What Is a Common Mode Inductor?

Ray Hawk

A common mode inductor, or common mode choke, is a device that is used to filter out electrical noise that can enter a circuit, known as common mode noise. Usually this is a result of electromagnetic interference (EMI) from nearby power sources and other electrical equipment. Another widespread source of interference comes from radio frequencies used by cellular and cordless phones, television broadcasts, radio transmissions, and more.

Man with hands on his hips
Man with hands on his hips

The attempt to attenuate or prevent electromagnetic interference must be regulated, as too much blockage of EMI will also cause the primary circuit itself to stop functioning. Where interference is coming from radio frequencies, a common mode inductor uses a differential choke that can block these much higher frequencies while having minimal effect on the standard frequencies that electrical circuits typically produce. A common mode choke coil is usually designed to have leakage inductance as well to filter noise at levels that don't significantly impede the circuit's functioning.

Ferrite, which is composed of iron and other compounds in a ceramic state, is one of the most frequent materials used in making a common mode choke coil. The ferrite compounds surround wiring or are used as a protective layer for circuit components. As the material is magnetized by the current flowing into the circuit, it creates a magnetic barrier against outside interference. This also provides protection, as the ferrite compound itself is a poor electrical conductor, and, therefore, serves as a form of insulator from outside signals.

Radio frequency interference is usually dampened by ferrite-based common mode inductor designs. Different ferrite compositions are made to handle various radio frequency bands. A common mode inductor may be best suited to suppressing ultra-high frequencies (UHF) in the 300 megahertz to 3 gigahertz televisions bands, very-high frequencies (VHF) in the 30 megahertz to 300 megahertz television and frequency modulation (FM) radio bands, or medium frequencies (MF) in the 300 kilohertz to 3 megahertz amplitude modulation (AM) radio bands.

Another routine use for a common mode inductor is as a common mode transformer. This resembles an ordinary transformer in a circuit design, with the function being that it is built to block alternating current (AC) flow to regions of the circuit and allow direct current (DC) flow through. A variation on this type of common mode inductor is a DC-to-DC converter. Some circuits will take input DC voltage that must then be converted to AC voltage and back to DC. In this case, the common mode current or stray capacitance is contained entirely within the confines of the common mode inductor device itself.

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