Speaking of capacitance, all kinds of way can make people dizzy, bypass capacitor
, decoupling capacitor, filter capacitance, etc. , in fact, no matter what name, its principle is the same, it is using the ac signal to present the characteristic of low impedance, it can be seen through the equivalent capacitance impedance formula: an Xcap = 1/2 л fC, the higher the working frequency, the greater the capacitance value, the smaller the impedance of the capacitor.
If capacitance in circuit, the main function is to provide low impedance to ac signal pathway, is called a bypass capacitor;
If is mainly in order to increase the power and the exchange coupling, reduce the influence of ac signal of power supply, and can be referred to as the decoupling capacitor;
If used in the filter circuit, and can be referred to as the filter capacitor;
But in practice, often capacitance effect is various, we don't need to spend a lot of thoughts to consider how to define.
Capacitance is the nature of exchanges, dc, in theory the power filter capacitor is bigger, the better.
日本由于引线和pcb布线原因,实时上电容是电感和电容的水平联电路,这就引入了谐振此时率概念:ω= 1 / (
1/2 under resonant frequency capacitance is capacitive, resonance frequency capacitance is perceptual.
This also explains why the same capacity value of STM encapsulation capacitor filter frequency is higher than DIP package.
As to exactly how capacitance, it is a reference.
5 to 50.
1μF 8 160.
01μf 25 80 pF 501000 160100 pF 250 50010 pF 800 1。
But only a reference, in the words of the old engineer -
Mainly by experience.
More reliable, it is two capacitors in parallel, one large and one small difference in general requirements of more than two orders of magnitude in order to obtain larger filter frequency band.
Capacitance value and is inversely proportional to the square of the filter frequency you want.
The choice of specific capacitance can use the formula C = 4 Pi * Pi/(
R * * f)
How to select power filter capacitance, grasp the essence and the method, actually not difficult also.
In theory, the ideal capacitance impedance decrease with the increase of frequency (
1 / jwc)
, but due to the inductive effect of pins on both ends of the capacitor, the capacitor should be considered a LC series resonant circuit, device from the resonance frequency of FSR parameters, which means that the frequency is greater than the FSR, capacitance becomes a inductance, if the capacitance to earth filtering, when frequency beyond the FSR of interference suppression is compromised, so I need a smaller capacitance in parallel to ground.
Due to the small capacitance, SFR value is big, the high frequency signal, provides a pathway to, so we often understand it in the power supply filter circuit, large capacitance, low frequency capacitance, high frequency fundamental reason lies in the SFR (
Values are different.
So in actual design, we often have a question, how do I know how much is the SFR capacitance?
Even if I know the value of SFR, how can I choose different SFR value of capacitance value?
SFR value of the capacitance and the capacitance, inductance and capacitance of the pin, so the same values of 0402060 3, or into the capacitive SFR value is not the same, of course, access to the SFR value has two, 1)
Device Data sheet, such as 22 pf0402 SFR values of capacitance around 2 g, 2)
Know the capacitance value of SFR, using software simulation, such as RFsim99, choose one or two circuit is whether you have the working frequency band of the power supply circuit has enough noise rejection ratio.
滤波电容性选举取与计算网上两种工程常用的计算方法:一,当要求不是一些简单的故事,可爱以根据负载计算,每ma, 2 uf。
二,下图rc时间常数近似等归于估算电源半周期3 ~ 5倍。
奖金出现一个例子:负载情况:直流1 a, 12 v。
桥式整流:rc = 3 (
C = 3 (
x / R = 3 (
/ 12 = 2500 (
Desirable in 2200 mu F, because there is no 2500 mu F this specification.
If want to ripple, according to the five times.
反反复复,t是电源的周期,50 hz时,t =。
Full-wave rectification results, but the half-wave rectifier, double time constant.
It also reflects the filter capacitance calculation relationship.
Filter capacitance is bigger, the better the effect of filter in theory, the output voltage is more smooth, but at the instant of the circuit switched on, the impact of current in the circuit factors cannot be ignored, this is because, almost all of the electronic components has its by the maximum current value, so, when choosing electronic components, must consider the impact brought about by the current maximum of instantaneous electric current flows through the related components, the greater the impact current, the higher requirement for electronic components, circuit costs will increase.
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