As one of the passive components, the capacitor has nothing more than the following:
1. It is used in power circuits to realize the functions of bypass, decoupling, filtering and energy storage. The following categories are detailed:
1. Bypass
The bypass capacitor is an energy storage device that provides energy for the local device. It can make the output of the voltage regulator uniform and reduce the load demand. Just like a small rechargeable battery, the bypass capacitor can be charged and discharged to the device. To minimize the impedance, the bypass capacitor should be as close as possible to the power supply pin and ground pin of the load device. This can well prevent ground potential rise and noise caused by excessive input value. The ground bounce is the voltage drop when the ground connection passes through a large current glitch.
2. Decoupling
Decoupling, also known as decoupling. From the circuit, it can always be distinguished as the driving source and the driven load. 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 steeper, the current is relatively large, so the drive current will absorb a large power supply current. The inductance and resistance (especially the inductance on the chip pins will rebound), this current is actually a kind of noise relative to the normal situation, which will affect the normal operation of the front stage. .
The decoupling capacitor acts as a "battery" to satisfy the change of the drive circuit current and avoid coupling interference.
Combining bypass capacitors and decoupling capacitors will be easier to understand. The bypass capacitor is actually decoupled, but the bypass capacitor generally refers to a high-frequency bypass, that is, a low-impedance leakage prevention method for high-frequency switching noise. The high-frequency bypass capacitor is generally small, generally 0.1μF, 0.01μF, etc. according to the resonance frequency;
The capacity of the decoupling capacitor is generally larger, which may be 10µF or greater, depending on the distribution parameters in the circuit and the change in drive current.
Bypass is to filter the interference in the input signal, while 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
In theory (that is, the capacitor is assumed to be a pure capacitor), the larger the capacitance, the smaller the impedance and the higher the frequency of passage. But in fact, the capacitors exceeding 1µF are mostly electrolytic capacitors, which have a large inductance component, so the impedance will increase after high frequency. Sometimes I see an electrolytic capacitor with a larger capacitance connected in parallel with a small capacitor. At this time, the large capacitor passes low frequency and the small capacitor passes high frequency. The function of the capacitor is to pass high resistance and low resistance, and pass high frequency to block low frequency. The larger the capacitance, the easier the low frequency passes, and the larger the capacitance, the easier the high frequency passes. Specifically used in filtering, a large capacitor (1000µF) filters low frequencies, and a small capacitor (20pF) filters high frequencies.
Someone once vividly compared the filter capacitor to a "pond". Since the voltage at both ends of the capacitor does not change suddenly, 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 there will be no change in the amount of water caused by the addition or evaporation of a few drops of water. It converts the voltage change into a current change. 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 charge through the rectifier and transfers the stored energy to the output end of the power supply through the converter lead. Aluminum electrolytic capacitors with a voltage rating of 40 to 450 VDC and a capacitance between 220 and 150 000 µF (such as EP43's B43504 or B43505) are more commonly used. According to different power supply requirements, devices are sometimes used in series, parallel, or a combination of them. For power supplies with a power level exceeding 10 KW, a larger volume spiral-shaped terminal capacitor is usually used.
2. Applied to signal circuits, mainly to complete the functions of coupling, oscillation / synchronization and time constant:
1. Coupling
For example, the emitter of a transistor amplifier has a self-sufficient bias resistor, which at the same time causes the signal to generate a voltage drop and feeds back to the input to form an input and output signal coupling. A capacitor is connected in parallel with the terminal. Because a capacitor of an appropriate capacity has a small impedance to the AC signal, this reduces the coupling effect caused by the resistance, so this capacitor is called a decoupling capacitor.
2. Oscillation / synchronization
The load capacitance including RC, LC oscillator and crystal all belong to this category.
3. Time constant
This is the common integral circuit composed of R and C connected in series. When the input signal voltage is applied to the input terminal, the voltage on the capacitor (C) gradually rises. The charging current decreases as the voltage rises. The characteristics of current through resistance (R) and capacitance (C) are described by the following formula: i = (V / R) e-(t / CR)
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