Capacitance in lighting power

Capacitors, which are one of the passive components, function in the following ways:

1. It is applied to the power supply circuit to realize the functions of capacitors in bypass, decoupling, filtering and energy storage. The following classification details:

1) Filtering

Filtering is an important part of the function of a capacitor. It is used in almost all power circuits. Theoretically (that is, assuming the capacitor is a pure capacitor), the larger the capacitance, the smaller the impedance and the higher the frequency of passing. However, in fact, most of the capacitors exceeding 1 uF are electrolytic capacitors, which have a large inductance component, so the impedance will increase after the frequency is high. Sometimes you will see a small capacitor with a larger capacitance and a small capacitor. At this time, the large capacitor passes through the low frequency and the small capacitor passes through the high frequency. The function of the capacitor is to pass the high impedance and low frequency. The larger the capacitance, the easier the low frequency passes, and the higher the capacitance, the easier it is to pass. Specifically used in filtering, large capacitor (1000uF) filter low frequency, small capacitor (20pF) filter high frequency.

Some netizens have compared filter capacitors to "water ponds." Since the voltage across the capacitor does not change, 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 water quantity will not be changed due to the addition or evaporation of a few drops of water. It converts the change in voltage into a change in current. The higher the frequency, the larger the peak current, which buffers the voltage. Filtering is the process of charging and discharging.

2) Bypass

The bypass capacitor is an energy storage device that supplies energy to the local device, which equalizes the output of the regulator and reduces the load requirements. Like a small rechargeable battery, the bypass capacitor can be charged and discharged to the device. To minimize impedance, the bypass capacitor should be as close as possible to the power supply and ground pins of the load device. This can well prevent ground potential elevation and noise caused by excessive input values. The ground bounce is the voltage drop at which the ground connection is passed through a large current glitch.

3) Go to squat

Going to squat, also known as defamatory. From the circuit, it can always be distinguished as the source of the drive and the load being driven. If the load capacitance is relatively large, the drive circuit must charge and discharge the capacitor to complete the signal transition. When the rising edge is steep, the current is relatively large, so the driven current will absorb a large supply current, due to the circuit. The inductance, the resistance (especially the inductance on the chip pin, will produce a rebound), this current is actually a kind of noise compared to the normal situation, which will affect the normal operation of the previous stage. This is the coupling. The decoupling capacitor acts as a battery to satisfy the change of the drive circuit current and avoid mutual coupling interference. Combining bypass and decoupling capacitors will be easier to understand. The bypass capacitor is actually decoupled, except that the bypass capacitor generally refers to the high-frequency bypass, which is to improve the low-frequency leakage prevention path for high-frequency switching noise. The high-frequency bypass capacitor is generally small. According to the resonant frequency, it is generally 0.1u, 0.01u, etc., and the decoupling capacitor is generally large, 10uF or larger, which is determined according to the distributed parameters in the circuit and the variation of the driving current. Bypass is to filter the interference in the input signal, and decoupling is to filter the interference of the output signal to prevent the interference signal from returning to the power supply. This should be their essential difference.

4) Energy storage

The energy storage capacitor collects charge through the rectifier and transfers the stored energy through the converter lead to the output of the power supply. Aluminum electrolytic capacitors (such as EPCOS B43504 or B43505) with a voltage rating of 40 to 450 VDC and a capacitance between 220 and 150 000 uF are more commonly used. Depending on the power requirements, the devices are sometimes used in series, in parallel, or a combination thereof. For power supplies with power levels above 10 kW, bulky screw-type terminal capacitors are typically used.

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

1) Coupling

For example, the emitter of a transistor amplifier has a self-biasing resistor, which at the same time causes the voltage drop of the signal to be fed back to the input to form an input-output signal coupling. This resistor is the component that produces the coupling. Parallel connection of a capacitor, because the capacitor of the appropriate capacity has a small impedance to the AC signal, thus reducing the coupling effect caused by the resistor, so the capacitor is called a decoupling capacitor.

2) Oscillation / synchronization

Load capacitors including RC, LC oscillators and crystals fall into this category.

3) Time constant

This is the common integration circuit of R and C connected in series. When the input signal voltage is applied to the input, the voltage across the capacitor (C) gradually rises. The charging current decreases as the voltage rises. The characteristics of the current through the resistor (R) and capacitor (C) are described by the following formula:

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

We know the role of the capacitor, let's talk about the precautions of the capacitor in use.

