The working principle of capacitors and the selection of capacitors? What is a capacitor? What is the unit of capacitance? This article will give you a detailed answer! One of the capacitors: the role of the capacitor as one of the passive components of the capacitor, its role is no more than the following: 1, applied to the power circuit, to achieve bypass, decoupling, filtering and energy storage. The working principle of capacitors and the selection of capacitors? What is a capacitor? What is the unit of capacitance? This article will give you a detailed answer!

One of the words capacitors: the role of capacitors

As one of the passive components of the capacitor, its role is no more than the following:

1, applied to the power supply circuit, to achieve bypass, decoupling, filtering and energy storage. The following categories are detailed:

1) Bypass

The bypass capacitor is an energy storage device that provides energy to the local device, which can homogenize the output of the regulator and reduce the load demand. Like a small rechargeable battery, the bypass capacitor can be charged and discharged to the device. To minimize impedance, the bypass capacitor should be as close to the power supply pin and ground pin of the load device as possible. This can well prevent the ground potential elevation and noise caused by too large input value. Ground bounce is the voltage drop when the ground connection passes through a large current burr.

2) Remove lotus root

Remove the lotus root, also known as solving the lotus root. In terms of the circuit, it is always possible to distinguish between the driving source and the driven load. If the load capacitance is relatively large, the drive circuit to charge the capacitor, discharge, in order to complete the signal jump, when the rising edge is relatively steep, the current is relatively large, so that the drive current will absorb a large power current, due to the inductance in the circuit, resistance (especially the inductance on the chip pin, will produce rebound), This current is actually a noise relative to normal conditions, which will affect the normal operation of the preceding stage, which is the so-called "coupling".

Decoupling capacitor is to play a "battery" role, to meet the change of the drive circuit current, to avoid mutual coupling interference. Combining bypass capacitors and de-coupling capacitors will be easier to understand. The bypass capacitor is actually de-coupled, but the bypass capacitor generally refers to the high-frequency bypass, that is, to the high-frequency switching noise a low impedance leakage path. The high-frequency bypass capacitance is generally small, and the resonant frequency is generally 0.1μF, 0.01μF, etc. The capacity of the decoupling capacitor is generally large, perhaps 10μF or more, depending on the distributed parameters in the circuit and the change in the drive current.

Bypass is to filter the interference in the input signal as the object, and decoupling is to filter the interference in the output signal as the object to prevent the interference signal from returning to the power supply. This should be their essential difference.

3) Filtering

In theory (that is, if the capacitor is a pure capacitor), the larger the capacitor, the smaller the impedance, and the higher the frequency of passing. But in fact, more than 1μF capacitors are mostly electrolytic capacitors, have a large inductance component, so the impedance will increase after high frequency. Sometimes it will be seen that there is a large capacitance electrolytic capacitor in parallel with a small capacitor, at which time the large capacitor passes the low frequency and the small capacitor passes the high frequency. The role of the capacitor is to pass high and low resistance, and pass high frequency and low frequency. The smaller the capacitor, the easier the low frequency passes through, and the larger the capacitor, the easier the high frequency passes through. Specifically used in filtering, large capacitance (1000μF) filter low frequency, small capacitance (20pF) filter high frequency. Some netizens have likened the filter capacitor to a "pond". Since the voltage at both ends of the capacitor does not change, it can be seen that the higher the signal frequency, the greater the attenuation, it can be said that the capacitor is like a pond, and the water amount will not change due to the addition or evaporation of a few drops of water. It converts changes in voltage into changes in current, and the higher the frequency, the greater the peak current, thereby buffering the voltage. Filtering is the process of charging and discharging.

4) Energy storage

The energy storage capacitor collects the charge through the rectifier and transmits the stored energy to the output of the power supply through the converter lead. Aluminum electrolytic capacitors with voltage ratings of 40 ~ 450VDC and capacitance values between 220 ~ 150000 μF (such as EPCOS B43504 or B43505) are more commonly used. Depending on the power requirements, the device will sometimes be in series, parallel or a combination of the form, for power levels of more than 10KW power supplies, usually use a larger volume of can-shaped spiral terminal capacitors.

2, applied to the signal circuit, mainly to complete the coupling, oscillation/synchronization and time constant role:

1) Coupling

For example, the transistor amplifier emitter has a self-bias resistance, which at the same time makes the signal produce voltage drop feedback to the input end to form the input and output signal coupling, this resistance is the coupling element, if the resistance at both ends of a capacitor, due to the appropriate capacity of the capacitor to the AC signal smaller impedance, This reduces the coupling effect generated by the resistance, so it is called the decoupling capacitor.

2) Oscillation/synchronization

Load capacitors including RC, LC oscillators and crystals belong to this category.

