Detailed analysis of the working principle of capacitors and the selection and application of capacitors

How does the capacitor work and the choice of capacitor? What is a capacitor? What is the unit of capacitance? This article will answer you in detail!

One of the capacitors: the role of the capacitor

Capacitors, which are one of the passive components, function in the following ways:

1. It is applied to the power supply circuit to realize the functions of bypass, decoupling, filtering and energy storage. The following classification details:

1) Bypass

The bypass capacitor is an energy storage device that supplies energy to the local device, which equalizes the output of the regulator and reduces the load requirements. 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 as possible to the power supply and ground pins of the load device. This can well prevent ground potential elevation and noise caused by excessive input values. The ground bounce is the voltage drop when the ground connection is passed through a large current glitch.

2) Go to è—•

Going to squat, also known as defamatory. From the circuit, it can always be distinguished as the source of the drive and the load being driven. If the load capacitance is relatively large, the drive circuit must charge and discharge the capacitor to complete the signal transition. When the rising edge is steep, the current is relatively large, so that the driven current will absorb a large supply current due to the circuit. The inductance, the resistance (especially the inductance on the chip pin, will produce 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 called "coupling". .

The tantalum capacitor acts as a "battery" to satisfy the change of the drive circuit current and avoid mutual coupling interference. Combining bypass and decoupling capacitors will be easier to understand. The bypass capacitor is actually decoupled, except that the bypass capacitor generally refers to the high-frequency bypass, which is a low-impedance venting path for high-frequency switching noise. The high-frequency bypass capacitor is generally small, and generally takes 0.1μF, 0.01μF, etc. according to the resonant frequency; and the capacity of the decoupling capacitor is generally large, which may be 10μF or more, depending on the distributed parameters in the circuit and the variation of the driving current. to make sure.

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

3) Filtering

Theoretically (that is, assuming the capacitor is a pure capacitor), the larger the capacitance, the smaller the impedance and the higher the frequency of passing. However, in fact, the capacitance of more than 1μF is mostly an electrolytic capacitor, which has a large inductance component, so the impedance will increase after the frequency is high. Sometimes you will see a small capacitor with a larger capacitance and a small capacitor. At this time, the large capacitor passes through the low frequency and the small capacitor passes through the high frequency. The function of the capacitor is to pass the high impedance and low frequency. The smaller the capacitance, the easier it is to pass the low frequency. The higher the capacitance, the easier the high frequency passes. Specifically used in filtering, large capacitor (1000μF) filter low frequency, small capacitor (20pF) filter high frequency. Some netizens have compared the filter capacitor to a "water pond". Since the voltage across 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 quantity will not be changed due to the addition or evaporation of a few drops of water. It converts the change in voltage into a change in current. The higher the frequency, the larger the peak current, which buffers the voltage. Filtering is the process of charging and discharging.

4) Energy storage

The energy storage capacitor collects charge through the rectifier and transfers the stored energy through the converter lead to the output of the power supply. Aluminum electrolytic capacitors (such as EPCOS B43504 or B43505) with a voltage rating of 40 to 450 VDC and a capacitance between 220 and 150 000 μF are more commonly used. Depending on the power requirements, the devices are sometimes used in series, in parallel, or a combination thereof. For power supplies with power levels greater than 10 kW, bulky screw-type terminal capacitors are typically used.

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

1) Coupling

For example, the emitter of a transistor amplifier has a self-biasing resistor, which at the same time causes the voltage drop of the signal to be fed back to the input to form an input-output signal coupling. This resistor is the component that produces the coupling. Parallel connection of a capacitor, because the capacitor of the appropriate capacity has a small impedance to the AC signal, thus reducing the coupling effect caused by the resistor, so the capacitor is called a decoupling capacitor.

2) Oscillation / synchronization

Load capacitors including RC, LC oscillators, and crystals fall into this category.

