Electrolytic capacitors contain an electrolyte material that stores charge. They have positive and negative polarities, similar to batteries, and cannot be connected in reverse. The positive electrode is a metal substrate with an oxide film, and the negative electrode is connected to the electrolyte (solid or non-solid) through a metal plate.
Non-polar (bipolar) electrolytic capacitors employ a double oxide film structure, similar to two polarized electrolytic capacitors connected by their two negative terminals. Their two electrodes are two connected metal plates (both coated with oxide films), with the electrolyte positioned between the two oxide films. Polarized electrolytic capacitors typically function in power supply circuits or intermediate/low-frequency circuits for power filtering, decoupling, signal coupling, time constant setting, and DC blocking. Non-polarized electrolytic capacitors are commonly used in audio crossover circuits, television S-correction circuits, and starting circuits for single-phase motors .
Applications of electrolytic capacitors
1. DC blocking: Its function is to prevent DC from passing through while allowing AC to pass through.
2. Bypass (decoupling): Provides a low-impedance path for certain parallel components in an AC circuit.
3. Coupling: As a connection between two circuits, it allows AC signals to pass through and be transmitted to the next stage of the circuit.
4. Filtering: This is very important for DIY projects; the capacitors on graphics cards are basically used for this purpose.
5. Temperature compensation: This compensates for the effects of insufficient temperature adaptability of other components, thereby improving the stability of the circuit.
6. Timing: Capacitors and resistors are used together to determine the time constant of the circuit. The time constant is t = RC.
7. Tuning: Systematically tuning frequency-related circuits, such as mobile phones, radios, and televisions.
8. Rectification: Opening or closing a semi-closed conductor switching element at a predetermined time.
9. Energy storage: Storing electrical energy for release when necessary. Examples include camera flashes and heating devices.
The characteristics of capacitors required vary depending on the application, as shown in Table 1. For example, as air conditioner indoor and outdoor units for white goods become smaller and more efficient, miniaturization of electronic components is crucial. At the same time, with the increasing demand for high efficiency in equipment, the charging/discharging of capacitors is also important. Furthermore, since the demand for electricity differs between day and night, there are requirements for voltage fluctuations, necessitating high voltage withstand capability.
For automobiles, bullet trains, and high-speed trains, capacitors need to be able to withstand high temperatures, as some capacitors are placed in the engine compartment of a car, where the temperature exceeds 100°C. Also, because they are moving objects, the products must be shock-resistant. Thirdly, there are low-temperature requirements, as outdoor temperatures can sometimes reach -20°C.
In terms of power supply, new energy sources, including photovoltaics and solar power, are increasingly recognized and accepted. These devices and products are often installed in remote areas such as plateaus, seas, and deserts due to strong winds and sunlight. A key characteristic of these devices is that subsequent maintenance is extremely costly in terms of manpower and resources. Therefore, long lifespan and maintenance-free operation are generally required. Furthermore, unlike nuclear power, photovoltaic and wind power have inherent instability, which can load the power grid. High loads result in low voltage, and vice versa, requiring capacitor products to withstand significant voltage fluctuations.
In information and communication equipment, low ESR (equivalent series resistance) is particularly needed because the efficiency of semiconductor devices is increasing, but the voltage is not increasing, so the current is greater than before. Therefore, capacitor products must be resistant to high temperatures and instability. At the same time, as our living spaces become smaller, these communication devices need to be smaller and more portable, so capacitor products must become smaller and smaller.
It is evident that high/low temperature resistance, ultra-long lifespan, ultra-miniaturization, and high voltage withstand are the key technical challenges for capacitors. Nichicon, a professional capacitor manufacturer, primarily provides aluminum electrolytic capacitors and film capacitors. According to Mao Jidong, Director and Deputy General Manager of Nichicon Electronics Trading (Shanghai) Co., Ltd., the company has integrated technological resources in aluminum etched foil manufacturing, electrolyte development, and thin film deposition.
