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Common Mode Choke Impedance

Calculate common mode choke impedance, insertion loss, and Q factor at any frequency. Design EMC filters for CISPR 25 conducted emissions compliance.

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Formula

Z=2π×f×L,IL=20⋅log10(∣100Ω+RDC+jZ∣/100Ω)(50Ωsource,50Ωload,CISPR17)Z = 2π × f × L, IL = 20·log₁₀(|100 Ω + R_DC + jZ| / 100 Ω) (50 Ω source, 50 Ω load, CISPR 17)

Reference: CISPR 17; Paul, Introduction to Electromagnetic Compatibility, 2nd ed.

L— Inductance (H)
f— Frequency (Hz)

How It Works

The Common Mode Choke Calculator computes impedance and insertion loss for differential signal and power line filtering — essential for CISPR 32 conducted emissions compliance, USB/Ethernet EMC, and mains filter design. EMC engineers use this to achieve 20-40 dB common-mode attenuation while passing differential signals with <1 dB loss.

Per Henry Ott's 'EMC Engineering,' a CMC is a dual-winding inductor where differential currents (equal magnitude, opposite direction) produce canceling magnetic flux, presenting near-zero impedance to the wanted signal. Common-mode currents (same direction on both conductors) see the full inductance L, presenting impedance Z_CM = 2 x pi x f x L. A 1 mH CMC provides 942 ohm impedance at 150 kHz (CISPR lower limit).

This calculator takes insertion loss in the CISPR 17 test system, a 50-ohm source and a 50-ohm load: the choke in series adds its impedance Z = DCR + j x 2 x pi x f x L to the 100-ohm loop, so IL = 20 x log10(|100 + DCR + jX| / 100). The reactance adds to the loop resistance as a phasor, not arithmetically: X = 100 ohm gives 3.01 dB, and the 942-ohm reactance of 1 mH at 150 kHz gives 19.5 dB. In a product the source and load are a LISN and the cable's common-mode impedance, so the attenuation in circuit differs from this figure. CISPR 32 Class B typically requires 15-25 dB CM attenuation at 150 kHz — achievable in this system with a 0.7-2 mH CMC.

Q factor Q = Z_CM/DCR indicates loss versus reactivity. High-Q CMCs (Q > 50) are reactive and can resonate with cable capacitance; low-Q CMCs (Q < 10, using lossy ferrite) provide broadband suppression without resonance issues. Per Wurth application notes, power-line CMCs use lossy ferrite; signal-line CMCs use high-permeability, low-loss ferrite for minimal differential-mode attenuation.

Worked Example

Problem: Select a CMC for a USB 2.0 port showing 75 dBuV CM noise at 150 kHz against a CISPR 32 limit of 66 dBuV. Insertion loss is taken in the CISPR 17 test system: 50-ohm source, 50-ohm load.

Solution per Ott:

  1. Required attenuation: 75 - 66 + 6 dB margin = 15 dB at 150 kHz
  2. IL = 20 x log10(|100 + jX|/100) with DCR neglected; 15 dB needs |100 + jX| = 100 x 10^0.75 = 562 ohm, so X = sqrt(562^2 - 100^2) = 553 ohm
  3. Required inductance: L = X/(2 x pi x f) = 553/(2 x pi x 150000) = 587 uH; use 680 uH standard value
  4. Verify IL: X at 150 kHz = 2 x pi x 150000 x 680e-6 = 641 ohm; IL = 20 x log10(|100 + j641|/100) = 16.2 dB (meets 15 dB requirement)
  5. Differential signals see only the leakage inductance, which this calculator does not model; check it against the datasheet's differential-mode impedance curve at 240 MHz, the USB 2.0 high-speed fundamental

Select: Wurth 744272102 (1 mH, 1A, 0.3 ohm DCR, USB-rated) provides 19.5 dB at 150 kHz in this system.

Practical Tips

  • ✓Place CMC within 10mm of connector — CM currents enter at cable attachment point; placing CMC far from connector allows noise to radiate from internal wiring before filtering per Johnson/Graham.
  • ✓For USB 3.0 SuperSpeed (5 Gbps): select CMC with differential impedance <3 ohm at 2.5 GHz to prevent eye closure — standard power-line CMCs have excessive differential loss per USB-IF design guide.
  • ✓Add parallel Y-capacitors (1-4.7 nF) to ground on both sides of CMC — capacitors provide low-impedance CM path at high frequencies where CMC inductance is bypassed by parasitic capacitance.

Common Mistakes

  • ✗Using 100 MHz datasheet impedance to extrapolate to 150 kHz — ferrite permeability varies 10x across frequency range. Per Wurth, impedance vs frequency curve is essential; a CMC with 2000 ohm at 100 MHz may have only 200 ohm at 150 kHz.
  • ✗Ignoring DCR voltage drop — a 1 ohm DCR CMC at 5A load drops 5V, unacceptable for 5V USB power. Per TDK guidelines, select DCR < 2% of supply voltage divided by load current.
  • ✗Saturating core with DC bias — CMC inductance drops 30-50% at rated DC current. For 2A load, select CMC rated >3A to maintain specified inductance per Murata saturation curves.

Frequently Asked Questions

CMC has two coupled windings — differential currents cancel (low impedance to signal), CM currents add (high impedance to noise). Ferrite bead is single-winding — attenuates both differential and common-mode equally. Use CMC on differential pairs (USB, Ethernet, power lines); use ferrite beads on single-ended signals and supply rails per Ott's EMC Engineering.
Two mechanisms per TDK: (1) Ferrite permeability drops above material's characteristic frequency (typically 10-100 MHz), reducing inductance; (2) Inter-winding capacitance (1-10 pF) creates a bypass path that shorts CM currents around the inductance. CMC impedance peaks at self-resonant frequency then falls. Always verify impedance at highest problem frequency.
Yes, but select carefully per USB-IF: (1) Differential impedance <3 ohm at 5 Gbps frequencies (2.5 GHz fundamental); (2) CM impedance >100 ohm at 30-200 MHz (EMC problem band); (3) Low capacitance (<0.5 pF) to prevent impedance mismatch. Dedicated USB 3.0 CMCs (e.g., TDK ACM series) are designed for this application.
Depends on noise level and load impedance. Rule of thumb: 0.5-2 mH for mains EMI filters (provides 500-2000 ohm at 150 kHz); 100-500 uH for DC power lines; 10-100 uH for signal lines. Calculate: L = (required Z_CM) / (2 x pi x 150000). Add 50% margin for production and temperature variation.
The winding sense must be correct — both windings must be wound in the same direction so differential currents cancel. Incorrect winding polarity makes the CMC act as a differential-mode inductor, attenuating the wanted signal instead of CM noise. Per Wurth datasheets, follow the dot convention showing correct polarity. Swapping L and N connections on a correctly wound CMC does not affect performance.

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