Filter wave capacitance is having very main effect in switch power source, how to choose filter wave capacitance correctly, the choice that outputs filter wave capacitance especially is the problem that personnel of every project technology cares very. We can see various capacitance on circuit of wave of power source filter, 100uF, 10uF, 100nF, the look with different 10nF is worth, so how do these parameter decide?

 

The general set up an electric circuit that 50Hz uses in labour frequency circuit solves capacitor, its are pulsatile voltage frequency is 100Hz only, filling discharge time is millisecond amount class. To obtain smaller pulsatile coefficient, needs capacitance is as high as tens of 10 thousand μF, because aluminium of this common low frequency electroanalysises,the target of capacitor is give priority to in order to raise capacitance, value of tangent of horn of the capacitance of capacitor, loss and leakage current are the main parameter that differentiates its actor bad. And the output filter wave in switch power source electroanalysises capacitor, its are toothed wave voltage frequency is as high as tens of KHz, it is tens of MHz even, at this moment capacitance is not his main index, measure high frequency aluminium to electroanalysis the standard of capacitance actor bad is “ impedance – frequency ” is characteristic, the requirement should have inferior equivalent impedance inside the working frequency of switch power source, the signal of high frequency aiguille of the generation when working to semiconductor device at the same time has good filter wave action.

 

Common low frequency electroanalysises capacitor begins to present sensibility in 10kHz left and right sides, cannot satisfy the use requirement of switch power source. And appropriative of switch power source is high frequency aluminium electroanalysises capacitor has 4 terminal, positive electrode is aluminous piece two end part derivative the anode that serves as capacitor, negative pole is aluminous piece two end also part derivative as negative pole. Electric current is being carried of capacitance from 4 end flow into, through capacitance interior, also flow into from a of capacitance negative end from another electricity that carrying; of flow direction load to be returned from load again, lose end from power source of another flow direction that lose end again.

 

Because 4 upright capacitance have good high frequency character, the pulsatile heft to reduce tension and restrained switch aiguille noise to provide very advantageous measure. High frequency aluminium electroanalysises the form that capacitor still has much core, be about to aluminum foil is divided into shorter a certain number of paragraph, with much more derivative a shunt-wound join in order to reduce the impedance component in capacitive reactance. And use the data that low resistance leads serve as derivative terminal, raised capacitor to bear the ability of big electric current.

 

Digital circuit should run stability reliable, power source must ” clean “ , and energy complements must seasonable, namely filter wave goes Ou is close friends certainly. What is filter wave goes Ou, when saying simply not to need electricity in chip namely, store energy, when you need electricity I can seasonable complementary capabilities. Do not say with me this duty is not of DCDC, LDO, yes, in low frequency when they can be done calm, but high-speed number system is different.

 

Will see capacitance first, the action of capacitance says simply is memory charge. We know to want to add capacitance filter wave in power source, the capacitance that places a 0.1uF in the power source foot of every chip goes Ou. Etc, how I see the capacitance on the side of the power source base that wants some of bat chip is 0.1uF or of 0.01uF, what is paid attention to. Should do know this real character that way way is about to understand electric capacity. Ideal capacitance the memory that it is a charge only, namely C. And making the capacitor that come out actually is not so simple however, following plan institute show our commonly used when analysing power source integrality capacitance model.

 

 

The ESR in the graph is the resistor of series connection equivalent of capacitance, ESL is the inductance of series connection equivalent of capacitance, c just is real ideal electric capacity. ESR and ESL are decided by the workmanship of capacitance and material, do not have a law to eliminate. These two things have what effect to circuit. ESR affects the ripple of power source, ESL affects the character of filter wave frequency of capacitance.

 

We know the capacitive reactance Zc=1/ωC of capacitance, the inductance Zl=ωL of inductance, (ω=2πf) , of practical electric capacity answer impedance is Z=ESR+jωL-1/jωC= ESR+j2πf L-1/j2πf C. Should be capacitance acts well when frequency is very low it is thus clear that, and the action of inductance cannot be ignored when it is certain that frequency arrives high, again tall when inductance has dominant effect. Capacitance loses the action of filter wave. Remember so, high frequency when capacitance is not pure capacitance. Following plan institute show the filter wave curve of practical electric capacity.

 

   

The inductance of equivalent series connection that capacitance said above is the workmanship of capacitance and material decision, the actual ESL that sticks a capacitance of pottery and porcelain arrives from zero NH a few NH, enclose smaller ESL smaller.

 

From above we still see on the filter wave curve of capacitance is not evenness, it resembles a ’V’ , have the character that choose frequency that is to say, it is better that in moment we hope it is smoother (before the wave of board class filter of class) , and it is better to hope it exceeds a top more occasionally (filter wave or defect wave) . Those who affect this character is the quality factor Q of capacitance, q=1/ωCESR, ESR is bigger, q is smaller, the curve is flatter, conversely ESR is smaller, q is bigger, the curve is more pointed. Normally tantalum capacitance and aluminium electroanalysis have smaller ESL, and ESR is big, so tantalum capacitance and aluminium electroanalysis have very wide effective frequency range, the wave of board class filter of the class before suiting very much. Be in namely DCDC or the input class of LDO often is used relatively the tantalum capacitance of high capacity comes filter wave. And the electric capacity that puts a few 10uF and 0.1uF in the place that stands by chip will go Ou, capacitance of pottery and porcelain has very low ESR.

 

Said so much, that after all we place 0.1uF or 0.01uF in the pin point that approachs chip, the row comes out to consult to everybody below.

 

 

So, do not see whats put the electric capacity of 0.1uF later, the capacitance of these 0.1uF in system of some high speed cannot have effect at all.