Aluminum electrolytic capacitors are polar capacitors that use etched high-purity aluminum foil as the anode and tissue paper or cloth soaked in electrolyte as the cathode. \x0d\\x0d\Advantages: large capacity, high voltage resistance, cheap price\x0d\\x0d\Disadvantages: large leakage current, large error, poor stability, lifespan decreases quickly with temperature increase\x0d\\x0d\ Aluminum electrolytic capacitors used in digital circuits are generally used for power smoothing and filtering. In addition to well-known parameters such as capacity, withstand voltage, capacity error, operating temperature, and package size, there are also several important parameters related to capacitor quality, including loss angle. Tangent, leakage current, equivalent series resistance ESR, allowable ripple current, service life, etc. These parameters are not marked on the outer shell of the finished package, but are only reflected in the product specifications. However, these parameters may be the key to circuit performance. \x0d\Capacity and rated operating voltage\x0d\\x0d\The capacity and withstand voltage marked on the aluminum electrolytic capacitor body are very important and are the most basic content for selecting capacitors. \x0d\\x0d\In actual capacitor selection, capacitors with larger capacities should be used in places where the current changes rapidly. However, the larger the capacity, the better. First, as the capacity increases, the cost and volume may increase. In addition, The larger the capacitor, the greater the charging current and the longer the charging time. These are all things to consider in practical application selection. \x0d\\x0d\Rated operating voltage: Within the specified operating temperature range, the capacitor can work reliably for a long time and the maximum DC voltage it can withstand. In AC circuits, it should be noted that the maximum value of the AC voltage applied cannot exceed the DC working voltage value of the capacitor. Commonly used fixed capacitor working voltages are 6.3V, 10V, 16V, 25V, 50V, 63V, 100V, 2500V, 400V, 500V, 630V. The voltage that the capacitor actually withstands in the circuit cannot exceed its withstand voltage value. \x0d\\x0d\In the filter circuit, the withstand voltage value of the capacitor should not be less than 1.42 times the AC effective value. Another issue that should be noted is the operating voltage margin, which is generally above 15%. For example, the rated voltage of a capacitor is 50V. Although the surge voltage may be as high as 63V, generally only a maximum voltage of 42V will be applied. \x0d\\x0d\ Let the capacitor's rated voltage have more margin, which can reduce the internal resistance, reduce leakage current, reduce the loss angle, and increase the life. Although it is said that using 50V aluminum electrolytic capacitors with a working voltage of 48V will not cause problems in the short term, the lifespan may be reduced after being used for a long time. \x0d\\x0d\Dielectric loss\x0d\\x0d\The energy consumed by a capacitor under the action of an electric field is usually expressed by the ratio of the loss power to the reactive power of the capacitor, that is, the tangent of the loss angle (in the equivalent circuit of the capacitor , the ratio of the series equivalent resistance ESR to the capacitive reactance 1/ωC ??is called Tanδ. The ESR here is the value calculated at 120Hz. Obviously, Tanδ becomes larger as the measurement frequency increases, and as the measurement temperature decreases. and increase). The larger the loss angle, the greater the loss of the capacitor. Capacitors with large loss angles are not suitable for working at high frequencies. The dissipation factor (DF) exists in all capacitors, and sometimes the DF value is expressed as the loss angle tanδ. The lower this parameter is, the better. However, this parameter of aluminum electrolytic capacitors is relatively high. \x0d\\x0d\Whether the DF value is high or low, for capacitors of the same brand and series, is related to temperature, capacity, voltage, and frequency; when the capacity is the same, the DF value with higher withstand voltage is The lower. In addition, the higher the temperature, the higher the DF value, and the higher the frequency, the higher the DF value. \x0d\\x0d\External dimensions\x0d\\x0d\External dimensions are related to weight and pin type. Singleended is a radial lead type, screw is a locking screw type, and there are also chip aluminum electrolytic capacitors. As for the weight, if you compare two capacitors with the same capacity and voltage but different brands, the weight will definitely be different; and the external dimensions are more related to the shell planning. Generally speaking, for capacitors with the same diameter and capacity, a capacitor with a lower height can be replaced by a capacitor with a higher height. However, when a capacitor with a high length is replaced by a capacitor with a low height, the issue of mechanical interference must be considered. \x0d\\x0d\A capacitor will produce various impedances and inductive reactances due to its structure. ESR equivalent series resistance and ESL equivalent series inductance are a pair of important parameters - this is the basis of capacitive reactance. A capacitor with a small equivalent series resistance (ESR) absorbs the peak (ripple) current during fast switching very well compared to a larger external capacitor. It is more cost effective to use capacitors with large ESR in parallel. However, this requires a compromise between PCB area, component count and cost. \x0d\Ripple current and ripple voltage\x0d\\x0d\In some materials, they are called ripple current and ripple voltage, but they are actually ripplecurrent and ripplevoltage. The meaning is the ripple current/voltage value that the capacitor can withstand. Ripple voltage is equal to the product of ripple current and ESR. \x0d\\x0d\When the ripple current increases, the ripple voltage will increase exponentially even if the ESR remains unchanged. In other words, when the ripple voltage increases, the ripple current also increases, which is why the capacitor is required to have a lower ESR value.

