Why is a large capacitor connected in parallel with a small capacitor

Beijing capacitor manufacturers believe that because of the large capacity of large capacitors, they are generally large in volume, and they are usually made by multi-layer winding (it should be very experienced to remove aluminum electrolytic capacitors by hand, and several different capacitors can be taken to see if they have not been removed). This leads to the comparison of distributed inductance of large capacitors.

As we all know, the impedance of inductance to high frequency signal is very large, so the high frequency performance of large capacitance is not good. On the other hand, some small capacity capacitors are just the opposite. Because of their small capacity, the volume can be very small (shorten the lead wire, reduce the ESL, because a piece of wire can also be regarded as an inductance), and often use the structure of flat capacitor, so that the small capacity capacitor has a very small ESL, so it has a very good high frequency performance, but due to the small capacity, it can reduce the ESL The impedance of frequency signal is large.

Therefore, if we want to make the low-frequency and high-frequency signals pass through well, we should use a large capacitor and a small capacitor. When the frequency is higher, smaller capacitors can be connected in parallel. In digital circuits, a 0.1uF capacitor is usually connected to the power pin of each chip to the ground (this capacitance is called decoupling capacitor, which can also be understood as power filter capacitor. The closer it is to the chip, the better), because the signals in these places are mainly high-frequency signals, so a smaller capacitor filter can be used.

Due to the large capacity, the large capacitance is generally large in volume, and it is usually made by multi-layer winding, which leads to the large distributed inductance of large capacitance.

The impedance of inductance to high frequency signal is very large, so the high frequency performance of large capacitance is not good. On the other hand, some small capacity capacitors are just the opposite. Due to their small capacity, the volume can be very small (shorten the lead wire, reduce the ESL, because a piece of wire can also be regarded as an inductance). Moreover, the structure of flat capacitor is often used. In this way, the small capacity capacitor has a very small ESL, so it has good high-frequency performance, but due to the small capacity, it can reduce the ESL No. 1 has a high impedance.

Large capacitance in parallel with small capacitor is widely used in power filter. The fundamental reason lies in the self resonance characteristic of capacitor. The combination of large and small capacitors can well suppress the power interference signal from low frequency to high frequency. Small capacitance filters high frequency (high self resonance frequency) and large capacitance filters low frequency (low self resonance frequency), which complement each other.