As we all know, the switching power supply is the main power supply of today's information appliances, making an indelible contribution to the miniaturization and lightening of electronic equipment. With the continuous miniaturization, light weight and high efficiency of switching power supply, it is used more and more in electronic equipment, and the penetration rate is getting higher and higher. Correspondingly, electrolytic capacitors are required to be small and large-capacity, resistant to ripple current, low-impedance at high frequency, long-life at high temperature, and more suitable for high-density assembly.

Capacitance and volume

Since most electrolytic capacitors use a winding structure, it is easy to expand the volume, so the capacitance per unit volume is very large, several to dozens of times larger than other capacitors. However, the acquisition of large capacitance comes at the expense of volume expansion. Modern switching power supplies require higher and higher efficiency and smaller volume. Therefore, it is necessary to seek new solutions to obtain large capacitance and small size. volume of capacitors.

Once the active filter circuit is used on the primary side of the switching power supply, the use environment of aluminum electrolytic capacitors becomes more severe than before:

(1) The high-frequency pulse current is mainly a pulsating current of 20 kHz to 100 kHz, and it is greatly increased;

(2) The main switch tube of the converter heats up, causing the surrounding temperature of the aluminum electrolytic capacitor to rise;

(3) Converters mostly use boost circuits, so high-voltage aluminum electrolytic capacitors are required. As a result, since the aluminum electrolytic capacitors manufactured by the conventional technology have to absorb a larger pulsating current than the conventional ones, a large-sized capacitor has to be selected. As a result, the size of the power supply is made large, and it is difficult to use it in a miniaturized electronic device. In order to solve these problems, it is necessary to research and develop a new type of electrolytic capacitor, which is small in size, can withstand high voltage, and allows a large amount of high-frequency pulse current to flow. In addition, this electrolytic capacitor must work in a high temperature environment, and the working life must be relatively long.

Withstand temperature and life

In the design process of switching power supply, it is inevitable to select suitable capacitors. For medium and large-capacity products above 100μF, aluminum electrolytic capacitors are the most widely used so far because they are cheap. However, there has been a significant change in recent years and the avoidance of aluminum electrolytic capacitors is increasing. One reason for this change is that the lifespan of aluminum electrolytic capacitors tends to be the weak link in the overall device. An engineer from a power module manufacturer said: "If you can't use a component with a limited life such as an aluminum electrolytic capacitor, try not to use it." Because the electrolyte inside the aluminum electrolytic capacitor will evaporate or produce chemical changes, resulting in reduced electrostatic capacity or Equivalent series resistance (ESR) increases, and capacitor performance will definitely degrade over time.

The life of an electrolytic capacitor is directly related to the ambient temperature in which the capacitor works for a long time. The higher the temperature, the shorter the life of the capacitor. Ordinary electrolytic capacitors are damaged when the ambient temperature is 90°C. But now there are many kinds of electrolytic capacitors whose working environment temperature is very high. When the ambient temperature is 90℃, when the ratio of the alternating current through the electrolytic capacitor to the rated pulse current is 0.5, the life is still 10000h, but if the temperature rises to 95℃ When the electrolytic capacitor is damaged. Therefore, when choosing a capacitor, it should be selected according to the specific ambient temperature and other parameters. If the influence of the ambient temperature on the life of the capacitor is ignored, the reliability and stability of the power supply will be greatly reduced, and even damage the equipment. and instruments. In general, electrolytic capacitors can generally meet the requirements of 10000h life when the ambient temperature is 80 °C.

On the other hand, the life of electrolytic capacitors is also related to the AC current and rated pulse current of the capacitors working for a long time (generally refers to the test value at an ambient temperature of 85°C, but some electrolytic capacitors with high temperature resistance are tested at 125°C data) ratio. Generally speaking, the larger the ratio, the shorter the life of the electrolytic capacitor. When the current flowing through the electrolytic capacitor is 3.8 times the rated current, the electrolytic capacitor is generally damaged. Therefore, electrolytic capacitors have their safe working area. For general applications, when the ratio of AC current to rated pulse current is less than 3.0 times, the requirements for life have been met. Effects of ambient temperature and ripple current on electrolytic capacitors.

Frequency Characteristics and Impedance

For small and medium output power switching power supply, except a few still use 20~40kHz due to price constraints, most of them are above 50kHz; most of DC/DC power modules are above 300kHz; the switching frequency of high-power switching power supply is affected by the main switch ( Generally, the switching speed of IGBT is limited and generally 20~40kHz. Although the switching frequency is different, the function of the output rectifier filter capacitor of the switching power supply is basically the same, 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 not the same as 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 required capacitance is as high as hundreds of thousands of microfarads. Therefore, the general purpose of manufacturing ordinary aluminum electrolytic capacitors for low frequency is to increase the capacitance. The capacitance of the capacitor, the loss tangent value and the Leakage current is the main parameter to identify its pros and cons. It is used as an electrolytic capacitor for output filtering in switching regulated power supplies. Since most switching power supplies work in the state of square wave or rectangular wave, it contains extremely rich high-order harmonic voltage and current, and the frequency of sawtooth wave voltage on it. Up to tens of kilohertz, or even tens of megahertz, its requirements are different from those of low-frequency applications. Capacitance is not the main indicator. What measures its quality is its impedance frequency characteristics.