3) Time constant

This is a common R and C series integrated circuit. When the input signal voltage is applied to the input, the voltage on the capacitor (C) gradually rises. The charging current decreases with the increase of voltage. The characteristics of current through resistance (R) and capacitance (C) are described by the following formula:

i = (V / R)e - (t / CR)

The second word capacitor: the choice of capacitor

In general, how should we choose a suitable capacitor for our circuit? The author believes that the following considerations should be based on:

1, electrostatic capacity;

2, rated pressure;

3, tolerance error;

4, the capacitance change under DC bias;

5, noise level;

6, the type of capacitor;

7, capacitor specifications.

So, is there a shortcut? In fact, as a peripheral component of the device, the capacitor Datasheet or Solutions of almost every device clearly indicate the selection parameters of the peripheral components, that is, the basic device selection requirements can be obtained, and then further refined. In fact, when choosing capacitors, it is not only to look at the capacity and packaging, but also to look at the environment used by the product, and special circuits must use special capacitors.

The following chip capacitor is classified according to the dielectric constant of the dielectric. The dielectric constant directly affects electricity

Stability of the road.

NP0 or CH (K < 150) : The electrical performance is the most stable, basically does not change with the change of temperature, voltage and time, and is suitable for high-frequency circuits with high stability requirements. Due to the small K value, it is difficult to have large capacity capacitors in 0402, 0603, 0805 packages. Such as 0603, the maximum is below 10nF. X7R or YB (2000 < K < 4000) : The electrical performance is relatively stable, and the performance change is not significant when the temperature, voltage and time change. C < ±10%). It is suitable for straight separation, coupling, bypass and full frequency identification circuit with low capacity stability requirement. Y5V or YF (K > 15000) : The capacity stability is worse than that of X7R. C < +20% ~ -80%), the capacity and loss are more sensitive to temperature, voltage and other test conditions, but because of its larger K value, it is suitable for some occasions with higher capacity requirements.

The third word capacitor: the classification of capacitors

Capacitors are classified in many ways and types. Based on the material characteristics of capacitors, they can be divided into the following categories:

1, aluminum electrolytic capacitor

Capacitive capacity range is 0.1μF ~ 22000μF, high pulsating current, long life, large capacity of choice, widely used in power filtering, decoupling and other occasions.

2, thin film capacitance

Capacitance ranges from 0.1pF to 10μF, with small tolerances, high capacity stability and very low piezoelectric effect, so it is the first choice for X and Y safety capacitors and EMI/EMC.

3. Tantalum capacitor

Capacitance ranges from 2.2μF to 560μF with low equivalent series resistance (ESR) and low equivalent series inductance (ESL). Pulsation absorption, transient response and noise suppression are better than aluminum electrolytic capacitors, which is an ideal choice for high stability power supply.

4, ceramic capacitor

The capacitive capacity ranges from 0.5pF to 100μF, and the crystallization of unique material and film technology caters to today's "lighter, thinner and more energy efficient" design concept.

5. Supercapacitor

Capacitance capacity range is 0.022F ~ 70F, very high capacity, so it is also called "gold capacitance" or "farrah capacitance". The main features are: high capacity, good charge/discharge characteristics, suitable for energy storage and power backup. The disadvantage is low pressure resistance and narrow operating temperature range.

Capacitor four: Multi-layer ceramic capacitor (MLCC)

For capacitors, miniaturization and high capacity are eternal trends. Among them, the development of multi-layer ceramic capacitors (MLCC) is the fastest.

Multilayer ceramic capacitors are widely used in portable products, but the technological progress of digital products in recent years has put forward new requirements for them. For example, mobile phones require higher transmission rates and higher performance; Baseband processor requires high speed and low voltage. The LCD module requires low thickness (0.5mm) and large capacity capacitance. The harshness of the automotive environment has special requirements for multi-layer ceramic capacitors: first, it is high temperature resistance, and the multi-layer ceramic capacitors placed in it must be able to meet the operating temperature of 150℃; The second is the need for short-circuit fail-safe design on the battery circuit.

In other words, miniaturization, high speed and high performance, resistance to high temperature conditions, and high reliability have become the key characteristics of ceramic capacitors.

The capacity of ceramic capacitor varies with the change of DC bias voltage. Dc bias voltage reduces the dielectric constant, so it is necessary to reduce the dependence of dielectric constant on voltage and optimize the characteristics of DC bias voltage from the aspect of material.

The more common application is the X7R (X5R) class multi-layer ceramic capacitor, its capacity is mainly concentrated in more than 1000pF, the main performance index of this kind of capacitor is the equivalent series resistance (ESR), in the high ripple current power decoupling, filtering and low frequency signal coupling circuit low power performance is more prominent.

