The function of capacitor

1. Bypass

Bypass capacitors are energy storage devices that provide energy for local devices, which can homogenize the output of the voltage regulator and reduce load requirements. Like a small rechargeable battery, the bypass capacitor can be charged and discharged into the device. To minimize impedance, the bypass capacitor should be as close as possible to the power supply pin and ground pin of the load device. This can effectively prevent ground potential elevation and noise caused by excessive input values. The ground potential is the voltage drop when the ground connection passes through a large current burr.

2. Decoupling

Decoupling, also known as decoupling. From a circuit perspective, 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 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 that the drive 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 pin, it will generate a rebound), this current is actually a kind of noise compared to normal conditions, It affects the normal operation of the previous stage, which is called "coupling".

Decoupling capacitors act as a "battery" to meet the changes in the driving circuit current and avoid mutual coupling interference.

Combining bypass capacitance and decoupling capacitance will be easier to understand. The bypass capacitor is actually decoupled, but the bypass capacitor generally refers to high-frequency bypass, which provides a low impedance leakage path for high-frequency switching noise. The high-frequency bypass capacitance is generally small, and is generally taken as 0.1 based on the resonant frequency μ F、0.01 μ F, etc; The capacity of the decoupling capacitor is generally large, possibly 10 μ F or greater, determined based on the distribution parameters in the circuit and the variation of the driving current. Bypass refers to using the interference in the input signal as a filtering object, while decoupling refers to using the interference in the output signal as a filtering object to prevent the interference signal from returning to the power supply. This should be their essential difference.

3. Filtering

Theoretically (assuming that the capacitance is pure), the larger the capacitance, the smaller the impedance, and the higher the frequency of passage. But actually more than 1 μ The capacitance of F is mostly electrolytic, with a large inductance component, so the impedance increases when the frequency is high. Sometimes it can be seen that a large electrolytic capacitor is 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 role of capacitance is to pass high resistance low, pass high frequency resistance low frequency. The larger the capacitance, the easier it is to pass at low frequencies. Specifically used in filtering, large capacitance (1000 μ F) Filter low frequency, small capacitance (20pF) filter high frequency. Some netizens have vividly compared the filter capacitor to a "pond". Since the voltage at both ends of the capacitor does not suddenly change, 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, which does not cause changes in water volume due to the addition or evaporation of a few drops of water. It converts a change in voltage into a change in current. The higher the frequency, the greater the peak current, thereby buffering the voltage. Filtering is the process of charging and discharging.

4. Energy storage

The energy storage capacitor collects electric charges through a rectifier and transmits the stored energy to the output terminal of the power supply through the converter lead. The voltage rating is 40 to 450VDC, and the capacitance value is 220 to 150 000 μ Aluminum electrolytic capacitors between F (such as B43504 or B43505 from EPCOS) are commonly used. Depending on different power supply requirements, devices may sometimes be in series, parallel, or combinations thereof. For power supplies with a power level of more than 10 KW, large tank shaped spiral terminal capacitors are typically used