Capacitors are the most common and commonly used devices in circuit design, and they are one of passive components. Active devices are simply devices that require energy (electricity) sources and are called active devices. Devices that do not require energy (electricity) sources are passive devices. . Capacitors also often play an important role in high-speed circuits.
There are generally many functions and uses of capacitors. Such as: the role in bypassing, decoupling, filtering, energy storage; in the completion of oscillation, synchronization and time constant...
Let's analyze it in detail below:
1. DC blocking: The function is to prevent DC from passing and allow AC to pass.
2. Bypass (decoupling): Provide a low-impedance path for some parallel components in the AC circuit.
Bypass capacitor: Bypass capacitor, also known as decoupling capacitor, is an energy storage device that provides energy for a device. It uses the frequency impedance characteristic of the capacitor (the frequency characteristic of an ideal capacitor increases with the frequency, and the impedance decreases) , Like a pond, it can make the output voltage output uniform and reduce load voltage fluctuations. The bypass capacitor should be as close as possible to the power supply pin and ground pin of the load device. This is an impedance requirement. Special attention should be paid when drawing the PCB. Only when it is close to a certain component can the voltage or other output signals be suppressed due to excessive voltage. The ground potential rise and noise, to put it plainly, is to couple the AC component in the DC power supply to the power supply ground through a capacitor, which plays a role in purifying the DC power supply. As shown in the figure, C1 is a bypass capacitor, and it should be as close as possible to IC1 when drawing the picture.
Decoupling capacitor: The decoupling capacitor takes the interference of the output signal as the filtering object. The decoupling capacitor is equivalent to the battery and uses its charge and discharge so that the amplified signal will not be interfered by the sudden change of the current. Its capacity is determined by the frequency of the signal and the degree of ripple suppression. The decoupling capacitor acts as a "battery" to meet the change of the drive circuit current and avoid mutual coupling interference.
The bypass capacitor is actually decoupling, but the bypass capacitor generally refers to high-frequency bypass, that is, to improve a low-impedance leakage prevention method for high-frequency switching noise. High-frequency bypass capacitors are generally relatively small, generally 0.1F, 0.01F, etc. according to the resonance frequency; and the capacity of decoupling capacitors is generally larger, which may be 10F or greater, depending on the distribution parameters in the circuit and the change in drive current to make sure. Figure C3 is the decoupling capacitor
The difference between them: Bypass is to take the interference in the input signal as the filtering object, while decoupling is to take the interference of the output signal as the filtering object to prevent the interference signal from returning to the power supply.
3. Coupling: As a connection between two circuits, it allows AC signals to pass through and be transmitted to the next circuit.
The purpose of using capacitors as coupling components is to transmit the signals of the previous stage to the next stage, and to cut off the influence of the direct current of the previous stage on the latter stage, so that the circuit debugging is simple and the performance is stable.
The AC signal amplification will not change if no capacitor is added, but the working points of all levels need to be redesigned. Due to the influence of the front and back stages, the debugging of the working points is very difficult, and it is almost impossible to achieve it in multiple stages.
4. Filtering: This is very important to the circuit, and the capacitors behind the CPU basically do this.
That is, the larger the frequency f, the smaller the impedance Z of the capacitor. At low frequencies, because the impedance Z of the capacitor C is relatively large, useful signals can pass through smoothly; at high frequencies, the capacitor C is already very small due to the impedance Z, which is equivalent to short-circuiting high-frequency noise to GND.
Energy storage: store electrical energy and release it when necessary.
Such as camera flash, heating equipment, etc. (Nowadays, the energy storage level of some capacitors is close to the level of lithium batteries, and the electric energy stored by a capacitor can be used for a mobile phone for a day.
Energy storage function: Generally, electrolytic capacitors will have the function of energy storage. For special energy storage capacitors, the mechanism of capacitor energy storage is electric double-layer capacitor and Faraday capacitor. Its main form is supercapacitor energy storage. Supercapacitors are capacitors that use the principle of electric double-layer. When an external voltage is applied to the supercapacitor When the two plates of the capacitor are the same as ordinary capacitors, the positive electrode of the plate stores positive charges, and the negative plate stores negative charges. Under the action of the electric field generated by the charges on the two plates of the supercapacitor, there is a gap between the electrolyte and the electrodes. Opposite charges are formed on the interface to balance the internal electric field of the electrolyte. This positive charge and negative charge are arranged on the opposite position with a very short gap between the positive and negative charges on the contact surface between the two different phases. This charge distribution layer is called an electric double layer, so the capacitance is very large.
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