[Repost] Analysis of what faults may occur in switching power supplies due to capacitor damage
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At present, switching power supply has gradually entered our daily life and production. With the advantages of energy saving, environmental protection and high cost performance, it quickly replaced the traditional "linear power supply" which was both bulky and inefficient, and was quickly accepted by people. Capacitors are one of the most important and most prone to failure components in switching power supplies, and the failure phenomenon is not easy to identify, making maintenance more difficult. This article explains the principle of capacitors in switching power supplies, common fault analysis and maintenance methods. 1. The role of capacitors in switching power supplies 1.1 Filtering Filtering is a very important part of the function of capacitors. It is used in almost all power circuits. The filter capacitor is like a "pool", which converts electrical energy into water in the pool and can restore water to electrical energy. 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 larger than 1 uF are electrolytic capacitors, which have a large inductance component, so the impedance will increase at high frequencies. Sometimes you will see a large electrolytic capacitor connected in parallel with a small capacitor. At this time, the large capacitor passes low frequencies and the small capacitor passes high frequencies. The function of the capacitor is to pass high frequencies and resist low frequencies, 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 (1000 uF) filter low frequencies, and small capacitors (20 pF) filter high frequencies. 1.2 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 to the power supply pins and ground pins of the load device as possible. This can effectively prevent ground potential increase and noise caused by excessive input values. 1.3 Decoupling From the circuit point of view, it can always be distinguished between the driving source and the driven load. If the load capacitance is relatively large, the drive circuit needs to charge and discharge the capacitance to complete the signal jump. When the rising edge is relatively steep, the current is relatively large, so the drive 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 pin, 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 the coupling effect. 1.4 Energy Storage Energy storage capacitors collect charge through rectifiers and transmit the stored energy to the output of the power supply through converter leads. Aluminum electrolytic capacitors with a voltage rating of 40 to 450VDC and a capacitance value of 220 to 150000 uF (such as B43504 or B43505 from EPCOS) are more commonly used. Depending on the power supply requirements, devices are sometimes connected in series, in parallel or in combination. For power supplies with a power level exceeding 10 KW, larger can-shaped spiral terminal capacitors are usually used. 2. Fault phenomena of capacitor damage in switching power supplies There are several types of capacitor damage and failure: 25)]1) Short circuit or open circuit damage inside the capacitor. The fault phenomenon is burning the switch tube and other current limiting components, such as the current limiting resistor in the insurance and switching power supply. Capacitor short circuit or open circuit damage. When the voltage of the filter capacitor working in high voltage and high current (such as the switching power supply and line output circuit of the color TV) rises for some reason and exceeds its withstand voltage value, it will be broken down and short-circuited and damaged. Or, after the rectifier diode is damaged, the polar electrolytic capacitor is equivalent to working in an AC circuit, and it will heat up and short-circuit and damage under a large reverse leakage current. Since the current flowing through the capacitor is very large during a short circuit, the capacitor will generally burst or its sealing plug will bulge out. After the filter capacitor is short-circuited, the fuse or current limiting resistor is burned out, the power thick film block or the switch tube, the rectifier tube is broken down, and other faults often occur. The main manifestation is the "three no" of the whole machine. This fault is common in various types of switching power supplies. 2) Inefficiency or slight leakage caused by reduced capacitor capacity. The fault phenomenon is "S"-shaped distortion of the TV image or asynchronous line phenomenon. For the current TVs using IIC bus, some special fault phenomena occur. If the synchronization card is in a critical state due to the impact, the accompanying sound may affect the quality of the TV, making the accompanying sound appear at any time. The main reason is that the parameters of the capacitor have changed, but it has not completely failed. It still works to a certain extent, but it does not achieve the desired effect, which causes the existing fault phenomenon. Moreover, this type of fault is difficult to judge and eliminate. 3) Failure, complete leakage or explosion caused by the disappearance of capacitor capacity is the most difficult fault to identify and repair after the capacitor in the power supply fails. This is because when measuring capacitor components, everything is normal when tested with a multimeter, but after the capacitor is installed in the circuit, the capacity of the capacitor disappears completely. This is one of the most difficult soft faults to repair in the circuit, that is, the components cannot withstand voltage, and once there is voltage, it is easy to disappear completely. Types of explosion:25)]There are two types of capacitors: input capacitor explosion and output capacitor explosion. For input capacitors, they are capacitors with large volume, high voltage rating and high power supply circuit, which filter the received current. Input capacitor explosion is related to the input current of the power supply and the quality of the capacitor itself. Excessive glitch voltage, high peak voltage and unstable current will cause the capacitor to charge and discharge too frequently. The internal temperature of the capacitor in such working environment for a long time will rise quickly. Explosion will occur when the tolerance limit of the explosion vent is exceeded. For the output capacitor, the current adjusted by the power module is filtered and stored. Here, the current 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. There are many reasons for capacitor explosion, such as current greater than the allowed steady wave 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, an explosion occurs. Therefore, temperature is the direct cause of capacitor explosion. The designed service life of capacitors is about 20,000 hours, and it 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°C 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 life of the capacitor. In order to ensure the stability of the capacitor, the capacitor must be tested in a high-temperature environment for a long time before being plugged into the board. Even at 100°C, high-quality capacitors can work for thousands of hours. At the same time, the life of the capacitor mentioned refers to the fact that the capacitance will not exceed 10% of the standard range during the use of the capacitor. The life of the capacitor refers to the problem of the capacitance of the capacitor, 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 board capacitor will explode, which is a problem of capacitor quality. In addition, abnormal use may also cause the capacitor to explode. 