Expert technical explanation: Two key points of capacitor filtering in EMC design

Capacitors are very important in EMC design and are also commonly used filtering components! People are not very clear about the use of capacitors! Here are two key points of capacitor filtering :

1. Capacitor filtering has a frequency band. 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 determined by the resonant frequency of the capacitor. The capacitor has the best filtering effect at the resonant frequency point! There is a better filtering effect within a frequency band centered on the resonance point, and the filtering effect is not good in other parts! The resonance point of the capacitor is related to the capacitance value of the capacitor and ESL (equivalent series inductance). You can check the online information and the theoretical analysis of the series resonant circuit in the conference school to know! Usually we recommend adding UF-level capacitors to the power port to filter the differential mode interference between hundreds of KHZ and 5MHZ. The reason is that the resonance point of the UF-level capacitor is around 1MHZ. In addition, we recommend adding 1nF chip capacitors to high-frequency digital circuits. The reason is that the resonant frequency of the 1nf capacitor is between 100MHZ, and the resonant frequency of different manufacturers is different. This is better for filtering interference from tens of MHZ to 200MHZ, which is beneficial to solving EMI problems!

2. Choosing a good capacitor does not mean that interference can be filtered out! When the river overflows and reaches a high water level, we often add a ditch to divert the water. The place where it is diverted must be a low water level. If it is diverted to a reservoir with a high water level, it will cause water to flow back and raise the water level. Capacitor filtering is the same as water control. The capacitor only plays the role of a ditch. Whether it can filter depends on the size of the interference on the ground connected to the capacitor. We often find that engineers have no effect in solving interference problems by adding capacitors. To a large extent, the interference on the ground itself is very large! Instead, the interference on the ground is directed to the signal or power supply! Everyone needs to pay attention that the small ground interference in some cases is not the smallest! Therefore, we emphasize that there is an important basis for filtering, that is, the ground connected to the ground should have small interference, which is usually called "quiet ground".

Therefore, when we use capacitor filtering to achieve the filtering effect, we must select suitable capacitors and connect to a ground with relatively small interference! The capacitor can be selected according to the device manual and experience, and the ground with small interference can be used by instrument methods during debugging. Experienced engineers should consider this in the early schematics and PCB.