1. The meaning of capacitance
Capacitance, also known as "capacitance", refers to the storage capacity of charge under a given potential difference, denoted as C, and the international unit is farad (F). Generally speaking, charges will move under force in an electric field. When there is a medium between conductors, it will hinder the movement of charges and cause charges to accumulate on the conductors, resulting in the accumulation and storage of charges. The amount of stored charges is called capacitance.
The formula for capacitance is: C=εS/4πkd. Among them, ε is a constant, S is the facing area of the capacitor plate, d is the distance of the capacitor plate, and k is the constant of electrostatic force. A common parallel plate capacitor has a capacitance of C=εS/d (ε is the dielectric constant of the medium between the plates, S is the area of the plates, and d is the distance between the plates).
When the reference direction of the voltage u across the capacitive element is given, if q is used to represent the amount of charge on the reference positive potential plate, the relationship between the amount of charge and the voltage of the capacitive element satisfies q=Cu. The current is equal to the amount of charge passing through a certain cross-section per unit time, so I=dq/dt is obtained, so the relationship between current and capacitance is I=dq/dt =C(du/dt). This formula shows that the magnitude and direction of the current depend on the rate of change of the voltage to time. When the voltage increases, du/dt>0, then dq/dt>0, i>0, the charge on the plate increases, and the capacitor charges; the voltage decreases When du/dt < 0, then dq/dt < 0, i < 0, the charge on the plate decreases, and the capacitor discharges in reverse. When the voltage does not change with time, du/dt=0, then the current I=0, at this time the current of the capacitive element is equal to zero, which is equivalent to an open circuit. Therefore, the capacitive element has the effect of cutting off the direct current.
2. Capacitance value
The symbol of capacitance is C. In the International System of Units, the unit of capacitance is farad, referred to as law, and the symbol is F. Since the unit of farad is too large, the commonly used capacitance units are millifarad (mF), microfarad (μF), Nanofarad (nF) and picofarad (pF), etc., the conversion relationship is as follows:
1 farad (F) = 1000 millifarad (mF) = 1000000 microfarad (μF)
1 microfarad (μF) = 1000 nanofarad (nF) = 1000000 picofarad (pF)
3. Capacitance parameters
1. Nominal capacitance and error
Capacitance is the ability to store charge after the capacitance is added to the charge. Capacitance error refers to the deviation between the actual capacity and the nominal capacity, usually ±10%, ±20%, and the small error capacitance of ±0.5% and ±0.75% used in PI matching in radio frequency circuits.
2. Rated voltage
The rated working voltage is the maximum DC voltage (also known as withstand voltage) that the capacitor can work reliably in the circuit for a long time without being broken down. It is related to the structure of the capacitor, the dielectric material and the thickness of the medium. Generally speaking, for capacitors with the same structure and medium and equal capacity, the higher the withstand voltage value, the larger the volume.
When a voltage is applied between the two plates of the capacitor, the electrolyte between the plates is in the electric field. It is a neutral dielectric. Due to the external electric field force, the positive and negative charges in the medium molecules will be slightly offset in space. Shift (such as the negative charge moves against the direction of the electric field), forming a so-called electric dipole, that is, an electric field appears inside the medium, destroying the original electric neutral state. This phenomenon is called polarization of the electrolyte. It can be seen that the medium in the polarized state is negatively charged, but these charges are still bound by the medium itself and cannot move freely. The insulation performance of the medium has not been damaged, and only a few charges are unbound to form a small leakage current. If the applied voltage continues to increase, a large amount of polarized charges will eventually be released from the shackles, causing a large increase in leakage current, so the insulation performance of the medium is destroyed, the two plates are short-circuited, and the role of capacitance is completely lost. This phenomenon is called dielectric breakdown. After dielectric breakdown, the capacitor is destroyed. Therefore, the working voltage of the capacitor must be limited and cannot be increased arbitrarily.
3. Temperature coefficient
The change of capacitor capacitance with temperature is expressed by temperature coefficient (in a certain temperature range, every time the temperature changes by 1°C, the relative change value of capacitance) is expressed, which is the same as resistance.
4. Insulation resistance
The size of capacitor leakage is measured by insulation resistance. The smaller the leakage of the capacitor, the better, that is, the larger the insulation resistance, the better. Generally, the insulation resistance of small capacitors is very large, which can reach hundreds of megohms or several gigaohms. The insulation resistance of electrolytic capacitors is generally small. the
5. Loss
Under the action of an electric field, the energy consumed by the capacitor to generate heat per unit time is called the loss of the capacitor. Ideal capacitors should not consume energy in the circuit, but in reality, capacitors consume more or less energy, and their energy consumption is mainly composed of dielectric loss and metal part loss, usually expressed by the loss tangent value. the
6. Frequency characteristics
The frequency characteristic of a capacitor usually refers to the property that the electrical parameters of the capacitor (such as capacitance, loss tangent, etc.) change with the frequency of the electric field. For capacitors operating at high frequencies, since the dielectric constant is smaller at high frequencies than at low frequencies, the capacitance will decrease accordingly. At the same time, its loss will increase with frequency. In addition, when working at high frequency, the distribution parameters of the capacitor, such as the resistance of the electrode, the contact resistance of the lead and the electrode, the self-inductance of the electrode, the inductance of the lead, etc., will affect the performance of the capacitor. Due to the influence of these factors, the capacitor's Frequency of use is limited.
7. Medium
The parameters describe the type of dielectric material used in the capacitor, temperature characteristics, and error parameters. Different values also correspond to a certain range of capacitance. For example, X7R is often used in capacitors with a capacity of 3300pF~0.33uF. This type of capacitor is suitable for filtering, coupling and other occasions. The dielectric constant is relatively large. When the temperature changes from 0°C to 70°C, the change in capacitance is ±15%. ;
Y5P and Y5V are commonly used for capacitors with a capacity of 150pF~2nF, and the temperature range is relatively wide. As the temperature changes, the range of capacitance capacity changes is ±10% or +22%/-82%.
For the relationship between other codes and temperature characteristics, you can refer to Table 4-1. For example, X5R means that the normal operating temperature of the capacitor is -55°C~+85°C, and the corresponding capacitance change is ±15%.
8. Package size
Mainly for chip capacitors, the size of the package is the same as that of the resistor.
Fourth, the classification of capacitors
There are also many types of capacitors in different ways. The following summarizes several common types.
1. According to the material, there are different types of mica capacitors, electrolytic capacitors, ceramic capacitors, tantalum capacitors, etc.;
2. According to the purpose, there are filter capacitors, bypass capacitors, coupling capacitors, load capacitors, etc.;
3. According to the polarity, there are non-polarized capacitors and polarized capacitors.