3) Time constant

This is the common integration circuit of R and C connected in series. When the input signal voltage is applied to the input, the voltage across the capacitor (C) gradually rises. The charging current decreases as the voltage rises. The characteristics of the current through the resistor (R) and capacitor (C) are described by the following formula:

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

Say the second of the capacitor: the choice of capacitor

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

1. Electrostatic capacity;

2, rated pressure;

3. Tolerance error;

4. The amount of capacitance change under DC bias;

5, noise level;

6, the type of capacitor;

7, the specifications of the capacitor.

So, is there a shortcut to find? In fact, as the peripheral components of the device, almost every device's Datasheet or Solutions clearly specify the selection parameters of the peripheral components, that is, the basic device selection requirements can be obtained, and then further refined. It. In fact, when choosing a capacitor, it is not only about the capacity and the package. It depends on the environment in which the product is used. Special circuits must use special capacitors.

The following is the classification of the dielectric of the dielectric according to the dielectric constant of the dielectric. The dielectric constant directly affects the electricity.

Road stability.

NP0 or CH (K “150”): The electrical performance is the most stable, basically does not change with temperature, voltage and time, and is suitable for high-frequency circuits with high stability requirements. In view of the small K value, it is difficult to have a large capacity capacitor in the 0402, 0603, and 0805 packages. Such as 0603 is generally the largest 10nF or less. X7R or YB (2000 “K” 4000): Electrical performance is relatively stable, and performance changes are not significant when temperature, voltage, and time change (?C “±10%”). Suitable for DC blocking, coupling, bypass and full frequency identification circuits that do not require high capacity stability. Y5V or YF(K 》 15000): The capacity stability is worse than X7R (?C "+20% ～ -80%"). The capacity and loss are sensitive to the test conditions such as temperature and voltage, but because of its large K value, Applicable to some occasions with high capacitance requirements.

Say the third of the capacitor: the classification of the capacitor

There are many types and types of capacitors. Based on the material properties of capacitors, they can be divided into the following categories:

1, aluminum electrolytic capacitor

The capacitance range is from 0.1μF to 22000μF. It is the best choice for high ripple current, long life and large capacity. It is widely used in power supply filtering and decoupling.

2, film capacitor

With capacitances ranging from 0.1pF to 10μF, with small tolerances, high capacity stability and extremely low piezoelectricity, it is the first choice for X, Y safety capacitors and EMI/EMC.

3, tantalum capacitor

Capacitance ranges from 2.2μF to 560μF, low equivalent series resistance (ESR), and low equivalent series inductance (ESL). Pulsating absorption, transient response and noise suppression are superior to aluminum electrolytic capacitors, making them ideal for highly stable power supplies.

4, ceramic capacitor

With capacitances ranging from 0.5pF to 100μF, the unique materials and crystallization of thin film technology cater to today's “lighter, thinner, more energy-efficient” design philosophy.

5, super capacitor

Capacitance capacity ranges from 0.022F to 70F, which is extremely high capacitance, so it is also called "gold capacitor" or "farad capacitor". The main features are: high capacitance, good charge / discharge characteristics, suitable for electrical energy storage and power backup. The disadvantage is that the withstand voltage is low and the operating temperature range is narrow.

Say the fourth of the capacitor: multilayer ceramic capacitor (MLCC)

For capacitors, miniaturization and high capacity are eternal trends. Among them, the number of multilayer ceramic capacitors (MLCC) is the fastest growing.

Multi-layer ceramic capacitors are widely used in portable products, but in recent years, technological advances in digital products have placed new demands on them. For example, mobile phones require higher transmission rates and higher performance; baseband processors require high speed and low voltage; LCD modules require low thickness (0.5mm) and large capacitance. The harshness of the automotive environment has special requirements for multilayer ceramic capacitors: first, high temperature resistance, multilayer ceramic capacitors placed in it must meet the operating temperature of 150 ° C; secondly, short circuit failure protection design is required on the battery circuit. .

That is to say, miniaturization, high speed and high performance, high temperature resistance and high reliability have become key characteristics of ceramic capacitors.

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

The most common application is the X7R (X5R) multilayer ceramic capacitor, whose capacity is mainly concentrated above 1000pF. The main performance index of this type of capacitor is equivalent series resistance (ESR), decoupling and filtering at high ripple current. The low-power performance of the low-frequency signal coupling circuit is outstanding.

