A common-mode choke is two windings on a single magnetic core, wound so that differential currents (signal or power flowing out one wire and back the other) see almost zero impedance, while common-mode currents (garbage flowing the same direction on both wires, returning through ground or parasitic capacitance) see a huge impedance. It's the single most effective tool for suppressing conducted EMI on power cords, USB cables, CAN bus, Ethernet, and motor leads.

How the magic works: The two windings are phased so their fluxes cancel for differential currents. The core sees no net flux, doesn't saturate, and presents only leakage inductance (typically 0.5–2% of the common-mode value). But common-mode currents flow the same direction in both windings — their fluxes add, and the core presents the full inductance, often several millihenries.

Real-world example — offline AC input filter: A 100 W flyback supply generates switching noise that couples through the Y-caps to earth ground, then flows back on both L and N as common-mode current. A 10 mH common-mode choke (say a Würth 744824210) in series with the AC input presents ~6 kΩ at 100 kHz to that noise, but only ~60 Ω of leakage inductance to the 60 Hz mains current — invisible to normal operation. Combined with the Y-caps forming an LC filter to earth, you get 40+ dB of common-mode attenuation.

Design rules of thumb:

• Impedance target: aim for Z_CM ≥ 1 kΩ at your worst-offending noise frequency. Z = 2πfL, so 1 kΩ at 150 kHz needs about 1 mH.
• Current rating: the differential current heats the copper. Derate to 70% of the datasheet rating for continuous use.
• Saturation: because differential flux cancels, DC bias doesn't saturate the core — but any current imbalance (e.g. leakage to earth) does. Nanocrystalline and ferrite cores tolerate a few mA of imbalance; check the datasheet.
• Leakage inductance is a feature, not a bug — it works with X-caps to filter differential-mode noise too. Bifilar winding minimizes it; sectional winding maximizes it.

Placement matters: put the choke as close to the noise source (or connector) as possible. On DC power lines feeding switching converters, put it on the input side, before the bulk capacitor. On signal lines like CAN or USB 2.0, use a smaller "data line" choke (~90 Ω @ 100 MHz) right at the connector, with TVS diodes on the board side.