How do filter capacitors and common mode inductors eliminate electromagnetic interference?

Last Update Time: 2019-08-12 10:30:07

Filter capacitors and common mode inductors are common in EMC design circuits and are a powerful tool for eliminating electromagnetic interference. For the role of the two in the circuit, I believe that many engineers are confused. In this paper, from the design and design, the principle of eliminating two major weapons of EMC is analyzed in detail.

Common mode inductance

Since the problem facing EMC is mostly common mode interference, common mode inductor is also one of the commonly used powerful components. The common mode inductor is a common mode interference suppression device with ferrite as the core. It is made up of two identical sizes. The coils with the same number of turns are symmetrically wound on the same ferrite toroidal core to form a four-terminal device, which has a suppressing effect on the common mode signal and a small inductance. Leakage inductance hardly works. The principle is that the magnetic fluxes in the magnetic ring are superimposed on each other when the common mode current flows, so that the inductance is relatively large, and the common mode current is suppressed, and when the two coils flow through the differential mode current, the magnetic in the magnetic ring by canceling each other, there is almost no inductance, so the differential mode current can pass without attenuation. Therefore, the common mode inductor can effectively suppress the common mode interference signal in the balanced line, and has no effect on the differential mode signal that the line normally transmits.

The common mode inductor should meet the following requirements when it is manufactured:

 1) The coil should be wound as much as possible, which can reduce the parasitic capacitance of the coil and enhance the ability of the coil to instantaneous overvoltage.

2) The wires wound on the coil core should be insulated from each other to ensure that no breakdown short circuit occurs between the turns of the coil under the action of instantaneous overvoltage.

       3) When the coil flows through a large instantaneous current, the core does not appear to be saturated.

       4) The core in the coil should be insulated from the coil to prevent breakdown between the two under transient overvoltage.


Under normal circumstances, at the same time pay attention to select the frequency band required for filtering, the larger the common mode impedance, the better. Therefore, we need to look at the device data when selecting the common mode inductor, mainly based on the impedance frequency curve. In addition, pay attention to the influence of differential mode impedance on the signal, mainly focusing on differential mode impedance, paying special attention to high-speed ports.


Filter capacitor

Although the resonance of the capacitor is undesirable from the viewpoint of filtering out high frequency noise, the resonance of the capacitor is not always harmful. When the noise frequency to be filtered is determined, the capacitance can be adjusted to make the resonance point just fall on the disturbance frequency.

In actual engineering, the frequency of electromagnetic noise to be filtered is often as high as several hundred MHz or even more than 1 GHz. For such high frequency electromagnetic noise, a feedthrough capacitor must be used to effectively filter out. The reason why ordinary capacitors cannot effectively filter out high-frequency noise is because of two reasons. One reason is that the capacitor lead inductance causes capacitance resonance, which has a large impedance to high-frequency signals and weakens the bypass effect on high-frequency signals. Another reason is that the parasitic capacitance between the wires couples the high frequency signals, reducing the filtering effect.

The reason why the core capacitor can effectively filter out the high frequency noise is because the core capacitor not only has no problem that the capacitor resonance frequency is too low due to the lead inductance, but also the core capacitor can be directly mounted on the metal panel, and the metal panel is used to be high the role of frequency isolation. However, when using a feedthrough capacitor, the problem to be aware of is the installation problem. The biggest weakness of the feedthrough capacitor is the fear of high temperature and temperature shock, which causes great difficulty in soldering the feedthrough capacitor to the metal panel. Many capacitors are damaged during the soldering process. Especially when a large number of feedthrough capacitors need to be mounted on the panel, as long as there is a damage, it is difficult to repair, because when the damaged capacitor is removed, damage to other capacitors is caused.

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