CommonModeChokes

Common mode choke is a passive electrical component that reduces common‑mode noise and electromagnetic interference (EMI) in electronics. It typically consists of two or more windings wound on a shared magnetic core. The core material is usually ferrite or powdered iron, chosen for high magnetic permeability and low core loss at the frequencies of interest. The windings are designed to carry signals with opposite polarity so that differential‑mode currents are largely unaffected, while common‑mode currents induce counter‑rotating magnetic fields that cancel each other and are therefore strongly attenuated.

The choke functions by presenting a high impedance to common‑mode currents. When such a current flows, the magnetizing flux of the core increases, inducing a voltage that opposes the current. Because the core material saturates at high fluxes, the impedance rises sharply above a certain current level, providing a natural protection against surges. On the other hand, differential‑mode signals, which flow in opposite directions through the windings, cancel each other’s magnetic fields and experience minimal voltage drop across the choke.

Common mode chokes are widely used in power supplies, USB and Ethernet interfaces, and battery chargers to meet regulatory EMI requirements. They are also employed in automotive electronics, medical equipment, and industrial control systems where noise immunity is critical. Advantages include small size, passive operation, and ability to block both conducted and radiated emissions. Disadvantages may include limited attenuation at very low frequencies, dependence on core material quality, and potential high in‑band loss if not carefully designed. Advanced chokes incorporate toroidal cores and interleaved windings to increase attenuation bandwidth while maintaining low insertion loss for differential signals.