The role of capacitance in the power supply

As one of the passive components, the capacitor has the following functions: it is applied in the electronic circuit to realize filtering, bypass, de-coupling, and energy storage.

The filter

Filtering is an important part of the role of capacitance. It is used in almost all power supply circuits. Theoretically (that is, assuming the capacitance is pure), the greater the capacitance, the smaller the impedance, and the higher the frequency. However, in fact, most capacitors over 1uF are electrolytic capacitors with large inductance components, so the impedance will increase when the frequency is high. Sometimes you will see a large capacitance electrolytic capacitor connected in parallel with a small capacitor, then the large capacitor through the low frequency, the small capacitor through the high frequency. The role of capacitance is through high resistance low, through high frequency resistance low frequency. The larger the capacitance, the easier it is to pass low frequencies, and the larger the capacitance, the easier it is to pass high frequencies. Specifically used in filtering, large capacitance (1000uF) filters low frequency, small capacitance (20pF) filters high frequency.

Some netizens once compared the filter capacitor to a "pond". Since the voltage at both ends of the capacitor will not change suddenly, it can be known that the higher the signal frequency, the greater the attenuation. It can be very vividly said that the capacitor is like a pond, not due to the addition or evaporation of a few drops of water caused by the change of water. It converts changes in voltage into changes in current, and the higher the frequency, the greater the peak current, thus buffering the voltage. Filtering is the process of charging and discharging.

The bypass

The by-pass capacitor is an energy storage device that provides energy for local devices. It can equalize the output of the voltage regulator and reduce the load demand. Like a small rechargeable battery, the by-pass capacitor can be charged and released into the device. To minimize impedance, the by-pass capacitance should be as close as possible to the power supply pin and ground pin of the load device. This is a good way to prevent ground potential elevation and noise caused by large input values. The ground projectile is the voltage drop when the ground connection passes through a large current burr.

Go to lotus root

To the lotus root, also known as the lotus root. From the point of view of the circuit, it is always possible to distinguish between the driven source and the driven load. If the load capacitance is large, driver circuit to the capacitor charging, discharge, to complete the signal jump, in the rise is steep, the current is large, this will absorb a lot of drive current of the power supply current, because in the circuit inductance, resistance (especially the inductance on the chip pins, can produce a rebound), the current relative to normal, in fact is a kind of noise, affect the normal work of the former level. That's the coupling. Debunking capacitor is to play the role of a battery, to meet the driving circuit current changes, to avoid mutual coupling interference. It is easier to understand by combining bypass capacitors with de-coupling capacitors. Bypass capacitance is actually de-coupled, but bypass capacitance generally refers to the high-frequency bypass, that is, to the high-frequency switching noise to improve a low impedance leakage path. The high frequency bypass capacitance is generally small, generally 0.1u, 0.01u, etc., according to the resonance frequency, while the uncoupled capacitance is generally large, 10uF or more, according to the distribution parameters in the circuit, as well as the change of the driving current. By-pass is to take the interference in the input signal as the filter object, while decoupling is to take the interference in the output signal as the filter object to prevent the interference signal from returning to the power supply. This should be the essential difference between them.

Energy storage

The energy-storage capacitor collects charge through the rectifier and transmits the stored energy to the output of the power supply through the converter lead. Aluminum electrolytic capacitors (such as EPCOS B43504 or B43505) with voltage ratings of 40 ~ 450VDC and capacitance between 220 ~ 150 000uF are commonly used. Depending on different power requirements, devices are sometimes used in series, parallel connection, or combination of them. For power sources exceeding 10KW, larger can spiral terminal capacitors are usually used.