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Evolution of aluminum electrolytic capacitors

  • Categories:Industry News
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  • Time of issue:2020-06-22 10:05
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(Summary description)The 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 important devices in the primary and secondary loop filter circuits of the switching power supply, aluminum electrolytic capacitors also follow the evolution of the switching power supply.

Evolution of aluminum electrolytic capacitors

(Summary description)The 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 important devices in the primary and secondary loop filter circuits of the switching power supply, aluminum electrolytic capacitors also follow the evolution of the switching power supply.

  • Categories:Industry News
  • Author:
  • Origin:
  • Time of issue:2020-06-22 10:05
  • Views:
Information
The 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 important devices in the primary and secondary loop filter circuits of the switching power supply, aluminum electrolytic capacitors also follow the evolution of the switching power supply.
Now let's elaborate on their "evolutionary trend":
1 Large capacity and small volume
We all know that most electrolytic capacitors are wound structures, so that the volume becomes larger and becomes a bruise. The trouble is that the switching power supply has a smaller and smaller volume when the same capacity is required. Using an active filter circuit 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 increases significantly;
(2) The main switch tube of the converter heats up, causing the surrounding temperature of the aluminum electrolytic capacitor to rise;
 (3) Most of the converters use booster circuits, so high-voltage resistant aluminum electrolytic capacitors are required.
 In this way, aluminum electrolytic capacitors manufactured using conventional technology have to select large-sized capacitors because they have to absorb a larger ripple current than before. This choice will cause the power supply to be too bulky and difficult to apply to miniaturized electronic equipment. In order to solve these problems, it is necessary to develop a new electrolytic capacitor with a small volume, high voltage resistance, and a large amount of high-frequency pulse current. In addition, this electrolytic capacitor works in a high temperature environment, and the working time must be relatively long.  
For medium and large-capacity products above 100μF, aluminum electrolytic capacitors with the most price advantage are the most widely used. But in recent years, the situation of avoiding the use of aluminum electrolytic capacitors is increasing. The reason is stuck on the short board of life. "For components with limited lifespan, such as aluminum electrolytic capacitors, you can use them if you don’t need them" ) Increase, the capacitor performance will definitely deteriorate over time. A
In fact, the life of an electrolytic capacitor is inversely related to the long-term ambient temperature of the capacitor.
Ordinary electrolytic capacitors have been damaged when the ambient temperature is 90°C, but there are many types of electrolytic capacitors whose working ambient temperature is higher than 90°C. When the ratio of the AC 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 ℃, the electrolytic capacitor has been damaged.
On the other hand, the life of the electrolytic capacitor is also related to the AC current and rated pulse current of the capacitor for a long time (generally refers to the test value at an ambient temperature of 85 ℃, but some high temperature resistant electrolytic capacitors are tested at 125 ℃ Data). Generally speaking, the greater the ratio, the shorter the life. When the current flowing through the electrolytic capacitor is 3.8 times the rated current, the electrolytic capacitor will generally be damaged.
3 High frequency and low impedance
Although the switching frequency is different, the output rectification 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 to filter its ripple voltage component. A
The filter capacitor used at the output 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 100 Hz, and the charge and discharge time is On 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 goal of manufacturing low-frequency ordinary aluminum electrolytic capacitors is to increase the capacitance. The capacitance of the capacitor, the tangent of the loss angle, and Leakage current is the main parameter to distinguish its advantages and disadvantages.
As an electrolytic capacitor for output filtering in a switching power supply, because most switching power supplies work in a square wave or rectangular wave state, which contains rich high-order harmonic voltages and currents, the frequency of the sawtooth wave voltage is as high as several Ten kilohertz, or even tens of megahertz, its requirements are different from those used in low-frequency applications. The capacitance is not the main indicator, and its impedance frequency characteristic is what measures it. To reduce the impedance of the output filter capacitor at high frequencies, the equivalent series resistance must be reduced.
4 Ripple current resistance
One of the most important parameters that affect the performance of electrolytic capacitors is the ripple current problem. The impact of ripple current on aluminum electrolytic capacitors is mainly due to the power dissipation in the ESR, which causes the aluminum electrolytic capacitors to heat up, thereby shortening the service life. The loss of ripple current on ESR is proportional to the square of the effective value of ripple current, so as the ripple current increases, the hourly life curve is similar to the parabolic function curve.
Generally speaking, the switching conversion current generated by the flyback converter is relatively maximum. Take the flat-panel TV as an example, in order to withstand large currents, the ESR of the capacitor needs to be further reduced.
If the ESR is small, the amount of drop in the output voltage of the capacitor is also small when a large current flows. With the increase in current, the requirement to reduce ESR may become the main reason for advancing the capacitor replacement process. Compared with the ESR of aluminum electrolytic capacitors of nearly 1Ω, the ESR of multilayer ceramic capacitors is very small, less than 10mΩ. The ESR of a conductive polymer capacitor is usually several tens of mΩ, and the ESR is relatively small at 10mΩ or less. Aluminum electrolytic capacitors are also developing products with relatively small ESR, whose ESR is about 1/2 to 1/3 of the general products.
5 High reliability
As an input electrolytic filter and smoothing aluminum electrolytic capacitor, its quality and reliability directly affect the reliability of the switching power supply. Once the aluminum electrolytic capacitor fails, it will cause failure of the switching power supply.
The failure modes of aluminum electrolytic capacitors used in switching power supplies include breakdown failure, open circuit failure, liquid leakage failure, and electrical parameter out-of-tolerance failure.
Among them, breakdown failure is divided into dielectric breakdown and thermal breakdown. For electrolytic capacitors for switching power supplies with high power and high current output, thermal breakdown failure often accounts for a certain proportion; electrical corrosion causes aluminum lead strips to break and capacitor cores dry up. , The main failure mode of open circuit failure of aluminum electrolytic capacitors for switching regulators; leakage is a common failure mode of aluminum electrolytic capacitors for switching regulators. Due to the harsh environment and working conditions, leakage failures often occur; the switch is stable The most common failure modes of aluminum electrolytic capacitors used in power supply are reduced capacitance, increased leakage current and loss angle

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