Selection of capacitors in the circuit

Capacitors are passive devices that we often use in circuits. Several types of aluminum electrolytic capacitors, tantalum capacitors, and ceramic capacitors are commonly seen. Since the characteristics of each type of capacitor determine the corresponding use of different occasions. Let's compare the differences and characteristics of several types of capacitors and summarize how to select capacitors in an actual circuit.

(a) Aluminum electrolytic capacitors As the name implies, the main components of aluminum electrolytic capacitors are aluminum foil and electrolyte. A simple understanding of the production process of aluminum electrolytic capacitors, that is rolled into a column of aluminum foil, into the liquid electrolyte, and then lead out the positive and negative terminals, and then seal the capacitor core material in the metal housing. A certain proportion of water exists in the liquid electrolyte. When a leakage current flows through the capacitor, the water can be decomposed into hydrogen and oxygen. The oxygen can form a new oxide film with the anode through the oxidation reaction, and the hydrogen can pass through the rubber plug of the capacitor. discharge. This will not damage the capacitor. The simple production process and low cost are a feature of aluminum electrolytic capacitors. In addition, aluminum electrolytic capacitors have the following features:

a. Because the sealing shell is not completely sealed, the electrolyte will dry out easily, so the service life of the aluminum electrolytic capacitor is limited. b. Due to the presence of water in the electrolyte, the performance of the aluminum electrolytic capacitor in high temperature and low temperature environment is affected. c. Due to the nature of the process characteristics, the ESR and ESL of aluminum electrolytic capacitors are relatively difficult to make small, so the self-resonant frequency of aluminum electrolytic capacitors is usually low, roughly in the range of tens of kHz to several MHz. d. The capacity of aluminum electrolytic capacitors and the size of aluminum foil Positive correlation, capacity can be done very much, the larger the capacity, the larger the capacitor size.

According to the above characteristics, aluminum electrolytic capacitors are widely used in low-frequency filtering applications, especially in the environment of tens of KHz to several MHz, such as the output filtering of power supplies. We often see aluminum electrolytic capacitors. When using aluminum electrolytic capacitors, it should be noted that the capacitor withstand voltage meets the circuit requirements. In addition, in the case of other requirements are not strict, the capacity can be chosen as large as possible, the larger the capacity is, the smaller the ESR is, and it is easier to meet the requirements of the circuit target impedance. In certain high-temperature environments, try to avoid the use of small-capacity aluminum electrolytic capacitors to avoid the effect of capacitance failure due to the excessively high temperature of the electrolyte causing the entire circuit to work.

(b) Tantalum Capacitors Tantalum capacitors are another widely used type of capacitor. Like aluminum electrolytic capacitors, tantalum capacitors are electrolytic capacitors. The main process is to press the powder into a porous solid block, anodize to form an oxide film, then cover the solid electrolyte, then coat with a layer of graphite and lead-tin coating, and finally encapsulate with resin. Solid tantalum capacitors. Tantalum capacitors have the following characteristics:

Unlike aluminum electrolytic capacitors, tantalum capacitors have a solid electrolyte, so there is no problem of electrolyte drying up and the lifetime will be improved. b. Due to the solid electrolyte used, its capacity and temperature characteristics are relatively stable, so the temperature of the capacitor capacity Less impact, high temperature performance than aluminum electrolytic capacitors c. Tantalum electrolytic capacitors can be achieved in a larger package smaller package size, so ESR, ESL can be controlled is relatively small, the self-resonant frequency is higher than the aluminum electrolytic capacitor d. The process is more complex than the aluminum electrolytic capacitor, and the cost is high. By comparison, we can find that the tantalum capacitor has many advantages that are not available in the aluminum electrolytic capacitor. In some filtering applications, the tantalum capacitor can be a good substitute for the aluminum electrolytic capacitor. However, there are several points to pay attention to: Since the structure of the tantalum capacitor is determined, the tantalum capacitor's withstand voltage is generally not high. In the actual circuit, it is important to pay attention to the tantalum capacitor's withstand voltage requirement and leave a certain margin. The tantalum capacitors do not have excellent aluminum electrolytic capacitors in dealing with large current surges and large voltage transients during power-on. Temperature has little effect on the tantalum capacitor. In practice, we can ignore the effect of temperature on the tantalum capacitor.

(3) Ceramic capacitor Ceramic capacitor is the capacitor that we use most in the actual circuit. Its structure is relatively simple, that is, the ceramic plates are alternately laminated and sintered. Small size and low cost, widely used. Its features are as follows:

a. Small size. Ceramic capacitor structure is simple, the size can be done very small, 0402 or even 0201 package ceramic capacitor is widely used in mobile phones such as the size of the strict requirements of the occasion b. The electrical performance is stable, the temperature effect is less c.ESR, ESL value Very low, high self-resonant frequency, can be applied to filter requirements from several MHz to 1 GHz on PCBs. d. Due to the structural characteristics of multilayers and packaging reasons, the resistance to bending is poor. PCB bending deformation may cause capacitor cracking failure. The device features, in addition to the filtering scene, the ceramic capacitor is also widely used in applications such as blocking, coupling, and bypassing. Its operating frequency is much higher than that of electrolytic capacitors, and it can satisfy application rings of several MHz to 1 GHz.