Required attenuation: 82 - 66 = 16 dB, plus 6 dB margin = 22 dB at 150 kHz
  2. For second-order LC: A = 40 x log10(f/f0); 22 = 40 x log10(150/f0); f0 = 150/10^0.55 = 42 kHz
  3. Characteristic impedance: Z0 = 50 ohm to match LISN
  4. L = Z0/(2 x pi x f0) = 50/(2 x pi x 42000) = 189 uH; use 220 uH standard value
  5. C = 1/(2 x pi x f0 x Z0) = 1/(2 x pi x 42000 x 50) = 76 nF; use 100 nF X2 capacitor
  6. Verify f0: f0 = 1/(2 x pi x sqrt(220e-6 x 100e-9)) = 34 kHz (lower, provides more attenuation)
  7. Attenuation at 150 kHz: A = 40 x log10(150/34) = 40 x 0.64 = 26 dB (meets 22 dB requirement)
Components: 220 uH inductor (rated >1A DC, saturation current >2A), 100 nF X2 capacitor (250VAC rated). Total filter cost approximately $2-5.

Practical Tips

  • ✓Use off-the-shelf EMI filter modules for mains applications — they include safety-certified X/Y capacitors and meet UL/IEC leakage current limits (<3.5mA per IEC 60950). Custom designs require safety certification.
  • ✓Place filter at power entry point (IEC inlet or DC jack) — filtering after internal wiring allows noise to couple to internal cables before reaching filter per Henry Ott's layout guidelines.
  • ✓Measure with filter installed to verify no resonances — LC filter Q can create peaking at f0 that worsens emissions. Add damping resistor (R = Z0/3 to Z0) in parallel with C if peaking observed.

Common Mistakes

  • ✗Neglecting inductor saturation current — ferrite-core inductors lose 50-80% inductance at saturation, shifting f0 upward and reducing attenuation by 10-20 dB. Per Wurth application notes, select I_sat > 2x peak operating current.
  • ✗Using electrolytic capacitors for EMI filtering — electrolytics have 0.1-1 ohm ESR and 5-20 nH ESL, limiting effectiveness above 100 kHz. Use X2/Y2 film or MLCC capacitors per CISPR 32 filter design guidelines.
  • ✗Designing filter for exact required attenuation — per Ott, add 6-10 dB margin for production variation, temperature drift, and measurement uncertainty. Pre-compliance setups have +/-6 dB typical uncertainty.

Frequently Asked Questions

40 dB/decade above cutoff frequency. A filter with f0 = 30 kHz provides: 26 dB at 150 kHz (5x f0), 40 dB at 300 kHz (10x f0), 54 dB at 1 MHz (33x f0). For higher attenuation, use Pi-filter (C-L-C) topology which achieves 60 dB/decade, or cascade multiple filter stages per CISPR filter design guide.
CISPR 32/22 conducted emissions start at 150 kHz (CISPR Band B). CISPR 11 Class A/B also starts at 150 kHz. FCC Part 15 conducted limits start at 150 kHz for Class B. Radiated limits: CISPR 32 starts at 30 MHz; FCC Part 15 starts at 30 MHz. Filter design must provide adequate attenuation at 150 kHz as the critical frequency.
Per IEC 60384-14: X capacitors connect across mains (L-N), suppressing differential-mode noise — rated for 250-400VAC, fail-safe open (won't cause shock). Y capacitors connect from L or N to earth/chassis, suppressing common-mode noise — rated for lower capacitance (<4.7 nF typical) to limit earth leakage current to <3.5mA per IEC 60950/62368.
Two constraints: (1) Inductance for required f0: L = 1/(4 x pi^2 x f0^2 x C); (2) Current rating: I_sat > 2x peak load current to prevent saturation. Per Wurth/Coilcraft, common-mode chokes (for CM filtering) have 1-10 mH inductance; differential-mode inductors have 10-500 uH. Inductor DCR should cause <2% voltage drop at full load.
Yes — standard practice per Ott. A complete EMI filter includes: (1) Common-mode choke (1-10 mH) for CM noise; (2) X capacitors (100 nF-1 uF) across L-N for DM noise; (3) Y capacitors (1-4.7 nF) to earth for CM noise. The CMC's leakage inductance (1-5% of CM inductance) provides additional DM filtering. Commercial filter modules integrate all elements.

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