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Differential Mode EMI Filter

Design a differential mode LC EMI filter. Calculate corner frequency, attenuation, and impedance for SMPS output noise suppression.

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Formula

f0=1/(2π√LDMCDM)f₀ = 1/(2π√L_DM C_DM)

How It Works

The Differential Mode Filter Calculator designs LC low-pass filters for SMPS output ripple and mains-input EMI filtering — essential for CISPR 32 conducted emissions compliance and clean power delivery to sensitive loads. Power electronics engineers use this to achieve 20-40 dB differential-mode attenuation at switching frequencies while maintaining stable power conversion.

Per Henry Ott's 'EMC Engineering,' differential-mode (DM) noise flows symmetrically between L and N conductors (or + and - power rails), distinguished from common-mode noise which flows in the same direction on both conductors. A second-order LC low-pass filter provides A = 40 x log10(f/f0) dB attenuation above cutoff f0 = 1/(2 x pi x sqrt(L x C)). Characteristic impedance Z0 = sqrt(L/C) should match source/load impedance for minimum reflection.

Per CISPR 32, conducted emissions are measured from 150 kHz to 30 MHz using a LISN presenting 50-ohm impedance. Typical SMPS produces 60-90 dBuV DM noise at switching frequency harmonics; Class B limits are 66-56 dBuV. Required attenuation is therefore 20-35 dB at 150 kHz, increasing at higher frequencies where limits are tighter.

For Pi-filter topology (C-L-C), attenuation is 60 dB/decade above cutoff — 20 dB better than single-stage LC. Per Ott, Pi-filters are preferred when >40 dB attenuation is required, but require careful damping to avoid resonant peaking. T-filters (L-C-L) provide same rolloff with better output impedance for voltage-source loads.

Worked Example

Problem: Design DM filter for 100 kHz SMPS showing 80 dBuV emissions at 150 kHz fundamental. CISPR 32 Class B limit is 66 dBuV. 50-ohm LISN reference.

Solution per Ott:

  1. Required attenuation at 150 kHz: 80 - 66 + 6 dB margin = 20 dB
  2. For second-order LC: A = 40 x log10(f/f0); 20 = 40 x log10(150/f0)
  3. Solve: f0 = 150/10^0.5 = 47 kHz
  4. Match to 50 ohm: L = 50/(2 x pi x 47000) = 169 uH; use 180 uH
  5. C = 1/(2 x pi x 47000 x 50) = 68 nF; use 68 nF X2 capacitor
  6. Verify: f0 = 1/(2 x pi x sqrt(180e-6 x 68e-9)) = 45.5 kHz; A at 150 kHz = 40 x log10(150/45.5) = 21 dB
  7. Inductor requirements: I_sat > 2 x load current (e.g., 3A load needs 6A sat); DCR < 100 mohm for <2% efficiency loss
Components: 180 uH inductor (Wurth 744771118), 68 nF X2 film capacitor. For additional margin, upgrade to Pi-filter (add second 68 nF capacitor at output).

Practical Tips

  • ✓Use Pi-filter (C-L-C) when >40 dB attenuation needed — per Ott, Pi-filter achieves 60 dB/decade rolloff versus 40 dB/decade for single LC stage. Critical for SMPS with high ripple or sensitive downstream loads.
  • ✓Add damping resistor if filter Q > 5 — per Ott, undamped LC filters can have resonant peaking of 10-20 dB at f0, worsening emissions at that frequency. Add R approximately Z0/3 in series with output capacitor to damp resonance.
  • ✓Measure with filter to verify no resonances — per Ott, filter resonances can create new emission peaks not present without filter. Scan full CISPR band (150 kHz - 30 MHz) after adding filter to verify no unintended consequences.

Common Mistakes

  • ✗Confusing DM and CM filtering — per Ott, DM filter (LC between L and N) only addresses noise that flows differentially. Common-mode noise (L and N in phase to earth) requires CMC plus Y-capacitors. Complete EMI filter addresses both; DM-only filter fails CM tests.
  • ✗Choosing large X capacitor without safety rating — per IEC 60384-14, X capacitors across mains must be safety-rated (X1, X2) and fail-safe open. Standard ceramic or film capacitors are not mains-safe and can create shock hazard if they short.
  • ✗Ignoring inductor saturation with DC bias — per Wurth, ferrite-core inductors lose 50-80% inductance at saturation current, shifting filter f0 upward and reducing attenuation by 10-20 dB. Select I_sat > 2x peak load current.

Frequently Asked Questions

Per Ott: DM filter (inductor + capacitor) attenuates noise flowing in opposite directions on L and N — the normal power current path. CMC attenuates noise flowing in the same direction on L and N — current that returns through earth/chassis. Complete EMI filter needs both: X-capacitors and inductors for DM; CMC and Y-capacitors for CM.
Per CISPR design practice: f0 typically 20-50 kHz to provide 20-30 dB attenuation at 150 kHz (CISPR lower limit). For SMPS with 100 kHz switching, f0 should be below 50 kHz. Lower f0 gives more attenuation but requires larger/heavier components. Balance attenuation requirements against size/cost constraints.
Yes — per Ott, CMC leakage inductance (1-5% of CM inductance) provides differential-mode filtering. A 10 mH CMC with 2% leakage has 200 uH DM inductance — often sufficient for basic DM filtering. Dedicated DM inductor adds cost but provides better control over DM attenuation independent of CM design.
Per Wurth/Coilcraft: (1) Ferrite drum core — compact, low cost, but saturates easily; (2) Powdered iron toroid — handles DC bias well, higher saturation current; (3) Sendust/MPP toroid — best saturation performance, lower core loss, higher cost. For SMPS output filters with DC bias, powdered iron or Sendust preferred. For AC mains filters, ferrite acceptable since no DC bias.
Per Ott: (1) Determine required attenuation A at lowest problem frequency f; (2) Calculate f0 = f/10^(A/40) for second-order LC; (3) Choose Z0 to match source/load impedance (50 ohm for LISN measurements); (4) L = Z0/(2 x pi x f0); C = 1/(2 x pi x f0 x Z0); (5) Verify inductor handles DC load current without saturation; (6) Verify capacitor is rated for operating voltage and meets safety requirements.

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