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Differential Mode EMI Filter

Design a differential mode LC EMI filter: calculate corner frequency, attenuation, and characteristic impedance for SMPS output filtering.

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Formula

f₀ = 1/(2π√L_DM C_DM)

How It Works

A differential-mode (DM) EMI filter is an LC low-pass filter that attenuates differential-mode noise — noise that flows symmetrically in opposite directions on the two power conductors (line and neutral). In SMPS output filters, differential-mode noise arises from the switching ripple current. The filter uses a series inductor L and shunt capacitor C: f₀ = 1/(2π√LC), with attenuation A = −40·log₁₀(f/f₀) dB above f₀. The characteristic impedance Z₀ = √(L/C) should be matched to the source/load impedance for maximum energy transfer efficiency and minimal reflections. For CISPR conducted emissions, the filter must provide adequate attenuation at 150 kHz and above. The same LC structure is used for both SMPS output ripple filters and mains input EMI filters.

Worked Example

Problem
Design a differential-mode filter with corner frequency 50 kHz for a 50 Ω load. What attenuation does it provide at 150 kHz?
Solution
1. Assume L = 47 μH and C = 0.47 μF: 2. f₀ = 1/(2π√(47×10⁻⁶ × 0.47×10⁻⁶)) = 1/(2π × 4.70×10⁻⁶) = 1/(2.95×10⁻⁵) = 33.8 kHz 3. Attenuation at 150 kHz: freq ratio = 150/33.8 = 4.44; A = −40·log₁₀(4.44) = −40 × 0.647 = −25.9 dB 4. Characteristic impedance: Z₀ = √(47×10⁻⁶/0.47×10⁻⁶) = √100 = 10 Ω Result: The filter provides −26 dB at 150 kHz with a characteristic impedance of 10 Ω. For a 50 Ω system, adding damping may be needed to prevent resonant peaking at 33.8 kHz.

Practical Tips

  • ✓Use a Π-filter topology (C-L-C) for differential-mode filtering when greater than 40 dB attenuation is required at the lowest problem frequency.
  • ✓Add a small damping resistor (R ≈ Z₀/5) in series with a second capacitor across the main filter capacitor to prevent resonant peaking.
  • ✓Measure conducted emissions after installing the filter to verify it doesn't create resonances that worsen emissions at specific frequencies.

Common Mistakes

  • ✗Confusing differential-mode and common-mode filters — a DM filter only addresses noise between L and N; a CMC is needed for L/N to PE common-mode noise.
  • ✗Choosing C too large without safety certification — X2 capacitors for across-the-mains must be IEC/UL safety-rated; standard ceramic capacitors are not mains-safe.
  • ✗Ignoring the saturation current of the series inductor — in SMPS applications the inductor must handle the full load current plus peak ripple current without saturating.

Frequently Asked Questions

A DM filter (inductor + capacitor) attenuates noise that appears between the two power conductors with equal and opposite currents. A common-mode choke presents high impedance to noise flowing in the same direction on both conductors simultaneously. Complete conducted emissions suppression typically requires both.
For CISPR conducted emissions starting at 150 kHz, the filter corner frequency is typically 20–50 kHz, providing 20–30 dB attenuation at 150 kHz. The exact value depends on the level of differential-mode noise from the SMPS.
Yes — this is done in practice. A common-mode choke presents high impedance to common-mode currents while passing differential-mode currents with only a small leakage inductance. The leakage inductance acts as a differential-mode inductor, providing partial DM filtering. In many designs the CMC leakage inductance is deliberately designed to provide a specific DM inductance.

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