RFrftools.io
EMC

Conducted Emissions LC Filter

Design an LC filter to meet CISPR 22/FCC conducted emissions limits by calculating required L and C values for a target attenuation.

Loading calculator...

Formula

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

How It Works

Conducted emissions are high-frequency noise currents that flow back through the power mains cable, potentially interfering with other equipment on the same supply. CISPR 22 Class B limits conducted emissions between 150 kHz and 30 MHz using a LISN (line impedance stabilisation network) measurement. The limit is typically 66 dBμV at 150 kHz, falling to 56 dBμV at 500 kHz, then flat at 60 dBμV to 30 MHz. An LC low-pass filter on the mains input provides the required attenuation. For a second-order filter, the attenuation is A = 40·log₁₀(f/f₀) dB, so the required corner frequency is f₀ = f/10^(A/40). The required component values are C = √(1/((2πf₀)² × Z_L)) and L = Z_L × C, where Z_L is the impedance reference (50 Ω for LISN measurements).

Worked Example

Problem
A SMPS shows a conducted emission of 80 dBμV at 150 kHz. CISPR 22 Class B limit is 66 dBμV. Design an LC filter for 14 dB attenuation at 150 kHz with a 50 Ω reference.
Solution
1. Required attenuation: A = 80 − 66 = 14 dB 2. Add margin: use 20 dB total attenuation 3. Required f₀: f₀ = 150,000 / 10^(20/40) = 150,000 / 10^0.5 = 150,000 / 3.162 = 47.4 kHz 4. LC product: LC = 1/(2π × 47,400)² = 1/(8.86×10¹⁰) = 1.13×10⁻¹¹ 5. C = √(1.13×10⁻¹¹/50) = √(2.26×10⁻¹³) = 475 nF ≈ 470 nF 6. L = 50 × 475×10⁻⁹ = 23.75 μH ≈ 22 μH (standard value) Result: Use a 22 μH inductor and 470 nF capacitor for the conducted emissions filter. Verify the inductor's current rating and the capacitor's mains voltage rating.

Practical Tips

  • ✓Use a standard off-the-shelf EMI filter module — they integrate common-mode choke, X2 capacitors, and Y1/Y2 capacitors in a tested, safety-certified package.
  • ✓Place the mains EMI filter at the point of entry into the product (power inlet connector) before any other circuitry to prevent noise from entering the product's internal wiring.
  • ✓Measure conducted emissions with and without the filter at multiple frequencies — some filters create resonances that worsen emissions at certain frequencies.

Common Mistakes

  • ✗Using electrolytic capacitors for conducted emissions filters — they have insufficient high-frequency performance above 100 kHz; use X2 (across-the-mains) film capacitors rated for AC mains.
  • ✗Sizing the filter for CISPR limit exactly — always add 6–10 dB margin to account for production variation, temperature, and measurement uncertainty.
  • ✗Ignoring the differential-mode component — conducted emissions have both common-mode and differential-mode components; the LC filter targets differential mode while a common-mode choke targets common mode.

Frequently Asked Questions

X capacitors are placed across the mains (line to line) and suppress differential-mode noise; they are rated for high AC voltage and must be safe-fail (they cannot short-circuit to create a shock hazard). Y capacitors connect from line or neutral to earth/chassis; they suppress common-mode noise and must be low-leakage for safety standards.
The applicable standard depends on the product category and market. Common standards: CISPR 22/EN 55022 (now CISPR 32/EN 55032) for IT equipment, CISPR 11/EN 55011 for industrial/scientific equipment, CISPR 25 for vehicle components, and FCC Part 15 Class B for consumer products in the USA.
A common-mode choke alone suppresses only common-mode noise. For full compliance you typically need both a common-mode choke and X/Y capacitors to address both common-mode and differential-mode noise. A complete EMI filter module addresses both.

Shop Components

Affiliate links — we may earn a commission at no cost to you.

EMC Shielding Tape

Copper foil tape for EMI shielding and grounding

DigiKey →Mouser →

Ferrite Beads

SMD ferrite beads for suppressing high-frequency noise

DigiKey →Mouser →

Related Calculators

EMC

LC EMI Filter

Design an LC low-pass EMI filter for conducted emissions suppression — calculate inductance, capacitance, filter order, and attenuation at the stop band.

EMC

Ripple Filter

Calculate LC filter attenuation and output ripple voltage for power supply EMC filtering. Find the resonant frequency and ripple rejection.

EMC

CMC Impedance

Calculate common mode choke impedance, insertion loss, and Q factor at a given frequency for EMC filter design.

EMC

RF Shielding

Calculate electromagnetic shielding effectiveness of conductive enclosures

EMC

Ferrite Bead

Calculate ferrite bead filter effectiveness, impedance at frequency, and insertion loss for EMI suppression

EMC

ESD TVS Diode

Calculate TVS diode clamping voltage, breakdown voltage, peak pulse current, and power rating for ESD protection circuit design.