How to select common mode inductors？

The common mode inductor is a coil with the same name and the same winding direction and the same number of turns on a closed magnetic ring.

1. Classification

a. Use on power line: high circuit voltage, high current, and wide filtering frequency band;

b. General signal line use: f<10MHz, such as RS485, CAN, MVB, etc.;

c. High-speed signal cable: f>10MHz, Ethernet, USB2.0, DDR differential clock/data, HDMI, USB3.0, DVI, etc.

2. Technical parameters

Common mode impedance, differential mode impedance (to be considered on the signal line), inductance value, DC resistance, rated current and voltage, withstand voltage (voltage between line group and line group), insulation resistance, temperature range.

3. Selection points

a. Determine the parameters of CMC according to the electrical characteristics of the application circuit. Satisfy the inductor rated voltage and rated current are greater than the operating voltage and current.

b. Determination of common mode impedance or inductance value:

Power supply circuit: There is no need to consider differential mode impedance. When used to solve the problem of low-frequency conduction, generally choose a device with a larger filter impedance from 0.01MHz to tens of MHz, with an inductance of mH and a common mode impedance range of hundreds of Ω to hundreds of KΩ; used to solve the problem of radiation interference, Generally choose a device with a large filter impedance of 10MHz~1GHz, and its common mode impedance should be as large as possible under normal conditions;

Signal circuit: In the signal working frequency range, the differential mode impedance should be as small as possible to not affect the normal signal transmission; it is generally recommended that the differential mode impedance be less than 50 Ω at the signal frequency point.

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