What is common mode inductance?

Common mode choke, also called common mode choke, is often used in computer switching power supplies to filter common mode electromagnetic interference signals. In the board design, the common mode inductance also plays the role of EMI filtering, which is used to suppress the electromagnetic wave generated by the high-speed signal line from radiating to the outside. Why can common mode inductors prevent EMI? To figure this out, we need to start with the analysis of the structure of the common-mode inductor. In the filter circuit of the common mode inductance, La and Lb are the common mode inductance coils. The two coils are wound on the same iron core with the same number of turns and phase (winding in opposite directions). In this way, when the normal current in the circuit flows through the common-mode inductance, the currents generate opposite magnetic fields in the inductance coils wound in the same phase and cancel each other. At this time, the normal signal current is mainly affected by the coil resistance (and a small amount of leakage due to leakage). When a common-mode current flows through the coil, due to the isotropic nature of the common-mode current, a magnetic field in the same direction will be generated in the coil, which will increase the inductive reactance of the coil, causing the coil to exhibit high impedance. Strong damping effect, in order to attenuate the common mode current, to achieve the purpose of filtering. In fact, connect one end of this filter circuit to the interference source and the other end to the interfered device, then La and C1, Lb and C2 form two sets of low-pass filters, which can control the common mode EMI signal on the line to a very low level. On the level. The circuit can not only suppress the incoming of external EMI signals, but also attenuate the EMI signals generated by the line itself, which can effectively reduce the intensity of EMI interference. A domestically produced small common mode inductor, adopts high-frequency noise suppression countermeasures, common mode choke coil structure, no signal attenuation, small size, easy to use, good balance, easy to use, high quality and other advantages. Widely used in double-balanced tuning devices, multi-frequency transformers, impedance transformers, balanced and unbalanced conversion transformers...etc. There is also a common mode filter inductor/EMI filter inductor that uses a ferrite core, two-wire parallel winding, good noise suppression countermeasures, high common mode noise suppression and low differential mode noise signal suppression, low differential mode noise signal suppression interference The source is difficult to deform in high-speed signals, has the advantages of small size, good balance, easy use, and high quality. It is widely used to suppress EMI noise of electronic equipment, USB lines of personal computers and peripheral equipment, DVC, IEEE1394 lines of STB, liquid crystal display panels, low-voltage differential signals... etc.