The most basic functions of the components of science and technology capacitors

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:

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.

Go to squat:

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 split, but 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.

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 larger the capacitance, the easier the low frequency passes, and the smaller 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 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.

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 coupling, oscillation / synchronization and time constant role:

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.

Oscillation / Synchronization:

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

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)

In addition, there is a big misunderstanding of common capacitors:

The misunderstanding of tantalum capacitor instead of electrolytic capacitor:

The general view is that tantalum capacitors perform better than aluminum capacitors because the tantalum capacitor is a tantalum pentoxide produced by anodization. Its dielectric capacity (usually expressed as ε) is higher than that of aluminum capacitors. .

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, 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 that of 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), pay attention to the effect of the change in capacity on the filter performance.