Capacitance function in the hottest lighting power

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Capacitance in lighting power supply

don't underestimate small capacitors. He plays a great role. Have you ever used his electronic products. There are everywhere. If it is not used well, it will die ugly, so first introduce the role of capacitors

as one of the passive components, the function of capacitance is no more than the following:

first, it is applied to the power circuit to realize the function of capacitance in bypass, decoupling, filtering and energy storage

the following classification is detailed:

1) filtering

filtering is a very important part of the role of capacitance. Almost all power circuits will be used. Theoretically (i.e. assuming that the capacitance is pure capacitance), the larger the capacitance, the smaller the impedance, and the higher the passing frequency. But in fact, most capacitors over 1uF are electrolytic capacitors, which have a large inductance component, so the impedance will increase when the frequency is high

sometimes you can see a large electrolytic capacitor connected in parallel with a small capacitor. At this time, the large capacitor is connected to the low frequency and the small capacitor to the high frequency. The function of capacitance is to connect high resistance and low resistance, and connect high resistance and low frequency. The larger the capacitance, the easier it is to pass through the low frequency, and the larger the capacitance, the easier it is to pass through the high frequency. Specifically used in filtering, large capacitance (1000uf) filters low frequency and small capacitance (20pF) filters high frequency

some friends once compared filter capacitors to "ponds". 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 vividly said that the capacitor is like a pond, which will not cause the change of water volume 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, thus buffering the voltage. Filtering is the process of charging and discharging

2) bypass

bypass capacitor is an energy storage device that provides energy for local devices, which can homogenize the output of the voltage regulator 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 to the power supply pin and ground pin of the load device as possible. This can well prevent the ground potential rise and noise caused by excessive input value. The ground spring is the voltage drop when the ground connection passes through a large current burr

3) lotus root removal

lotus root removal, also known as lotus root removal. In terms of circuit, it can always be divided into driving source and driven load. If the load capacitance is relatively large, the driving circuit needs to charge and discharge the capacitance to complete the signal jump. When the rising edge is steep, the current is relatively large, so the driving current will absorb a large power supply current. Due to the inductance and resistance in the circuit (especially the inductance on the chip pin, it will rebound), this current is actually a kind of noise compared with normal conditions, It will affect the normal work of the previous stage. This is coupling

decoupling capacitor is to play the role of a battery to meet the current change of the driving circuit and avoid mutual coupling interference. It will be easier to understand by combining bypass capacitance and decoupling capacitance. The bypass capacitance is actually decoupled, but the bypass capacitance generally refers to the high-frequency bypass, which is to improve a low impedance leakage way for high-frequency switching noise

the high-frequency bypass capacitance is generally small. According to the resonant frequency, it is generally 0.1U, 0.01u, etc., while the decoupling capacitance is generally large, which is 10uF or the three gangs do not commit a crime at ordinary times. It is determined according to the distribution parameters in the circuit and the change of driving current. Bypass takes the interference in the input signal as the filtering object, while decoupling takes 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

4) energy storage

......................... (1) the energy storage capacitor collects the charge through the rectifier and transmits the stored energy to the output end of the power supply through the converter lead. Aluminum electrolytic capacitors (such as b43504 or b43505 of EPCOS) with a voltage rating of 40-450vdc and a capacitance of 220-150 000uf are commonly used. According to different power supply requirements, devices sometimes adopt the form of series connection, parallel connection or their combination. For power supply with power level more than 10kW, tank shaped spiral terminal capacitor with larger volume is usually used

second, it is applied to signal circuits, mainly to complete the functions of coupling, oscillation/synchronization and time constant:

1) coupling

for example, the emitter of transistor amplifier has a self-contained bias resistance, which makes the signal generate voltage drop and feed back to the input end to form input-output signal coupling. This resistance is the element that generates coupling. If a capacitor is connected in parallel at both ends of this resistance, Due to the small impedance of the capacitor with appropriate capacity to the AC signal, the coupling effect caused by the resistance is reduced, so this capacitor is called decoupling capacitor

2) oscillation/synchronization

including RC, LC oscillators and crystal load capacitors 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 with the increase of voltage. The characteristics of current passing through resistance (R) and capacitance (c) are described by the following formula:

I = (v/r) e- (t/cr)

after we know the role of capacitance, let's talk about the precautions of

capacitance in use

A. what is a good capacitance

(1) the larger the capacitance, the better the applicability

many people tend to use high-capacity capacitors in the replacement of capacitors. We know that the larger the capacitance is, the stronger the current compensation ability for IC is. Not to mention that the increase in capacitance will lead to a larger volume, which will not only increase the cost, but also affect the air flow and heat dissipation. The key is that there is parasitic inductance on the capacitor, and the capacitor discharge circuit will resonate at a certain frequency point. 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 supplementing energy is also 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 begins to decline. The larger the capacitance value of the capacitor, the lower the resonant frequency, and the smaller the frequency range in which the capacitor can effectively compensate the current. From the perspective of ensuring the capacity of capacitors to provide high-frequency current, the view that the larger the capacitance, the better is wrong. There is a reference value in general circuit design

