[Repost] Popular Science of Components: The Most Basic Functions of Capacitors
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As one of the passive components, the capacitor has the following functions: 1. It is used in power supply circuits to realize bypass, decoupling, filtering and energy storage. The following categories are detailed: 1) Bypass The bypass capacitor is an energy storage device that provides energy to local devices. It can even out the output of the regulator and reduce load requirements. Like a small rechargeable battery, the bypass capacitor can be charged and discharged to the device. To minimize impedance, the bypass capacitor should be as close to the power supply pin and ground pin of the load device as possible. This can effectively prevent the ground potential from being raised and causing noise due to excessive input values. Ground bounce is the voltage drop at the ground connection when a large current glitch passes through it. 2) Decoupling Decoupling is also called decoupling. From the circuit point of view, it can always be distinguished into the driving source and the driven load. If the load capacitance is relatively large, the driving circuit must charge and discharge the capacitor to complete the signal jump. When the rising edge is relatively steep, the current is relatively large, In this way, the driving current will absorb a large amount of power supply current. Due to the inductance and resistance in the circuit (especially the inductance on the chip pins, which will rebound), this current is actually a kind of noise relative to normal conditions, which will affect the normal operation of the previous stage. This is the so-called "coupling". The decoupling capacitor plays the role of a "battery" to meet the changes in the current of the driving circuit and avoid mutual coupling interference. It will be easier to understand if the bypass capacitor and the decoupling capacitor are combined. The bypass capacitor is actually also decoupling, but the bypass capacitor generally refers to high-frequency bypass, that is, to provide a low-impedance leakage protection path for high-frequency switching noise. High-frequency bypass capacitors are generally small, generally 0.1μF, 0.01μF, etc. according to the resonant frequency; while the capacity of the decoupling capacitor is generally larger, which may be 10μF or larger, determined by the distributed parameters in the circuit and the change in the driving current. Bypass is to filter out the interference in the input signal, while decoupling is to filter out the interference in the output signal to prevent the interference signal from returning to the power supply. This should be their essential difference. 3) Filtering Theoretically (assuming that the capacitor is a pure capacitor), the larger the capacitance, the smaller the impedance and the higher the frequency. But in fact, most capacitors over 1μF are electrolytic capacitors, which have a large inductance component, so the impedance will increase at high frequencies. Sometimes you will see a large electrolytic capacitor connected in parallel with a small capacitor. At this time, the large capacitor passes low frequencies and the small capacitor passes high frequencies. The function of a capacitor is to pass high frequencies and block low frequencies, and to pass high frequencies and block low frequencies. The larger the capacitance, the easier it is for low frequencies to pass through, and the smaller the capacitance, the easier it is for high frequencies to pass through. Specifically used in filtering, large capacitors (1000μF) filter low frequencies, and small capacitors (20pF) filter high frequencies. Some netizens have vividly compared filter capacitors to "ponds". Since the voltage across the capacitor will not change suddenly, it can be seen that the higher the signal frequency, the greater the attenuation. It can be said that a capacitor is like a pond, and the amount of water will not change due to the addition or evaporation of a few drops of water. It converts voltage changes into current changes. The higher the frequency, the greater the peak current, thereby buffering the voltage. Filtering is the process of charging and discharging. 4) Energy storage [color=rgb(25, 25, Energy storage capacitors collect charge through a rectifier and transfer the stored energy to the output of the power supply through the converter leads. Aluminum electrolytic capacitors with a voltage rating of 40 to 450 VDC and a capacitance of 220 to 150,000 μF (such as EPCOS's B43504 or B43505) are more commonly used. Depending on the requirements of different power supplies, devices are sometimes connected in series, in parallel, or in combination. For power supplies with a power level exceeding 10KW, larger can-shaped spiral terminal capacitors are usually used. 2. Applied to signal circuits, it mainly completes the functions of coupling, oscillation/synchronization and time constant: 1) Coupling For example, the emitter of a transistor amplifier has a self-biased resistor, which also causes the signal to generate a voltage drop and feedback to the input end to form input-output signal coupling. This resistor is the element that generates coupling. If a capacitor is connected in parallel across this resistor, the coupling effect generated by the resistor is reduced due to the small impedance of the capacitor with appropriate capacity to the AC signal. Therefore, this capacitor is called a decoupling capacitor. 