Another type of multilayer ceramic capacitor is C0G type, its capacity is more than 1000pF. The main performance index of this type of capacitor is the loss tangent value tgδ(DF). The conventional noble metal electrode (NME) has a DF value range of (2.0 to 8.0) × 10-4, while the technically innovative base metal electrode (BME) has a DF value of (1.0 to 2.5) × 10-4. , about 31 to 50% of the former. These products have significant low-power characteristics in GSM, CDMA, cordless phones, Bluetooth, and GPS systems carrying T/R module circuits. More used in various high frequency circuits, such as oscillator / synchronizer, timer circuit and so on.

Say the fifth of the capacitor: tantalum capacitor

The common misconception of replacing electrolytic capacitors is that tantalum capacitors perform better than aluminum capacitors because the tantalum capacitor is a tantalum pentoxide produced by anodization, its dielectric capacity (usually represented by ε) is higher than that of aluminum capacitors. The two aluminum medium is high.

Therefore, in the case of the same capacity, the volume of the tantalum capacitor can be made smaller than that of the aluminum capacitor. (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. Otherwise, the volume needs to be made larger) The nature of tantalum is relatively stable, so tantalum capacitor performance is generally considered to be better than aluminum capacitors.

However, this method of judging the performance of the capacitor by the anode is outdated. The key to determining the performance of the electrolytic capacitor is not the anode but the electrolyte, that is, the cathode. Because different cathodes and different anodes can be combined into different types of electrolytic capacitors, their performance is also very different. The capacitance of the same anode can vary greatly depending on the electrolyte. In general, the effect of the anode on the performance of the capacitor is much smaller than that of the cathode. Another view is that tantalum capacitors perform better than aluminum capacitors, mainly because they are significantly better than aluminum electrolyte capacitors after adding manganese dioxide cathode. If the cathode of the aluminum electrolyte capacitor is replaced with manganese dioxide, its performance can actually be improved.

To be sure, ESR is one of the main parameters for measuring a capacitor's characteristics. However, to select a capacitor, you should avoid the ESR as low as possible, and the higher the quality, the better. To measure a product, we must consider it in all directions and from multiple angles. We must not exaggerate the role of capacitors intentionally or unintentionally.

---The above quotes the experience summary of some netizens.

The structure of a common electrolytic capacitor is an anode and a cathode and an electrolyte, the anode is passivated aluminum, and the cathode is pure aluminum, so the key is at the anode and the electrolyte. The quality of the anode is related to the problem of resistance to piezoelectric coefficient.

In general, the ESR of tantalum electrolytic capacitors is much smaller than aluminum electrolytic capacitors of the same capacity and withstand voltage, and the high frequency performance is better. If that capacitor is used in a filter circuit (such as a 50 Hz bandpass filter at the center), however, this requires a trade-off between PCB area, device count, and cost.

Say the sixth of the capacitor: the electrical parameters of the electrolytic capacitor

The electrolytic capacitor here mainly refers to the aluminum electrolytic capacitor, and its basic electrical parameters include the following five points:

1, the capacitance value

The capacitance of an electrolytic capacitor depends on the impedance exhibited when operating at an alternating voltage. Therefore, the capacitance value, that is, the value of the AC capacitor, varies with the operating frequency, voltage, and measurement method. In the standard JISC 5102, the capacitance of the aluminum electrolytic capacitor is measured under the conditions of a frequency of 120 Hz, a maximum AC voltage of 0.5 Vrms, and a DC bias voltage of 1.5 to 2.0 V. It can be asserted that the capacity of the aluminum electrolytic capacitor decreases as the frequency increases.

2, loss tangent tan δ

In the equivalent circuit of the capacitor, the ratio of the series equivalent resistance ESR to the capacitance of 1/ωC ​​is called Tan δ, where ESR is the value calculated at 120 Hz. Obviously, Tan δ becomes larger as the measurement frequency increases, and increases as the measurement temperature decreases.

3, impedance Z

At a specific frequency, the resistance that blocks the passage of the alternating current is the so-called impedance (Z). It is closely related to the capacitance value and inductance value in the capacitor equivalent circuit, and is also related to ESR.

