Basic knowledge of circuits: Common sense concepts of circuits - principles and functions of capacitors

The so-called capacitor is an electronic component that holds and releases electric charge. The basic working principle of the capacitor is charging and discharging, of course, there are also rectification, oscillation and other functions. In addition, the structure of the capacitor is very simple, mainly composed of two positive and negative electrodes and an insulating medium sandwiched in between.

As one of the passive components, the capacitor has the following functions:

1. Applied to power circuits to achieve bypass, decoupling, filtering and energy storage

1) Bypass

Bypass capacitors are energy storage devices that provide energy to local devices, which can even out the output of the regulator and reduce load requirements. Like a small rechargeable battery, bypass capacitors can be charged and discharged to the device. To minimize impedance, bypass capacitors should be as close to the power supply pins and ground pins of the load device as possible. This can effectively prevent ground potential increase and noise caused by excessive input values. Ground bounce is the voltage drop at the ground connection when a large current glitch passes through it.

2) Remove the lotus root

Decoupling, also known as decoupling. From the circuit point of view, it can always be divided into the driving source and the driven load. If the load capacitance is relatively large, the driving circuit must charge and discharge the capacitor to complete the signal jump. When the rising edge is relatively steep, the current is relatively large, so the driving 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 pins, which will rebound), this current is actually a kind of noise relative to normal conditions, which will affect the normal operation of the previous stage. This is coupling. The decoupling capacitor plays the role of a battery to meet the change of the driving circuit current and avoid mutual coupling interference. It will be easier to understand by combining the bypass capacitor and the decoupling capacitor. The bypass capacitor is actually also decoupling, but the bypass capacitor generally refers to high-frequency bypass, that is, to provide a low-impedance leakage protection path for high-frequency switching noise. High-frequency bypass capacitors are generally small, generally 0.1u, 0.01u, etc. according to the resonant frequency, while decoupling capacitors are generally large, 10uF or larger, determined according to the distributed parameters in the circuit and the change in the driving current.

In general, bypassing is to filter out the interference in the input signal, while decoupling is to filter out the interference in the output signal to prevent the interference signal from returning to the power supply. This should be their essential difference.

3) Filtering

Theoretically (assuming the capacitor is a pure capacitor), the larger the capacitance, the smaller the impedance and the higher the frequency. But in fact, most capacitors above 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 in parallel. At this time, the large capacitor passes low frequencies and the small capacitor passes high frequencies. The function of the capacitor is to pass high frequencies and resist low frequencies, and pass high frequencies and resist low 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. Specifically used in filtering, large capacitors (1000uF) filter low frequencies, and small capacitors (20pF) filter high frequencies. 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. It converts the change of voltage into the change of current. The higher the frequency, the greater the peak current, thereby buffering the voltage. Filtering is the process of charging and discharging. In the power supply circuit , the rectifier circuit converts the AC into pulsating DC, and a large-capacity electrolytic capacitor is connected after the rectifier circuit to use its charging and discharging characteristics to make the rectified pulsating DC voltage become a relatively stable DC voltage. In practice, in order to prevent the power supply voltage of each part of the circuit from changing due to load changes, electrolytic capacitors of tens to hundreds of microfarads are generally connected to the output end of the power supply and the power input end of the load. Since large-capacity electrolytic capacitors generally have a certain inductance and cannot effectively filter out high-frequency and pulse interference signals, a capacitor with a capacity of 0.001--0.1pF is connected in parallel at both ends to filter out high-frequency and pulse interference.

4) Energy storage

Energy storage capacitors collect charge through rectifiers and transmit 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 spiral terminal capacitors are usually used.

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

1) Decoupling

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

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. 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)

Finally, let’s talk about the precautions for using electrolytic capacitors :

1. Since electrolytic capacitors have positive and negative polarities, they cannot be connected in reverse when used in circuits. In the power supply circuit, when outputting positive voltage, the positive pole of the electrolytic capacitor is connected to the power supply output terminal and the negative pole is grounded. When outputting negative voltage, the negative pole is connected to the output terminal and the positive pole is grounded. When the polarity of the filter capacitor in the power supply circuit is reversed, the filtering effect of the capacitor is greatly reduced. On the one hand, it causes fluctuations in the power supply output voltage. On the other hand, the electrolytic capacitor, which is equivalent to a resistor, heats up due to reverse power supply. When the reverse voltage exceeds a certain value, the reverse leakage resistance of the capacitor will become very small. In this way, the capacitor will explode and be damaged due to overheating soon after power-on.

