Tips for fast selection of capacitors in switching power supplies

Capacitance is a common device in switching power supplies. It can be used to reduce ripple noise and can be used to improve power supply stability and transient responsiveness. However, there are many types of capacitors, and how to quickly select them by skill. And the product reliability is high, and the performance is stable?

1, the understanding of the type of capacitor

A general understanding of the type of capacitor helps to quickly filter the type of capacitor when selecting a capacitor.

There are many types of capacitors, as shown in Figure 1. According to the package, there are chip capacitors and plug-in capacitors. According to the medium, there are ceramic capacitors, tantalum capacitors, electrolytic capacitors, mica capacitors, film capacitors, etc. According to the structural situation, there are fixed capacitors. , semi-fixed capacitors, variable capacitors.

figure 1

There are many types of capacitors, which makes people susceptible to syndrome selection, but don't worry. In switching power supplies, we use ceramic capacitors, electrolytic capacitors and tantalum capacitors. Understand the types of capacitors. Next, we will understand some performance parameters of capacitors. .

2, the understanding of the key parameters of the capacitor

Understand the key parameters of the capacitor in order to quickly select and use it reliably. The key parameters of all capacitors are the same, including the capacitance value, the withstand voltage of the capacitor, the ESR of the capacitor, the capacitance tolerance, and the allowable operation of the capacitor. temperature range.

3. The selection of the capacitor in the switching power supply should understand the characteristics of the capacitor itself.

In switching power supply design, the most frequently used capacitors are ceramic capacitors, electrolytic capacitors, and tantalum capacitors. You need to understand the difference in characteristics to quickly select them.

The ceramic capacitor has small capacitance, good high frequency characteristics, wide operating temperature range, small ESR compared with electrolytic capacitor, small volume and no polarity;

The capacity of the electrolytic capacitor can be made larger, but the operating temperature range is narrower, the ESR is larger, and there is polarity;

The tantalum capacitor ESR is the smallest, the capacitance value can be larger than the ceramic capacitor, the polarity is poor, the safety performance is poor, and it is easy to catch fire.

Understand the characteristics of the above three capacitors, you can use them when you use them.

4, the selection of the capacitor in the switching power supply must also understand the use of the environment

The environment in which the capacitor is used is also divided into the internal environment of the circuit and the external environment of the circuit. The internal environment of the circuit includes the frequency, the voltage value, the current value, and the main role of the capacitor in the circuit. The type of the 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 role in the circuit can be used to refer to the capacitance of the selected capacitor;

According to the external environment of the circuit, the capacitor can be selected, the ambient temperature of the product, safety requirements, etc. can be used to narrow the capacitor selection range.

5, some examples of circuit capacitor selection in switching power supply

(1) Capacitor selection of the absorbing circuit, as shown in Figure 2 below. In the switching power supply, it is a very common circuit to parallel the capacitance absorption peak on the MOS tube or diode. How to choose this capacitor? First of all, considering the circuit environment, that is, the maximum voltage stress that can be withstood on the capacitor, the voltage withstand value of the capacitor can be determined. Secondly, as the absorption circuit, the capacitance value cannot be too large, too large will affect the efficiency of the whole machine, so the capacitance value is generally PF. Class capacitors, capacitors have high frequency characteristics. According to these characteristics, ceramic capacitors are the most ideal. However, ceramic capacitors are also divided into NPO and COG materials. According to the temperature characteristics, X5R, X7R, etc. are also included. No need to worry, as long as the circuit environment is known. For the accuracy requirements and temperature requirements of the capacitors used, these can be solved. In this absorption circuit, the accuracy of the capacitor is not high, and the capacitance of the general COG material can be satisfied.

figure 2

(2) Start circuit, take chip 3843 as an example, start with RC, C value generally uses uF level capacitor, and the temperature characteristics are better, otherwise the temperature value changes, the capacitance is not enough, it will start badly, you can choose here Tantalum capacitors and ceramic capacitors; however, tantalum capacitors have a problem that does not meet the safety requirements. If you want to pass the safety certification, you can only choose ceramic capacitors.


image 3

(3) Filter circuit capacitor selection, according to the voltage across the capacitor in the circuit to determine the withstand voltage value, such as output voltage 24V, you can not choose 15V or 25V withstand voltage capacitor, the maximum current in the circuit can be used to determine the capacitance, current In order to get a smaller ripple, it is necessary to select a large-capacity capacitor. In the filter circuit, the capacitance value can be used together with a pure ceramic, a pure electrolytic capacitor, or an electrolytic capacitor mixed ceramic capacitor.

Figure 4

6, automotive grade power supply capacitor selection example

The selection of automotive-grade power supply capacitors involves the screening of capacitors in the use environment. The operating temperature of automotive-grade power supplies is higher than that of general industrial-grade power supplies. The operating temperature requirements are 125 °C. Therefore, the selection of capacitors is to select temperature characteristics and resistance. High temperature, good safety performance capacitors.

The selection technique of the capacitor is nothing more than understanding the commonly used capacitor characteristics and the application environment of the capacitor in the circuit and the environment in which the product is used.

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