Two main points of capacitor filter

Capacitance is very important in EMC design, and it is also our common filter element! But in the process of my training, we found that the use of capacitance is not very clear! Here I will introduce two points of capacitance filtering:

1. Capacitance filtering has frequency band. Many people think that the larger the capacitance, the better. In fact, each capacitance has a certain filtering frequency band. Large capacitance filters low frequency and small capacitance filters high frequency. It is mainly determined by the resonance frequency point of the capacitance. The capacitance has the best filtering effect at the resonance frequency point! There is a better filtering effect within a frequency band centered on the resonance point, and others Part of the filtering effect is not good! The resonance point of the capacitor is related to the capacitance value and ESL (equivalent series inductance). For details, you can check the online data and the theoretical analysis of the series resonance circuit learned by the conference school! Generally, we suggest adding a UF level capacitor to the power port to filter the differential mode interference between several hundred kHz and 5MHz, because the UF level capacitor resonance point About 1MHz. In addition, it is recommended to add 1NF chip capacitor to high frequency digital circuit. The reason is that the resonance frequency of 1NF capacitor is between 100MHz, and the resonance frequency points of different manufacturers are different. In this way, it is better to filter the interference of tens of MHz to 200MHz, which is beneficial to solve the EMI problem!

2. If the capacitance is selected, it doesn't mean that the interference can be filtered out! When the river overflows and reaches the high water level, we will often add a ditch for drainage. Then the place to be led must be a low water level. If it is led to a high water level reservoir, it will cause the water to pour back and raise the water level. Capacitance filtering is the same as water control. Capacitance only plays a role of a ditch. Whether it can be filtered depends on the magnitude of interference on the ground connected to the capacitance. 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 introduced to the signal or power supply! We need to pay attention to that the ground interference is not the smallest in some cases! So we emphasize that there is an important basis for filtering, that is, the ground to be connected should have small interference, which is usually called "static ground".

Therefore, in order to achieve the filtering effect, we must select the appropriate capacitance and the ground with small interference! The capacitance can be based on the device manual and experience, and the ground with small interference can adopt the instrument method when debugging, and the experienced engineers should consider the schematic diagram and PCB in the early stage.