Common Mode Choke Calculator

Turns	AL (nH/N²)	Freq (kHz)	Inductance (mH)	Impedance (Ω)
8	1800	100	0.1152	72.38
12	2500	150	0.36	336.15
16	3200	250	0.8192	1286.8

Formula Used

Inductance: L = AL × N²

Inductive reactance: XL = 2πfL

Capacitive reactance: XC = 1 / (2πfC)

Total impedance estimate: Z = √(R² + (XL - XC)²)

Self resonant frequency: fsr = 1 / (2π√(LC))

Winding resistance: R = ρl / A

Magnetic field strength: H = NI / le

Flux density estimate: B = μ0μrH

Stored energy: E = ½LI²

This tool uses practical first-pass equations for common-mode choke sizing. Real parts also vary with frequency, leakage, winding symmetry, temperature, and manufacturer core data.

How to Use This Calculator

Enter the target noise frequency where suppression matters most.
Set turns and the core AL value from your intended magnetic core.
Provide wire size and mean turn length for DC resistance estimation.
Enter parasitic capacitance to approximate high-frequency behavior.
Fill in Ae, le, and μr for the magnetic field estimate.
Input noise voltage and operating current for attenuation and energy results.
Click the calculate button to place results above the form.
Review impedance, resonance, and insertion loss before shortlisting parts.
Use the graph and CSV or PDF exports for documentation.

FAQs

1. What does a common-mode choke do?

It blocks unwanted common-mode noise while allowing intended differential current to pass. The choke increases impedance to noise currents that flow in the same direction on both conductors.

2. Why is AL value important?

AL links the core and winding geometry to inductance. A higher AL value gives more inductance for the same number of turns, increasing low-frequency common-mode impedance.

3. Why include parasitic capacitance?

Parasitic capacitance reduces high-frequency performance and creates a self-resonant point. Above resonance, impedance can stop rising and may drop, so it matters in EMI work.

4. Is the impedance result exact?

No. It is a practical estimate. Real choke impedance depends on material losses, leakage inductance, winding balance, test setup, temperature, and manufacturer frequency curves.

5. Why calculate winding resistance?

Resistance affects copper loss, heating, and overall impedance. Lower resistance is generally better for efficiency, especially when line current is continuous.

6. What frequency should I enter?

Use the dominant EMI frequency or the compliance trouble spot you are targeting. You can also test several frequencies to compare candidate designs.

7. Can this calculator predict saturation?

It gives a simple flux density estimate, not a full saturation model. Always compare against core material limits and bias curves from the datasheet.

8. When should I trust manufacturer data over this tool?

Always use manufacturer curves for final design decisions. This calculator is best for pre-selection, tradeoff studies, and quick engineering checks.