I. Overview of Failure Modes and Factors of Aluminum Electrolytic Capacitors
The positive electrode of an aluminum electrolytic capacitor, the anode lead, and the outer casing are all made of high-purity aluminum. The dielectric material 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. However, only the anodized layer on the positive electrode is formed, and no electrochemical reactions occur. Theoretically, the current should be zero, but a small leakage current arises due to impurities in the electrode and 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 has no quality issues, failures often arise from application conditions. Key factors include ambient temperature, heat dissipation methods, voltage, and current parameters. Short circuits and open circuits are considered "catastrophic" or "fatal" failures that cause the capacitor to lose functionality. Other types of failures are typically classified as "degradation" or "depletion" failures.
II. Failure Mechanism of Aluminum Electrolytic Capacitors
Depletion Failure (1)
The end of an electrolytic capacitor's life is usually determined by its capacitance dropping below 80% of the rated value. Early capacitors had filled electrolytes, leading to a slow decrease in capacitance at the beginning. As the working electrolyte repairs itself during use, the electrolyte gradually decreases due to damage from impurities in the anodic oxide film. In later stages, evaporation causes the electrolyte to thicken, reducing contact with the corroded aluminum foil, thus decreasing the effective plate area and causing a sharp drop in capacitance. This decline is mainly due to electrolyte evaporation, often caused by high temperatures.
Depletion Failure (2)
Heating in aluminum electrolytic capacitors occurs when they operate in rectification filters, including high-frequency rectification and filtering in switching power supplies. The ESR of the capacitor generates losses, which convert into heat. When the electrolyte evaporates more and becomes thicker, its resistivity increases, raising the equivalent series resistance (ESR) and significantly increasing the capacitor's loss and loss angle. For example, at 105°C, if the core temperature exceeds 125°C, the electrolyte's viscosity rises dramatically, increasing ESR tenfold. This leads to more heat, further evaporation, and a sharp drop in capacitance, potentially causing an explosion.
Depletion Failure (3)
Excessive leakage current often results in failure. High applied voltage and elevated temperatures can increase leakage current.
Pressure Release Device Action
To prevent explosions caused by internal gas buildup due to high-temperature boiling 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 internal pressure is reached, releasing gas and rendering the capacitor ineffective.
Electrochemical Process Causes Pressure Release
Leakage current is an electrochemical process that generates gas, increasing internal pressure and triggering the pressure release device.
High Temperature Causes Pressure Release
If the ambient temperature is too high, such as near a heating element or in a hot environment, it can cause overheating. Additionally, high core package temperatures from excessive ripple current can generate significant heat, leading to electrolyte boiling and pressure release.
Instantaneous Over-Temperature
A 10°C increase in core temperature doubles the core's lifespan. However, reaching the maximum allowable temperature can drastically increase ESR, causing permanent damage and failure. In high-temperature and high-ripple-current applications, careful attention to cooling is essential.
Instantaneous Overvoltage Generation
During power-on, inductors release stored energy into the filter capacitor, causing transient overvoltage.
Prevention of Capacitor Overvoltage Failure
Capacitors are prone to breakdown under overvoltage conditions. Transient high voltages are common in practical applications. Selecting capacitors with good transient overvoltage performance is crucial. Some RIFA capacitors provide instantaneous overvoltage values.
Electrolyte Drying Is the Main Cause of Failure
Electrolyte naturally evaporates, especially at higher temperatures. Sealing quality also affects evaporation. Electrochemical effects from leakage current consume the electrolyte, reducing the capacitor's lifespan. Leakage current increases with temperature and applied voltage, significantly impacting performance.
Life of the Capacitor Is Determined by Electrolyte Drying
Factors Affecting Life (Temperature 1)
Aluminum electrolytic capacitors have different maximum operating temperatures: 85°C for general use, 105°C for high-temperature use, 125°C for special high-temperature use, and 140–150°C for engine compartments.
Factors Affecting Life (Hours of Rated Life)
Capacitors can be categorized by their rated life hours: 1000 hours for general use, 2000+ hours for long-term use, and longer for industrial-grade models.
Factors Affecting Life (Temperature 2)
For every 10°C increase in temperature, the life is halved.
Factors Affecting Life (Electrolyte)
The electrolyte solution determines the capacitor’s lifespan.
Factors Affecting Life (Application Conditions)
High temperature, high ripple current, and high operating voltage shorten the capacitor's life.
Parameters and Application Conditions Affecting Life
Relationship between operating voltage and leakage current, relationship between temperature and leakage current, and the effect of temperature, voltage, and ripple current on life.
III. Calculation Method for the Life of Aluminum Electrolytic Capacitors
Simple Life Calculation
Estimate life based on ESR, thermal resistance, and ripple current. Estimate life based on temperature, ripple current, and lifetime.
Simple Life Calculation Method 1
This method uses the "10°C rule," where every 10°C increase halves the life. It applies under zero ripple current conditions but may not be accurate with large ripple currents.
Simple Life Calculation Method 2
Some domestic brands recommend this method. Japanese brands suggest similar approaches.
Basic Idea
At rated voltage, life can be calculated using a formula. L and L0 represent life at actual ambient temperature T and rated maximum temperature T0. The service life decreases with every 10°C increase, following the "10°C rule." Lower temperatures extend the capacitor's life.
Graphical Calculation
The above methods apply to storage and low-ripple-current operations. For high-ripple-current scenarios, consider the ripple current effect.
