How is the life of electrolytic capacitors affected? How to calculate the life of electrolytic capacitors

How is the life of electrolytic capacitors affected? How to calculate the life of electrolytic capacitors

The life of electrolytic capacitors is one of the issues that everyone is concerned about, because the life of electrolytic capacitors directly affects the normal use and replacement of electrolytic capacitors. For this reason, we must improve our understanding of the life of electrolytic capacitors, and how to reduce the life of electrolytic capacitors.

1. Introduction to electrolytic capacitors

Electrolytic capacitors are capacitors made with an oxide film formed by electrolysis as a medium. The aluminum electrolytic capacitor uses high-purity aluminum as the anode, and a paste composed of ethylene glycol, glycerol, boron, and ammonia as the electrolyte. The electrolysis in the electrolyte produces a very thin layer on the aluminum surface. The aluminum oxide film is a capacitor made of dielectric. Electrolytic capacitors can be made very thin because the dielectric film can be made into a capacitor with a small volume and a large capacity, which is the main part of a large-capacity capacitor. However, electrolytic capacitors have many shortcomings, such as poor frequency well characteristics and temperature characteristics, and large leakage current and dielectric loss. In addition, when the polarity is reversed or the voltage applied at both ends exceeds the specification, its safety will be destroyed, and the electrolyte will be vaporized and exploded (commonly known as the so-called breakdown).

There are many external environmental factors that can cause the deterioration of electrolytic capacitor performance, such as temperature, humidity, air pressure, and vibration. Electrical effects include voltage, ripple, current, and charging and discharging. Among the environmental factors, temperature has a great influence on the life of the capacitor, and will reduce the electrostatic capacity and increase the loss. In addition, aluminum electrolytic capacitors have greater loss (internal resistance) than other capacitors, and the heat caused by the ripple current will also have a great impact on the life. In addition, the high bias current required by some pure Class A amplifiers will cause high heat, which can easily lead to a serious shortening of the service life of all related parts. Among them, the electrolytic capacitors are important power supplies. High heat will rapidly degrade the performance of the electrolytic capacitor, shortening its life span and electrostatic capacity to only a fraction of the original value. As a result, the filter capacitor will lose its function, and the machine will easily fail.

Two, analysis of the life of electrolytic capacitors

Aluminum electrolytic capacitors are now widely used components, but we will more or less hear about the failure of electrolytic capacitors. Then, when we design the circuit, we have to consider the following issues.

1. Why do aluminum electrolytic capacitors fail?

2. What factors are related to the failure of aluminum electrolytic capacitors?

3. How to calculate the life of electrolytic capacitor?

Then look at these issues in turn.

(1) Why do aluminum electrolytic capacitors fail?

This question has to mention the structure of aluminum electrolytic capacitors.

The basic structure of a general-purpose aluminum electrolytic capacitor is a foil-wound structure. The anode is aluminum metal foil, the medium is the valve metal oxide film Al2O3 formed on the surface of the anode metal foil by electrochemical methods, and the cathode is porous electrolytic paper. Adsorbed working electrolyte.

It is generally understood that capacitors are composed of two conductive plates arranged side by side. The positive electrode is aluminum (anode foil), the intermediate medium is non-conductive aluminum oxide (anodic oxide film), and the negative electrode is electrolyte.

The main reason for the failure of the electrolytic capacitor is the electrolyte. This electrolyte will decompose itself to produce gas, and it will also chemically react with the oxide film to produce gas. Of course, under normal use, this rate is very slow, otherwise it would be useless.

Therefore, the length of the electrolysis life is mainly determined by the electrolyte. When the electrolyte is gone, the capacitor also fails.

(2) Factors affecting the failure of aluminum electrolytic capacitors

The failure rate of aluminum electrolytic capacitors is related to the volatilization rate of the electrolyte, and the volatilization rate mainly depends on the temperature. The temperature inside the electrolytic capacitor depends on the ambient temperature and ripple current.

If it is used in a situation where the power supply ripple is relatively small, the temperature of the electrolytic capacitor is mainly determined by the ambient temperature.

If it is used in a situation where the ripple current is relatively large, the ESR of the electrolytic capacitor cannot be ignored. For example, when used in a switching power supply, the electrolytic capacitor will actively generate heat due to the existence of ESR.

(3) Calculation of the life of aluminum electrolytic capacitors

As the temperature rises, the chemical reaction rate (lifetime consumption) increases. Generally speaking, when the ambient temperature rises by 10°C, the chemical reaction rate (K value) will increase by 2-10 times, that is, the operating temperature of the capacitor increases by 10 ℃, the life of the capacitor is doubled, and the service life of the capacitor is doubled every time the operating temperature of the capacitor drops by 10 ℃.