2. The voltage applied to the two ends of the electrolytic capacitor cannot exceed its allowable working voltage. When designing the actual circuit, a certain margin should be left according to the specific situation. When designing the filter capacitor of the voltage-stabilized power supply, if the AC power supply voltage is 220~, the rectified voltage of the secondary of the transformer can reach 22V. At this time, a 25V electrolytic capacitor can generally meet the requirements. However, if the AC power supply voltage fluctuates greatly and may rise to more than 250V, it is best to choose an electrolytic capacitor with a withstand voltage of more than 30V.

3. The electrolytic capacitor should not be placed near high-power heating elements in the circuit to prevent the electrolyte from drying up due to heat.

4. For filtering of signals with positive and negative polarity, two electrolytic capacitors with the same polarity can be connected in series as a non-polar capacitor.

Filter capacitors are used in power rectifier circuits to filter out AC components and make the output DC smoother. Decoupling capacitors are used in amplifier circuits where AC is not required to eliminate self-excitation and make the amplifier work stably. Bypass capacitors are used when there is a resistor connected, connected across the resistor to allow AC signals to pass smoothly.

1. Understanding the energy storage function of decoupling capacitors

1) Decoupling capacitors are mainly used to remove interference from high-frequency signals such as RF signals. Interference enters through electromagnetic radiation. In fact, the capacitors near the chip also have the function of storing energy, which is secondary. You can think of the total power supply as the Miyun Reservoir. Every household in our building needs water supply. At this time, the water does not come directly from the reservoir, as the distance is too far. By the time the water comes, we are already thirsty. The actual water comes from the water tower on the top of the building. The water tower actually acts as a buffer.

If we look at it microscopically, when high-frequency devices are working, their current is discontinuous and the frequency is very high. There is a distance between the device VCC and the main power supply. Even if the distance is not long, at a very high frequency, the impedance Z=i*wL+R, and the inductance of the line will have a very large impact, which will cause the device to not be supplied with current in time when it needs it. The decoupling capacitor can make up for this deficiency. This is one of the reasons why many circuit boards place small capacitors at the VCC pin of high-frequency devices (a decoupling capacitor is usually connected in parallel to the VCC pin, so that the AC component is grounded from this capacitor.)

2) The high-frequency switching noise generated by active devices during switching will propagate along the power line. The main function of the decoupling capacitor is to provide a local DC power supply to the active device to reduce the propagation of switching noise on the board and guide the noise to the ground.

2. The difference between bypass capacitors and decoupling capacitors

Decoupling: Removes RF energy from high-frequency devices into the power distribution network when the device is switched. Decoupling capacitors can also provide a localized DC voltage source for the device, which is particularly useful in reducing cross-board surge current.

Bypass: Divert unwanted common-mode RF energy away from components or cables. This is primarily done by creating an AC bypass to eliminate unintentional energy from entering sensitive parts, and can also provide baseband filtering (bandwidth limited).

We often see that a decoupling capacitor is connected between the power supply and the ground. It has three functions: first, it serves as the energy storage capacitor of the integrated circuit; second, it filters out the high-frequency noise generated by the device and cuts off its path of propagation through the power supply circuit; third, it prevents the noise carried by the power supply from interfering with the circuit.

In electronic circuits, decoupling capacitors and bypass capacitors both play an anti-interference role. The names are different depending on the location of the capacitor. For the same circuit , the bypass capacitor takes the high-frequency noise in the input signal as the filter object and filters out the high-frequency clutter carried by the previous stage, while the decoupling capacitor, also known as the decoupling capacitor, takes the interference of the output signal as the filter object. The role and application principle of large capacitors in parallel with small capacitors

Large capacitors are usually large in size due to their large capacity, and are usually made using a multi-layer winding method, which results in a large distributed inductance (also called equivalent series inductance, ESL in English). The impedance of inductance to high-frequency signals is very large, so the high-frequency performance of large capacitors is not good. Some small-capacity capacitors are just the opposite. Due to their small capacity, their size can be made very small (shortening the lead reduces the ESL, because a section of wire can also be regarded as an inductor), and they often use a flat capacitor structure, so that small-capacity capacitors have very small ESL, so they have very good high-frequency performance, but due to their small capacity, they have a large impedance to low-frequency signals. Therefore, if we want to allow both low-frequency and high-frequency signals to pass well, we use a large capacitor and a small capacitor in conjunction.

The commonly used small capacitor is a 0.1uF ceramic capacitor. When the frequency is higher, a smaller capacitor can be connected in parallel, such as a few pF or hundreds of pF. In digital circuits, a 0.1uF capacitor is generally connected in parallel to the ground on the power pin of each chip (this capacitor is called a decoupling capacitor, of course, it can also be understood as a power filter capacitor, the closer to the chip, the better), because the signals in these places are mainly high-frequency signals, and a smaller capacitor can be used for filtering.