SMD capacitors are used as passive components, and their functions are often used in power circuits: to achieve bypass, decoupling, filtering and energy storage, etc. The main functions in signal circuits are coupling, oscillation/synchronization and time constant role. Qihe pick and place will explain each of these separately.
Filtering function in the power circuit
Filtering is a very important part of the function of chip capacitors. It is used in almost all power supply circuits. Theoretically (that is, assuming that the capacitor is a pure capacitor), the larger the capacitor, the smaller the impedance, and the higher the passing frequency. But in fact, most of the capacitors exceeding 1uF are electrolytic capacitors, which have a large inductance component, so the impedance will increase when the frequency is high.
Sometimes it is seen that there is a large electrolytic capacitor connected in parallel with a small capacitor. At this time, the large capacitor passes the low frequency, and the small capacitor passes the high frequency. The function of the capacitor is to pass high resistance to low, pass high frequency to block low frequency. The larger the capacitance, the easier it is for low frequencies to pass, and the larger the capacitance, the easier it is for high frequencies to pass. Specifically used in filtering, a large capacitor (1000uF) filters low frequencies, and a small capacitor (20pF) filters high frequencies.
A friend once compared the filter capacitor to a “pond”. Since the voltage across the capacitor will 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 the water volume will not change due to the addition or evaporation of a few drops of water. It converts changes in voltage into changes in current. The higher the frequency, the greater the peak current, thereby buffering the voltage. Filtering is the process of charging and discharging.
The bypass capacitor
The bypass capacitor is an energy storage device that provides energy for local devices. It can even out the output of the voltage regulator and reduce the load demand. Like a small rechargeable battery, the bypass capacitor can be charged and discharged to the device. To minimize impedance, bypass capacitors should be placed as close as possible to the power and ground pins of the load device. This is a good protection against ground potential rise and noise caused by excessively large input values. Ground bounce is the voltage drop across a ground connection when a large current spike is passed through it.
Remove decoupling, also known as decoupling solution. From a circuit point of view, it is always possible to distinguish between the source of the drive and the load being driven. If the load capacitance is relatively large, the drive circuit needs to charge and discharge the capacitance to complete the signal transition. When the rising edge is relatively steep, the current is relatively large, so that the driving current will absorb a large power supply current. The inductance and resistance (especially the inductance on the chip pins will bounce back), this current is actually a kind of noise compared to the normal situation, which will affect the normal operation of the front stage. This is coupling.
The decoupling capacitor is to play the role of a battery to meet the change of the drive circuit current and avoid mutual coupling interference. It will be easier to understand by combining bypass capacitors and decoupling capacitors. The bypass capacitor is actually decoupling, but the bypass capacitor generally refers to a high-frequency bypass, that is, to provide a low-impedance leakage prevention path for high-frequency switching noise. High-frequency bypass capacitors are generally small, generally 0.1u, 0.01u according to the resonant frequency, and decoupling capacitors are generally relatively large, 10uF or larger, determined according to the distribution parameters in the circuit and the change of the driving current. Bypass is to filter out the interference in the input signal, while decoupling is to filter out the interference in the output signal to prevent the interference signal from returning to the power supply. This should be their essential difference.
The energy storage capacitor
The energy storage capacitor collects the charge through the rectifier, and transmits the stored energy to the output terminal of the power supply through the converter lead. Aluminum electrolytic capacitors with a voltage rating of 40-450VDC and a capacitance value between 220-150 000uF are more commonly used. According to different power requirements, devices are sometimes connected in series, in parallel or in combination. For power supplies with a power level exceeding 10KW, generally larger pot-shaped screw-terminal capacitors are used.Read more: What is the purpose of a chip capacitor?
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