Z = √ [ESR2 + (XL - XC)2 ]

Where XC = 1 / ωC = 1 / 2πfC

XL = ωL = 2πfL

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

4, leakage current

The dielectric of the capacitor has a great hindrance to DC current. However, since the aluminum oxide film medium is immersed in the electrolyte, when a voltage is applied, a small current called a leakage current is generated when the oxide film is reformed and repaired. Generally, the leakage current increases as the temperature and voltage increase.

5, ripple current and ripple voltage

In some materials, the two are called "chopper current" and "chopper voltage", which is actually ripple current, ripple voltage. The meaning is that the capacitor can withstand the ripple current / voltage value. They are closely related to ESR and can be expressed by the following formula:

Urms = Irms × R

Where Vrms represents the ripple voltage

Irms represents ripple current

R represents the ESR of the capacitor

It can be seen from the above that when the ripple current increases, the chopping voltage is multiplied even when the ESR remains unchanged. In other words, as the ripple voltage increases, the ripple current also increases, which is why the capacitor is required to have a lower ESR value. After the ripple current is added to the stack, heat is generated due to the equivalent series resistance (ESR) inside the capacitor, which affects the life of the capacitor. In general, the ripple current is proportional to the frequency, so the ripple current is also low at low frequencies.

Say the seventh of the capacitor: the basic formula of the capacitor parameters

1, capacity (Fala)

Inch: C = ( 0.224 × K · A) / TD

Metric: C = ( 0.0884 × K · A) / TD

2. Energy stored in the capacitor

E = 1/2 CV2

3, the linear charge of the capacitor

I = C (dV/dt)

4, the total impedance of the capacitor (ohms)

Z = √ [ RS

2 + (XC – XL)2 ]

5, capacitive reactance (ohm)

XC = 1/(2Ï€fC)

6, phase angle Ф

Ideal capacitor: advanced current voltage 90o

Ideal inductor: lag current voltage 90o

Ideal resistor: the same phase as the current voltage

7, dissipation coefficient (%)

DF = tan δ (loss angle)

= ESR / XC

= (2Ï€fC)(ESR)

8, quality factors

Q = cotan δ = 1/ DF

9, equivalent series resistance ESR (ohms)

ESR = (DF) XC = DF/ 2Ï€fC

10, power consumption

Power Loss = (2Ï€fCV2) (DF)

11, power factor

PF = sin δ (loss angle) – cos Ф (phase angle)

12. Root mean square

Rms = 0.707 × Vp

13, kVA KVA (kW)

KVA = 2πfCV2 × 10-3

14, the temperature coefficient of the capacitor

TC = [ (Ct – C25) / C25 (Tt – 25) ] × 106

15, capacity loss (%)

CD = [ (C1 – C2) / C1 ] × 100

16, the reliability of ceramic capacitors

L0 / Lt = (Vt / V0) X (Tt / T0)Y

17, the capacitance value in series

n capacitors in series: 1/CT = 1/C1 + 1/C2 + ,,. + 1/Cn

Two capacitors in series: CT = C1 · C2 / (C1 + C2)

18, the capacitance value in parallel

CT = C1 + C2 + ,,. + Cn

19. Againg Rate

AR = % â–³C / decade of time

The symbols in the above formula are as follows:

K = dielectric constant

A = area

TD = insulation thickness

V = voltage

t = time

RS = series resistance

f = frequency

L = inductance inductive coefficient

δ = loss angle

Ф = phase angle

L0 = service life

Lt = test life

Vt = test voltage

V0 = operating voltage

Tt = test temperature

T0 = ​​operating temperature

X , Y = effect index of voltage and temperature.

