Application of capacitors in circuit design

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Application of capacitors in circuit design [Copy link]

Background: In order to coordinate the more comprehensive knowledge points of circuit design, I will introduce various commonly used devices and design concepts in circuit design in the near future, such as basic components resistance, capacitance, inductance, diode protection, memory devices SDRAM, FLASH, PCB design process DCDC power supply, PCB board wiring design process, etc., hoping to provide some reference for everyone.

Note: It is mainly used in circuits such as power filtering, signal filtering, signal coupling, resonance, filtering, compensation, charging and discharging, energy storage, and DC isolation. The following is a detailed introduction to the application of each function based on the common functions of capacitors.

Capacitor characteristics: pass AC and block DC, pass high frequency and block low frequency!

Capacitors are indispensable in our daily circuit design, but many people have a misunderstanding about the use of capacitors, that is, the larger the capacitance, the better the filtering effect. In fact, this is not the case. Simply put, large-capacitance capacitors filter low-frequency noise, and small-capacitance capacitors filter high-frequency noise.

The essence of the work of a capacitor is the process of charging and discharging. Assuming that the capacitor does not store any electricity in the initial state, a large-capacitance capacitor needs to charge more to achieve voltage balance in the circuit, which requires a longer time. Low-frequency noise can meet its time requirements, but if it is placed in a circuit with high-frequency noise, the frequency is high, and the charging and discharging of large-capacitance capacitors cannot react in time, and the purpose of filtering cannot be achieved. At this time, a small-capacitance capacitor must be used. A small-capacitance capacitor has a short charging and discharging time and can meet the purpose of filtering. In short, the frequency of filtering decreases as the capacitance value increases. Therefore, when using it, you should select the appropriate capacitance value according to the needs of your own circuit to achieve the desired filtering purpose and reduce costs.

The most common use of capacitors in circuits is "decoupling capacitors" and "bypass capacitors". As one of the passive components, capacitors have the following functions:

1. Applied to power circuits to achieve bypass, decoupling, filtering and energy storage capacitors. The following categories are detailed:

① Filtering

Filtering is a very important part of the function of capacitors. It is used in almost all power supply circuits. Theoretically (assuming that the capacitor is a pure capacitor), the larger the capacitance, the smaller the impedance and the higher the frequency. But in fact, most capacitors over 1uF are electrolytic capacitors, which have a large inductance component, so the impedance will increase at high frequencies. Sometimes you will see a large electrolytic capacitor in parallel with a small capacitor. At this time, the large capacitor passes low frequencies and the small capacitor passes high frequencies. The larger the capacitance, the easier it is for low frequencies to pass, and the smaller the capacitance, the easier it is for high frequencies to pass.

Take the above figure as an example: C5 will filter out the low-frequency components in the output of the previous stage U2, C7 will filter out its high-frequency components, and C3 and C4 are similar.

Some netizens have compared the filter capacitor to a "pond". Since the voltage at both ends of the capacitor will 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 the amount of water will not change due to the addition or evaporation of a few drops of water. The capacitor converts the change of voltage into the change of current. The higher the frequency, the greater the peak current, thus buffering the voltage. Note: filtering is the process of charging and discharging.

② Bypass

Bypass capacitors are generally connected between the signal terminal and the ground. Their main function is to create an AC branch to eliminate unnecessary energy that enters the susceptible area.

Bypass capacitors are generally used as high-frequency bypass devices to reduce the transient current demand on power modules. Usually, aluminum electrolytic capacitors and tantalum capacitors are more suitable for bypass capacitors. Their capacitance value depends on the transient current demand on the PCB board, generally in the range of 10 to 470μF. If there are many integrated circuits, high-speed switching circuits and power supplies with long leads on the PCB board, large-capacity capacitors should be selected. Bypass capacitors are energy storage devices that provide energy to local devices. They can even out the output of the regulator and reduce load requirements. Like a small rechargeable battery, the bypass is charged and discharged to the device.

Note: To minimize impedance, the bypass capacitor should be as close to the power supply pin and ground pin of the load device as possible. This can effectively prevent the input value from being too large and causing ground potential increase and noise.

③ Remove the lotus root

The decoupling capacitor is actually named according to the actual effect of its use. It is generally connected between the power line and the ground line. It plays two main roles: filtering and energy storage.

The specific role is explained in combination with the following points:

1. When the power supply is introduced into the circuit, the voltage of the power supply is not constant, but in a relatively stable state, which contains a lot of noise. If these noises are allowed to enter the circuit, they will affect the circuit, especially for voltage-sensitive devices, which have higher requirements for the stability of the circuit voltage, and one end used as a reference voltage, which affects its accuracy. Therefore, adding capacitors can ensure the linear relationship of the circuit. (A simple understanding is that if the voltage is too much, I absorb it, and if it is too little, I supplement it, to maintain a balanced state)

2. When the active device is switched, it generates high-frequency switching noise, which will propagate along the power line. At this time, the capacitor provides a local DC power supply to the active device to reduce the propagation of the switching noise in the power line and connect the noise to the ground.

3. There are many electromagnetic waves in space, which often interfere with the stability of chip operation. The decoupling capacitors around the chip can filter out these interferences very well. On the other hand, in high-frequency circuits, the inductance effect generated by the wires has a great hindering effect on the current, which will lead to insufficient current. If the device happens to need sufficient current drive at this time, it cannot be supplied in time. At this time, the energy stored in the decoupling capacitor can make up for these deficiencies in time to ensure the normal operation of the device.

Note: In the circuit, decoupling capacitors and bypass capacitors both play an anti-interference role. The different positions of the capacitors give them different names. Bypass filters out interference in the input signal, while decoupling filters out interference in the output signal to prevent the interference signal from returning to the power supply. This is their essential difference.

④ Energy storage

Energy storage capacitors collect charge through rectifiers and transfer the stored energy to the output of the power supply through converter leads. Aluminum electrolytic capacitors with voltage ratings of 40 to 450 VDC and capacitance values between 220 and 150 000 uF (such as B43504 or B43505 from EPCOS) are more commonly used. Depending on the requirements of different power supplies, devices are sometimes connected in series, in parallel, or in combination. For power supplies with power levels exceeding 10KW, larger can-shaped screw terminal capacitors are usually used.

2. Applied to signal circuits, mainly to complete the functions of coupling, oscillation/synchronization and time constant:

① Coupling

For example, the emitter of a transistor amplifier has a self-biased resistor, which causes the signal to generate a voltage drop and feedback to the input end to form input-output signal coupling. This resistor is the element that generates coupling. If a capacitor is connected in parallel across this resistor, the coupling effect generated by the resistor is reduced due to the smaller impedance of the capacitor with appropriate capacity to the AC signal. Therefore, this capacitor is called a decoupling capacitor.

② Oscillation/Synchronization

Load capacitance including RC, LC oscillators and crystals falls into this category.

③ Time constant

This is the common integration 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 increases. And its charging current decreases as the voltage increases. The characteristics of the current passing through the resistor (R) and capacitor (C) are described by the following formula:

i = (V/R)e-(t/CR)

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Capacitors and resistors are the most used in circuits. Good sharing.