The Function of Capacitors[Linkeycon]

I. Functions of Capacitors

1. Bypassing
   A bypass capacitor is an energy - storing device that provides energy for local components. It can make the output of the voltage regulator more uniform and reduce the load demand. Similar to a small rechargeable battery, a bypass capacitor can be charged and then discharge to supply power to components.
   To minimize impedance, the bypass capacitor should be placed as close as possible to the power - supply pins and ground pins of the load device. This can effectively prevent the elevation of the ground potential and noise caused by excessive input values. Ground bounce refers to the voltage drop at the ground connection when a large - current spike passes through.
2. Decoupling
   Decoupling, also known as de - coupling. In a circuit, it can always be divided into the driving source and the driven load.
   If the load capacitance is relatively large, the driving circuit needs to charge and discharge the capacitor to complete the signal transition. When the rising edge is steep, the current is large. In this case, the driving current will draw a large amount of power - supply current. Due to the inductance and resistance in the circuit (especially the inductance on the chip pins, which can cause a rebound), this current is actually a kind of noise compared to the normal situation, which will affect the normal operation of the previous stage. This is the so - called "coupling".
   A decoupling capacitor acts like a "battery" to meet the current changes of the driving circuit and avoid mutual coupling interference. It is easier to understand by combining the concepts of bypass and decoupling capacitors.
   In fact, a bypass capacitor also serves a decoupling function. Generally, a bypass capacitor refers to high - frequency bypass, which provides a low - impedance path for high - frequency switching noise to be discharged.
   High - frequency bypass capacitors are usually small. According to the resonance frequency, they are often 0.1 μF, 0.01 μF, etc. The capacitance of decoupling capacitors is usually larger, perhaps 10 μF or even larger, which is determined based on the distribution parameters in the circuit and the magnitude of the driving - current change.
   The difference is that bypassing filters out the interference in the input signal, while decoupling filters out the interference in the output signal to prevent the interference signal from returning to the power supply.
3. Filtering
   Theoretically (assuming the capacitor is a pure capacitor), the larger the capacitance, the smaller the impedance, and the higher the frequency that can pass through. However, in reality, most capacitors larger than 1 μF are electrolytic capacitors, which have a large inductive component. Therefore, the impedance will increase at higher frequencies.
   Sometimes, a large - capacity electrolytic capacitor is connected in parallel with a small capacitor. In this case, the large capacitor allows low - frequency signals to pass through, and the small capacitor allows high - frequency signals to pass through. The function of a capacitor is to pass high - frequency signals and block low - frequency signals. The larger the capacitor, the easier it is for low - frequency signals to pass through, and the larger the capacitor, the easier it is for high - frequency signals to pass through.
   Specifically in filtering, a large capacitor (1000 μF) is used to filter low - frequency signals, and a small capacitor (20 pF) is used to filter high - frequency signals.
   Some netizens vividly compare the filtering capacitor to a "water pond". Since the voltage across the capacitor does not change suddenly, it can be known that the higher the signal frequency, the greater the attenuation. Just like a water pond, the amount of water in the capacitor does not change easily due to a small amount of charge or discharge.
   It converts the change in voltage into a change in current. The higher the frequency, the larger the peak current, thus buffering the voltage. Filtering is a process of charging and discharging.
4. Energy Storage
   Energy - storage capacitors collect charges through rectifiers and transmit the stored energy to the output terminal of the power supply through converter leads. Aluminum electrolytic capacitors with a rated voltage of 40 - 450 VDC and a capacitance value between 220 - 150,000 μF are commonly used.
   According to different power - supply requirements, components sometimes use series, parallel, or a combination of these connection methods. For power supplies with a power level exceeding 10 KW, larger tank - shaped screw - terminal capacitors are usually used.
   When applied in signal circuits, capacitors mainly perform the functions of coupling, oscillation/synchronization, and acting as a time - constant element:
5. Coupling
   For example, in a transistor amplifier, the emitter has a self - bias resistor. This resistor not only causes a voltage drop in the signal but also feeds it back to the input terminal, forming the coupling between the input and output signals. This resistor is the component that causes coupling.
   If a capacitor is connected in parallel across this resistor, due to the relatively small impedance of a capacitor with an appropriate capacity to AC signals, the coupling effect caused by the resistor is reduced. Therefore, this capacitor is called a decoupling capacitor.
6. Oscillation/Synchronization
   This category includes RC, LC oscillators, and the load capacitors of crystals.
7. Time - Constant
   This is the common integral circuit formed by the series connection of R and C. When an input - signal voltage is applied to the input terminal, the voltage across the capacitor (C) gradually rises.
   The charging current decreases as the voltage rises. The characteristics of the current passing through the resistor (R) and the capacitor (C) are described by the following formula: i=(V/R)e−(t/CR)

