Common mode inductor (Common mode Choke), also called common mode choke coil, magnetic flux flowing through the principle of superposition of the common mode current, so as to have a considerable inductance, inhibition play a common mode current , when the two differential mode current flowing through the coil, the magnetic flux cancel each other, almost no inductance, the differential mode current can pass through unattenuated; Therefore common mode choke coil can be effectively suppressed in the balance line common mode interference signals, no effect on normal differential mode signal transmission line.
In the electronic card design, since the common mode inductance EMI filtering effect for suppressing the electromagnetic radiation generated by high speed signal lines outwardly, then specifically how to design a circuit, please refer to Example HDMI follows:
Why can prevent EMI to clarify this point, we need to start its analysis of the structure;? Common mode filter circuit inductance, La, and Lb is the common mode inductance coil, two coils wound on the same core, and the number of turns have the same phase (reverse wound), normal current flowing through the circuit when the common mode choke coil, reverse current generated in the inductor magnetic field winding of the same phase cancel each other, when the main current is the normal signal when a sense current flowing through the common-mode coil, since an isotropic common mode current will produce a magnetic field in the same direction within the coil increases coil; affected the coil resistance (and a small amount of damping caused by the leakage inductance) anti, showed a high impedance coil, produce a strong damping effect, in order to attenuate the common mode current filtering purposes.
Common mode inductance to be noted that when the selection of the desired frequency band filter, the common mode impedance the better, so we need to look at the device selection information is common mode choke coil, primary impedance frequency according to the selected curve. Also note that consider differential mode impedance effect on the signal, the main focus differential mode impedance, pay special attention to the choice of high-speed ports. It is noted that, when the rated current is large, the common mode choke coil diameter should be increased accordingly in order to withstand large currents. In addition, due to increase inductance, low frequency attenuation characteristics can be improved.
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