The capacitance value refers to the charge storage under a given potential difference, which is recorded as C. The SI unit isFarad（F）。In general,chargeIt will move under force in the electric field. When there is a medium between conductors, it will prevent the movement of charges and make the charges accumulate on the conductors, resulting in the accumulated storage of charges. The amount of stored charges is called capacitance.
Capacitance refers to the capacity to hold electric field.Any electrostatic field is composed of many capacitors. Where there is electrostatic field, there is capacitance. Capacitance is described by electrostatic field.It is generally believed that isolated conductors form capacitance with infinity, and conductor grounding is equivalent to being connected to infinity and connected to the earth as a whole.
Capacitance (or electric capacity) is a physical quantity that represents the ability of a capacitor to hold electric charges.From a physical point of view, capacitance is a static charge storage medium, which may permanently exist. This is its feature. It has a wide range of applications, and is indispensable in the field of electronics and powerElectronicsComponent.It is mainly used in power filter, signal filter, signal coupling, resonance, filter, compensation, charge discharge, energy storage, DC isolation and other circuits[1]。
definition
Announce
edit
The ratio of the charged quantity Q of the capacitor to the voltage U between the two poles of the capacitor is called the capacitance of the capacitor.In circuit science, given the potential difference, the capacity of a capacitor to store charge is called capacitance, which is marked as C.useInternational System of Units, the unit of capacitance is farad, marked with F.
A capacitor, if the power with 1 bank is between two stagesPotential differenceIt is 1 volt, and the capacitance of this capacitor is 1 farad, that is, C=Q/U.However, the capacitance is not determined by Q (charged quantity) or U (voltage), that is, the determinant of capacitance is: C=ε S/4 π kd.Where, ε is a constant, S is the positive area of the capacitor plate, d is the distance of the capacitor plate, and k iselectrostatic force constant 。commonParallel plate capacitorThe capacitance value is C=ε S/d (ε is the dielectric constant of the medium between the plates, S is the area of the plates, and d is the distance between the plates).
The bypass capacitor provides energy for local devicesEnergy storageDevice, which can make the output of voltage regulator uniform and reduceloadDemand.Like a small rechargeable battery, the bypass capacitor can be charged and discharged to the device.To minimize the impedance, the bypass capacitor should be as close as possible to the power supply pin and ground pin of the load device.This can well prevent ground potential rise and noise caused by excessive input value.The ground potential is the voltage drop when the ground connection passes through a large current burr.
Decoupling is also called decoupling.In terms of the circuit, it can always be distinguished into the source of the drive and the load being driven.If the load capacitance is relatively large, the drive circuit needs to charge and discharge the capacitance to complete the signal jump. When the rising edge is relatively steep, the current is relatively large, so the drive current will absorb a large power supply current. Because of the inductance in the circuit,resistance(Especially the inductance on the chip pin) will rebound. This current is actually a kind of noise relative to the normal situation, which will affect the normal operation of the front stage. This is called "coupling".
Decoupling capacitorIt acts as a "battery" to meet the current changes of the drive circuit, avoid mutual coupling interference, and further reduce the high-frequency interference impedance between the power supply and the reference ground in the circuit.
The combination of bypass capacitance and decoupling capacitance will be easier to understand.The bypass capacitor is actually decoupled, but generally refers tohigh frequencyBypass, that is, to provide a low impedance discharge path for high-frequency switching noise.The high-frequency bypass capacitance is generally small, and it is generally 0.1 μ F, 0.01 μ F, etc. according to the resonant frequency;The capacity of decoupling capacitor is generally large, which may be 10 μ F or more, and it is determined according to the distribution parameters in the circuit and the change of driving current.Bypass refers to the interference in the input signal as the filtering object, while decoupling refers to the interference in the output signal as the filtering object to prevent the interference signal from returning to the power supply.This should be their essential difference[2]。
3) Filtering
Theoretically (assuming that the capacitance is pure), the larger the capacitance, the smaller the impedance, and the higher the passing frequency.However, in fact, most of the capacitors above 1 μ F are electrolytic capacitors with large inductance, so the impedance will increase when the frequency is high.Sometimes it can be seen that an electrolytic capacitor with a large capacitance is connected in parallel with a small capacitor. At this time, the large capacitor filters the low frequency and the small capacitor filters the high frequency.The function of capacitor is to connect AC and isolate DC, connect high-frequency resistance and low-frequency resistance.The larger the capacitance, the easier the high frequency pass.Specifically used in filtering, large capacitance (1000 μ F) filters low frequency and small capacitance (20pF) filters high frequency.Some netizens once vividly compared the filter capacitor to a "pond".Since the voltage at both ends of the capacitor will not change suddenly, it can be seen that the higher the signal frequency is, the greater the attenuation will be. It can be said that the capacitor is like a pond and will not change the water volume due to the addition or evaporation of a few drops of water.It converts the change of voltage into the change of current. The higher the frequency, the greater the peak current, thus buffering the voltage.Filtering is the process of charging and discharging.
The energy storage capacitor collects the charge through the rectifier, and transmits the stored energy to the output terminal of the power supply through the converter lead.The rated voltage is 40~450VDC, and the capacitance is 220~150 000 μ FAluminum electrolytic capacitorIt is commonly used.According to different power supply requirements, devices may sometimes be in series, parallel or combination. For power supply with power level more than 10KW, tank shaped spiral terminal capacitor with large volume is usually used[2]。
