Capacitance is very important in EMC design, and it is also our common filter component! The use of capacitor is not very clear! Here are two points of capacitor filtering:
1, capacitor filtering is a frequency band, many people think that the larger the capacitor, the better, in fact, each capacitor has a certain filter frequency band, large capacitor filter low frequency, small capacitor filter high frequency, mainly based on the resonant frequency of the capacitor It is decided that the capacitor has the best filtering effect at the resonance frequency point! It has a good filtering effect in a frequency band centered on the resonance point, and the other part of the filtering effect is not good! The resonance point of the capacitor and the capacitance value of the capacitor and ESL (equivalent series inductance) is related, you can check the online information, and the theoretical analysis of the series resonant circuit in the conference school will know! Usually we recommend adding UF level capacitors at the power port to filter between several hundred KHZ to 5MHZ. The differential mode interference is due to the resonance point of the UF level capacitor being around 1 MHz. In addition, it is recommended to add 1nF chip capacitor on high frequency digital circuit. The reason is that the resonant frequency of 1nf capacitor is between 100MHZ. The resonance frequency of different manufacturers is different, so it is better to filter tens of MHZ to 200MHZ interference. Solve problems with EMI!
2. If the capacitor is selected, it does not mean that the interference can be filtered out! The river overflows and reaches the high water level. At this time, we often add a ditch drainage, so the place must be a low water level, if it leads to a high water level. If it is a reservoir, it will cause water to be poured and raise the water level. Capacitor filtering is the same as the problem of water management. The capacitor only acts as a ditch. Whether it can be filtered depends on the size of the ground interference connected to the capacitor. We often find that engineers solve the interference problem and the capacitor has no effect. To a large extent, the ground interference itself is very large! Instead, the ground interference is led to the signal or the power supply! Everyone needs to be aware that the ground interference is not the smallest in some cases! We emphasize that there is an important basis for filtering, that is, the ground to be connected is small, which is usually called "quiet land."
Therefore, when we adopt capacitive filtering, we must achieve the filtering effect. We must select the appropriate capacitor and the ground with relatively small interference. The capacitor can be used according to the device manual and experience. The interference can be taken when debugging. Consideration should be given to the previous schematic and PCB.
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