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The role of capacitors in power supply

2024-06-26 10:28

As one of the passive components, capacitors have the following functions: applied in electronic circuits to achieve filtering, bypass, decoupling, and energy storage.

wave filtering

Filtering is a crucial part of the function of capacitors. Almost all power circuits will use it. In theory (assuming that the capacitor is a pure capacitor), the larger the capacitor, the smaller the impedance, and the higher the frequency of passage. However, in reality, most capacitors exceeding 1uF are electrolytic capacitors with a large inductance component, so the impedance increases when the frequency is high. Sometimes you may see a large electrolytic capacitor connected in parallel with a small capacitor, where the large capacitor is connected to the low frequency and the small capacitor is connected to the high frequency. The function of a capacitor is to pass high resistance and low resistance, and to pass high frequency resistance and low frequency resistance. The larger the capacitance, the easier it is to pass through low frequencies, and the larger the capacitance, the easier it is to pass through high frequencies. Specifically used in filtering, a large capacitor (1000uF) filters low frequencies and a small capacitor (20pF) filters high frequencies.

Some netizens once compared the filtering capacitor to a "pond". Due to the fact that the voltage at both ends of the capacitor does not suddenly change, it can be inferred that the higher the signal frequency, the greater the attenuation. It can be vividly described 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 changes in voltage into changes in current, and the higher the frequency, the greater the peak current, thereby buffering the voltage. Filtering is the process of charging and discharging.

bypass

A bypass capacitor is an energy storage device that provides energy to local devices, which can homogenize the output of the voltage regulator and reduce load demand. Just like a small rechargeable battery, a bypass capacitor can be charged and discharged into the device. To minimize impedance, the bypass capacitor should be placed as close as possible to the power supply pin and ground pin of the load device. This can effectively prevent ground potential rise and noise caused by excessive input values. Ground bounce refers to the voltage drop when the ground connection passes through a high current burr.

Removing lotus roots

Removing lotus roots, also known as removing lotus roots. 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 driving circuit needs to charge and discharge the capacitor in order to complete the signal jump. When the rising edge is steep, the current is relatively high, so the driving current will absorb a large amount of power current. Due to the inductance and resistance in the circuit (especially the inductance on the chip pins, it will rebound), this current is actually a kind of noise compared to normal situations, which will affect the normal operation of the front stage. This is coupling. The decoupling capacitor serves as a battery, satisfying the changes in driving circuit current and avoiding mutual coupling interference. Combining bypass capacitors and decoupling capacitors will be easier to understand. The bypass capacitor is actually decoupled, but it generally refers to a high-frequency bypass, which provides a low impedance leakage path for high-frequency switching noise. High frequency bypass capacitors are generally relatively small, based on resonant frequencies of 0.1u, 0.01u, etc., while decoupling capacitors are generally larger, 10uF or larger, determined by the distributed parameters in the circuit and the magnitude of changes in driving current. Bypass is to filter out the interference in the input signal, while decoupling is to filter out the interference in the output signal, preventing the interference signal from returning to the power source. This should be their fundamental difference.

Energy storage

Energy storage capacitors collect charges through rectifiers and transmit the stored energy to the output of the power supply through converter leads. Aluminum electrolytic capacitors with a voltage rating of 40-450VDC and a capacitance value between 220-150 000uF (such as EPCOS B43504 or B43505) are commonly used. According to different power supply requirements, devices may sometimes be used in series, parallel, or their combination. For power supplies with power levels exceeding 10KW, larger can shaped spiral terminal capacitors are usually used.

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