Say the capacitor eight: X, Y safety capacitor at the input of the power supply

At the AC input, it is generally necessary to add three capacitors to suppress EMI conducted interference. The input of the AC power supply can generally be divided into three lines: FireWire (L) / Neutral (N) / Ground (G). The capacitor connected between the live and ground lines and between the neutral and ground lines is generally referred to as a Y capacitor. The position of the two Y capacitors is relatively critical, and must comply with relevant safety standards to prevent leakage of electronic equipment or the casing is charged, which is likely to endanger personal safety and life. Therefore, they are all safety capacitors, and the capacitance value cannot be too large. The withstand voltage must be high. Generally, a machine operating in a subtropical zone requires that the earth leakage current should not exceed 0.7 mA; working in a temperate machine requires that the earth leakage current should not exceed 0.35 mA. Therefore, the total capacity of the Y capacitor generally cannot exceed 4700pF.

Special note: The Y capacitor is a safe capacitor and must be certified by a safety inspection agency. The voltage tolerance of Y capacitors is generally marked with a safety certification mark and AC250V or AC275V, but its true DC withstand voltage is as high as 5000V or more. Therefore, the Y capacitor cannot be used arbitrarily using a nominal voltage of AC250V or a common capacitor such as DC400V.

A capacitor connected in parallel between the live line and the neutral line is generally referred to as an X capacitor. Since the location of this capacitor connection is also critical, it also needs to meet safety standards. Therefore, the X capacitor is also one of the safety capacitors. The capacitance of the X capacitor is allowed to be larger than the Y capacitor, but a safety resistor must be connected in parallel with the X capacitor to prevent the power cord plug from being charged for a long time due to the charging and discharging process of the capacitor. The safety standard stipulates that when the working power cord is unplugged, the voltage charged at both ends of the power cord plug (or ground potential) must be less than 30% of the original rated operating voltage within two seconds. Similarly, the X capacitor is also a safety capacitor and must be certified by a safety inspection agency. The X capacitor's withstand voltage is generally marked with the safety certification mark and AC250V or AC275V, but its true DC withstand voltage is as high as 2000V or more. Do not use the nominal voltage AC250V or DC400V to replace it. .

X capacitors generally use polyester film capacitors with large ripple current. These capacitors are generally large in size, but they allow a large amount of current for instantaneous charge and discharge, and their internal resistance is relatively small. The common capacitor ripple current has a low index and a high dynamic internal resistance. Replacing the X capacitor with a common capacitor, in addition to the withstand voltage conditions can not be met, the general ripple current index is also difficult to meet the requirements.

In fact, it is not possible to rely solely on the Y and X capacitors to completely filter out conducted interference signals. Because the spectrum of the interfering signal is very wide, it covers the frequency range from tens of KHz to several hundred MHz, even thousands of MHz. Generally, the filtering of the low-end interference signal requires a large-capacity filter capacitor, but due to the safety conditions, the capacity of the Y-capacitor and the X-capacitor cannot be used large; the filtering of the high-end interference signal, the filtering of the large-capacity capacitor The performance is extremely poor, especially the high-frequency performance of the polyester film capacitor is generally poor, because it is produced by the winding process, and the high-frequency response characteristics of the polyester film medium is far from the ceramic or mica, generally The polyester film media has an adsorption effect, which reduces the operating frequency of the capacitor. The operating frequency range of the polyester film capacitor is about 1 MHz, and the impedance will increase significantly beyond 1 MHz. Therefore, in order to suppress the conducted interference generated by the electronic device, in addition to the Y capacitor and the X capacitor, multiple types of inductive filters are also selected, and the interference is filtered together. Inductance filters are mostly low-pass filters, but there are many types of inductive filters, such as differential mode, common mode, and high frequency and low frequency. Each type of inductor is mainly used for filtering out interference signals of a certain small frequency, and the filtering effect of interference signals of other frequencies is not large. Generally, an inductor with a large inductance has a large number of turns, and the distributed capacitance of the inductor is also large. The high frequency interference signal will be bypassed by the distributed capacitance. Moreover, a magnetic core with a high magnetic permeability has a lower operating frequency. At present, the operating frequency of a large number of inductive filter cores is mostly below 75 MHz. For applications where the operating frequency is relatively high, high-frequency toroidal cores must be used. The magnetic permeability of high-frequency toroidal cores is generally not high, but the leakage inductance is particularly small, such as amorphous alloy cores, permalloys, etc.

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