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Talk about several uses of capacitors. Talk about one of capacitors: the role of capacitors

Published on:2021-05-28

Talking about one of capacitors: the role of capacitors
  Capacitor as one of the power components, its role is nothing more than the following:
1. Used in power circuits to realize the functions of bypass, decoupling, filtering and energy storage.
The following categories are detailed:
1) Bypass
Capacitors that can bypass high-frequency components in alternating current mixed with high-frequency currents and low-frequency currents are called "bypass capacitors." For the same circuit,
The bypass capacitor takes the high-frequency noise in the input signal as the filtering object, and filters the high-frequency clutter carried by the previous stage, and the decoupling capacitor takes the interference of the output signal as the filtering object.

The main function of the bypass capacitor is to generate an AC shunt to eliminate the unnecessary energy that enters the susceptible area, that is, when the signal mixed with high frequency and low frequency is amplified by the amplifier,
It is required that only low-frequency signals are allowed to enter the next stage when passing through a certain stage, and high-frequency signals are not required to enter, so add an appropriate size grounding capacitor at the input of this stage, so that higher-frequency signals can easily pass through this capacitor It is bypassed (this is because the capacitor has a small high-frequency impedance), and the low-frequency signal is sent to the next stage of amplification due to the large impedance of the capacitor to it.
The bypass capacitor is an energy storage device that provides energy for the local device. It can uniformize the output of the regulator and reduce the load demand. Just like a small rechargeable battery, the bypass capacitor can be charged,
And discharge 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 low voltage and noise caused by the input value being too large. The ground bounce is the voltage drop when the ground is connected through a large current glitch.
  2) Go to lotus root
     De-lotus, also known as de-lotus. From the circuit point of view, it can always be divided into the source of the drive and the load of the drive. 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 power supply will absorb a large power supply current. The inductance and resistance (especially the inductance on the chip pins will bounce). Compared with the normal situation, this current is actually a kind of noise, which will affect the normal operation of the previous stage. This is the so-called "coupling ". The decoupling capacitor acts as a "battery" to meet the change of the drive circuit current 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, a low-impedance leakage prevention path for high-frequency switching noise. High-frequency bypass capacitors are generally relatively small, 0.1uF according to the resonance frequency, etc.; while the decoupling capacitor has a larger capacity, which may be 10uF or greater, depending on the distribution parameters in the circuit and the change in drive current.  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. On the one hand, the energy storage capacitor of the integrated circuit, on the other hand, bypasses the high-frequency noise of the device. The typical decoupling capacitor value in digital circuits is 0.1u. The typical value of the distributed inductance of this capacitor is 5nH.
The 0.1uF decoupling capacitor has a distributed inductance of 5nH, and its parallel zone oscillation frequency is about 7MHz. That is to say, it has a good decoupling effect for noise below 10MHz, and it has almost no effect on noise above 40MHz. 1Uf, 10uf capacitors, the parallel resonance frequency is above 20MHz, and the effect of removing high-frequency noise is better. For every 10 or so integrated circuits, add a charge and discharge capacitor, or an energy storage capacitor, which can be about 10uF. It is best not to use electrolytic capacitors. Electrolytic capacitors are rolled up with two layers of film. This rolled up structure behaves as an inductance at high frequencies. Use tantalum capacitors or polycarbonate capacitors. The choice of decoupling capacitor is not strict, you can press C=1/F, that is, 0.1uf for 10MHz and 0.01uF for 100MHz.
3) Filtering
   In theory (that is, assuming that the capacitor is a pure capacitor), the larger the capacitance, the smaller the resistance, 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 after 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 impedance and low impedance, and pass high frequency to block low frequency. The larger the capacitance, the easier it is to pass low frequencies, and the smaller 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 have vividly compared the filter capacitor to a "pond." Since the voltage at both ends of the capacitor does not change suddenly, it can be seen that the higher the signal frequency, the greater the attenuation. It can be 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, which buffers the voltage. Filtering is the process of charging and discharging.

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. The voltage rating is 40-450VDC. Aluminum electrolytic capacitors with a capacitance value of 220-150000UF are more commonly used. According to different power requirements, devices sometimes adopt series, parallel or combination forms. For power supplies with a power level of more than 10KV, larger tank-shaped screw terminal capacitors are usually used.

2. Used in signal circuits, mainly to complete the functions of coupling, oscillation, synchronization and time constant:
   1) 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 the resistor. Since a capacitor of appropriate capacity has a small impedance to the AC signal, the coupling effect generated by the resistor is reduced, so this capacitor is called a decoupling capacitor.
   2) Oscillation, synchronization
  Including the load capacitance of RC, LC oscillators and crystals belong to this category.
   3) 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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