key points of aluminum electrolytic capacitor selection: capacity, withstand voltage, temperature range, component packaging form and size, ripple current, leakage current, esr, dissipation factor, impedance / frequency characteristics, capacitor life, actual needs, performance and cost, etc.
aluminum electrolytic capacitor is a polar capacitor composed of etched high-purity aluminum foil as anode and thin paper or cloth soaked with electrolyte as cathode. advantages: large capacity, high pressure resistance and low price; disadvantages: large leakage current, large error, poor stability, and the service life decreases rapidly with the increase of temperature.
capacity and rated operating voltage
the capacity and withstand voltage marked on the body of aluminum electrolytic capacitor are very important and the most basic content of capacitor selection. in the actual capacitor selection, the capacitor with larger capacity should be used where the current change rhythm is fast, but the larger the capacity is, the better. firstly, the larger the capacity is, the cost and volume may rise. in addition, the larger the capacitance is, the greater the charging current is and the longer the charging time will be. these are to be considered in practical application selection.
（在电路设计中，电容的容量大小直接关系到电路的稳定性，例如：根据公式“c＝i/(△v/△t)”，假设某电路平均电流为 6a，△v=50mv，△t=10μs，就可计算出此处对电容总容量的要求为1200μf。如果选用1000uf 可能在短期内不会出现问题，但长时间运行就会出现电容暴浆等故障。在电路设计过程中，并不是电容越大，滤波效果越好，这要看具体电路，在低频电路中，电容值越大，对纹波的滤除效果就越好，但如果有高频信号，就不一定了。在高频段要选择合适的电容值和电容类型，一般采用云母电容和高频瓷片电容，电容值一般都比较小。）
(in circuit design, the capacity of the capacitor is directly related to the stability of the circuit. for example, according to the formula "c = i / (△ v / △ t)", suppose that the average current of a circuit is 6a, △ v = 50mv, △ t = 10 μ s. it can be calculated that the requirement for the total capacitance here is 1200 μ f。 if 1000uf is selected, there may be no problem in the short term, but faults such as capacitor slurry explosion will occur in long-term operation. in the process of circuit design, it is not that the larger the capacitance, the better the filtering effect, which depends on the specific circuit. in the low-frequency circuit, the larger the capacitance, the better the ripple filtering effect, but if there is a high-frequency signal, it is not necessarily. appropriate capacitance value and type shall be selected in the high-frequency band. mica capacitance and high-frequency ceramic chip capacitance are generally used, and the capacitance value is generally small.)
rated working voltage: the maximum dc voltage that the capacitor can withstand when it works reliably for a long time within the specified working temperature range. in the ac circuit, it shall be noted that the maximum ac voltage applied shall not exceed the dc working voltage of the capacitor. the actual voltage to be borne by the capacitor in the circuit shall not exceed its withstand voltage value. in the filter circuit, the withstand voltage value of the capacitor shall not be less than 1.42 times of the ac effective value. another problem to pay attention to is the working voltage margin, which is generally more than 15%. for example, the rated voltage of a capacitor is 50v. although the surge voltage may be as high as 63v, generally only 42v will be applied at most. the rated voltage of the capacitor has more margin, which can reduce the internal resistance, reduce the leakage current, reduce the loss angle and increase the service life.
the energy consumed by the capacitor under the action of electric field is usually expressed by the ratio of loss power to reactive power of the capacitor, that is, the tangent of loss angle. the capacitor with large loss angle is not suitable for working at high frequency.
the external dimension is related to the weight and pin type. single ended is radial lead type, screen is lock screw type, in addition, there are smd aluminum electrolytic capacitors, etc. as for the weight, if you compare two capacitors with the same capacity and withstand voltage but different brands, the weight must be different; the external dimension is more related to the shell planning. generally speaking, for capacitors with the same diameter and capacity, those with low height can replace those with high height, but when replacing low capacitance with those with high length, the problem of structural interference should be considered.
a capacitor with a small equivalent series resistance can absorb the peak (ripple) current during fast conversion compared with an external capacitor with a large capacity. parallel connection with capacitors with large esr is more cost-effective. however, this requires a compromise between pcb area, number of devices and cost.
ripple current and ripple voltage
其实就是 ripple current，ripple voltage，含义就是电容器所能耐受纹波电流/电压值。纹波电压等于纹波电流与esr的乘积。当纹波电流增大的时候，即使在esr保持不变的情况下，纹波电压也会成倍提高。换言之，当纹波电压增大时，纹波电流也随之增大，这也是要求电容具备更低 esr值的原因。叠加入纹波电流后，由于电容内部的等效串连电阻引起发热，从而影响到电容器的使用寿命。一般的，纹波电流与频率成正比，因此低频时纹波电流也比较低。额定纹波电流是在最高工作温度条件下定义的数值。而实际应用中电容的纹波承受度还跟其使用环境温度及电容自身温度等级有关。规格书通常会提供一个在特定温度条件下各温度等级电容所能够承受的最大纹波电流。甚至提供一个详细图表以帮助使用者迅速查找到在一定环境温度条件下要达到某期望使用寿命所允许的电容纹波量。
in fact, it means ripple current and ripple voltage that the capacitor can withstand. the ripple voltage is equal to the product of ripple current and esr. when the ripple current increases, the ripple voltage will increase exponentially even when 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 adding ripple current, the equivalent series resistance inside the capacitor will cause heating, which will affect the service life of the capacitor. generally, the ripple current is directly proportional to the frequency, so the ripple current is also relatively low at low frequency. the rated ripple current is defined at the maximum operating temperature. in practical application, the ripple tolerance of the capacitor is also related to the ambient temperature and the temperature level of the capacitor itself. the specification usually provides the maximum ripple current that each temperature grade capacitor can withstand under specific temperature conditions. a detailed chart is even provided to help users quickly find out the allowable capacitance ripple to achieve a certain expected service life under certain ambient temperature conditions.
the dielectric of the capacitor has a great hindrance to the dc current. however, because the aluminum oxide film medium is immersed in electrolyte, a small current called leakage current will be generated when the voltage is applied, and when the oxide film is re formed and repaired. usually, the leakage current increases with the increase of temperature and voltage. generally speaking, the higher the capacitor capacity, the greater the leakage current. it can be seen from the formula that the higher the rated voltage is, the greater the leakage current is. therefore, reducing the working voltage can also reduce the leakage current.
there are many reasons affecting the service life of capacitors, 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 volatilization loss of electrolyte. it should be noted that the temperature here does not refer to the ambient or surface temperature, but refers to the working temperature of aluminum foil.