Another kind of multilayer ceramic capacitor is C0G class, its capacity is mostly below 1000pF, the main performance index of this kind of capacitor is the loss Angle tangent value tgδ(DF). The DF value range of the C0G product of the traditional precious metal electrode (NME) is (2.0 ~ 8.0) × 10-4, while the DF value range of the C0G product of the technologically innovative base metal electrode (BME) is (1.0 ~ 2.5) × 10-4, which is about 31 ~ 50% of the former. The low-power characteristics of these products are more significant in GSM, CDMA, cordless telephone, Bluetooth and GPS systems with T/R module circuits. It is widely used in various high frequency circuits, such as oscillators/synchronizers, timer circuits, etc.

The fifth capacitor: tantalum capacitor

The common view is that the performance of tantalum capacitors is better than that of aluminum capacitors, because the medium of tantalum capacitors is tantalum pentoxide generated after anodizing, and its dielectric capacity (usually expressed by ε) is higher than that of aluminum capacitors.

Therefore, in the case of the same capacity, the volume of tantalum capacitors can be smaller than that of aluminum capacitors. (The capacitance of the electrolytic capacitor depends on the dielectric capacity and volume of the medium, in the case of a certain capacity, the higher the dielectric capacity, the smaller the volume can be made, and on the contrary, the larger the volume needs to be made) coupled with the properties of tantalum is relatively stable, so it is generally believed that the performance of tantalum capacitors is better than that of aluminum capacitors.

However, this method of judging the capacitance performance by the anode is outdated, and the key to determining the performance of electrolytic capacitors is not the anode, but the electrolyte, that is, the cathode. Because different cathodes and different anodes can be combined into different kinds of electrolytic capacitors, their performance is also very different. Due to the different electrolytes, the performance of the capacitor using the same anode can be very different, in short, the effect of the anode on the capacitance performance is far less than that of the cathode. There is also a view that the performance of tantalum capacitors is better than that of aluminum capacitors, mainly because the performance of tantalum and manganese dioxide anode is significantly better than that of aluminum electrolyte capacitors. If the cathode of the aluminum electrolyte capacitor is replaced with manganese dioxide, its performance can actually be improved a lot.

To be sure, ESR is one of the main parameters to measure the characteristics of a capacitor. However, when choosing capacitors, the lower the ESR, the better, and the higher the quality, the better. To measure a product, we must consider it from a full range and multiple angles, and we must not exaggerate the role of capacitors intentionally or unintentionally.

-- The above quotes the experience of some netizens.

The structure of ordinary electrolytic capacitors is anode and cathode and electrolyte, anode is passivated aluminum, cathode is pure aluminum, so the key is in the anode and electrolyte. The quality of anode is related to the voltage dielectric coefficient and so on.

In general, the ESR of tantalum electrolytic capacitors is much smaller than that of aluminum electrolytic capacitors with the same capacity and pressure resistance, and the high frequency performance is better. If the capacitor is used in a filter circuit (such as a bandpass filter with a center of 50Hz), it is beneficial. However, this requires you to find a compromise between the PCB area, the number of devices and the cost.

The eighth part of the capacitor: the electrical parameters of the electrolytic capacitor

Electrolytic capacitors here mainly refer to aluminum electrolytic capacitors, and their basic electrical parameters include the following five points:

1. Capacitance value

The capacitance of an electrolytic capacitor depends on the impedance presented when operating at AC voltage. Therefore, the capacitance value, that is, the AC capacitance value, changes with the operating frequency, voltage, and measurement method. According to the standard JISC 5102, the measurement conditions of the capacitance of the aluminum electrolytic capacitor are carried out at the frequency of 120Hz, the maximum alternating current voltage of 0.5Vrms, and the DC bias voltage of 1.5-2.0V. It can be asserted that the capacity of aluminum electrolytic capacitors decreases with the increase of frequency.

2, loss Angle tangent value Tan δ

In the equivalent circuit of the capacitor, the ratio of the series equivalent resistance ESR to 1/ωC is called Tan δ, where ESR is calculated at 120Hz. Obviously, Tanδ increases with the increase of measurement frequency and increases with the decrease of measurement temperature.

3. Impedance Z

At a specific frequency, the resistance that prevents the AC current from passing through is the so-called impedance (Z). It is closely related to the capacitance value and inductance value in the capacitance equivalent circuit, and also has a relationship with ESR.

Z = √ [ESR2 + (XL - XC)2 ]

Where XC = 1 / ωC = 1/2 πfC

XL = ωL = 2πfL

The capacitive reactance (XC) of the capacitor gradually decreases with the increase of frequency in the low frequency range, and the reactance (XL) decreases to the value of ESR when the frequency continues to increase and reaches the medium frequency range. When the frequency reaches the high frequency range, the inductive reactance (XL) becomes dominant, so the impedance increases with the increase of frequency.

4. Leakage current

The medium of the capacitor has a great hindering effect on the DC current. However, due to the fact that the aluminum oxide film medium is impregnated with an electrolyte, the re-formed as well as the reformed when a voltage is applied