Application in power supply

The function of capacitance applied to the power supply circuit
As an important passive component, capacitance is widely used. This paper will introduce the role of capacitors in power supply circuits, achieving bypass, decoupling, filtering and energy storage.
As one of the passive components, the capacitance has many functions.
It is applied to power supply circuit to realize the function of capacitance in bypass, decoupling, filtering and energy storage.
1) filtering
Filtering is a very important part of capacitance. Almost all power circuits are used. Theoretically, assuming capacitance is pure capacitance, the greater the capacitance, the smaller the impedance, and the higher the frequency of passing through. But in fact, more than 1uF capacitors are mostly electrolytic capacitors, and have a large inductance component, so the impedance will increase when the frequency is high. Sometimes there is a larger capacitance than the electrolytic capacitor in parallel with a small capacitor. At that time, the large capacitor goes through the low frequency, and the small capacitor goes through the high frequency. The function of capacitance is to pass through high resistance and low frequency. The higher the capacitance is, the easier it is to pass through the low frequency. The higher the capacitance is, the easier it is to pass the high frequency. Specifically used in filtering, large capacitance (1000uF) filter low frequency, small capacitance (20pF) filter high frequency.
A net friend used to compare the filter capacitor to the "pond". Because the voltage at both ends of the capacitor does not change, it can be seen that the higher the frequency of the signal, the greater the attenuation, and the more image the capacitance is like a pond, which will not cause the change of water due to the addition or evaporation of several drops of water. It transforms the change of voltage into the change of current. The higher the frequency, the greater the peak current, thus buffering the voltage. Filtering is the process of charging and discharging.
2) bypass
The bypass capacitor is an energy storage device that provides energy for local devices. It can make the output of the regulator even and reduce the load demand. Like a small rechargeable battery, the bypass capacitor can be charged and discharged to the device. To minimize impedance, bypass capacitors should be close to the power supply pins and ground pins of the load devices. This can prevent ground potential elevation and noise caused by excessive input value. The earth bomb is the voltage drop of the earth connection when it is burr through high current.
3) remove the lotus root
The lotus root is also called the lotus root. From the circuit, it can always be divided into driving source and driven load. If the load capacitance is large, the driving circuit will charge and discharge the capacitor to complete the jump of the signal. When the rising edge is more steep, the current is larger, so that the current will absorb a lot of power current, because the inductance in the circuit, especially the inductance on the chip foot, will produce the inverse. This kind of current is actually a noise relative to the normal condition, which will affect the normal work of the former stage. This is the coupling. The decoupling capacitor acts as a battery to meet the changes in the current of the drive circuit and avoid coupling interference. The combination of bypass capacitance and decoupling capacitance will be easier to understand. The bypass capacitor is actually the decoupling, but the bypass capacitor usually refers to the high frequency bypass, which is to improve the switching noise of high frequency a low impedance way. High frequency bypass capacitance is generally small, according to the resonance frequency is generally 0.1U, 0.01u and so on, and the decoupling capacitance is generally larger, is 10uF or larger, according to the circuit distribution parameters, and the change of the driving current change size to determine. The bypass is to filter the interference in the input signal, and decoupling is the interference of the output signal as a filter object, so as to prevent the interference signal from returning to the power supply. This should be the essential difference between them.
4) energy storage
The energy storage capacitor collects the charge through the rectifier and transmits the stored energy through the converter lead to the output end of the power supply. Electrolytic capacitors with a voltage rating of 40 to 450VDC and capacitance values between 220~150 000uF are quite common. According to different power requirements, devices are sometimes used in series, parallel or combination. For power supply more than 10KW, a large tank type spiral terminal capacitor is usually used.

After we know the function of the capacitor, let's talk about the matters needing attention in the use of the capacitor.
A. what is a good capacitance.

