Distinguish:
1 Use an impedance meter to test whether it is within the qualified impedance
2 Whether the white paper such as SM/md is used for disassembly [generally yellow paper]
3 Whether high frequency low resistance electrolyte is selected

With the miniaturization and portability of switching power supply equipment and the rapid development of integrated circuits, this paper analyzes the requirements of the development of switching power supply on the performance of electrolytic capacitors as an important component of switching power supply, that is, miniaturization, high-frequency low impedance, long life and ripple current resistance must also be achieved.

Electrolytic capacitor is one of the most important devices in the primary and secondary circuit filter circuit of switching power supply. In general, the equivalent circuit of electrolytic capacitor can be considered as the series connection of ideal capacitor, parasitic inductance and equivalent series resistance. As we all know, the equivalent circuit of electrolytic capacitor is the main power supply of information household appliances today, making an indelible contribution to the miniaturization and portability of electronic equipment. With the continuous miniaturization, lightweight and high efficiency of switching power supply, it is used more and more in electronic equipment, and its popularity is higher and higher. Correspondingly, electrolytic capacitors are required to be small, large capacity, ripple current resistant, high-frequency and low impedance, high temperature and long life, and more suitable for high-density assembly.

High frequency low impedance
For switching power supply with medium and small output power, the operating frequency is 50 kHz, except that a few of them still use 20~40 kHz due to price restrictions

above; Most DC/DC power modules are above 300kHz; The switching frequency of high-power switching power supply is controlled by the main switch (IGBT is generally used）The switching speed is limited to 20~40kHz. Although the switching frequency is different, the output rectifier filter capacitor of the switching power supply has basically the same function, mainly by using the filter capacitor to absorb the current component of the switching frequency and its higher harmonic frequency and filter its ripple voltage component. The filter capacitor used at the output end of the switching power supply is different from the filter capacitor used in the power frequency circuit. The common electrolytic capacitor used for filtering in the power frequency circuit has a pulsating voltage frequency of only 100Hz, and the charging and discharging time is in the order of milliseconds. In order to obtain a small pulsation coefficient, the capacitance required is as high as hundreds of thousands of micro methods. Therefore, the common aluminum electrolytic capacitor is generally used for low frequency.

The objective is to improve the capacitance, and the capacitance, loss tangent and leakage current of the capacitor are the main parameters to identify its advantages and disadvantages.

As the electrolytic capacitor used for output filtering in the switching power supply, most switching power supplies work in the state of square wave or rectangular wave, and contain extremely rich high-order harmonic voltage and current. The frequency of sawtooth wave voltage on the capacitor is as high as tens of kilohertz, or even tens of megahertz. Its requirements are different from those in low-frequency applications. The capacitance is not the main indicator, but its impedance frequency characteristics are used to measure its quality,

Some 47 μ The impedance frequency characteristic of F/350V aluminum electrolytic capacitor decreases and increases with the increase of frequency. When the capacitive reactance is equal to the inductive reactance and cancels each other, the frequency is the resonant frequency of the aluminum electrolytic capacitor. At this time, the impedance is the lowest, leaving only ESR. If ESR is zero, then the impedance is zero; If the frequency continues to rise, the inductive reactance starts to be greater than the capacitive reactance. When the inductive reactance is close to the ESR, the impedance frequency characteristic starts to rise and become inductive. From this frequency, the capacitor is an inductor in time at the frequency above. Due to the manufacturing process,

The larger the capacitance is, the larger the parasitic inductance is, the lower the resonant frequency is, and the lower the inductive frequency of the capacitor is. This requires it to have a low equivalent impedance within the operating frequency band of the switching power supply. At the same time, for the internal part of the power supply, because the semiconductor devices start to work and produce hundreds of kilohertz of peak noise, it can also have a good filtering effect. Generally, when the low frequency is about 10kHz, the impedance of the common electrolytic capacitor begins to appear inductive, which cannot meet the requirements of switching power supply. Electrolytic capacitors used for output rectification of switching regulated power supply are required to have impedance frequency characteristics that do not show an upward trend at 300 kHz or even 500 kHz. Electrolytic capacitors have low ESR, which can effectively filter high-frequency ripple and peak voltage in switching power supply. However, ordinary electrolytic capacitors began to show an upward trend after 100kHz, and the filtering effect of switching power supply output rectifier is relatively poor. In the experiment, the author found that 4700 μ F. 16V electrolytic capacitor, the ripple and peak used for switching power supply output filtering are no better than CD03HF 4700 μ F. The 16V high-frequency electrolytic capacitor is low, and the temperature rise of ordinary electrolytic capacitor is relatively high. When the load is sudden change, the transient response of ordinary electrolytic capacitor is far less than that of high-frequency electrolytic capacitor. In order to achieve high efficiency, switching power supply improves the high frequency of working frequency, especially in small high output switching power supply, the input filter capacitor requires high ripple, and the output terminal has low impedance. In order to reduce the impedance of output filter capacitor at high frequency, the equivalent series resistance must be reduced.