II. Selection of Capacitors

1. Capacitance value
2. Rated withstand voltage
3. Capacitance - value error
4. Capacitance change under DC bias voltage
5. Noise level
6. Capacitor type
7. Capacitor specifications
   Is there a shortcut? In fact, as a peripheral component of a device, almost every device's Datasheet or Solutions clearly indicates the selection parameters of peripheral components. That is, the basic requirements for device selection can be obtained accordingly, and then further refined.
   In fact, when selecting a capacitor, it is not just about looking at the capacitance and package. It depends on the application environment of the product. Special circuits require special capacitors.
   The following is the classification of chip capacitors according to the dielectric constant of the dielectric. The dielectric constant directly affects the stability of the circuit.
   NP0 or CH (K < 150):
   They have the most stable electrical performance, basically remaining unchanged with variations in temperature, voltage, and time. They are suitable for high - frequency circuits with high - stability requirements. Given the small K value, it is difficult to obtain large - capacity capacitors in 0402, 0603, and 0805 packages. For example, in the 0603 package, the maximum capacitance is generally less than 10 nF.
   X7R or YB (2000 < K < 4000):
   They have relatively stable electrical performance. The performance change is not significant (?C < ±10%) when the temperature, voltage, and time change. They are suitable for DC - blocking, coupling, bypassing, and full - frequency - identification circuits with not - too - high requirements for capacitance stability.
   Y5V or YF (K > 15000):
   The capacitance stability is worse than that of X7R (?C < +20% - -80%). The capacitance loss is more sensitive to test conditions such as temperature and voltage. However, due to their large K value, they are suitable for some occasions with high capacitance - value requirements.

III. Classification of Capacitors

1. Aluminum Electrolytic Capacitors
   The capacitance range is 0.1 μF - 22000 μF. They are the best choice for high - ripple current, long - life, and large - capacity applications and are widely used in power - supply filtering, decoupling, and other scenarios.
2. Film Capacitors
   The capacitance range is 0.1 pF - 10 μF. They have small tolerances, high capacitance stability, and extremely low piezoelectric effects. Therefore, they are the first choice for X and Y safety capacitors and EMI/EMC applications.
3. Tantalum Capacitors
   The capacitance range is 2.2 μF - 560 μF. They have low equivalent series resistance (ESR) and low equivalent series inductance (ESL). Their ripple - absorption, transient - response, and noise - suppression capabilities are better than those of aluminum electrolytic capacitors, making them an ideal choice for high - stability power supplies.
4. Ceramic Capacitors
   The capacitance range is 0.5 pF - 100 μF. They are the result of unique materials and thin - film technology, meeting the current design concept of "lighter, thinner, and more energy - efficient".
5. Supercapacitors
   The capacitance range is 0.022 F - 70 F. With extremely high capacitance values, they are also known as "gold capacitors" or "farad capacitors". Their main features are extremely high capacitance and good charging/discharging characteristics, making them suitable for electrical - energy storage and power - supply backup. However, they have a relatively low withstand voltage and a narrow operating - temperature range.

IV. Multilayer Ceramic Capacitors

V. Misconceptions about Replacing Electrolytic Capacitors with Tantalum Capacitors

VI. Application Issues of Bypass Capacitors

VII. Equivalent Series Resistance (ESR) of Capacitors

The common view is that a relatively large - capacity external capacitor with a small equivalent series resistance (ESR) can effectively absorb the peak (ripple) current during rapid conversion.
However, sometimes such a choice can easily cause instability in voltage regulators (especially linear voltage regulators LDO). Therefore, it is necessary to reasonably select the capacitance values of small - capacity and large - capacity capacitors. Always keep in mind that a voltage regulator is an amplifier, and it can exhibit all the problems that an amplifier may have.
Since the response speed of DC/DC converters is relatively slow, the output decoupling capacitor plays a dominant role in the initial stage of a load step. Therefore, additional large - capacity capacitors are needed to slow down the rapid conversion relative to the DC/DC converter, and high - frequency capacitors are used to slow down the rapid conversion relative to the large capacitor.
Generally, the equivalent series resistance of large - capacity capacitors should be selected appropriately to ensure that the peak value and spike of the output voltage are within the specified range in the device's Datasheet.
In high - frequency conversions, small - capacity capacitors in the range of 0.01 μF to 0.1 μF can well meet the requirements. Surface - mounted ceramic capacitors or multilayer ceramic capacitors (MLCC) have smaller ESR.
In addition, at these capacitance values, their volume and BOM cost are relatively reasonable. If local low - frequency decoupling is insufficient, the input voltage will decrease when converting from low frequency to high frequency. The voltage - drop process may last for several milliseconds, and the duration mainly depends on the regulation gain of the voltage regulator and the time it takes to provide a large load current.
Using capacitors with a large ESR in parallel is more cost - effective than using a single capacitor with an extremely low ESR. However, this requires finding a balance among the PCB area, the number of components, and the cost.

