Capacitors are very important in EMC design, and they are also commonly used filter components! Everyone is not very clear about the use of capacitors! Here are two key points of capacitor filtering:
1. Capacitor filtering has frequency bands. Many people think that the larger the capacitor, the better. In fact, each capacitor has a certain filtering frequency band. Large capacitors filter low frequencies and small capacitors filter high frequencies. It is mainly based on the resonance frequency of the capacitor. It is decided that the capacitor has the best filtering effect at the resonance frequency! It has a good filtering effect in a frequency band centered on the resonance point, and the filtering effect of other parts is not good! The resonance point of the capacitor and the capacitance value of the capacitor and ESL (Equivalent Series Inductance) is related. For details, you can check the online information and the theoretical analysis of the series resonant circuit in the conference school! Usually we recommend adding UF-level capacitors to the power port to filter between hundreds of KHZ and 5MHZ The reason for the differential mode interference is that the resonance point of the UF-level capacitor is about 1MHZ. In addition, it is recommended to add a 1nF chip capacitor to the high-frequency digital circuit. The reason is that the resonant frequency of the 1nf capacitor is between 100MHZ. The resonant frequency of different manufacturers is different, so it is better to filter dozens of MHZ to 200MHZ interference, which is beneficial. Solve the problem with EMI!
2. If the capacitor is selected, it does not mean that the interference can be filtered out! The river is flooded and reaches a high water level. At this time, we will often add a ditch for drainage, so the lead must be a low water level, if it is led to a high water level In the case of a large reservoir, it will cause water inflow and raise the water level. Capacitor filtering is the same as the problem of water control. Capacitors only serve as a ditch, and whether they can be filtered also depends on the magnitude of the ground interference connected to the capacitor. We often find that engineers can solve the interference problem and add capacitance to no effect. To a large extent, the ground interference itself is very large! Instead, the ground interference is introduced to the signal or power supply! Everyone needs to pay attention that the ground interference is not the smallest in some cases! So We emphasize that filtering has an important basis, that is, the ground to be connected should have little interference, which is usually referred to as "quiet ground".
Therefore, to achieve the filtering effect when we adopt capacitor filtering, we must select a suitable capacitor and connect to a ground with less interference! Capacitors can be based on the device manual and experience, and the ground with less interference can be debugged by instrument methods. Experienced engineers are in It is necessary to consider the preliminary schematic diagram and PCB.
