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Conducted Emissions LC Filter

Design an LC filter for CISPR 22/FCC conducted emissions compliance. Calculate required inductor and capacitor values for target attenuation.

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Formula

f0=f/10(A/40),L=Z√(1/(2πf0)2/Z)f₀ = f / 10^(A/40), L = Z√(1/(2πf₀)²/Z)

How It Works

The Conducted Emissions Filter Calculator designs LC filters for CISPR 32/22 compliance — essential for power supplies, motor drives, and any product connected to AC mains or DC power buses. EMC engineers use this to suppress switching noise 20-40 dB below regulatory limits (66-56 dBuV from 150 kHz to 30 MHz).

Per Henry Ott's 'EMC Engineering' and CISPR 32, conducted emissions are measured with a LISN (Line Impedance Stabilization Network) presenting 50-ohm impedance from 150 kHz to 30 MHz. Emissions appear as both common-mode (CM: L and N in phase relative to earth) and differential-mode (DM: L opposite to N). Typical SMPS produces 70-90 dBuV emissions; Class B limits are 66 dBuV (150-500 kHz), 56 dBuV (0.5-5 MHz), 60 dBuV (5-30 MHz).

A second-order LC filter provides A = 40 x log10(f/f0) dB attenuation above cutoff f0 = 1/(2 x pi x sqrt(L x C)). For 20 dB at 150 kHz, f0 = 150/10^0.5 = 47 kHz. Per Ott, design margin should be 6-10 dB to account for measurement uncertainty (+/-6 dB typical for pre-compliance) and production variation.

X capacitors (across L-N) filter differential-mode noise; Y capacitors (L/N to earth) filter common-mode noise. Per IEC 60950/62368, Y-capacitor total leakage current must be <3.5 mA for Class I equipment — limiting Y capacitance to approximately 4.7 nF at 50 Hz mains. This leakage constraint makes common-mode filtering harder than differential-mode.

Worked Example

Problem: Pre-compliance scan shows SMPS with 82 dBuV DM emission at 200 kHz. Design filter for CISPR 32 Class B (limit 66 dBuV at 200 kHz). Assume 50-ohm LISN.

Solution per Ott:

  1. Required attenuation: 82 - 66 = 16 dB, plus 6 dB margin = 22 dB at 200 kHz
  2. Second-order filter: A = 40 x log10(f/f0); 22 = 40 x log10(200/f0)
  3. Solve: f0 = 200/10^0.55 = 56 kHz
  4. Component values for 50-ohm match: L = 50/(2 x pi x 56000) = 142 uH; use 150 uH
  5. C = 1/(2 x pi x 56000 x 50) = 57 nF; use 68 nF X2 capacitor
  6. Verify: f0 = 1/(2 x pi x sqrt(150e-6 x 68e-9)) = 50 kHz; A at 200 kHz = 40 x log10(200/50) = 24 dB
  7. Inductor: 150 uH, I_sat > 3A (for 1A load with 2x margin), DCR < 0.2 ohm
  8. Capacitor: 68 nF X2 safety-rated, 275VAC for 230V mains
BOM: DM inductor (Wurth 744771115, 150 uH/4A), X2 capacitor (EPCOS B32922, 68 nF/275VAC). Cost: approximately $1.50.

Practical Tips

  • ✓Use off-the-shelf EMI filter modules — they include safety-certified X/Y capacitors, CMC, and meet UL/IEC leakage limits. Custom designs require safety certification (6-12 month delay). Module cost: $5-20.
  • ✓Place filter at power entry before any internal wiring — noise on internal wires can couple back past the filter. Per Ott, filter should be within 20mm of IEC inlet or power connector.
  • ✓Measure emissions with filter installed to detect resonances — LC filter Q can create peaking at f0, worsening emissions at specific frequencies. Add damping resistor (R approximately sqrt(L/C) / 3) if Q > 5.

Common Mistakes

  • ✗Using electrolytic capacitors — ESR (0.1-1 ohm) and ESL (5-20 nH) limit HF performance. Per Ott, use X2 film capacitors (ESR <10 mohm) for mains filters or MLCC for DC applications. Electrolytics are only useful for bulk storage below 10 kHz.
  • ✗Designing to exact CISPR limit without margin — measurement uncertainty is +/-6 dB for pre-compliance, +/-3 dB for accredited lab. Per CISPR 16-4-2, add 6 dB minimum margin to ensure production units pass. Temperature and aging add another 2-3 dB variation.
  • ✗Ignoring DM vs CM separation — conducted emissions have both components; an LC filter addresses DM only. Per Ott, measure CM and DM separately using current probes or CM/DM separator. A CMC is needed for CM noise even with perfect DM filter.

Frequently Asked Questions

Per IEC 60384-14: X capacitors connect L to N (differential-mode filtering), rated 275-400VAC, fail-safe open. X2 class is most common for consumer products. Y capacitors connect L or N to earth/chassis (common-mode filtering), limited to <4.7 nF total to maintain leakage current <3.5 mA per IEC 60950/62368. Y1 class for double insulation, Y2 for basic insulation.
Depends on product category and market. CISPR 32/EN 55032: IT/multimedia equipment. CISPR 11/EN 55011: industrial/scientific/medical. CISPR 25: automotive components. FCC Part 15 Class B: consumer products in USA. Medical devices: IEC 60601-1-2 (references CISPR 11). All use similar limits (66-56 dBuV) and LISN measurement method.
No — CMC only addresses common-mode noise. Per Ott, typical SMPS produces both CM and DM noise; DM often dominates at lower frequencies (150-500 kHz), CM at higher frequencies (1-30 MHz). Complete filter requires CMC for CM, plus X-capacitors for DM. Y-capacitors supplement CM filtering at high frequencies where CMC inductance is bypassed.
Per CISPR 16-1-2: (1) Use CM/DM separator network between LISN and spectrum analyzer; (2) Use current probes — CM appears as sum of L and N currents, DM as difference; (3) If adding X capacitor reduces emissions, DM dominates; if adding CMC reduces emissions, CM dominates. Most products have both — test each filtering element separately.
Per Wurth/Coilcraft guidelines: I_sat > 2x RMS load current for continuous operation. For pulsed loads (motor drives, SMPS startup), I_sat > peak current. Saturation causes 50-80% inductance drop, shifting filter f0 upward and reducing attenuation by 10-20 dB. At I_sat, inductance drops to approximately 70% of nominal — select 1.5x margin on I_sat rating.

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