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The effect of capacitance in lighting power supply

Do not underestimate the small capacitance oh. His role is very large, do you see if he used his electronic products. Where there is a poor use of the dead ugly, so first introduced the role of capacitance.

As one of the passive components of the capacitor, its role is nothing more than the following:

First, applied to the power supply circuit, bypass, to remove the lotus, filtering and energy storage capacity of the role of.

The following classification:

1) filtering

Filtering is an important part of the function of the capacitor. Almost all power circuits are used. In theory, that is, assuming that the capacitance is pure capacitance, the greater the capacitance, the smaller the impedance and the higher the frequency of the pass. But in fact, more than 1uF of the capacitors are mostly electrolytic capacitors, there is a large inductance component, so after the high frequency impedance will increase.

Sometimes we can see that there is a large capacitor with a small capacitor connected in parallel, then the large capacitance through the low frequency, small capacitance through high frequency. The function of the capacitor is to pass through the high resistance and low frequency. The greater the capacitance, the lower the frequency is easier to pass through, the higher the capacitance, the higher the frequency is. Used in the filter, large capacity (1000uF) filter low frequency, small capacitance (20pF) filter high frequency.

Users have been compared to the filter capacitor pond". Since the two ends of the capacitor voltage will not change, it can be seen, the higher the signal frequency, the greater the attenuation, it can be said that the image of a capacitor like a pond, not because of the addition of a few drops of water or evaporation caused by changes in water. It changes the voltage into the change of the 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 the local device. It can make the output of the voltage regulator to be uniform and reduce the load demand. 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 is a good way to prevent the ground potential rise and noise caused by too large input value. The ground bounce is a voltage drop that is connected to the ground through a high current glitch.

3) lotus root

Lotus root, also known as lotus root solution. From the circuit, can always be distinguished as the source of the drive and driven load. If the load capacitance is relatively large, the driving circuit to the capacitor charging and discharging, to complete the signal jump at the rising edge of relatively steep, relatively large current, this current driven power will absorb a large current, the inductance circuit, resistance (especially inductor, chip pins on the causes the current rebound), compared to the normal situation is actually a kind of noise, will affect the normal work of the previous stage. This is coupling.

To remove the lotus capacitor is to play a role in the battery to meet the driving circuit current changes, to avoid mutual interference between the. It will be easier to combine the bypass capacitor and the decoupling capacitor. Bypass capacitor is actually to go together, but the bypass capacitor is generally referred to as high-frequency bypass, which is to increase the frequency of switching noise to improve a low impedance leakage prevention approach.

Generally relatively small high-frequency bypass capacitor according to the resonance frequency is 0.1U, 0.01u, and the decoupling capacitor is relatively large, is 10uF or more, according to the distribution parameters of the circuit, and the driving current changes to determine the size of. The bypass is the interference of the input signal as the filter object, and the decoupling is the interference of the output signal as the filter object, to prevent interference signal return power. This should be the essential difference between them.

4) energy storage

The energy storage capacitor is charged by a rectifier, and the stored energy is transmitted to the output end of the power supply through the converter lead wire. Voltage rating of 40 ~ 450VDC, the capacitance value of between 220 ~ 150 000uF aluminum electrolytic capacitors (such as EPCOS's B43504 or B43505) is more commonly used. According to the different power requirements, the device will sometimes be used in series, in parallel or in the form of a combination, for the power level of more than 10KW of the power supply, usually the larger the size of the tank screw terminal capacitors.

Two, applied to the signal circuit, mainly to complete the coupling, oscillation / synchronization and the role of time constant:

1) coupling

For example, a transistor amplifier emitter self bias resistor, it also makes the signal from the pressure drop back to the input form coupled input and output signals, the resistance is a coupling element, if the resistance is connected in parallel with a capacitor, the capacitor impedance of suitable capacity of AC signal small, it can reduce the coupling effect of resistance, so that this capacitance decoupling capacitor.

2) oscillation / synchronization

Including RC, LC oscillator and crystal load capacitance belong to this category.

(3) time constant

This is the common R, C series integrated circuit. When the input signal voltage is added to the input, the voltage on the capacitor (C) rises gradually. The charging current decreases with the increase of the voltage. The characteristics of the current through the resistor (R) and capacitance (C) are described by the following formula:

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

Let's talk about the role of the capacitor

Precautions in use of capacitors

A. what is a good capacitor?

(1) the larger the capacitance, the better.

Many people tend to use large capacity capacitors in the replacement of capacitors. We know that the greater the capacitance, the stronger the ability to provide current compensation for the IC. Not to mention the increase in the capacity of the capacitor brought about by large volume, increase the cost of air flow and heat dissipation. The key lies in the parasitic inductance on the capacitor, the capacitor discharge circuit will resonate at a certain frequency. At the resonance point, the impedance of the capacitor is small.

