CommonModeSpulen

CommonModeSpulen, or common-mode chokes, are passive components used to suppress high-frequency interference in electrical and electronic systems. They typically consist of two windings on a common magnetic core and are installed in series with the lines whose noise must be reduced. The goal is to impede unwanted noise without affecting the desired signal or power.

In normal operation, live and neutral currents are equal and opposite, producing minimal net flux in the core and thus low impedance to differential-mode currents. Noise that travels in the same direction on both conductors, however, generates flux that adds in the core, increasing impedance and attenuating the common-mode noise. This makes CommonModeSpulen effective EMI filters in power and signal lines.

CommonModeSpulen are usually wound on ferrite cores, with two windings on a single core. They may be toroidal or box-shaped, and windings are often bifilar or paired to ensure the currents produce the intended flux pattern. Some designs use shielded housings to reduce radiated emissions.

Applications include input filters for switched-mode power supplies, mains EMI filters, and equipment in communications, audio, video, and automotive systems. They are chosen to meet regulatory EMI limits and to minimize insertion loss and inrush effects.

Key specifications include the impedance of the choke to common-mode signals at target frequencies, the current rating, DC resistance, and saturation current, as well as package size and environmental ratings.