A capacitor is a basic electronic component whose core property is the ability to store charge and energy in the form of electric field energy under potential difference (voltage). In the International System of Units, capacitance is measured in farads (F). # The formula for calculating capacitance size # is:

Among them:

• CC is capacitance (unit: farad, F);

• QQ is the amount of charge stored on the two plates of the capacitor (unit: coulomb, C);

• UU is the electric potential difference (voltage, in volts, V) between the two plates.

Capacitor structure and plate area, spacing

The basic structure of a capacitor consists of two conductors (plates) that are insulated from each other and a dielectric in between. Capacitance size is closely related to the following physical characteristics:

• Plate area: According to the definition of capacitance, the larger the surface area of the two plates, the wider the charge distribution area that can be accommodated, thus increasing the capacitance. Therefore, increasing the capacitor plate area is a direct way to improve the capacitance value.
• Plate spacing: Capacitance is inversely proportional to plate spacing. When the two plates are closer together, the mutual attraction between the charges increases, so that more charges can be stored at the same voltage, and the capacitance increases. Conversely, increasing the plate spacing will reduce the capacitance value. In practical applications, high capacitance densities can be achieved by reducing the plate spacing through precision manufacturing processes.

Working principle

The operation of capacitors is based on the distribution of electric charges in the electric field. When two conductors (such as the two metal plates of a parallel-plate capacitor) are close to each other but not in contact, if a voltage is applied between them, charges will accumulate on the two plates respectively, and the same sex repels and the opposite sex attracts, forming a stable charge distribution. This separation and storage process of charge is the basic function of capacitance. The medium (air, insulation material, electrolyte, etc.) inside the capacitor determines the difficulty of charge separation, that is, the relationship between charge density and voltage, which in turn affects the size of the capacitor.

Unit and conversion: The international standard unit of capacitance is Farad (Farad, F), which is a larger unit, and its derivative units are often seen in practical applications, such as micromethod (μF), nanomethod (nF) and skin method (pF). The relationship between them is as follows:

Action of capacitance

Capacitors have a variety of applications in electronic circuits, including but not limited to:

• Energy storage: Capacitors can temporarily store electrical energy in the circuit, release energy when the power is disconnected, and maintain the transient stability of the circuit.

• Signal coupling: The voltage signal is transmitted in the AC circuit, the DC component is isolated, and the AC signal is connected between the circuits.

• Filter: In the power supply circuit, it is used to smooth voltage fluctuations and reduce noise; In a signal processing circuit, it is used to remove unwanted frequency components and retain or suppress signals in a specific frequency range.

• Timing and oscillation: together with resistors and inductors, they form RC or LC circuits to produce oscillations of specific time constants or specific frequencies.

• Protection: Such as power decoupling capacitors can prevent the impact of voltage mutations on sensitive circuits and suppress electromagnetic interference (EMI).

Material selection of capacitors: dielectric materials and permittivity

The choice of dielectric material has a significant effect on the capacitance value. The role of the dielectric is to separate the two plates and determine the ability of charge storage. The main parameter that affects the capacitance size is the dielectric constant (ε), which represents the capacitance enhancement effect of the dielectric relative to the vacuum. The relationship between capacitance and permittivity is as follows:

Where AA is the plate area and dd is the plate spacing. It can be seen that the selection of materials with high dielectric constant (such as ceramics, polyester film, manganese dioxide in tantalum capacitors, etc.) can obtain a larger capacitance under the same size and structure.

Capacitor type and package form

The design parameters of the capacitor, such as the type (electrolytic capacitor, ceramic capacitor, thin film capacitor, etc.) and the package form, also determine its capacitance size.

1. Capacitor type: Different types of capacitors have different capacitance ranges due to their internal structure, working principle and the different dielectric used. For example, electrolytic capacitors use oxide film as dielectric, and the plate area is large, so the capacitance value is high, often used in filtering, energy storage and other occasions; Ceramic capacitors are suitable for circuits with high frequency and high stability requirements due to the use of high dielectric constant ceramic materials.
2. Packaging form: Capacitor packaging not only affects its external size, installation mode, but also indirectly determines the plate area and dielectric thickness. Miniaturization and flat packaging are beneficial to reduce plate spacing and increase effective area, thus increasing capacitance density. Advanced packaging technologies such as multi-layer ceramic capacitors (MLCC) and laminated chip capacitors enable high capacitance values in limited Spaces.

Capacitor temperature, frequency and voltage analysis

Capacitance size is not constant, it will be adjusted with the change of environmental conditions:

• Temperature: The permittivity and conductivity of a dielectric material usually change with temperature, resulting in a change in the capacitance value. Some capacitors (such as tantalum capacitors and aluminum electrolytic capacitors) have a positive temperature coefficient, that is, the capacitance increases when the temperature rises. Other types, such as ceramic capacitors, may exhibit a negative temperature coefficient. Capacitance stability over the operating temperature range should be considered in the design.
• Frequency: The capacitance value of the capacitor will decrease at high frequencies due to factors such as skin effect and dielectric loss, which is called the frequency response of the capacitor. Selecting capacitors with good high frequency characteristics or appropriate compensation design is crucial to ensure the normal operation of the circuit in the wide frequency range.
• Voltage: The operating voltage of a capacitor has an important effect on its life, reliability and capacitance value stability. Exceeding the rated voltage may lead to dielectric breakdown, capacitor value plummeting or even failure. Reasonable selection and voltage margin are the key to ensure the long-term stable operation of capacitors.

Capacitor size Select the application policy

In various industry applications, the selection of capacitor size needs to consider the circuit function requirements, cost, volume, power consumption, stability and other factors. The following are some typical scenarios for capacitor size selection strategies:

1. Power filter: In the power supply circuit, large-capacity capacitors (such as electrolytic capacitors) are used to filter out low-frequency ripple, and small-capacity ceramic capacitors are responsible for suppressing high-frequency noise. The selection must be based on power quality requirements, load characteristics, and working frequency.
2. Resonant circuit: In LC resonant circuit, capacitor and inductor jointly determine the resonant frequency. The required capacitance is calculated according to the design target frequency and the inductance value.
3. Timing circuit: In RC timing circuit, capacitance and resistance determine the time constant. According to the required delay time and resistance value, calculate the required capacitance value.
4. Energy storage applications: such as new energy vehicles and energy storage of power systems, large-capacity capacitors are required to quickly absorb and release a large amount of energy. At this time, the capacitance size should be determined according to the energy storage demand, charge and discharge rate, system voltage level and other factors.

Combined with the above, capacitor size is closely related to many factors such as plate area, spacing, dielectric material, capacitor type and package form, operating environment (temperature, frequency, voltage), and specific application requirements. Understanding these relationships helps engineers accurately select and optimize designs to improve the performance of electronic devices and systems.