Therefore, the impedance of the discharge circuit is the smallest, and the effect of supplementary energy is the best. However, when the frequency exceeds the resonance point, the impedance of the discharge circuit begins to increase, and the capacity of the capacitor to provide current decreases. The greater the capacitance value of the capacitance, the lower the resonant frequency, the smaller the frequency range of the capacitor can compensate the current effectively. From the point of view of the capacity of the capacitor to provide high frequency current, the larger the capacitance is, the better the viewpoint is, and there is a reference value in the circuit design.

(2) the same capacity of the capacitor, the smaller the smaller the better shunt

Withstand voltage, temperature resistance, capacitance, ESR (equivalent resistance) is a few important parameters of the capacitor, the lower the better the nature of ESR. ESR and capacitance capacity, frequency, voltage, temperature and so on. When the voltage is fixed, the larger the capacity, the lower the ESR. In the board design using a small number of capacitors and even more is out of the PCB space constraints, so that some people think that the more the parallel small resistance, the lower the ESR, the better the effect. In theory, it is necessary to take into account the impedance of the capacitor pin joint, the use of multiple small capacitors in parallel, the effect is not necessarily prominent.

(3) the lower the ESR, the better the effect.

Combined with the above mentioned power supply circuit, the capacity of the input capacitor is larger than the input capacitance. Relative capacity requirements, the requirements of ESR can be properly reduced. Because the input capacitor is mainly voltage, followed by the absorption of MOSFET switching pulse. For the output capacitor, the voltage requirements and capacity can be properly reduced a little. The ESR requirement is a little higher, because there is a sufficient amount of current to be guaranteed. But it should be noted that the ESR is not as low as possible, low ESR capacitor will cause the switch circuit oscillation. However, the complexity of the damping circuit will increase the cost. Board design, where there is a general reference value, as a component selection parameters, to avoid the vibration circuit caused by the increase in costs.

(4) good capacitance represents high quality.

"Only capacitance theory" has been very popular, some manufacturers and media also deliberately put this thing into a selling point. In the board design, circuit design is the key. And some manufacturers can use two phase power supply than some manufacturers use four phase power supply more stable products, blindly using high capacity, not necessarily able to make good products. To measure a product, we must consider the full range of multi angle, must not be the role of intentionally or unintentionally exaggerated.

Face interview with B. capacitors

Burst type: divided into two categories, the input capacitor burst and output capacitor explosion.

For the input capacitor, I mean that the C1, C1 on the current received by the power filter. The input capacitance is related to the quality of the input current. Too much burr voltage, peak voltage is too high, the current instability and so on, so that the capacitor is too charged and discharged too frequently, a long time in such a working environment of the capacitor, the internal temperature rise quickly. More than the explosion limit of the mouth of the explosion will occur.

For the output capacitor, I said that the C2, the power module adjusted after the current filter. Here the current through a filter, relatively stable, the possibility of the occurrence of relatively small explosion of a lot of. However, if the ambient temperature is too high, the capacitor is also prone to slurry. Burst, also reported. The use of garbage to explode naturally, retribution ah. For those who know the past, see its present fruit; to know the future fruit, see its present reason.

Reasons for electrolytic capacitor explosion:

There are many reasons for the capacitance explosion, such as the current is greater than the allowable steady wave current, the use of voltage beyond the operating voltage, reverse voltage, frequent charging and discharging, etc.. But the most direct reason is the high temperature. We know that the capacitance has an important parameter is the temperature resistance, refers to the capacitance of the internal electrolyte boiling point. When the internal temperature of the capacitor reaches the boiling point of the electrolyte, the electrolyte begins to boil, and the pressure inside the capacitor increases. So the temperature is the direct cause of the explosion.

Capacitor design life of about 20 thousand hours, the impact of environmental temperature is also great. The service life of the capacitor decreases with the increase of temperature. The experimental results show that the life of the capacitor can be reduced by 10 DEG c.. 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 before the board to go through a long period of high temperature environment test. Even at 100 DEG C, high quality capacitors can work for thousands of hours.

At the same time, we refer to the life of the capacitor refers to the capacitor in the process, the capacitance will not exceed the standard range of 10%. Capacitor life refers to the capacity of the problem, rather than the design life after the arrival of the slurry. Simply can not guarantee the capacitance of the design capacity of the standard.

Therefore, in a short period of time, the normal use of the board capacitance occurs on the case of plasma explosion, which is the quality of capacitors. In addition, the normal use of the situation may also occur in the case of capacitor explosion. For example, hot swappable computer accessories will also lead to local board

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