EMI Filter Designer

Size a starter EMI filter (Common-Mode + Differential-Mode) from your noise band, impedance, and target attenuation. Includes cutoff estimates, damping guidance, and Show Work.

How to Use

  1. Pick your filter type: DC input filter or AC mains-style front end.
  2. Enter your source/load impedance estimates (or leave defaults).
  3. Set a noise band (start & stop) and a target attenuation (dB).
  4. Choose a topology (π, T, or 2-stage) and enable CM choke if needed.
  5. Review suggested parts + cutoff and open “Show Work”.
Filter Lab View
Quick readout: CM/DM cutoff, estimated attenuation, and stability hints.
DM fc
—
CM fc
—
Atten @ f
—
Damping
—
Stability: —
Source Zs EMI Filter L C DM CM Choke CM Load Zl Diagram is conceptual. Use datasheets, safety-rated capacitors (X/Y), and compliance test gear for final design.
Inputs & Settings
Estimate values for a starter filter. Final design requires measurement, layout control, and component ratings.
AC mode shows X/Y capacitor guidance labels (still heuristic).
Choose CM+DM when common-mode emissions dominate.
EMI tests often start around 150 kHz (conducted).
Radiated often goes higher; this tool stays “starter values”.
Heuristic: 20–40 dB is common as a first pass.
DC supplies can be very low; unknown? try 0.1–1Ω.
Approximate at noise band (not DC). Use a guess if needed.
Used to propose C values for π / 2-stage.
Optional override. Leave blank for “solve L from fc + C”.
Show Work (step-by-step)
—
Work is shown in base units (Hz, Ω, F, H). This tool provides starter values only (measurement + compliance testing required).

Reference

  • Differential-mode (DM): Noise between line and return. Often reduced by DM L + X-cap (across the lines).
  • Common-mode (CM): Noise in the same direction on both conductors relative to chassis/earth. Often reduced by CM choke + Y-caps (to chassis/earth).
  • Cutoff (LC): For a simple 2nd order section, f_c ≈ 1 / (2π√(LC)) (heuristic).
  • Damping: Real filters can ring; damping networks (R in series with C, or ESR) can reduce peaking.
Safety note: For AC mains, use properly rated X/Y capacitors and follow applicable safety standards. This tool does not replace engineering review.

FAQ

Is this an “exact” EMI compliance design?

No. It’s a deterministic starter-value designer. Final EMI compliance depends heavily on layout, parasitics, cabling, grounding, and real impedance vs frequency.

When should I pick CM+DM?

If emissions appear similarly on both conductors (relative to chassis/earth) or your cable acts like an antenna, CM control is usually required.

What’s the biggest “gotcha” with input filters?

Damping and impedance interaction. A low-ESR input cap + inductor can create a high-Q resonance that destabilizes a converter. Add damping and validate with measurement.

Tool Info

Last updated:

Updates may include improved heuristics, more topologies, and export formats.