The role of capacitors

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1) Bypass

Bypass capacitors are energy storage devices that provide energy to local devices, homogenizing the output of the regulator and reducing load demand. Just 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 as possible to the power and ground pins of the load device. This is a good way to prevent ground potential elevation and noise caused by excessive input values. Ground potential is the voltage drop at the ground junction as it passes through a high-current glitch.

2) Decoupling

Decoupling, also known as decoupling. From the perspective of the circuit, it is always possible to distinguish between the source of the drive and the load being driven. If the load capacitance is relatively large, the drive circuit to charge and discharge the capacitor to complete the signal jump, in the rising edge is relatively steep, the current is relatively large, so that the drive current will absorb a large amount of power supply current, due to the inductance in the circuit, resistance (especially the inductance on the chip pin) will rebound, this current relative to the normal situation is actually a kind of noise, will affect the normal operation of the pre-stage, this is the so-called "coupling".
The decoupling capacitor plays the role of a "battery" to meet the change of the current of the driving circuit, avoid the coupling interference between each other, and further reduce the high-frequency interference impedance between the power supply and the reference ground in the circuit.

Combining bypass capacitors and decoupling capacitors will be easier to understand. The bypass capacitor is actually decoupled, but the bypass capacitor generally refers to the high-frequency bypass, that is, to provide a low-impedance discharge path for high-frequency switching noise. The high-frequency bypass capacitance is generally relatively small, and 0.1μF, 0.01μF, etc. are generally taken according to the resonant frequency; The capacity of the decoupling capacitor is generally large, perhaps 10μF or more, depending on the distribution parameters in the circuit and the change in the drive current. Bypass is to use the interference in the input signal as a filter object, while decoupling is to use the interference of the output signal as a filter object to prevent the interference signal from returning to the power supply. This is their essential difference.

3) Filtering

Theoretically (i.e., assuming that the capacitor is pure capacitance), the larger the capacitor, the smaller the impedance and the higher the frequency of passage. However, in fact, most capacitors over 1μF are electrolytic capacitors and have a large inductive component, so the impedance increases when the frequency is high. Sometimes you see a large capacitance electrolytic capacitor 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 the capacitor is to block AC DC, pass high frequency resistance low frequency. The larger the capacitance, the easier it is for high frequencies to pass through. Specifically used in filtering, large capacitance (1000μF) filters low frequency, small capacitor (20pF) filters 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 figuratively said that the capacitor is like a pond, and it will not cause changes in the amount of water caused by the addition or evaporation of a few drops of water. It converts the change in voltage into a change in current, and the higher the frequency, the greater the peak current, thus buffering the voltage. Filtering is the process of charging and discharging.

4) Energy storage

The energy storage capacitor collects charge through the rectifier and transfers the stored energy through the converter leads to the output of the power supply. Aluminum electrolytic capacitors with a voltage rating of 40~450VDC and a capacitance value between 220~150 000μF are more commonly used. Depending on the power requirements, devices are sometimes connected in series, parallel, or a combination thereof, and larger canister screw terminal capacitors are usually used for power supplies with power levels greater than 10KW.