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EMC

Common Mode Choke Impedance

Calculate common mode choke impedance, insertion loss, and Q factor at a given frequency for EMC filter design.

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Formula

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

L— Inductance (H)
f— Frequency (Hz)

How It Works

A common mode choke (CMC) is a two-winding inductor wound on a single toroidal or drum core such that differential-mode currents cancel magnetically while common-mode currents see the full inductance. At frequency f, the common-mode impedance is Z = 2πfL, where L is the inductance (typically 100 μH to 10 mH for power-line filters). The insertion loss in a 50 Ω system is IL = 20·log₁₀(Z/(Z+50)). The Q factor Q = Z/DCR shows how lossy the choke is; high Q means the choke is more reactive, while low Q (lossy ferrite) provides broadband suppression. CMCs are the primary defence against conducted common-mode noise on power lines, data cables, and USB interfaces, targeting CISPR 22/25 limits above 150 kHz.

Worked Example

Problem
A common-mode choke has L = 1 mH, DCR = 0.5 Ω. What is the impedance, insertion loss, and Q at 150 kHz?
Solution
1. Convert inductance: L = 1 mH = 1×10⁻³ H 2. Impedance: Z = 2π × 150,000 × 1×10⁻³ = 942 Ω 3. Insertion loss (50 Ω system): IL = 20·log₁₀(942/(942+50)) = 20·log₁₀(0.950) = −0.45 dB 4. Q factor: Q = 942 / 0.5 = 1884 Result: The choke provides 942 Ω at the CISPR lower limit of 150 kHz, representing good broadband suppression. The high Q indicates a low-loss, primarily reactive component at this frequency.

Practical Tips

  • ✓Choose a CMC whose rated DC current exceeds the peak load current by at least 25% to prevent saturation.
  • ✓For USB or signal lines, use a CMC with a cut-off differential impedance below 5 Ω to avoid distorting the signal waveform.
  • ✓Place the CMC as close to the cable connector as possible to intercept common-mode currents before they enter the PCB ground plane.

Common Mistakes

  • ✗Ignoring the DCR voltage drop under load current — a 1 Ω DCR at 5 A load causes a 5 V drop, unacceptable in 5 V systems.
  • ✗Using the 100 MHz datasheet impedance to extrapolate to 150 kHz — ferrite permeability falls steeply at high frequency; use the impedance-vs-frequency curve.
  • ✗Saturating the core with DC load current — check the rated DC current and verify the inductance remains specified at that current.

Frequently Asked Questions

A common-mode choke (CMC) has two coupled windings and specifically targets common-mode noise while passing differential-mode signals. A ferrite bead is a single-winding lossy inductor that attenuates both common and differential noise. CMCs are used on differential signal pairs (USB, Ethernet, power lines); ferrite beads are used on single power or signal lines.
At high frequencies, ferrite core permeability drops and inter-winding capacitance creates a bypass path. The impedance peaks at a self-resonant frequency and then falls. Always check the full impedance-vs-frequency curve from the datasheet.
Yes, but the CMC must have very low differential-mode impedance (< 3 Ω at 5 Gbps frequencies) and sufficient common-mode impedance at EMC problem frequencies. Look for CMCs specifically characterised for USB 3.0 or use a combination of a CMC plus individual ferrite beads.

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