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Capacitor selection knowledge and basic capacitance knowledge summary

Views:2     Author:Site Editor     Publish Time: 2020-06-04      Origin:Site

There are always sources of driving and loads being driven in the circuit. 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 Inductance, resistance (especially the inductance on the chip pin, will produce rebound), this current is a kind of noise, which will affect the normal operation of the front stage. This is coupling.

 

The decoupling capacitor functions as a battery, satisfying the change of the drive circuit current and avoiding the coupling interference between each other.

 

The bypass capacitor is actually decoupled, but the bypass capacitor generally refers to high-frequency bypass, which is to increase a low-impedance leakage prevention method for high-frequency switching noise. The high-frequency bypass capacitor is generally small, according to the resonant frequency is generally 0.1u, 0.01u, etc., and the decoupling capacitor is generally larger, 10u or more, according to the distribution parameters in the circuit, and the size of the drive current to determine.

 

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.

 

52solution's summary: the decoupling capacitor has two roles between the power supply and ground of the integrated circuit. On the one hand, it is the storage capacitor of the integrated circuit, and on the other hand, it bypasses the high-frequency noise of the device. A typical decoupling capacitor value in digital circuits is 0.1μF. The typical value of the distributed inductance of this capacitor is 5μH. The 0.1μF decoupling capacitor has a distributed inductance of 5μH, and its parallel resonance frequency is about 7MHz, that is to say, it has a good decoupling effect for noise below 10MHz, and has little effect on noise above 40MHz. 1μF, 10μF capacitors, parallel resonance frequency above 20MHz, the effect of removing high-frequency noise is better. Every 10 or so integrated circuits need to add a charge and discharge capacitor, or an energy storage capacitor, optional about 10μF. It is best not to use electrolytic capacitors. The electrolytic capacitors are rolled up with two layers of film. This rolled-up structure behaves as an inductance at high frequencies. To use tantalum capacitors or polycarbonate capacitors. The selection of decoupling capacitors is not strict, you can press C = 1 / F, that is, 10MHz to take 0.1μF, 100MHz to take 0.01μF.

 

Distributed capacitance refers to a distributed parameter formed by non-morphic capacitance. Generally refers to the capacitance formed between the line and the line, and between the upper and lower layers of the printed board on the printed board or other forms of circuit form. Some netizens of Cntronics have said that the capacity of this capacitor is very small, but it may have a certain impact on the circuit. This effect must be fully considered when designing printed boards, especially when the operating frequency is high. It also becomes a parasitic capacitance, which must occur during manufacturing, but it is only a matter of size. When laying high-speed PCBs, vias can reduce board capacitance but increase inductance.

 

Distributed inductance refers to the increase in impedance due to the self-inductance of the conductor as the frequency increases.

 

Attention to the selection and use of capacitors:

1. Generally, low-frequency coupling or bypass and low electrical characteristics require paper dielectric and polyester capacitors. In high-frequency and high-voltage circuits, mica capacitors or ceramic dielectric capacitors should be used. Optional electrolytic capacitors are available.

2. In the oscillation circuit, delay circuit, and tone circuit, the capacitor capacity should be as consistent as possible with the calculated value. In various filters and networks (frequency selection networks), the capacitor capacity is required to be precise; in decoupling circuits and low-frequency coupling circuits, the requirements for the same two levels of accuracy are not too strict.

3. The rated voltage of the capacitor should be higher than the actual working voltage, and there should be enough room. Generally, a capacitor with a withstand voltage of more than twice the actual working voltage is selected.

4. Give priority to capacitors with high insulation resistance and low loss, and pay attention to the environment.


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