2) Oscillation/synchronization Load capacitance including RC, LC oscillators and crystals all fall into this category. 3) Time constantThis is the common integration circuit composed of R and C in series. When the input signal voltage is applied to the input terminal, the voltage on the capacitor (C) gradually rises. Its charging current decreases as the voltage rises. The characteristics of current passing through resistance (R) and capacitance (C) are described by the following formula: i = (V / R)e - (t / CR) In addition, here is a major misunderstanding of commonly used capacitors: Misunderstandings about replacing electrolytic capacitors with tantalum capacitors It is generally believed that tantalum capacitors perform better than aluminum capacitors because the dielectric of tantalum capacitors is tantalum pentoxide generated by anodization, and its dielectric capacity (usually expressed as ε) is higher than the aluminum trioxide dielectric of aluminum capacitors. Therefore, under the same capacity, the volume of tantalum capacitors can be made smaller than that of aluminum capacitors. (The capacitance of electrolytic capacitors depends on the dielectric capacity and volume of the medium. Under the condition of a certain capacity, the higher the dielectric capacity, the smaller the volume can be made. Conversely, the volume needs to be made larger.) In addition, the properties of tantalum are relatively stable, so it is generally believed that tantalum capacitors perform better than aluminum capacitors. However, this method of judging capacitor performance by anode is outdated. Currently, the key to determining the performance of electrolytic capacitors is not the anode, but the electrolyte, that is, the cathode. Because different cathodes and different anodes can be combined into different types of electrolytic capacitors, their performance is also very different. Capacitors using the same anode can have very different performances due to different electrolytes. In short, the impact of the anode on capacitor performance is much smaller than that of the cathode. There is also a view that tantalum capacitors perform better than aluminum capacitors, mainly because tantalum plus manganese dioxide cathodes can significantly outperform aluminum electrolyte capacitors. If the cathode of an aluminum electrolyte capacitor is replaced with manganese dioxide, its performance can actually be improved a lot. It is certain that ESR is one of the main parameters for measuring the characteristics of a capacitor. However, when choosing a capacitor, we should avoid the misunderstanding that the lower the ESR, the better, and the higher the quality, the better. When measuring a product, we must consider it from all aspects and angles, and we must not exaggerate the role of the capacitor intentionally or unintentionally. The structure of an ordinary electrolytic capacitor is an anode, a cathode and an electrolyte. The anode is passivated aluminum and the cathode is pure aluminum, so the key lies in the anode and the electrolyte. The quality of the anode is related to issues such as the withstand voltage and dielectric constant. Generally speaking, the ESR of tantalum electrolytic capacitors is much smaller than that of aluminum electrolytic capacitors with the same capacity and the same voltage resistance, and their high-frequency performance is better. If that capacitor is used in a filter circuit (such as a bandpass filter with a center of 50Hz), pay attention to the impact of capacity changes on filter performance. Source: Internet, please delete if infringed 25)]Generally speaking, the ESR of tantalum electrolytic capacitors is much smaller than that of aluminum electrolytic capacitors with the same capacity and the same voltage resistance, and their high-frequency performance is better. If that capacitor is used in a filter circuit (such as a bandpass filter with a center of 50Hz), pay attention to the impact of capacity changes on filter performance. Source: Internet, please delete if infringed 25)]Generally speaking, the ESR of tantalum electrolytic capacitors is much smaller than that of aluminum electrolytic capacitors with the same capacity and the same voltage resistance, and their high-frequency performance is better. If that capacitor is used in a filter circuit (such as a bandpass filter with a center of 50Hz), pay attention to the impact of capacity changes on filter performance. Source: Internet, please delete if infringed
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