Evolution of aluminum electrolytic capacitors
Switching power supply is widely used in various communication equipment, household appliances, computers and other terminal equipment with the characteristics of small size, light weight and high efficiency. As one of the most important devices in the primary and secondary circuit filter circuit of switching power supply, aluminum electrolytic capacitor is also evolving with the evolution trend of switching power supply.
Now let's talk about their "evolutionary trends" in detail
1 large capacity and small volume
As we all know, electrolytic capacitors are mostly wound structure, so that the volume becomes larger and becomes a hard injury. The trouble is that the switching power supply has smaller and smaller size when it requires the same capacitance. When active power filter is used in the primary side of switching power supply, the operating environment of aluminum electrolytic capacitor becomes more severe than before
(1) The high frequency pulse current is mainly 20 kHz ~ 100 kHz pulsating current and increases greatly;
(2) The main switch tube of the converter is heated, which causes the temperature around the aluminum electrolytic capacitor to rise;
(3) The converter mostly uses boost circuit, so it requires aluminum electrolytic capacitor with high voltage resistance.
In this way, aluminum electrolytic capacitors made by using the previous technology have to choose large-size capacitors because they want to absorb more pulsating current than before. This choice will lead to the power supply volume is too large to be suitable for miniaturized electronic equipment. In order to solve these problems, it is necessary to develop a new type of electrolytic capacitor with small volume, high voltage resistance and large amount of high frequency pulse current. In addition, this kind of electrolytic capacitor, working in high temperature environment, has to have a longer working life.
2. High temperature and long life
For the medium and large capacity products above 100 μ F, the aluminum electrolytic capacitor with price advantage is the most widely used. However, in recent years, the situation of avoiding the use of aluminum electrolytic capacitors is increasing. The reason lies in the short life span. "For aluminum electrolytic capacitor, this kind of component with limited life, it can not be used if it can not be used", the engineer of power module manufacturer said that the electrolyte inside the aluminum electrolytic capacitor will evaporate or produce chemical changes, resulting in the reduction of electrostatic capacity or the increase of equivalent series resistance (ESR). As time goes on, the capacitance performance will definitely deteriorate. In fact, the life of electrolytic capacitor is negatively correlated with the long-term working environment temperature.
However, if the temperature of the electrolytic capacitor is higher than 90 ℃ when the temperature of the electrolytic capacitor is higher than 90 ℃, there are many kinds of capacitors that have been damaged when the temperature is higher than 90 ℃.
On the other hand, the service life of electrolytic capacitors is also related to the ratio of AC current and rated pulse current (generally refers to the test value at 85 ℃ ambient temperature, but some electrolytic capacitors with high temperature resistance are tested at 125 ℃). Generally speaking, the larger the ratio, the shorter the life. When the current flowing through the electrolytic capacitor is 3.8 times of the rated current, the electrolytic capacitor will be damaged.
3. High frequency and low impedance
Although the switching frequency is different, the function of the output rectifier filter capacitor is basically the same. The ripple voltage component is filtered by absorbing the current component of the switching frequency and its higher harmonic frequency by using the filter capacitor.
The filter capacitor used at the output end of switching power supply is different from the filter capacitor used in power frequency circuit. The pulse voltage frequency of ordinary electrolytic capacitor used as filter in power frequency circuit is only 100Hz, and the charge and discharge time is in the order of millisecond. In order to obtain a small pulse coefficient, the capacitance required is as high as hundreds of thousands of Micromethods. Therefore, ordinary aluminum electrolysis is used in low frequency The main purpose of vessel manufacturing is to improve the capacitance. The capacitance, loss tangent and leakage current are the main parameters to identify the advantages and disadvantages.
The electrolytic capacitor used as output filter in switching power supply, because most of the switching power supply work in the state of square wave or rectangular wave, which contains rich high-order harmonic voltage and current, the frequency of sawtooth wave voltage on it is as high as tens of kilohertz, or even tens of megahertz. Its requirements are different from those in low-frequency application. Capacitance is not the main index, but the measure of its quality is Its impedance frequency characteristic. In order to make the output filter capacitor low impedance at high frequency, the equivalent series resistance must be reduced.
4 ripple resistance
Ripple current is one of the most important parameters affecting the performance of electrolytic capacitors. The main effect of ripple current on aluminum electrolytic capacitor is to generate power consumption on ESR, which makes aluminum electrolytic capacitor heat up, thus shortening its service life. The loss on ESR caused by ripple current is directly proportional to the square of the effective value of ripple current. Therefore, with the increase of ripple current, the hourly life curve is similar to a parabolic function curve.
Generally speaking, the switching current generated by flyback converter is relatively maximum. For example, in order to withstand large current, ESR of capacitance should be further reduced.
If the ESR is small, the decrease of output voltage of capacitor is small when there is a large current flow. With the increase of current, the requirement of reducing ESR may be the main reason to promote the process of capacitor replacement. Compared with the ESR of nearly 1 Ω for aluminum electrolytic capacitor, the ESR of multilayer ceramic capacitor is very small, less than 10 m Ω. The ESR of conductive polymer capacitors is usually tens of M Ω, and the ESR of small ones is less than