After the ripple current is superimposed, the equivalent series resistance (ESR) inside the capacitor causes heat, thus affecting the service life of the capacitor. Generally, the ripple current is proportional to the frequency, so the ripple current is relatively low at low frequencies. \x0d\\x0d\The rated ripple current is a value defined under the maximum operating temperature condition. In actual applications, the ripple tolerance of a capacitor is also related to the ambient temperature of its use and the temperature grade of the capacitor itself. Data sheets usually provide a maximum ripple current that each temperature grade capacitor can withstand under specific temperature conditions. It even provides a detailed chart to help users quickly find the amount of capacitor ripple allowed to achieve a certain expected service life under certain ambient temperature conditions. \x0d\Leakage current\x0d\\x0d\The dielectric of the capacitor has a great hindrance to DC current. However, since the aluminum oxide film dielectric is immersed in electrolyte, when a voltage is applied, a small current called leakage current will be generated when the oxide film is re-formed and repaired. Typically, leakage current increases with temperature and voltage. Its calculation formula is roughly: I=K×CV. The unit of leakage current I is μA, and K is a constant. Generally speaking, the higher the capacitance of the capacitor, the greater the leakage current. It can be known from the formula that the higher the rated voltage, the greater the leakage current. Therefore, lowering the operating voltage can also reduce the leakage current. \x0d\\x0d\Lifespan\x0d\\x0d\First of all, it must be clear that aluminum electrolytic capacitors will definitely break, it is only a matter of time. There are many reasons that affect the life of the capacitor, such as overvoltage, reverse voltage, high temperature, rapid charge and discharge, etc. Under normal use, the biggest impact is temperature, because the higher the temperature, the faster the evaporation loss of the electrolyte. It should be noted that the temperature here does not refer to the ambient or surface temperature, but to the working temperature of the aluminum foil. Manufacturers usually label the capacitor life and test temperature on the capacitor itself. \x0d\\x0d\ Since the lifespan of the capacitor is halved for every 10°C increase in the operating temperature of the capacitor, do not think that an aluminum electrolytic capacitor with a lifespan of 2000 hours is better than one with a lifespan of 1000 hours. Pay attention to the test temperature to confirm the lifespan. Each manufacturer has formulas for calculating temperature and life. When designing capacitors, you should refer to actual data for calculations. What needs to be understood is that in order to improve the life of aluminum electrolytic capacitors, the first step is to lower the operating temperature and keep it away from heat sources on the PCB. The second step is to consider using capacitors with a high maximum operating temperature. Of course, the price will be higher. \x0d\\x0d\Impedance: \x0d\\x0d\At a specific frequency, the resistance that blocks the passage of alternating current is the so-called impedance. It is closely related to the capacitance value and inductance value in the capacitor equivalent circuit, and is also related to ESR. The capacitive reactance of the capacitor gradually decreases as the frequency increases in the low frequency range, and the reactance drops to the ESR value when the frequency continues to increase and reaches the mid-frequency range. When the frequency reaches the high frequency range, the inductive reactance becomes dominant, so the impedance increases with the increase of frequency. \x0d\\x0d\The output filter electrolytic capacitor in the switching power supply has a sawtooth wave voltage frequency as high as tens of kHz or even tens of MHz. At this time, the capacitance is not its main indicator. It is a measure of the quality of high-frequency aluminum electrolytic capacitors. The standard is the "impedance-frequency" characteristic, which requires a lower equivalent impedance within the operating frequency of the switching power supply, and at the same time has a good filtering effect on the high-frequency spike signals generated when the semiconductor device is working. \x0d\\x0d\Summary\x0d\\x0d\On the surface, the lower the DF, leakage current, and ESR, the higher the ripple current, the better the performance of the aluminum electrolytic capacitor, but the cost of improved performance is the size and price improvement.