(2) for capacitors with the same capacity, the more small capacitors connected in parallel, the better

withstand voltage value, temperature resistance value, capacitance value, ESR (equivalent resistance) are several important parameters of capacitance, and the lower the ESR, the better. ESR is related to capacitance, frequency, voltage, temperature, etc. When the voltage is fixed, the larger the capacity, the lower the ESR. In board design, using multiple small capacitors in parallel is the limitation of PCB space, so some people think that the more small resistors in parallel, the lower the ESR, the better the effect. Theoretically, it is so, but considering the impedance of the capacitor pin solder joint, the effect is not necessarily prominent when multiple small capacitors are connected in parallel

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

in combination with our improved power supply circuit above, the capacity of the input capacitor should be larger for the input capacitor. The requirements of relative capacity and ESR can be appropriately reduced. Because the input capacitor mainly withstands voltage, and then absorbs the switching pulse of MOSFET. For the output capacitor, the requirements and capacity of withstand voltage can be appropriately reduced. The requirement of ESR is a little higher, because what we need to ensure here is sufficient current flow. However, it should be noted that the lower the ESR is, the better. Low ESR capacitance will cause switching circuit oscillation. The complexity of vibration elimination circuit will lead to the increase of cost. In the board design, there is usually a reference value here, which is used as the component selection parameter to avoid the increase of cost caused by the vibration elimination circuit

(4) good capacitance represents high quality

"capacitance only theory" was once popular, and some manufacturers and media also deliberately made it a selling point. In board design, the level of circuit design is the key. Just as some manufacturers can use two-phase power supply to make products more stable than some manufacturers use four-phase power supply, blindly using high-priced capacitors may not make good products. To measure a product, we must consider it from all aspects and angles. We must not exaggerate the role of capacitance intentionally or unintentionally

b. face-to-face interview of capacitance explosion

types of explosion: there are two types, input capacitance explosion and output capacitance explosion

for the input capacitance, I mean C1, C1 filters the current received by the power supply. The burst of the input capacitor is related to the quality of the input current of the power supply. Too much burr voltage, too high peak voltage, unstable current, etc. all make the capacitor charge and discharge too frequently. The internal temperature of the capacitor that has been in this kind of working environment for a long time increases rapidly. If the bearing limit of the vent is exceeded, slurry explosion will occur

for the output capacitor, as I call C2, filter the current adjusted by the power module. The current here is relatively stable after one filtration, and the possibility of slurry explosion is relatively small. However, if the ambient temperature is too high, the capacitor is also prone to slurry explosion. Burst, report also. If you use garbage, it will explode naturally. It will be retribution. If you want to know the cause of the past, see its present result; If you want to know the future result, see its present cause

reasons for electrolytic capacitor slurry explosion:

there are many reasons for capacitor slurry explosion, such as the current is greater than the allowable stable wave current, the use voltage exceeds the working voltage, reverse voltage, frequent charge and discharge, etc. But the most direct reason is high temperature. We know that an important parameter of the capacitor 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, and the pressure inside the capacitor increases. When the pressure exceeds the bearing limit of the explosion vent, the slurry explosion occurs. Therefore, temperature is the direct cause of capacitor slurry explosion

the designed service life of the capacitor is about 20000 hours, which is also greatly affected by the ambient temperature. The service life of the capacitor decreases with the increase of temperature. The experiment shows that the service life of the capacitor will be halved when the ambient temperature increases by 10 ℃. The main reason is that the temperature accelerates the chemical reaction and makes the medium degenerate and fail with time, so the life of the capacitor ends. In order to ensure the stability of the capacitor, the capacitor must be tested in a high temperature environment for a long time before inserting the board. Even at 100 ℃, high-quality capacitors can work for thousands of hours

at the same time, the service life of the capacitor we mentioned means that the capacitance will not exceed 10% of the standard range during the use of the capacitor. Capacitor life refers to the problem of capacitor capacity, rather than slurry explosion after the design life is reached. It's just that it can't be guaranteed in the experiment that the experimenter sees the capacity standard of opening the oil delivery valve and then bulabula certifies the design of capacitance

therefore, in a short period of time, the normally used board capacitor will explode, which is the problem of capacitor quality. In addition, abnormal use may also lead to capacitor plasma explosion. For example, hot plugging computer accessories will also lead to drastic changes in the current and voltage of local circuits of the board, which will lead to capacitor failure

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