The role of capacitors

1) Bypass
The bypass capacitor is an energy storage device that provides energy to the local device, it can even out the output of the regulator and reduce the load demand. Like a small rechargeable battery, the bypass capacitor can be charged and discharged to the device. To minimize impedance, bypass capacitors should be placed as close as possible to the power supply and ground pins of the load device. This is a good protection against ground potential rise and noise caused by excessive input values. Ground potential is the voltage drop across the ground connection through a high current glitch.
2) Decoupling
Decoupling, also known as decoupling. From a circuit perspective, a distinction can always be made between the source being driven and the load being driven. If the load capacitance is relatively large, the driving circuit needs to charge and discharge the capacitance to complete the signal transition. When the rising edge is relatively steep, the current is relatively large, so the driving current will absorb a large power supply current. The inductance and resistance (especially the inductance on the chip pins, which will cause rebound), this current is actually a kind of noise compared to the normal situation, which will affect the normal operation of the front stage, which is the so-called "coupling" .
The decoupling capacitor acts as a "battery" to meet the change of the current of the driving circuit and avoid mutual coupling interference.
It will be easier to understand by combining bypass and decoupling capacitors. The bypass capacitor is actually decoupled, but the bypass capacitor generally refers to the high-frequency bypass, which is to improve a low-impedance leakage prevention way for the high-frequency switching noise. The high-frequency bypass capacitor is generally relatively small, and generally takes 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 more, depending on the distribution parameters in the circuit and the change of the driving current. to make sure. Bypass is to take the interference in the input signal as the filtering object, and decoupling is to take the interference of the output signal as the filtering object to prevent the interference signal from returning to the power supply. This should be their essential difference.
3) Filtering
Theoretically (that is, assuming that the capacitor is a pure capacitor), the larger the capacitor, the smaller the impedance and the higher the passing frequency. But in fact, most of the capacitors exceeding 1μF are electrolytic capacitors, which have a large inductance component, so the impedance will increase when the frequency is high. Sometimes it is seen that there is an electrolytic capacitor with a large capacitance connected in parallel with a small capacitor. At this time, the large capacitor is connected to the low frequency, and the small capacitor is connected to the high frequency. The function of the capacitor is to pass high resistance and low frequency, and pass high frequency and block low frequency. The larger the capacitor, the easier it is for low frequencies to pass. Specifically used in filtering, the large capacitor (1000μF) filters the low frequency, and the small capacitor (20pF) filters the high frequency. Some netizens have vividly compared the filter capacitor to a "pond". Since the voltage across the capacitor will not change abruptly, it can be seen that the higher the signal frequency, the greater the attenuation. It can be vividly said that the capacitor is like a pond, and the water volume will not change due to the addition or evaporation of a few drops of water. It converts changes in voltage into changes in current, and the higher the frequency, the greater the peak current, which buffers the voltage. Filtering is the process of charging and discharging.
4) Energy storage
The energy storage capacitor collects the charge through the rectifier and transfers the stored energy to the output of the power supply through the inverter leads. Aluminum electrolytic capacitors (such as B43504 or B43505 from EPCOS) with a voltage rating of 40 to 450 VDC and a capacitance of 220 to 150 000 μF are commonly used. Depending on the power supply requirements, devices are sometimes used in series, parallel, or a combination thereof. For power supplies with power levels exceeding 10KW, bulky can-shaped screw terminal capacitors are usually used.