VIII. Electrical Parameters of Electrolytic Capacitors

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

1. Capacitance Value The capacitance value of an electrolytic capacitor depends on the impedance it exhibits when operating under an alternating voltage. Therefore, the capacitance value, that is, the AC capacitance value, varies with the operating frequency, voltage, and measurement method. According to the standard JISC 5102, the measurement conditions for the capacitance of an aluminum electrolytic capacitor are a frequency of 120 Hz, a maximum AC voltage of 0.5 Vrms, and a DC bias voltage of 1.5 - 2.0 V. It can be affirmed that the capacitance of an aluminum electrolytic capacitor decreases as the frequency increases.
2. Tan δ (Dissipation Factor Tangent) In the equivalent circuit of a capacitor, the ratio of the series equivalent resistance ESR to the capacitive reactance 1/ωC is called Tan δ, where the ESR is calculated at 120 Hz. Obviously, Tan δ increases with the increase of the measurement frequency and also increases with the decrease of the measurement temperature.
3. Impedance Z At a specific frequency, the resistance that hinders the flow of alternating current is called impedance (Z). It is closely related to the capacitance value and inductance value in the capacitor's equivalent circuit and is also related to ESR. \(Z = \sqrt{[ESR^2+(X_L - X_C)^2]}\) In the formula, \(X_C = 1/ωC = 1/(2πfC)\) and \(X_L = ωL = 2πfL\). The capacitive reactance (\(X_C\)) of a capacitor gradually decreases as the frequency increases in the low - frequency range. When the frequency continues to increase and reaches the mid - frequency range, the inductive reactance (\(X_L\)) drops to the value of ESR. When the frequency reaches the high - frequency range, the inductive reactance (\(X_L\)) becomes dominant, so the impedance increases as the frequency increases.
4. Leakage Current The dielectric of a capacitor has a great resistance to direct current. However, since the aluminum oxide film dielectric is immersed in the electrolyte, when a voltage is applied, a very small current called leakage current is generated during the re - formation and repair of the oxide film. Usually, the leakage current increases with the increase of temperature and voltage.
5. Ripple Current and Ripple Voltage In some materials, these two are called "ripple current" and "ripple voltage", which are actually ripple current and ripple voltage. They represent the values of ripple current/voltage that a capacitor can withstand. They are closely related to ESR and can be expressed by the following formula: \(U_{rms}=I_{rms}×R\) In the formula, \(V_{rms}\) represents the ripple voltage, \(I_{rms}\) represents the ripple current, and R represents the ESR of the capacitor. It can be seen from the above that when the ripple current increases, even if the ESR remains unchanged, the ripple voltage will increase exponentially. In other words, when the ripple voltage increases, the ripple current also increases, which is why capacitors with a lower ESR value are required. When a ripple current is superimposed, the internal equivalent series resistance (ESR) of the capacitor generates heat, affecting the service life of the capacitor. Generally, the ripple current is proportional to the frequency, so the ripple current is relatively low at low frequencies.

