Detailed explanation of the role of capacitors in lighting power supplies

In daily life, we should not underestimate the small capacitor. It plays a big role. There are electronic products that use capacitors. There are capacitors everywhere. If they are not used well or well, they will be used badly. So first introduce the role of capacitors:

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

1. Applied to power circuits to achieve bypass, decoupling, filtering and energy storage. The role of capacitors is detailed in the following categories:

1) Filtering

Filtering is a very important part of the role of capacitors. It is used in almost all power circuits. Theoretically (assuming that the capacitor is a pure capacitor), the larger the capacitance, the smaller the impedance and the higher the frequency. But in fact, most capacitors over 1uF are electrolytic capacitors, which have a large inductance component, so the impedance will increase after the frequency is high. Sometimes you will see a large electrolytic capacitor in parallel with a small capacitor. At this time, the large capacitor passes low frequencies and the small capacitor passes high frequencies. The role of capacitors is to pass high frequencies and low impedance, and pass high frequencies and block low frequencies. The larger the capacitance, the easier it is for low frequencies to pass, and the larger 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.

Some netizens have compared filter capacitors to "ponds". Since the voltage across 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 voltage changes into current changes. The higher the frequency, the greater the peak current, thus buffering the voltage. Filtering is the process of charging and discharging.

2) Bypass

Bypass capacitors are energy storage devices that provide energy to local devices. They 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 as possible to the power supply pins and ground pins of the load device. This can effectively prevent the ground potential from being raised and noise caused by excessive input values. Ground bounce is the voltage drop at the ground connection when a large current burr passes through it.

3) De-coupling

De-coupling is also called de-coupling. 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 capacitance 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. 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.

4) Energy

storage Energy storage capacitors collect charge through rectifiers and transmit the stored energy to the output end of the power supply through converter leads. Aluminum electrolytic capacitors with voltage ratings of 40 to 450VDC and capacitance values ​​between 220 and 150 000uF (such as B43504 or B43505 from EPCOS) are more commonly used. Depending on different power supply requirements, devices are sometimes connected in series, in parallel, or in combination. For power supplies with power levels exceeding 10KW, larger can-shaped screw terminal capacitors are usually used.

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

1) Coupling

For example, the emitter of a transistor amplifier has a self-biased resistor, which also 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 at both ends of this resistor, the coupling effect generated by the resistor is reduced due to the small impedance of the capacitor with appropriate capacity to the AC signal. Therefore, this capacitor is called a decoupling capacitor.

2) Oscillation/synchronization

Including RC, LC oscillators and crystal load capacitors belong to this category.

3) Time constant

This is the common R, C series integral circuit. When the input signal voltage is applied to the input end, the voltage on the capacitor (C) gradually increases. And its charging current decreases as the voltage increases. The characteristics of current passing through the resistor (R) and capacitor (C) are described by the following formula:

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

After we know the role of capacitors, let's talk about the precautions for capacitors in use

. A. What is a good capacitor.

1. The larger the capacitance, the better.

Many people tend to use large-capacity capacitors when replacing 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 will increase the volume, increase the cost, and affect 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 point. At the resonance point, the impedance of the capacitor is small. Therefore, the impedance of the discharge loop is the smallest, and the effect of replenishing energy is the best. But when the frequency exceeds the resonance point, the impedance of the discharge loop begins to increase, and the current supply capability of the capacitor 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 for the current. From the perspective of ensuring the ability of the capacitor 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 of the same capacity, the more small capacitors in parallel, the better

Withstand voltage, temperature, capacitance, ESR (equivalent resistance), etc. are several important parameters of capacitors. For ESR, the lower the better. ESR is related to the capacitance, frequency, voltage, temperature, etc. of the capacitor. When the voltage is fixed, the larger the capacitance, the lower the ESR. In the board design, the use of multiple small capacitors in parallel is mostly due to the limitation of PCB space. Some people think that the more small resistors are connected in parallel, the lower the ESR and the better the effect. In theory, this is true, but considering the impedance of the capacitor pin solder joint, the effect of using multiple small capacitors in parallel is not necessarily outstanding.

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

Combined with the improved power supply circuit above, for the input capacitor, the input capacitor capacity should be larger. Relative to the capacity requirements, the ESR requirements can be appropriately reduced. Because the input capacitor is mainly for withstand voltage, and secondly for absorbing the switching pulse of MOSFET. For the output capacitor, the withstand voltage requirements and capacity can be appropriately reduced. The ESR requirements are higher, because here we need to ensure sufficient current flow. But it should be noted that the lower the ESR, the better. Low ESR capacitors will cause oscillation in the switching circuit. The complexity of the vibration elimination circuit will also lead to an increase in cost. In board design, there is usually a reference value, which is used as a parameter for component selection to avoid the increase in cost caused by the vibration elimination circuit.

4. Good capacitors represent high quality.

The "capacitor-only theory" was once very popular, and some manufacturers and media also deliberately made this 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 more stable products than some manufacturers use four-phase power supply, blindly using high-priced capacitors may not necessarily make good products. When measuring a product, you must consider it from all angles and must not exaggerate the role of capacitors intentionally or unintentionally.

B. A face-to-face discussion on capacitor explosion

Types of explosion:

There are two categories, input capacitor explosion and output capacitor explosion.

For the input capacitor, I am talking about C1, which filters the current received by the power supply. The explosion of the input capacitor is related to the quality of the input current of the power supply. Excessive burr voltage, high peak voltage, unstable current, etc. all cause the capacitor to charge and discharge too frequently. The internal temperature of the capacitor that is in this kind of working environment for a long time rises quickly. If the current exceeds the tolerance limit of the explosion vent, the capacitor will explode.

For the output capacitor, as I said, C2 filters the current adjusted by the power module. The current here is filtered once and is relatively stable, so the possibility of explosion is relatively small. However, if the ambient temperature is too high, the capacitor is also prone to explosion. Explosion is retribution. Using garbage will naturally explode, it's retribution. If you want to know the past cause, see the present result; if you want to know the future result, see the present cause.

Reasons for the explosion of electrolytic capacitors:

There are many reasons for the explosion of capacitors, such as current greater than the allowed steady-state current, voltage exceeding the working voltage, reverse voltage, frequent charging and discharging, etc. But the most direct reason is high temperature. We know that an important parameter of capacitors 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 increases, and when the pressure exceeds the tolerance limit of the explosion vent, the explosion occurs. Therefore, temperature is the direct cause of the capacitor explosion. The designed service life of the capacitor is about 20,000 hours, which is also greatly affected by the ambient temperature. The service life of capacitors decreases with the increase of temperature. Experiments have shown that the service life of capacitors will be halved for every 10℃ increase in ambient temperature. The main reason is that temperature accelerates chemical reactions and causes the medium to degrade and fail over time, thus ending the service life of capacitors. In order to ensure the stability of capacitors, capacitors must be tested in a high-temperature environment for a long time before plugging into the board. Even at 100℃, high-quality capacitors can work for thousands of hours. At the same time, the life of capacitors we mentioned means that during the use of capacitors, the capacitance will not exceed 10% of the standard range. Capacitor life refers to the problem of capacitance capacity, not the explosion after the design life is reached. It is just that the designed capacity standard of the capacitor cannot be guaranteed.

Therefore, in a short period of time, the normal use of board capacitors will explode, which is a problem of capacitor quality. In addition, abnormal use may also cause capacitor explosion. For example, hot-plugging computer accessories will also cause drastic changes in the current and voltage of the local circuit of the board, thereby causing capacitor failure.