Capacitors are the most common devices in switching Power Supplies. They can be used to reduce ripple noise and improve the stability and transient response of power supplies. However, there are many types of capacitors. How to quickly select them through tricks? And the product has high reliability and stable performance? 1. Understanding of the types of capacitors A general understanding of the types of capacitors helps to quickly screen the types of capacitors when choosing a capacitor. There are many types of capacitors. As shown in Figure 1, there are chip capacitors and plug-in capacitors according to the package, ceramic capacitors, tantalum capacitors, electrolytic capacitors, mica capacitors, and film capacitors according to the medium. According to the structure, there are fixed capacitors. , Semi-fixed capacitor, variable capacitor. figure 1 There are many types of capacitors, which make people easy to suffer from the choice syndrome, but don’t worry. In the switching power supply, we use ceramic capacitors, electrolytic capacitors and tantalum capacitors the most. After understanding the types of capacitors, the next step is to understand some of the performance parameters of capacitors. . 2. Understanding of the key parameters of capacitors Understanding the intrinsic key parameters of capacitors can quickly select models and use them reliably. The key parameters of all capacitors are the same, including capacitor capacitance, capacitor withstand voltage, capacitor ESR, capacitor capacitance accuracy, and capacitor allowable work temperature range. 3. The selection of capacitors in switching power supplies requires understanding of the characteristics of the capacitors themselves In the design of switching power supply, the capacitors with the highest frequency of use are ceramic capacitors, electrolytic capacitors, and tantalum capacitors. It is necessary to understand their characteristic differences to quickly make a selection. Ceramic capacitors have small capacitance, good high-frequency characteristics, and a wide operating temperature range. Compared with electrolytic capacitors, the ESR is smaller, smaller, and has no polarity; The capacitance value of electrolytic capacitor can be made larger, but the operating temperature range is narrow, the ESR is larger, and there is polarity; Tantalum capacitors have the smallest ESR and a larger capacitance value than ceramic capacitors. They have polarity, poor safety performance, and are easy to catch fire. Knowing the characteristics of the above three types of capacitors, you can use them with ease. 4. The selection of capacitors in switching power supplies should also understand the use environment The use environment of the capacitor is divided into the internal environment of the circuit and the external environment of the circuit. The internal environment of the circuit includes frequency, voltage value, current value, the main function of the capacitor in the circuit, etc.; the type of capacitor can be determined according to the circuit frequency; the selection can be determined according to the voltage value The withstand voltage value of the capacitor; the main function in the circuit can be used to refer to the capacitance value of the selected capacitor, etc.; The selection of capacitors according to the external use environment of the circuit, the ambient temperature of the product's work, and safety requirements, etc., can all be used to narrow the range of capacitor selection. 5. Examples of capacitor selection for some circuits in switching power supplies (1) The capacitor selection of the absorption circuit, as shown in Figure 2 below, in a switching power supply, it is a very common circuit to connect a capacitor in parallel with a MOS tube or a diode to absorb spikes, so how to choose this capacitor? First, considering the circuit environment, that is, the maximum voltage stress that the capacitor may bear, you can determine the withstand voltage value of the capacitor; secondly, as an absorption circuit, the capacitance value should not be too large, too large will affect the efficiency of the whole machine, so the capacitance value is generally PF The high-frequency characteristics of capacitors are better. According to these characteristics, ceramic capacitors are the most ideal. However, ceramic capacitors are also divided into NPO and COG materials. According to temperature characteristics, they are also divided into X5R, X7R, etc., no need to worry, as long as you understand the circuit environment These can be easily solved for the accuracy requirements and temperature requirements of the capacitors used. In this kind of absorption circuit, the requirements for the accuracy of the capacitors are not high, and the general COG capacitors can meet the requirements. figure 2 (2) Start-up circuit, take the chip 3843 as an example, use RC to start, the C value generally uses uF-level capacitors, and the temperature characteristics should be good, otherwise when the temperature changes, the capacitance value is not enough, it will start badly, here you can choose Tantalum capacitors and ceramic capacitors; however, there is a problem with tantalum capacitors that they do not meet safety requirements. If you want to pass safety certification, you can only choose ceramic capacitors. (3) Filter circuit capacitor selection. Determine the withstand voltage according to the voltage across the capacitor in the circuit. For example, if the output voltage is 24V, you cannot choose a capacitor with a withstand voltage of 15V or 25V. The maximum current in the circuit can be used to determine the capacitance. If you need a smaller ripple, you must choose a large-capacity capacitor. In the filter circuit, the capacitor value can be used with pure ceramics, pure electrolytic capacitors, or mixed ceramic capacitors with electrolytic capacitors. Figure 4 6. Examples of capacitor selection for automotive-grade power supplies The selection of automotive-grade power supply capacitors involves the selection of capacitors in the use environment. The working environment temperature of automotive-grade power supplies is higher than that of general industrial-grade power supplies, and the operating temperature is required to reach 125°C. Therefore, the selection of capacitors is to choose good temperature characteristics and resistance. Capacitors with high temperature and good safety performance. The skill of capacitor selection is nothing more than understanding the characteristics of several commonly used capacitors, as well as the application environment of the capacitor in the circuit, and the environmental characteristics of the product. Car Power Inverter,Mini Car Power Inverters,Car Power Inverter 200W,Car Power Inverter 600W GuangZhou HanFong New Energy Technology Co. , Ltd. , https://www.zjgzinverter.com
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