IX. Basic Formulas for Capacitor Parameters

X. X and Y Safety Capacitors at the Power Supply Input

At the input of an AC power supply, generally three capacitors are added to suppress EMI conducted interference.
The input of an AC power supply is usually divided into three wires: live wire (L), neutral wire (N), and ground wire (G). The capacitors connected in parallel between the live wire and the ground wire and between the neutral wire and the ground wire are generally called Y - capacitors.
The connection positions of these two Y - capacitors are crucial and must meet relevant safety standards to prevent electric leakage of electronic devices or electrification of the chassis, which may endanger personal safety. Therefore, they are safety capacitors. Their capacitance values should not be too large, and their withstand voltages must be high.
Generally, for machines operating in subtropical regions, the ground leakage current is required not to exceed 0.7 mA; for machines operating in temperate regions, the ground leakage current is required not to exceed 0.35 mA. Therefore, the total capacitance of Y - capacitors generally should not exceed 4700 pF.
Special reminder from "Hardware Notebook": Y - capacitors are safety capacitors and must be certified by a safety testing agency. The withstand voltage of Y - capacitors is generally marked with safety - certification marks and AC250V or AC275V, but their actual DC withstand voltage is as high as over 5000V. Therefore, Y - capacitors cannot be replaced casually with ordinary capacitors marked with a withstand voltage of AC250V or DC400V.
The capacitor connected in parallel between the live wire and the neutral wire is generally called an X - capacitor. Since the connection position of this capacitor is also crucial, it also needs to meet safety standards.
Therefore, the X - capacitor also belongs to one of the safety capacitors. The capacitance value of an X - capacitor is allowed to be larger than that of a Y - capacitor, but a safety resistor must be connected in parallel at both ends of the X - capacitor to prevent the power - supply plug from remaining charged for a long time due to the charging and discharging process of the capacitor when the power - supply wire is plugged or unplugged.
Safety standards stipulate that when the power - supply wire of an operating machine is unplugged, within two seconds, the voltage (or potential to the ground) at both ends of the power - supply plug must be less than 30% of the original rated operating voltage.
Similarly, X - capacitors are safety capacitors and must be certified by a safety testing agency. The withstand voltage of X - capacitors is generally marked with safety - certification marks and AC250V or AC275V, but their actual DC withstand voltage is as high as over 2000V. Do not use ordinary capacitors marked with a withstand voltage of AC250V or DC400V casually when using X - capacitors.
X - capacitors generally use polyester - film capacitors with a relatively large ripple current. These capacitors are generally large in volume, but they can allow a large instantaneous charging and discharging current, and their internal resistance is relatively small.
The ripple - current index of ordinary capacitors is very low, and their dynamic internal resistance is high. Using an ordinary capacitor to replace an X - capacitor cannot only not meet the withstand - voltage requirements but also generally cannot meet the ripple - current index requirements.
In fact, it is almost impossible to completely filter out conducted interference signals only relying on Y - capacitors and X - capacitors. Because the frequency spectrum of interference signals is very wide, basically covering the frequency range from tens of KHz to several hundred MHz or even over a thousand MHz.
Generally, filtering out low - end interference signals requires a large - capacity filter capacitor, but due to safety - condition limitations, the capacitance values of Y - capacitors and X - capacitors cannot be large. For filtering out high - end interference signals, the filtering performance of large - capacity capacitors is extremely poor, especially the high - frequency performance of polyester - film capacitors is generally poor.
Because polyester - film capacitors are produced by a winding process, and the high - frequency response characteristics of the polyester - film dielectric are far from those of ceramics or mica. Generally, polyester - film dielectrics have an adsorption effect, which reduces the operating frequency of the capacitor. The operating - frequency range of polyester - film capacitors is approximately around 1 MHz. When the frequency exceeds 1 MHz, their impedance will increase significantly.
Therefore, to suppress the conducted interference generated by electronic devices, in addition to selecting Y - capacitors and X - capacitors, multiple types of inductive filters should be selected simultaneously and combined to filter out the interference.
Inductive filters mostly belong to low - pass filters, but there are many specifications and types of inductive filters, such as differential - mode, common - mode, and high - frequency, low - frequency, etc. Each type of inductor mainly acts on filtering out interference signals in a certain small frequency segment and has little effect on filtering out interference signals of other frequencies.
Generally, an inductor with a large inductance has more turns of the coil, so its distributed capacitance is also large. High - frequency interference signals will be bypassed through the distributed capacitance. Moreover, a magnetic core with a high magnetic permeability has a relatively low operating frequency.
Currently, the operating frequency of the magnetic cores of most inductive filters in extensive use is below 75 MHz. For applications with high - frequency requirements, high - frequency toroidal magnetic cores must be selected. High - frequency toroidal magnetic cores generally have a low magnetic permeability but a very small leakage inductance, such as amorphous - alloy magnetic cores and permalloy magnetic cores.

XI. LINKEYCON Aluminum Electrolytic Capacitors

LINKEYCON was established in 2017. It focuses on the design, development, manufacturing, and sales of aluminum electrolytic capacitors and is an upgrade from SZWX established in 2005. The company's headquarters is located in Songshan Lake Science City, Dongguan, Guangdong Province. It has a provincial - level laboratory and has constructed a five - in - one R & D system covering the client - side application environment, materials, processes, products, and manufacturing technology, as well as a scientific management system built with new - generation information technology. It has become a strategic supplier for domestic and foreign industry leaders in frequency converters, inverters, lighting, and other fields. For more information,
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