Electrolytic capacitor selection: please pay attention to these '6 indicators'!

Electrolytic capacitors are one of the most important components in the primary and secondary loop filter circuits in switching power supplies. Generally, the equivalent circuit of an electrolytic capacitor can be considered as the series connection of an ideal capacitor, parasitic inductance, and equivalent series resistance, as shown in Figure 1. Figure 1: The equivalent circuit of an electrolytic capacitor is well known, and the switching power supply is the main power supply of today's information appliance equipment, making an indelible contribution to the miniaturization and portability of electronic equipment. Switching power supplies continue to be miniaturized, lightweight, and highly efficient, and are used more and more in electronic equipment, and their penetration rate is getting higher and higher. Correspondingly, electrolytic capacitors are required to be small and large-capacity, ripple current resistant, high frequency and low impedance, high temperature and long life, and more suitable for high-density assembly. 1 Capacitance and volume Since most electrolytic capacitors use a winding structure, it is easy to expand the volume, so the capacitance per unit volume is very large, several to several tens of times larger than other capacitors. However, the acquisition of large capacity is at the cost of volume expansion. Modern switching power supplies require higher and higher efficiency and smaller and smaller sizes. Therefore, it is necessary to seek new solutions to obtain large capacity and small size. Volumetric capacitors. Once the active filter circuit is used on the primary side of the switching power supply, the use environment of aluminum electrolytic capacitors becomes more severe than before: (1) The high-frequency pulse current is mainly the pulsating current of 20 kHz to 100 kHz, and it is greatly increased. (2) The main switch tube of the converter heats up, causing the surrounding temperature of the aluminum electrolytic capacitor to rise; (3) The converter mostly uses a booster circuit, so high-voltage aluminum electrolytic capacitors are required. As a result, aluminum electrolytic capacitors manufactured using conventional technology have to choose large-sized capacitors because they have to absorb a larger pulsating current than before. As a result, the power supply is bulky and difficult to use in miniaturized electronic devices. In order to solve these problems, it is necessary to research and develop a new type of electrolytic capacitor, which is small in size, withstands high voltage, and allows a large amount of high-frequency pulse current to flow. In addition, this kind of electrolytic capacitor, working in a high temperature environment, has to have a relatively long working life. 2 Withstand temperature and lifespan In the process of switching power supply design, it is inevitable to select suitable capacitors. As far as medium and large capacity products above 100μF are concerned, aluminum electrolytic capacitors are the most widely used so far because they are cheap. However, significant changes have taken place in recent years, and the avoidance of aluminum electrolytic capacitors is increasing. One reason for this change is that the life of aluminum electrolytic capacitors tends to become a weak link in the entire device. The engineer of the power module manufacturer said: 'For aluminum electrolytic capacitors, components with limited lifespan, if they can not be used, try not to use them.' Because the electrolyte inside the aluminum electrolytic capacitors will evaporate or produce chemical changes, resulting in a decrease in electrostatic capacity or As the equivalent series resistance (ESR) increases, the performance of the capacitor will definitely deteriorate over time. The life of an electrolytic capacitor is directly related to the ambient temperature in which the capacitor works for a long time. The higher the temperature, the shorter the life of the capacitor. Ordinary electrolytic capacitors have been damaged when the ambient temperature is 90°C. But now there are many types of electrolytic capacitors whose working environment temperature is already very high. When the ambient temperature is 90℃, when the ratio of the alternating current passing through the electrolytic capacitor to the rated pulse current is 0.5, the life is still 10000h, but if the temperature rises to 95℃ At that time, the electrolytic capacitor has been damaged. Therefore, when choosing a capacitor, it should be selected according to the specific ambient temperature and other parameters. If the influence of ambient temperature on the life of the capacitor is ignored, the reliability and stability of the power supply will be greatly reduced, and even the equipment will be damaged. And instruments. Generally speaking, when the electrolytic capacitor works at an ambient temperature of 80°C, it can generally reach the life expectancy of 10000h. On the other hand, the life of an electrolytic capacitor is also related to the AC current and rated pulse current of the capacitor for long-term operation (generally refers to the test value at an ambient temperature of 85 ℃, but some high temperature electrolytic capacitors are tested at 125 ℃ Data). Generally speaking, the larger the ratio, the shorter the life of the electrolytic capacitor. When the current flowing through the electrolytic capacitor is 3.8 times the rated current, the electrolytic capacitor is generally damaged. Therefore, the electrolytic capacitor has its safe working area. For general applications, when the ratio of the alternating current to the rated pulse current is below 3.0 times, the life expectancy requirement has been met. The influence of ambient temperature and ripple current on electrolytic capacitors is shown in Figure 2.
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