Large capacitance filter low frequency signal (II)

Series-parallel capacity formula of capacitor - series-parallel partial pressure formula of capacitor

1. Series formula: C = C1*C2/(C1 + C2) 2. Parallel formula C = C1+C2+C3

Supplementary part:

Series voltage divider ratio V1 = C2/(C1 + C2)*V ........ The larger the capacitance is, the smaller the voltage is. Under AC and DC conditions, the parallel shunt ratio is I1 = C1/(C1 + C2)* I........The larger the capacitance, the greater the current passed, of course, this is under AC conditions.

a small capacitor in parallel with a large capacitor

Due to its large capacity, large capacitors are generally large in size and are usually fabricated by multi-layer winding, which results in a large distributed capacitance of large capacitors (also called equivalent series inductance, referred to as ESL for short).

The impedance of the inductor to the high frequency signal is large, so the high frequency performance of the large capacitor is not good. Some small-capacitance capacitors are just the opposite. Due to the small capacity, the volume can be made small (the lead is shortened, the ESL is reduced, because a length of wire can also be regarded as an inductor), and the plate capacitor is often used. Structure, such a small capacity capacitor has a small ESL so that it has good high frequency performance, but due to the small capacity, the impedance to low frequency signals is large.

Therefore, if we can pass the low frequency and high frequency signals well, we use a large capacitor and a small capacitor.

A commonly used small capacitor with a 0.1uF CBB capacitor is good (the ceramic capacitor is also available). When the frequency is higher, a smaller capacitor can be connected in parallel, for example, a few pF, several hundred pF. In digital circuits, it is generally necessary to connect a 0.1uF capacitor to ground on each chip's power supply pin (this capacitor is called decoupling capacitor, of course, it can be understood as the power supply filter capacitor, the closer to the chip, the better) because The signals in these places are mainly high-frequency signals, which can be filtered with smaller capacitance.

The ideal capacitance, its impedance becomes smaller with increasing frequency (R=1/jwc), but the ideal capacitance does not exist. Due to the distributed inductance effect of the capacitor pin, the capacitor in the high frequency band is no longer a simple capacitor. It should be regarded as a series high-frequency equivalent circuit of capacitance and inductance. When the frequency is higher than its resonant frequency, the impedance shows the characteristic of increasing with increasing frequency, which is the inductance characteristic. At this time, the capacitance is like An inductor. In contrast, the inductor has the same characteristics.

Large capacitors and small capacitors are widely used in power supply filtering. The root cause lies in the self-resonance characteristics of capacitors. The combination of size and capacitance can well suppress low-frequency to high-frequency power interference signals, small capacitor filter high frequency (high self-resonant frequency), large capacitor filter low frequency (low self-resonant frequency), which complement each other.