1. the larger the capacity, the better.
Many people often use large capacity capacitors in the replacement of capacitors. We know that the greater the capacitance, the stronger the current compensation capability for IC. It is not to say that the increase of capacitance increases the volume and increases the cost, but also affects the air flow and heat dissipation. The key is that there is a parasitic inductance on the capacitor, and the capacitor discharge circuit will resonate at a certain frequency point. At the resonant point, the impedance of the capacitor is small. Therefore, the impedance of discharge circuit is the smallest, and the effect of replenish energy is the best. But when the frequency exceeds the resonance point, the impedance of the discharge circuit begins to increase, and the current capacity of the capacitor begins to decrease. The larger the capacitance is, the lower the resonant frequency is, and the smaller the frequency range of the capacitor can be effectively compensated. From the angle of ensuring the capacity of the capacitor to provide high frequency current, the view that the greater the better the capacitance is, is wrong, and there is a reference value in the general circuit design.
2. capacitors with the same capacitance, the smaller the capacitor, the better.
Withstand voltage, temperature tolerance, capacitance, ESR (equivalent resistance) are several important parameters of capacitance. The lower the ESR, the better. ESR is related to capacitance, frequency, voltage, temperature and so on. When the voltage is fixed, the larger the capacity, the lower the ESR. In the design of the board, the use of multiple small capacitors and even more is the limit of PCB space, so some people think that the more parallel small resistance, the lower the ESR, the better the effect. This is the theory, but considering the impedance of the capacitive pin soldered joints, using multiple small capacitors in parallel, the effect is not necessarily prominent.
The lower the 3.ESR, the better the effect.
Considering the power supply circuit above, input capacitance is larger for input capacitors. The requirement of ESR can be reduced appropriately by the requirement of relative capacity. Because the input capacitance is mainly voltage withstanding, followed by the absorption of MOSFET switching pulse. For output capacitors, the withstand voltage requirement and capacity can be reduced a little. The requirement of ESR is higher, because the amount of current passed is guaranteed. But what we should notice here is that ESR is not as low as possible. Low ESR capacitance will cause oscillation of switch circuit. The complexity of the vibration elimination circuit will also lead to the increase of cost. In the board design, there is generally a reference value, which is used as a component to select parameters and avoid vibration elimination circuit, resulting in an increase in cost.
4. good capacitors represent high quality.
"Capacitor theory" has been flourished, and some manufacturers and media have deliberately made it a selling point. In the design of board, the level of circuit design is the key. And some manufacture

Face talk on B. capacitor burst
Types of explosion slurry are classified into two categories: input capacitor explosion slurry and output capacitor explosion slurry.
For input capacitors, I mean C1, C1 filters the current received by the power supply. The input capacitor explosion is related to the quality of the power input current. Excessive burr voltage, high peak voltage, current instability and so on make the capacitor overcharge and discharge too frequently, the capacitance in this kind of working environment for a long time, the internal temperature rises very quickly. Explosion will occur if the limit of the vent is exceeded.
For the output capacitor, the C2 I used to filter the current after being adjusted by the power module. The current has been filtered for a relatively stable time, and the possibility of bursting is relatively small. However, if the ambient temperature is too high, the capacitance is also prone to burst. Detonating, newspaper also. The use of garbage is natural to explode, retribution. He who wants to know the past depends on his present fruit; he who wants to know the future will see his present cause.
The reasons of electrolytic capacitor burst:
There are many reasons for capacitor explosion. For example, the current is greater than the allowable steady wave current, the use voltage exceeds the working voltage, the reverse voltage, and the frequent charging and discharging. But the most direct reason is the high temperature. We know that an important parameter of capacitance is the temperature resistance value, which refers to the boiling point of the electrolyte inside the capacitor. When the internal temperature of the capacitor reaches the boiling point of the electrolyte, the electrolyte begins to boil, the pressure inside the capacitor rises, and the pulping occurs when the pressure exceeds the limit of the vent. Therefore, temperature is the direct cause of capacitance explosion. The design life of capacitance is about 20 thousand hours, and the influence of environmental temperature is also great. The service life of the capacitor decreases with the increase of temperature. The experiment proves that the life of the capacitor will be reduced by half if the ambient temperature rises by 10 degrees. The main reason is that the temperature accelerates the chemical reaction and causes the medium to degenerate with time. In order to ensure the stability of the capacitor, the capacitor must be tested for a long time in the high temperature environment before the plug-in board. Even at 100 degrees, high quality capacitors can work for thousands of hours. At the same time, the life of the capacitor mentioned is that the capacitance of the capacitor will not exceed 10% of the standard range during the use of the capacitor. Capacitor life refers to the problem of capacitance, rather than burst after the arrival of design life. Just can't guarantee the capacitance standard of capacitance design.
Therefore, in a short period of time, the normal use of the plate capacitor will explode slurry, which is the quality of capacitor. In addition, the abnormal usage may also cause capacitor explosion. For example, hot plug computer accessories will also lead to sharp changes in the current and voltage of the local circuit of the card, resulting in the failure of capacitor utilization.