Proper Capacitor Selection Methods in Electronic Design

What is a capacitor? How to choose a capacitor? Capacitance (Capacitance), also known as "capacitance", refers to the storage capacity of charge under a given potential difference, recorded 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 of charges. The amount of stored charges is called capacitance.

The formula of capacitance is: C=εS/4πkd. Among them, ε is a constant, S is the facing area of ​​the capacitor plate, d is the distance between the capacitor plates, and k is the electrostatic force constant. The capacitance of a common parallel plate capacitor is 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 represents the charge amount on the reference positive potential plate, then the relationship between the charge amount 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. Therefore, 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 voltage with time. When the voltage increases, du/dt》0, then dq/dt》0, i》0, the charge on the plate increases, 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 the reverse direction. 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 function of blocking DC.

2. Capacitance value of capacitor

The symbol of capacitance is C. In the International System of Units, the unit of capacitance is Farad, which is referred to as Farad. 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 millifarads (mF) = 1000000 microfarads (μF)

1 microfarad (μF) = 1000 nanofarad (nF) = 1000000 picofarad (pF)

3. Parameters of capacitor

1. Nominal capacitance and error

Capacitance is the ability of a capacitor to store charge after adding charge. Capacitance error refers to the deviation between its actual capacity and nominal capacity, usually ±10%, ±20%, and small error capacitors of ±0.5% and ±0.75% used in PI matching in radio frequency circuits.

2. Rated voltage

The rated operating voltage is the maximum DC voltage (also called withstand voltage) that the capacitor can withstand in a circuit for a long time and reliably without breakdown. 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, dielectric 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 an electric field. It is a neutral dielectric. Due to the external electric field force, the positive and negative charges in the dielectric molecules will be slightly biased in space. Shift (such as negative charges moving 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 electrically neutral state. This phenomenon is called electrolyte polarization. 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 destroyed, and only a few charges break away from the constraints to form a small leakage current. If the applied voltage continues to increase, a large number of polarized charges will eventually escape from their restraints, causing a large increase in leakage current. As a result, the insulation performance of the medium will be destroyed, causing the two plates to short-circuit, completely losing the function of the capacitor. 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 at will.

3.Temperature coefficient

The change in the capacitance of a capacitor with temperature is expressed by the temperature coefficient (the relative change in capacitance for every 1°C change in temperature within a certain temperature range), which is the same as that of a resistor.

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, up to several hundred megaohms or several thousand megaohms. The insulation resistance of electrolytic capacitors is generally small.

5.Loss

Under the action of the electric field, the energy consumed by the capacitor for heating per unit time is called the loss of the capacitor. An ideal capacitor should not consume energy in a circuit, but in reality, a capacitor consumes more or less energy. Its energy consumption is mainly composed of dielectric loss and metal part loss, which is usually expressed by the loss tangent value.

6. Frequency characteristics

The frequency characteristics 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 be reduced accordingly. At the same time, its losses will increase with frequency. In addition, when working at high frequency, the distribution parameters of the capacitor, such as the resistance of the pole piece, the contact resistance of the lead and the pole piece, the self-inductance of the pole piece, the lead inductance, etc., will affect the performance of the capacitor. Due to the influence of these factors, the capacitor's Frequency of use is limited.

7.Media

The parameters describe the type of dielectric material used in the capacitor, temperature characteristics, error and other parameters. Different values ​​also correspond to a certain range of capacitance capacity. For example, X7R is often used for 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 capacity is ±15%. ;

Y5P and Y5V are commonly used for capacitors with a capacity of 150pF~2nF. The temperature range is relatively wide. As the temperature changes, the capacitance capacity changes within a range of ±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 package size is the same as that of the resistor.

4. Classification of capacitors

There are many types of capacitors classified according to different methods. The following summarizes the common ones:

1. According to materials, there are different types of mica capacitors, electrolytic capacitors, ceramic capacitors, tantalum capacitors, etc.; 2. According to uses, there are filter capacitors, bypass capacitors, coupling capacitors, load capacitors, etc.; 3. According to polarity, There are non-polar capacitors and polarized capacitors. The above is the method for selecting capacitors. I hope it can help you.