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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((Z+50)/50)Z = 2π × f × L, IL = 20·log₁₀((Z+50)/50)
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).

Insertion loss IL = 20 x log10(Z_CM / (Z_CM + Z_load)). For Z_CM >> Z_load: IL approaches 20 x log10(Z_CM/Z_load). A 1000-ohm CMC in 50-ohm system provides IL = 20 x log10(1000/50) = 26 dB. CISPR 32 Class B typically requires 15-25 dB CM attenuation at 150 kHz — achievable with 0.5-2 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 CMC for USB 2.0 port showing 75 dBuV CM noise at 150 kHz against CISPR 32 limit of 66 dBuV. Load impedance 90 ohm (USB differential).

Solution per Ott:

  1. Required attenuation: 75 - 66 + 6 dB margin = 15 dB at 150 kHz
  2. IL = 20 x log10(Z_CM/Z_load) for Z_CM >> Z_load; 15 = 20 x log10(Z_CM/90); Z_CM = 90 x 10^0.75 = 506 ohm
  3. Required inductance: L = Z_CM/(2 x pi x f) = 506/(2 x pi x 150000) = 537 uH; use 680 uH standard value
  4. Verify IL: Z_CM at 150 kHz = 2 x pi x 150000 x 680e-6 = 641 ohm; IL = 20 x log10(641/90) = 17 dB (meets 15 dB requirement)
  5. Check differential attenuation: Leakage inductance approximately 2% = 13.6 uH; Z_diff = 2 x pi x 480e6 x 13.6e-6 = 41 ohm at 480 MHz (USB 2.0)
  6. Differential IL: 20 x log10((90+41)/90) = 1.7 dB — acceptable for USB 2.0 eye margin
Select: Wurth 744272102 (1 mH, 1A, 0.3 ohm DCR, USB-rated) provides 20 dB at 150 kHz with <2 dB differential loss.

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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