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What is the function of the capacitor?

by:GREEN TECH     2021-04-03

1, filtering

Filtering is a very important part of the role of capacitors. It is used in almost all power circuits. Theoretically (that is, assuming that the capacitor is a pure capacitor), the larger the capacitance, the smaller the impedance, and the higher the passing frequency.

But in fact, most of the capacitors over 1uF are electrolytic capacitors, which have a large inductance component, so the impedance will increase when the frequency is high. Sometimes you will see a large electrolytic capacitor with a small capacitor connected in parallel. 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 resistance, and pass high frequency to block 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, a large capacitor (1000uF) filters low frequencies, and a small capacitor (20pF) filters high frequencies.

Some netizens once compared filter capacitors to 'ponds.' Since the voltage across the capacitor does not change suddenly, 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 will not change the amount of water due to the addition or evaporation of a few drops of water.

It converts changes in voltage into changes in current. The higher the frequency, the greater the peak current, thus buffering the voltage. Filtering is the process of charging and discharging.

2, bypass

A bypass capacitor is an energy storage device that provides energy for local devices. It can make the output of the regulator uniform and reduce the load demand. Just like a small rechargeable battery, the bypass capacitor can be charged and discharged to the device. In order to minimize the impedance, the bypass capacitor should be as close as possible to the power supply pin and ground pin of the load device. This can well prevent the ground potential rise and noise caused by the input value is too large. The ground bounce is the voltage drop at the ground connection when a large current burr passes.

3, to remove the lotus root

Removing lotus root, also known as relieving lotus root. From the circuit point of view, it can always be divided into the source of the drive and the load being driven. If the load capacitance is relatively large, the drive circuit must 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 power supply current.

The inductance and resistance (especially the inductance on the chip pins will bounce). Compared with normal conditions, this current is actually a kind of noise, which will affect the normal operation of the previous stage. This is coupling. The decoupling capacitor acts as a battery to meet the current changes of the drive circuit and avoid mutual coupling interference.

Combining bypass capacitors and decoupling capacitors will make it easier to understand. The bypass capacitor is actually decoupling, but the bypass capacitor generally refers to high-frequency bypass, that is, to improve a low-impedance leakage prevention method for high-frequency switching noise. High-frequency bypass capacitors are generally relatively small. According to the resonance frequency, they are generally 0.1u, 0.01u, etc., while decoupling capacitors are generally larger, 10uF or greater, depending on the distribution parameters in the circuit and the magnitude of the drive current change. 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.

4, energy storage

The energy storage capacitor collects the charge through the rectifier and transfers the stored energy to the output terminal of the power supply through the lead of the converter. Aluminum electrolytic capacitors (such as B43504 or B43505 from EPCOS) with a voltage rating of 40-450VDC and a capacitance value of 220-150 000uF are more commonly used. According to different power requirements, the devices sometimes adopt the form of series, parallel or a combination thereof. For power supplies with a power level of more than 10KW, larger tank-shaped screw terminal capacitors are usually used.

5. Coupling

For example, the emitter of the transistor amplifier has a self-biased resistor, which at the same time causes the signal to generate a voltage drop and feedback to the input to form the input and output signal coupling. This resistor is the element that generates the coupling. A capacitor is connected in parallel at both ends of this resistor. Since a capacitor of appropriate capacity has a small impedance to the AC signal, which reduces the coupling effect produced by the resistor, this capacitor is called a decoupling capacitor.

6. Oscillation/Synchronization

The load capacitance including RC, LC oscillator and crystal all belong to this category.

7, time constant

This is the common integrating circuit composed of R and C in series. When the input signal voltage is applied to the input terminal, the voltage on the capacitor (C) gradually rises. The charging current decreases as the voltage rises.

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