【description】：capacitors are very important in emc design, and they are also commonly used filter elements. however, during my training, i found that the use of capacitors is not very clear! here i will introduce two key points of capacitor filtering:
capacitors are very important in emc design, and they are also commonly used filter elements. however, during my training, i found that the use of
capacitors is not very clear! here i will introduce two key points of
capacitor filtering:
1. if the capacitance is selected, it does not mean that the interference can be filtered out! when the river overflows and reaches the high water level, we often add a ditch for drainage, so the place to be led must be a low water level. if it is introduced to a reservoir with a high water level, it will cause water backflow and raise the water level. capacitance filtering is the same as water control. the capacitance only acts as a ditch. whether it can be filtered depends on the size of the ground interference connected with the capacitor. we often find that engineers have no effect in solving
the interference problem by adding capacitance. to a large extent, the ground interference itself is very large! instead, the ground interference is led to the signal or power supply! we need to note that the ground interference is small in some cases, and it is not the minimum! therefore, we emphasize that filtering has an important foundation, that is, the grounding interference should be small, which is commonly referred to as "static ground".
2. capacitor filtering has frequency band. many people think that the larger the capacitance, the better. in fact, each capacitor has a certain filtering frequency band. large capacitance filters low frequency and small capacitance filters high frequency. it is mainly determined by the resonant frequency point of the capacitor. the capacitor has the best filtering effect at the resonant frequency point! in the frequency band centered on the resonance point, the filter effect is better part of the filtering effect is not good! the resonance point of the capacitor is related to the capacitance value of the capacitor and esl (equivalent series inductance). for details, you can check the online information and the theoretical analysis of the series resonant circuit learned by the conference school. generally, we suggest adding uf class capacitor at the power port to filter the differential mode interference between hundreds of khz and 5mhz, and the reason is the resonance point of uf class capacitor about 1mhz. in addition, it is suggested to add 1nf chip capacitor to high-frequency digital circuit. the reason is that the resonant frequency of 1nf capacitor is between 100mhz. different manufacturers have different resonant frequency points. in this way, it is better to filter interference from dozens of mhz to 200mhz, which is beneficial to solve emi problems!