3. Repair method of capacitor damage in switching power supply. 1) For short-circuited and open-circuited capacitors, the quality of the component can be quickly measured with a multimeter. It mainly measures its charging performance rather than the charging and discharging performance. After determining the faulty capacitor component, during the replacement process, special attention should be paid to the quality of the replaced capacitor component. Good quality capacitor components should be selected, and the capacity and rated voltage must be consistent with the replaced capacitor or greater than the damaged capacitor. Before replacement, the quality of the capacitor component to be replaced must be judged again. Sometimes the newly purchased components also have quality problems. If they are installed without judgment, assuming that the newly purchased components really have problems, it will bring great difficulties to maintenance, because the replaced components are generally considered to have no problems, so when they are repaired again, the component will not be tested again, resulting in a very large human failure in the maintenance, making the maintenance more difficult. 2) When the capacitor is inefficient and fails, the most commonly used method is to use the substitution method. When the switching power supply protection appears, other key components are not faulty after measurement, and the capacitor cannot be found after measurement. The capacitor components are generally replaced, because the components with soft faults are of good quality when there is no voltage and current, but the quality of the components is problematic during the working process when there is voltage and current. This type of fault cannot be detected by conventional measurement methods. The substitution method can achieve very good results. Capacitor components often fail in switching power supplies, and some faults are not easy to judge. At the same time, how to use capacitors in switching power supplies without failures, I hope to gain some results through analysis. Source: Internet, please delete if infringed 25)]1) For short-circuited and open-circuited capacitors, a multimeter can quickly measure the quality of the components. It mainly measures its charging performance, not its charging and discharging performance. After determining the faulty capacitor, during the replacement process, special attention should be paid to the quality of the replaced capacitor. Good quality capacitors should be selected, and the capacity and rated voltage must be consistent with or greater than the damaged capacitor. Before replacement, the quality of the capacitor to be replaced must be determined again. Sometimes the newly purchased components also have quality problems. If they are installed without judgment, assuming that the newly purchased components really have problems, it will bring great difficulties to the maintenance, because the replaced components are generally considered to have no problems, so when they are repaired again, they will not be tested again, causing a very large human fault in the maintenance, making the maintenance more difficult.2）For capacitors that are inefficient or fail, the most commonly used method is to use the substitution method. When the switching power supply protection appears, other key components are not faulty after measurement, and the capacitor cannot be found to be faulty after measurement, the capacitor components are generally replaced. This is because the components with soft faults are of good quality when there is no voltage and current, but there are quality problems with the components during operation when there is voltage and current. This type of fault cannot be detected by conventional measurement methods, and the substitution method can achieve very good results. Capacitors often fail in switching power supplies, and some failures are not easy to judge. At the same time, how to use capacitors in switching power supplies without failures is hoped to gain some results through analysis. Source: Internet, if infringed, please delete 25)]1) For short-circuited and open-circuited capacitors, a multimeter can quickly measure the quality of the components. It mainly measures its charging performance, not its charging and discharging performance. After determining the faulty capacitor, during the replacement process, special attention should be paid to the quality of the replaced capacitor. Good quality capacitors should be selected, and the capacity and rated voltage must be consistent with or greater than the damaged capacitor. Before replacement, the quality of the capacitor to be replaced must be determined again. Sometimes the newly purchased components also have quality problems. If they are installed without judgment, assuming that the newly purchased components really have problems, it will bring great difficulties to the maintenance, because the replaced components are generally considered to have no problems, so when they are repaired again, they will not be tested again, causing a very large human fault in the maintenance, making the maintenance more difficult.2）For capacitors that are inefficient or fail, the most commonly used method is to use the substitution method. When the switching power supply protection appears, other key components are not faulty after measurement, and the capacitor cannot be found to be faulty after measurement, the capacitor components are generally replaced. This is because the components with soft faults are of good quality when there is no voltage and current, but there are quality problems with the components during operation when there is voltage and current. This type of fault cannot be detected by conventional measurement methods, and the substitution method can achieve very good results. Capacitors often fail in switching power supplies, and some failures are not easy to judge. At the same time, how to use capacitors in switching power supplies without failures is hoped to gain some results through analysis. Source: Internet, if infringed, please delete
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