Capacitor lifespan
The actual voltage that an electrolytic capacitor must withstand in a circuit must not exceed its rated voltage. In a filter circuit, the capacitor's rated voltage should not be less than 1.42 times the AC effective value. When using electrolytic capacitors, care must be taken to ensure that the positive and negative terminals are not reversed. Different types of capacitors should be selected for different circuits. Mica and high-frequency ceramic capacitors can be used for resonant circuits; paper, polyester, mica, electrolytic, and ceramic capacitors can be used for DC blocking; electrolytic capacitors can be used for filtering; and polyester, paper, ceramic, and electrolytic capacitors can be used for bypassing. Before installing a capacitor in a circuit, check it for short circuits, open circuits, and leakage, and verify its capacitance value. During installation, ensure that the capacitor's type, capacitance, and rated voltage are easily visible for verification.
Life expectancy: The duration for which an electrolytic capacitor can continue to operate at its highest operating temperature.
lx = lo * 2(to - ta) / 10
lx = Actual working life
lo = Guarantee lifespan
to = Maximum operating temperature (85℃-105℃)
ta = Actual ambient temperature of the capacitor
Example: Standard value 105℃/1000hrs
Lifespan estimate at 65℃: lx = 1000 * 2 (105 - 65) / 10
Actual working life: 16000hrs
High-temperature load life refers to the condition of an electrolytic capacitor after being subjected to its rated operating voltage at its maximum operating temperature for a specified duration. The capacitor must meet the following requirement: δcap: within 20% of its initial value.
The core technologies of capacitors are processes and materials.
During manufacturing, the first step is etching, primarily to increase the surface area of the aluminum foil, enabling the capacitor to be ultra-miniaturized. Second, Al2O3 is formed on the aluminum foil surface. Nichicon possesses a foil-making process capable of withstanding 1000 volts of DC voltage (Figure 2), and can produce the smallest product in the industry because Nichicon maximizes the surface area of the aluminum foil. Third is the electrolyte, which directly affects the capacitor's performance and its high-temperature resistance.
"Doing something is not difficult, but doing it well is very difficult," Mao Jidong said. To ensure quality, Nichicon adopts a vertically integrated production line, starting from scratch with the base material. In addition, it has introduced fully automated production lines to improve efficiency and reduce failure rates.
In terms of manufacturing processes, one of Nichicon's key technologies is aluminum foil. To achieve miniaturization, the aluminum foil needs to be made very thin while still being voltage-resistant (Figure 3). The main function of the Al2O3 etching layer is to store charge, so the deeper the etching, the better. For etching, the smooth, flat aluminum foil is first etched to form a blue Al2O3 layer on the etched surface. Then, it is rolled up, and the electrolyte adheres to the aluminum foil. Finally, it is encapsulated to form a thin-film capacitor.
Next comes the cutting of the aluminum foil. The challenge lies in cutting the aluminum foil needed for very short capacitors, requiring great precision. Additionally, since metal blades are used to cut aluminum, the metal edges will wear down. Nichicon's blades are manufactured by a professional overseas company and require regular inspection to prevent wear.
How to achieve a long lifespan? It mainly involves three aspects: 1. The substrate and purity of the aluminum foil, and the thickness, firmness, and strength of the Al2O3 in the process. 2. The electrolyte should be non-volatile, quick-repairing, and generate as little heat and gas as possible, thus reducing pressure inside the sealed casing. 3. Sealing materials—the sealing rubber is also very important. Previously, the aging concept of the entire rubber industry was 15 years. Nichicon has developed a higher-density rubber, increasing the lifespan to 40 years and improving sealing performance.
Furthermore, a wide variety of sizes and forms are required. For example, offshore wind power generation requires film capacitors, which are lighter and more efficient. Nichicon produces capacitors for industrial equipment ranging from 200V to 500V (note: some companies can only produce 400V) to improve efficiency. For instance, some customers require voltages of 960V; previously, if only 400V products were available, they would need three, but with 500V products, only two are needed, thus reducing the number of capacitors required per unit. For the automotive industry, Nichicon's aluminum foil is designed to prevent breakage, and the capacitors are made relatively small because they cannot afford to fail.
The biggest challenge is understanding the customer's needs.
Nichicon believes that the biggest challenge in making good capacitors is not the product itself, but understanding customer needs and recommending products that best meet those needs—neither too little nor too much.
The most challenging aspect is the need to predict products three to five years from now. Nichicon's chairman often reminds employees that Nichicon is currently a manufacturing company, but it needs to become an imaginative company—thinking about what will happen next, what application solutions to recommend to customers, and what specific products will be needed in those solutions.
