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 material in these capacitors is a thin layer of aluminum oxide formed on the surface of the positive electrode. The actual negative electrode is an electrolyte, making the capacitor function like an electrolytic cell during operation. However, only the anodized layer on the positive electrode is formed, and no electrochemical reaction occurs. Theoretically, the current should be zero, but a small leakage current arises due to impurities in the electrodes or electrolyte.
From a practical perspective, 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. These failures often result from environmental factors rather than inherent quality issues in the capacitor itself. Key application conditions affecting performance include ambient temperature, heat dissipation methods, and voltage and current parameters. Short circuits and open circuits are considered "catastrophic failures" that render the capacitor non-functional, while other types of failures, such as degradation or depletion, are classified as "degradation failures."
II. Failure Mechanisms of Aluminum Electrolytic Capacitors
Depletion Failure (1)
The end of an electrolytic capacitor's life is typically marked when its capacitance drops below 80% of the initial value. Early aluminum electrolytic capacitors had limited electrolyte filling, leading to a gradual decrease in capacitance over time. As the electrolyte repairs itself during operation, it gradually decreases due to damage caused by impurities in the anodic oxide film. In later stages, evaporation of the electrolyte increases its viscosity, making it harder to contact the roughened aluminum foil. This reduces the effective plate area, causing a sharp drop in capacitance. Thus, the primary cause of capacity reduction is electrolyte evaporation, which is most commonly triggered by high temperatures.
Depletion Failure (2)
Excessive heat generated by the capacitor during operation can also contribute to failure. When used in rectification filters or power supply bypass applications, the capacitor experiences losses in its Equivalent Series Resistance (ESR), which is converted into heat. As the electrolyte evaporates and becomes more viscous, its resistivity increases, raising the ESR and resulting in higher losses and a larger loss angle. For example, at 105°C, if the core temperature exceeds 125°C, the ESR may increase tenfold, generating even more heat and accelerating further evaporation. This creates a cycle that leads to a rapid decline in capacitance and potentially causes the capacitor to explode.
Depletion Failure (3)
An increase in leakage current often leads to failure. Excessive applied voltage and high temperatures can both cause leakage current to rise, further degrading the capacitor's performance.
Pressure Release Device Action
To prevent explosions caused by internal gas buildup due to high temperatures or electrochemical processes, aluminum electrolytic capacitors with a diameter of 8 mm or more are equipped with pressure release devices. These devices activate before dangerous pressure levels are reached, releasing gas and rendering the capacitor ineffective. This mechanism helps avoid catastrophic failures.
Electrochemical Processes and Pressure Release
Leakage current in an aluminum electrolytic capacitor is an electrochemical process that generates gas. Over time, this gas accumulation increases internal pressure, eventually triggering the pressure release device.
High Temperatures and Pressure Release
High ambient temperatures, such as those near heating elements or in high-temperature environments, can cause the capacitor to overheat. Additionally, excessive ripple current can generate too much heat, leading to boiling of the electrolyte and triggering the pressure release device.
Instantaneous Over-Temperature
A 10°C increase in core temperature can significantly reduce the lifespan of an aluminum electrolytic capacitor. The core, located at the center of the capacitor, is the hottest point. If the capacitor reaches its maximum allowable temperature, the electrolyte may be driven out, increasing the ESR by a factor of 10. This can cause permanent damage, especially under conditions of high temperature and large ripple current.
Instantaneous Overvoltage
During power-on, inductors can release stored energy into the filter capacitor, causing transient overvoltages. These overvoltages can exceed the capacitor's rated voltage, potentially damaging it. Selecting capacitors with good transient overvoltage performance is essential for reliable operation.
Electrolyte Drying as the Main Cause of Failure
The natural evaporation of the electrolyte and its consumption due to electrochemical effects are the primary reasons for failure. Sealing quality and operating temperature play significant roles in determining how quickly the electrolyte dries out.
Factors Affecting the Life of Aluminum Electrolytic Capacitors
The life of an aluminum electrolytic capacitor is influenced by several factors, including operating temperature, ripple current, and the quality of the electrolyte. For instance, every 10°C increase in temperature halves the capacitor's life. High ripple current and excessive voltage also shorten the service life of the capacitor.
Life Calculation Methods for Aluminum Electrolytic Capacitors
Simple Life Calculation
Estimating the life of an aluminum electrolytic capacitor based on ESR, thermal resistance, and ripple current is a common approach. The "10°C rule" states that for every 10°C increase in temperature, the life of the capacitor is halved. This rule applies primarily to capacitors without significant ripple current.
Graphical Life Estimation
Some manufacturers provide graphical data to estimate the life of their capacitors based on temperature and ripple current. These graphs can help users make informed decisions about capacitor selection and application conditions.
Advanced Life Estimation Considering Ripple Current
For capacitors with significant ripple current, the heat generated by ESR must be considered. The formula for calculating power loss and temperature rise is essential for accurate life estimation. However, determining the heat dissipation coefficient can be challenging, and results may vary depending on individual capacitor characteristics.
Importance of Manufacturer Data
Using the life curves provided by the manufacturer is the most accurate way to estimate the life of an aluminum electrolytic capacitor. Reputable foreign manufacturers often provide detailed information on the relationship between life, temperature, and ripple current.
Purchase Considerations for Aluminum Electrolytic Capacitors
Avoid Using Capacitors with Unknown Sources
Capacitors from unknown sources, such as disassembled parts or counterfeit products, should be avoided. These components often have reduced lifespans and may not meet safety standards. Disassembled capacitors, in particular, are unsuitable for use in new products due to their shortened life expectancy.
Voltage "Stealing" and Its Consequences
Some manufacturers use "voltage stealing" techniques to reduce costs by using lower-rated components. This practice can lead to increased leakage current and a significant reduction in capacitor life.
Refurbished and Downline Capacitors
Refurbished and downline capacitors may appear functional but often have hidden defects, such as higher leakage currents and reduced lifespans. These capacitors pose risks, especially in applications involving high ripple current or elevated temperatures.
Testing the Withstand Voltage of Aluminum Electrolytic Capacitors
A simple method to test the withstand voltage of an aluminum electrolytic capacitor involves connecting it in series with a 10kΩ resistor and gradually increasing the applied voltage until the current stabilizes. This method can help identify the breakdown voltage and ensure the capacitor is suitable for its intended application.
Guidelines for Purchasing Aluminum Electrolytic Capacitors
When purchasing aluminum electrolytic capacitors, it is advisable to choose reputable manufacturers or authorized agents. Avoid buying from unverified sources, such as electronics markets or under-represented agencies, to ensure product quality and reliability.
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