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 bypassing, decoupling, filtering and energy storage. The following classification details:

1) Bypass

The bypass capacitor is an energy storage device that powers 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 藕

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 the drive 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 decoupling capacitor acts as a “battery” to satisfy the change of the drive circuit current and avoid mutual coupling interference. Combining the bypass capacitor with the decoupling capacitor 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, and the higher the capacitance, the easier it is to pass.

Specifically used in filtering, large capacitor (1000μF) filter low frequency, small capacitor (20pF) filter high frequency. Some netizens have visually compared filter capacitors to “water ponds.” 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, parallel or a combination of them. For power supplies with power levels above 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. The tolerance value;

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 a peripheral component of the device, almost every device’s Datasheet or Solutions clearly indicates 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 capacitance according to the dielectric 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 changes in 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 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 very 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). Pulse 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

The capacitance range is 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.

Said 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 of power supply with high ripple current. The low-power performance of the low-frequency signal coupling circuit is outstanding.

Another type of multilayer ceramic capacitor is C0G, which has a capacity of less than 1000pF. The main performance index of this type of capacitor is the loss tangent 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 misunderstanding of replacing electrolytic capacitors is generally 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 has become obsolete. At present, 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 quite different. Capacitors using the same anode can vary greatly in performance due to differences in electrolytes. 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 to the 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, if you choose 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 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 an 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 value Tan δ

In the equivalent circuit of the capacitor, the ratio of the series equivalent resistance ESR to the capacitive reactance 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. When the intermediate frequency range is reached, the reactance (XL) drops to the value of ESR. 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