A. What is a good capacitor.

1. The larger the capacitance, the better.

Many people often prefer to use large-capacity capacitors in the replacement of capacitors. We know that the larger the capacitance, the stronger the current compensation capability provided to the IC. Not to mention that the increase in capacitance increases the volume, which increases the cost and also affects air flow and heat dissipation. The key is that there is parasitic inductance on the capacitor, and the capacitor discharge loop 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 replenishing energy is also the best. However, when the frequency exceeds the resonance point, the impedance of the discharge loop begins to increase, and the capacity of the capacitor to supply current begins to decrease. The larger the capacitance 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 ability of the capacitor to supply high-frequency current, the larger the better the capacitor is, the more accurate it is. There is a reference value in the general circuit design.

2. Capacitors of the same capacity, the more small capacitors in parallel, the better

The withstand voltage value, temperature resistance value, capacitance value, ESR (equivalent resistance), etc. are several important parameters of the capacitor, and the lower the ESR, the better. ESR is related to the capacity, frequency, voltage, and temperature of the capacitor. When the voltage is fixed, the larger the capacity, the lower the ESR. In the design of the board, multiple small capacitors are used and the connection space is limited. Therefore, some people think that the more parallel resistors, the lower the ESR and the better the effect. In theory, this is to take into account the impedance of the capacitor pin solder joints. The use of multiple small capacitors in parallel does not necessarily highlight the effect.

3. The lower the ESR, the better the effect.

Combined with our improved power supply circuit above, the input capacitor has a larger capacity for the input capacitor. Relative to the capacity requirements, the requirements for ESR can be appropriately reduced. Because the input capacitor is mainly withstand voltage, followed by the switching pulse of the MOSFET. For output capacitors, the withstand voltage requirements and capacity can be appropriately reduced. The requirements for ESR are higher, because there is enough current throughput to be guaranteed. However, it should be noted here that the ESR is not as low as possible, and the low ESR capacitor will cause the switching circuit to oscillate. The complexity of the vibration absorbing circuit also leads to an increase in cost. In the design of the board, there is generally a reference value here. This is used as a component selection parameter to avoid the increase of the cost caused by the vibration damping circuit.

4. Good capacitance represents high quality.

"Only Capacitance Theory" was once a spurt, and some manufacturers and the media also deliberately made this thing a selling point. In board design, the level of circuit design is key. Some manufacturers can use two-phase power supply to make products that are more stable than those of four-phase power supply. Some high-priced capacitors do not necessarily make good products. To measure a product, we must consider all aspects and angles, and we must not exaggerate the effect of the capacitor intentionally or unintentionally.

B. Face-to-face interview

Types of blasting: There are two types, the input capacitor blasting and the output capacitor blasting.

For the input capacitor, I mean C1, C1 filters the current received by the power supply. The input capacitor is related to the quality of the power input current. Excessive glitch voltage, high peak voltage, unstable current, etc. make the capacitor too charge and discharge too frequently. The capacitor in this kind of working environment for a long time, the internal temperature rises very quickly. Explosion will occur beyond the limit of the venting opening.

For the output capacitor, as I said C2, the current adjusted by the power module is filtered. Here, the current is filtered once and is relatively stable, and the possibility of bursting is relatively small. However, if the ambient temperature is too high, the capacitor is also prone to bursting. Explosive, newspaper also. The use of garbage is naturally explosive, retribution. For those who want to know the past, see their present results; for those who want to know the future, see their current causes.

The reason for the electrolytic capacitor explosion:

There are many reasons for capacitor explosion, such as the current is greater than the allowable steady-state current, the operating voltage exceeds the operating voltage, the reverse voltage, and the frequent charge and discharge. But the most direct reason is the 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, the pressure inside the capacitor rises, and when the pressure exceeds the end of the venting port, the slurry collapses. Therefore, temperature is the direct cause of capacitor explosion. Capacitor design has a service life of approximately 20,000 hours and is also greatly affected by ambient temperature. The lifetime of the capacitor decreases with increasing temperature. Experiments have shown that for every 10 °C increase in ambient temperature, the lifetime of the capacitor is halved. The main reason is that the temperature accelerates the chemical reaction and the medium deteriorates with time, so the life of the capacitor is terminated. In order to ensure the stability of the capacitor, the capacitor must pass the long-term high temperature environment test before inserting the board. Even at 100 ° C, high quality capacitors can operate for thousands of hours. At the same time, the life of the capacitor we mentioned refers to the capacitance of the capacitor does not exceed 10% of the standard range change during use. Capacitance life refers to the problem of capacitance capacity, rather than bursting after the design life has arrived. There is no guarantee of the capacity standard for the design of the capacitor.

Therefore, in a short period of time, the normal use of the board capacitor will explode, which is the quality of the capacitor. In addition, in the case of abnormal use, it is also possible to explode the capacitor. For example, hot-swapping computer accessories can also cause drastic changes in current and voltage of the board's local circuits, causing capacitor failure.