Definition and structure of aluminum electrolytic capacitors

Aluminum electrolytic capacitor is a kind of electrolytic capacitor, which is characterized by the use of aluminum material as an electrode and the storage of charge through the electrolyte inside the capacitor. Specifically, it consists of the following key components:

• Anode: The anode of aluminum electrolytic capacitors is made of aluminum foil, which is treated by electrochemical oxidation and forms a thin layer of aluminum oxide (Al₂O₃) film on its surface as a dielectric.

• Cathode: As opposed to the anode, the cathode usually consists of another piece of aluminum foil, which is in contact with the electrolyte. The electrolyte can be liquid, such as traditional aluminum electrolytic capacitors, or it can be solid, such as solid aluminum electrolytic capacitors that use conductive polymers as electrolytes.

• Electrolyte: The electrolyte is a key part of the capacitor that conducts current and can be a liquid electrolyte or a solid conducting polymer. In capacitors with liquid electrolytes, the electrolyte generally contains dissolved chemicals, such as boric acid and sulfate, to maintain the electrochemical reaction and provide ionic conductivity. Solid electrolytes provide better stability and longer service life.

• Housing and sealing: The entire assembly is enclosed in an aluminum housing, usually with a rubber plug or resin seal, to prevent electrolyte evaporation and maintain internal pressure stability.

Working principle

Aluminum electrolytic capacitor realizes the function of energy storage based on its unique structure. The basic principle is to use aluminum oxide film as a dielectric material to separate the anode made of aluminum cylinder and the cathode foil soaked with electrolyte. The positive electrode of the aluminum electrolytic capacitor is made of aluminum foil, which is anodized to form a very thin aluminum oxide (Al₂O₃) insulation film on its surface. This process is usually done by applying a direct current voltage in an electrolyte containing specific chemicals. The formed oxide film thickness is about 0.02~0.03 microns, as a dielectric, it can withstand a certain voltage without conducting electricity. The negative electrode (cathode) part is made of the original aluminum foil and a special paper soaked in electrolyte solution (electrolytic paper). The electrolyte usually contains boric acid, ammonia, ethylene glycol and other components, which can penetrate into the micropores of the anodized aluminum oxide film to form the environment required for electrochemical reactions.

The electrolyte not only acts as a conductive path, but also participates in the formation of electrochemical double layers, thereby increasing the capacitance effect. When an applied voltage is applied to both ends of the capacitor, the anode's free electrons move to the interface between the electrolyte and the oxide film, while the cathode attracts ions in the electrolyte, forming charge separation. The oxide film acts as an insulating medium that prevents the direct passage of charge but allows the presence of an electric field, thus storing energy.

Electrostatic capacity and calculation

The electrostatic capacity (also known as capacitance value) of an aluminum electrolytic capacitor refers to the capacity of the capacitor to store charge at a given voltage, and its calculation formula follows the basic capacitance definition:

Among them:

• C represents capacitance value (unit: farad, F);

• ε is the dielectric constant, for aluminum electrolytic capacitors, mainly depends on the dielectric properties of alumina (Al₂O₃) film;

• A is the effective area of the two electrodes (unit: square meters, m²);

• d is the distance between the two plates, that is, the thickness of the alumina film (unit: m, m).

For aluminum electrolytic capacitors, because the alumina layer is very thin (usually a few nanometers to tens of nanometers), relatively large capacitance values can be obtained even if the area is not large. In actual production, the capacity of capacitors is affected by a number of factors, including:

1. Dielectric constant: Although the dielectric constant of alumina is relatively fixed, different manufacturing processes and materials may slightly affect its value.

2. Plate area: Increasing the plate area can significantly increase the capacitance, which is why aluminum electrolytic capacitors often increase the capacity by increasing the surface area of the foil (for example, through etching technology to increase the surface area).

3. Oxide layer thickness: Theoretically, reducing the thickness of the oxide layer can increase the capacitance, but in practical applications, too thin oxide layer may not be able to withstand normal operating voltage, so it is necessary to find a balance between capacity and voltage resistance.

In the actual selection, the electrostatic capacity is usually provided directly by the manufacturer and does not require the user to calculate. The capacitor specification will clearly mark its rated capacity, operating voltage, temperature range and other key parameters, the designer can select according to these parameters. If you need to estimate or verify the capacitance capacity in a specific application, you can refer to the above formula, but the actual design should be based on the data provided by the manufacturer.

Selection and application precautions

In the design of switching power supply and other complex circuits, the correct selection of aluminum electrolytic capacitors is very important. The selection and application of aluminum electrolytic capacitors need to pay attention to the following key points:

1. When selecting a capacitor, its rated working voltage should be higher than the actual working voltage, and the actual working voltage is usually recommended to be 70% to 80% of the rated voltage of the capacitor. In the filter circuit, taking into account the voltage ripple, the voltage withstand value of the capacitor should not be less than 1.42 times the effective value of the AC, and for safety and extended life, it is recommended to leave at least 15% of the operating voltage margin.

2. Select a capacitor with appropriate capacity based on application requirements. Large capacity can provide better filtering and smoothing effects, but at the same time increases cost and size, and longer charging time. In high frequency applications, the equivalent series resistance (ESR) and equivalent series inductance (ESL) of aluminum electrolytic capacitors need to be considered and may need to be used with low ESR capacitors such as ceramic capacitors.

3. Confirm whether the working and storage temperature range of the capacitor meets the application requirements. High temperatures will shorten the life of the capacitor, so make sure that the maximum operating temperature of the capacitor is below its rating.

