I. Overview of Failure Modes and Factors of Aluminum Electrolytic Capacitors
The positive electrode of an aluminum electrolytic capacitor, the anode lead electrode, and the outer casing are all made of high-purity aluminum. The dielectric medium is a thin layer of aluminum oxide formed on the surface of the positive electrode. The actual negative electrode is the electrolyte, which functions like an electrolytic cell during operation, but only the anodized layer on the surface of the positive electrode has been formed, and no electrochemical reaction occurs. The theoretical current is zero, but a slight leakage current can occur due to impurities in the electrode or electrolyte.
Common failure phenomena and modes of aluminum electrolytic capacitors include electrolyte dryness, pressure relief device activation, short circuits, open circuits (no capacitance), and excessive leakage current. If the capacitor itself is of good quality, failures usually arise from the application environment. Design and application conditions such as ambient temperature, heat dissipation method, voltage, and current parameters play a significant role.
Short circuits and open circuits are considered "catastrophic failures" that cause the capacitor to lose its function. Other types of failure, such as degradation or depletion, are generally classified as "degradation failures."
II. Failure Mechanism of Aluminum Electrolytic Capacitors
Depletion Failure
(1) The end of the life of an electrolytic capacitor is typically judged by the capacitance falling below 80% of its rated value. Early aluminum electrolytic capacitors had electrolytes that gradually decreased in capacitance over time. As the working electrolyte was repaired during use, the electrolyte was gradually reduced due to damage caused by impurities. In the later stages, due to evaporation, the electrolyte became thicker, making it difficult for it to contact the roughened aluminum foil, reducing the effective area of the capacitor plates. This leads to a sharp drop in capacitance, primarily due to electrolyte evaporation, with high temperatures being the main cause.
(2) When the electrolyte evaporates and becomes thicker, the resistivity increases, leading to a higher equivalent series resistance (ESR) and increased loss angle. For example, at 105°C, if the core temperature exceeds 125°C, the ESR can increase by nearly ten times, producing more heat and accelerating evaporation. This creates a cycle where the capacitor's capacity drops sharply, potentially causing an explosion.
(3) An increase in leakage current often leads to failure. Excessive application voltage and high temperatures can cause leakage current to rise.
Pressure Release Device Action
To prevent explosions caused by internal gas buildup from high-temperature boiling or electrochemical processes, aluminum electrolytic capacitors with a diameter of 8 mm or more are equipped with pressure release devices. These activate before dangerous pressure levels are reached, releasing the gas and rendering the capacitor invalid.
Electrochemical processes can generate gas, leading to increased internal pressure and triggering the pressure release device. High temperatures, whether from the environment or excessive ripple current, can also cause this mechanism to activate.
Instantaneous Over-Temperature
The core temperature of an aluminum electrolytic capacitor rises by about 10°C for every 10°C increase in ambient temperature. If the capacitor overheats, especially under high ripple current, it may permanently increase the ESR, leading to failure. It is crucial to monitor temperature and ensure proper cooling in high-temperature applications.
Instantaneous Overvoltage Generation
During power-on, the filter inductor can release stored energy into the filter capacitor, causing an overvoltage transient. Selecting capacitors with good overvoltage performance is essential, as some models provide instantaneous overvoltage values.
Electrolyte drying is the primary cause of failure in aluminum electrolytic capacitors. The electrolyte naturally evaporates, and its consumption due to electrochemical effects from leakage current reduces the capacitor's lifespan. Higher temperatures and voltages increase leakage current, further shortening the life of the capacitor.
Factors Affecting the Life of Aluminum Electrolytic Capacitors
Temperature significantly impacts the life of these capacitors. For instance, every 10°C increase in temperature halves the life. The electrolyte solution determines the overall lifespan, and application conditions such as high temperature, high ripple current, and excessive operating voltage all shorten the life.
Parameters like operating voltage, leakage current, and temperature affect the capacitor's life. Graphs and formulas help estimate life based on these factors. However, accurate calculations require detailed thermal and ESR data, which are often provided by manufacturers.
III. Methods for Calculating the Life of Aluminum Electrolytic Capacitors
Simple Life Calculation
Estimating life based on ESR, thermal resistance, and ripple current is common. The "10°C rule" states that life halves with every 10°C increase in temperature. However, this applies mainly to low-ripple current scenarios.
Graphical methods and manufacturer-specific formulas offer more accurate estimates. Considering ripple current is essential, as it directly affects heat generation and thus the capacitor’s lifespan.
IV. Considerations When Purchasing Aluminum Electrolytic Capacitors
Avoid using capacitors from unknown sources, as they may be counterfeit, refurbished, or off-line parts. Disassembled components, especially electrolytic capacitors, have very limited lifespans and are unsuitable for new products.
Refurbished capacitors may appear functional but often have higher leakage currents and reduced lifespans. Foreign manufacturers sometimes produce lower-quality capacitors for specific markets, so it's important to purchase from reputable suppliers.
A simple test can check the withstand voltage of an aluminum electrolytic capacitor. By applying a small current and measuring the breakdown voltage, users can determine if the capacitor meets its rated specifications.
When purchasing, choose capacitors from trusted manufacturers or authorized agents to ensure quality and reliability. Avoid buying from unverified sources or electronics markets.
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