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A common mode inductor means that two coils are wound on the same iron core, with opposite windings, the number of turns and the same phase. Commonly used in switching power supplies to filter common-mode electromagnetic interference signals, EMI filters are used to suppress electromagnetic waves generated by high-speed signal lines from radiating outward. The common mode inductance at the input of the power module is usually to reduce radiation and high frequency common mode noise. However, a large common-mode inductance has a good suppression effect on low-frequency disturbance, and high frequency may become worse, but a small feeling has a poor suppression effect on low-frequency disturbance.

It has obvious suppression effect on common mode noise. The working principle is that when common mode current passes through the component, the inductances of the two inductors overlap. But for differential mode noise, the two inductances are equivalent to taking the difference, the inductance value decreases, and the suppression effect will be weakened.

The size of the common mode inductance will directly affect the EMC performance. The main function is to isolate the common mode signal and attenuate the external common mode interference, thereby reducing the impact on the power supply. It can also reduce the internal common mode signal and reduce the impact on the power grid. However, a large common-mode inductance has a good suppression effect on low-frequency disturbance, and high frequency may become worse, but a small feeling has a poor suppression effect on low-frequency disturbance.

What we commonly use at the input end of the power module are x capacity, y capacity and common mode inductance. The capacity has low impedance to the signal, which acts as a bypass and coupling signal. The inductance is high impedance to the signal and plays a role in reflecting and absorbing high-frequency interference signals.

The interference between the two power lines to the ground is called common mode interference, and the interference between the two power lines is called differential mode interference. When the inductance and capacitance are combined into a filter, the filtering effect is better, and the frequency band where the inductance and capacitance play a role. Also different. The Y capacitor and the Y capacitor play a role in filtering common mode interference, and the X capacitor mainly acts as a short-circuit signal, reducing the path through which the differential mode signal flows, thereby reducing the oscillation caused by the parasitic parameters in the circuit and causing high-frequency emission.

When the inductance or capacitance is subtracted in the design, the remaining part will still work, but the effect will be much worse. Generally speaking, the larger the common-mode impedance, the better. When choosing a common-mode inductor, the choice is mainly based on the impedance frequency curve. At the same time, attention should be paid to the influence of the differential mode impedance on the signal.