Life Estimation Considering Ripple Current
Heat generated by ESR causes temperature rise. The formula shows that temperature rise is proportional to the square of the ripple current and ESR, and inversely proportional to the capacitor's surface area. Therefore, ripple current magnitude determines heat generation and affects the capacitor's life.
Temperature Test of Aluminum Electrolytic Capacitor
Use test results and consider the life formula for ambient temperature and ripple current. The equations are used after testing the capacitor shell temperature, and the ripple current magnitude is known.
Problem
Determining the heat dissipation coefficient is challenging without manufacturer data, leading to potential deviations of at least 30%. Since internal temperature rise is hard to measure directly, the relationship between core and case temperature can be listed.
Problem
These formulas apply to ambient temperatures from +40°C to the maximum operating temperature range. Due to aging of sealing materials, actual estimated life is generally up to 15 years.
Problem
The expressions and calculations of equations 5.17 through 5.20 are cumbersome and inaccurate due to test differences and thermal conductivity variations. They are suitable for approximate estimates.
Problem
The best way to get a more accurate estimate is to use the temperature-life curve provided by the manufacturer. Responsible foreign manufacturers provide the relationship between life and ambient temperature and ripple current.
Estimate Life Based on ESR, Thermal Resistance, and Ripple Current
EPCOS specifications B relate aluminum electrolytic capacitor life to temperature and ripple current.
RIFA Life Estimation Method
Estimated life based on power loss, thermal resistance, and actual ESR.
To calculate the working life (LOP) of an aluminum electrolytic capacitor, it is necessary to know:
Operating voltage (Uapplied), the effective value (IRMS) of the ripple current, ambient temperature (Ta), and thermal resistance (Rth).
Related Formula
Calculation Method
First, find the ESR value corresponding to different frequencies and core package temperatures (Th) in the ESR matrix, then calculate the power loss (PLOSS) generated when the ripple current IRMS flows through the capacitor. If IRMS consists of multiple harmonics, calculate and add the power loss for each harmonic. Find the thermal resistance value of the capacitor winding core package to the ambient temperature in the thermal resistance matrix, allowing calculation of the actual core package temperature (Th). If the actual value does not match the previously selected ESR value, correct the hypothesis, re-detect the ESR value, and repeat the iterative calculation until the results agree.
This calculation is the most accurate!
Required Conditions:
Thermal resistance parameter of the capacitor
ESR parameters
Thermal resistance varies among different manufacturers.
IV. Things to Pay Attention to When Purchasing Aluminum Electrolytic Capacitors
Do Not Use Aluminum Electrolytic Capacitors with Unknown Sources (1)
Why not use capacitors with unknown sources? China's electronic components market has faced disassembled parts, parallel imports, and counterfeit goods. While this phenomenon is becoming less common, it still exists. Semiconductor devices can be disassembled, but aluminum electrolytic capacitors must not be used for disassembling parts. Their lifespan is the shortest among electronic components, and disassembled parts often have minimal remaining life, making them unsuitable for production.
Do Not Use Aluminum Electrolytic Capacitors with Unknown Sources (2)
Refurbishment is a common method for counterfeit capacitors. Usually, the sleeve is replaced with new heat-shrinkable tubes with rated voltages, and the printing level is faked.
"Voltage Stealing" of Aluminum Electrolytic Capacitors
Since the surge voltage of an aluminum electrolytic capacitor is 1.15 times the rated voltage, a 350V withstand voltage capacitor has a surge voltage of 402V, exceeding the 370V peak voltage of 220V+20%. Using a 300V-rated anode foil instead of a 400V one reduces costs, but it increases leakage current and reduces lifespan.
Consequences of "Voltage Stealing"
In general applications, reducing the use voltage helps avoid excessive leakage current and prolong the capacitor's life. However, stealing voltage can lead to excessive leakage current, greatly reducing the capacitor's lifespan.
Disassembled Parts
Disassembled parts are usually components from scrapped electronics abroad. While some can be used in general performance, aluminum electrolytic capacitors are generally unusable due to their short lifespan.
Refurbished Aluminum Electrolytic Capacitors
Refurbished capacitors are collected and re-dipped with electrolyte. Although detection is stricter, their performance is still not as good as genuine ones.
Downline Capacitors (1)
In the domestic market, you can see downline capacitors bought by irresponsible sellers. These capacitors are sold after removing defects. Their capacitance tests normally, but leakage current and loss factor are higher than genuine ones.
Downline Capacitors (2)
Using these capacitors as coupling capacitors can cause bias voltage shifts and unstable delay times. Their large leakage current shortens the actual lifespan significantly, sometimes only 2–3 years instead of 5.
Film Capacitors
Refurbishment is a common method for counterfeit capacitors. Usually, the sleeve is replaced with new heat-shrinkable tubes with rated voltages, and the printing level is faked.
Foreign Electrolytic Capacitor Manufacturers
Some foreign manufacturers supply high-quality capacitors to developed countries, but the ones supplied to China are often lower quality. This is due to cost considerations and market demands.
A Simple Method to Test the Withstand Voltage of Aluminum Electrolytic Capacitors
Possibility: A short-time small current breakdown does not damage the capacitor.
Basic Method: Connect the capacitor in series with a 10k resistor; slowly increase the voltage, keeping the charging current below 1mA (resistor voltage <10V). Stop when the voltage doesn't increase further. The corresponding voltage is the breakdown voltage, and 90% of this is the rated voltage. This method is only applicable to aluminum electrolytic capacitors.
Test Circuit
Guidelines for Purchasing Aluminum Electrolytic Capacitors
Purchase from reputable manufacturers or agents. Avoid electronics markets or under-represented agencies.
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