4. Assess the life expectancy of capacitors, especially for equipment that requires long periods of operation. The drying of the electrolyte will cause the capacitor performance to decline, and the choice of long-life or solid-state capacitors can improve reliability.

5. The electrolytic capacitor should be avoided near the heat source, and there should be no pad and hole directly below it to reduce thermal stress and mechanical stress. At the same time, ensure that the positive and negative terminals of the capacitor are correctly connected to avoid the direct application of reverse voltage and AC voltage.

6. For applications requiring low leakage current, special attention should be paid to the leakage current specification of the capacitor. Some high-end capacitors have a self-healing function that reduces the risk of failure due to local short circuits.

7. Choose a reputable manufacturer and avoid using used capacitors, as they may have aged or have potential failure, affecting the overall quality and safety of the product.

8. Ensure that capacitors comply with relevant industry standards and safety certifications, such as UL, CE, etc., especially in export or product design for specific application fields.

Common failure and failure causes of aluminum electrolytic capacitors

As an indispensable component in electronic equipment, the performance stability of aluminum electrolytic capacitors directly affects the reliable operation of the whole system. The faults often encountered by users in the process of use mainly include the following:

1. Electrolyte drying out: over time and temperature, the electrolyte will gradually evaporate, resulting in reduced capacitance or even complete failure. This is one of the most common failure modes of aluminum electrolytic capacitors.

2. Pressure release device action (explosion-proof valve open) : When the internal pressure increases due to overvoltage, high temperature or gas generated by electrolyte decomposition, the explosion-proof valve will open to release the internal pressure to prevent the capacitor explosion. Once the explosion-proof valve is opened, the capacitor will fail.

3. Short circuit: Due to internal metal particle migration, electrode damage or oxide film damage, may lead to direct positive and negative electrode conduction, resulting in short circuit fault.

4. Open circuit (no capacity) : The internal connection of the capacitor is broken or the oxide film is completely damaged, so that the capacitor cannot store charge, and the state is open.

5. Large leakage current: with the aging of the capacitor, the leakage current may gradually increase, resulting in increased power loss and reduced efficiency, which may affect the normal operation of the circuit in serious cases.

6. Deterioration of electrical parameters: The capacitance decreases with time, and the loss factor (i.e., the tangent value of the loss Angle) increases, because the electrolyte ages and the viscosity increases, resulting in an increase in the equivalent series resistance (ESR).

7. Overvoltage: The applied voltage exceeds the rated voltage of the capacitor, which may cause breakdown of the internal insulation layer and damage the capacitor.

8. Excessive ripple current: The ripple current exceeds the design limit, which will heat the electrolyte, accelerate aging, and reduce the capacitor life.

9. High temperature: long-term working in a high temperature environment, electrolyte evaporation accelerates, accelerates the aging of internal materials, and shortens the life of capacitors.

10. Excessive frequency: In high-frequency applications, the ESR and ESL effects of electrolytic capacitors are more significant, which may lead to heating and performance degradation.

11. Manufacturing defects: including insufficient amount of electrolyte, lax packaging, etc., may become the inducement of premature failure of capacitors.

In view of these failure causes, the design and use should be reasonable selection, to ensure that the working conditions are within the rated range of the capacitor, take heat dissipation measures when necessary, select the appropriate margin of the capacitor, and regularly check and replace to ensure the long-term stable operation of the system.

Common defects and circumvention methods of aluminum electrolytic capacitors

Common defect

1. Limited life: Due to electrolyte evaporation and aging, the life of aluminum electrolytic capacitors is usually shorter than other types of capacitors.

2. Temperature sensitive: sensitive to high and low temperature environments, high temperature accelerates electrolyte evaporation, low temperature may freeze the electrolyte, affecting performance.

3. High ESR (equivalent series resistance) : resulting in increased power loss, especially in high frequency applications.

4. Large leakage current: With the increase of time, the leakage current may gradually increase, affecting the circuit performance.

5. Poor stability: Capacitance values and performance may vary with time and temperature.

6. Polarity: The polarity must be connected correctly, otherwise it may cause capacitor damage.

7. Possible harmonic distortion: In switching power supply applications, improper design can cause loop instability and harmonic distortion.

Evasive method

1. Reasonable selection: According to the application needs to choose the appropriate rated voltage and capacity, reserve enough voltage and temperature margin.

2. Reduce the working temperature: Through the heat dissipation design, such as the use of heat sink, fan or choose the right working environment, to reduce the working temperature of the capacitor and extend the life.

3. Appropriate derating use: Use operating conditions lower than rated voltage and rated temperature to slow down the evaporation rate of the electrolyte.

4. Choose low ESR capacitors: For ESR sensitive applications, choose low ESR aluminum electrolytic capacitors or use a combination of solid state capacitors and ceramic capacitors.

5. Use high-quality electrolyte: Choose an electrolyte with excellent quality and high stability to reduce evaporation and aging of the electrolyte.

6. Design compensation circuit: In the design of switching power supply, the compensation network is reasonably designed to ensure the stability of the loop and reduce harmonic distortion.

7. Regular maintenance and replacement: For critical applications, check capacitor performance regularly and replace in advance if necessary.

8. Select long-life capacitors: For applications requiring high reliability, long-life capacitors with special designs, such as special electrolytes or solid electrolytes, can be selected.

9. Correct installation and layout: Ensure that capacitors are kept away from heat sources, avoid mechanical stress, and handle polarity correctly to prevent reverse connection.

Through the above methods, the common defects of aluminum electrolytic capacitors can be effectively avoided, and the reliability and stability of the system can be improved.