About parasitism capacitance must be read carefully
[introduction] power source ripple and the size that transient state norms can decide to need capacitor, the parasitism that also can restrict capacitor at the same time comprises a setting. Graph the 1 basic parasitism that shows a capacitor is comprised, its are established ties by equivalent resistor (ESR) with inductance of equivalent series connection (ESL) composition, and be shown with graph reveal 3 kinds of capacitor (capacitor of pottery and porcelain, aluminium electroanalysises character capacitor and aluminous polymer capacitor) the relation between impedance and frequency. Watch 1 showed those who be used at generating these curves to be worth individually. These values are low pressure (1V~2.5V) , moderate strength electric current (5A) the typical cost of synchronous step-down power source.
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Below low frequency, 3 kinds of all capacitor all did not show parasitism weight, because impedance is apparent related to capacitance. But, aluminium electroanalysises capacitor impedance stops to reduce, begin to show resistor character when opposite low frequency. Character of this kind of resistor increases ceaselessly, arrive continuously a certain relatively high frequency till (capacitor occurrence inductance) . Aluminous polymer capacitor is another kind of capacitor that nots agree with with ideal situation. Interesting is, it has low ESR, and ESL is apparent. Capacitor of pottery and porcelain also has low ESR, but because its crust size is smaller, its ESL is less than aluminous polymer and aluminium to electroanalysis capacitor.

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Graph 2 show run the power source of simulative of successive and synchronous adjuster below 500kHz to output capacitor weaveform. Its use graph shows the main impedance of 3 kinds of capacitor 1 times: Capacitance of pottery and porcelain; ESL of aluminous ESR; aluminous polymer.
Red line electroanalysises for aluminium capacitor, its by ESR dominant. Accordingly, ripple voltage and inductance ripple electric current are direct and relevant. Blue line represents the ripple voltage of capacitor of pottery and porcelain, its have small ESL and ESR. The ripple voltage of this kind of circumstance is the component that exports inductance ripple electricity. Because ripple electric current is linear, accordingly this causes a series of time square shares, and the appearance looks be like sinusoid.
Finally, green line represents ripple voltage, its capacitor impedance by its ESL dominant, for example: Aluminous polymer capacitor. Below this kind of circumstance, output filter inductance and ESL form a voltage divider. Of these weaveform relatively phasic what predict with us is same. When ESL dominant, ripple voltage conducts electricity of output filter inductance. When ESR dominant, ripple and electric current are inphase, and when capacitance dominant, its defer. Below feasibility condition, output ripple voltage is not include only in coming from these component one of voltage. Contrary, it is the sum of all 3 component voltage. Accordingly, can see in ripple voltage weaveform its are certain part.

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Graph 3 showed a deepness is turned over continuously stimulate or the weaveform of step-down adjuster, its output capacitor electric current to be able to be lost to be being mixed, and specific condition can change quickly ceaselessly. Red line is clear that the watch understands this kind of situation, its voltage is multiplied by this kind of electric current reach with ESR, the result is a kind of square wave. The voltage of capacitor component is the component of square wave. It is brought about linear charges and discharge, if what blue triangle weaveform shows. Finally, change during transfer when electric current only when, the voltage of capacitor ESL ability is apparent. This kind of voltage will be very tall, depend on output electric current rises time. Ask an attention, below this kind of circumstance, green line needs to divide with 10 (suppose 25 NS electric current transfers) . Aiguille is in these big inductance to be stimulated instead namely or one of numerous reasons of dual class filter often appear in step-down power source.

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Anyhow, the impedance that outputs capacitor conduces to improve ripple and transient state performance. Lift as power source frequency, the impact of parasitism problem is bigger, should not ignore more. Around 20kHz, aluminium electroanalysises the ESR of capacitor allows impedance to enough dominant report greatly. When 100kHz, capacitance of a few aluminous polymer shows inductance. When power source enters frequency of million hertz switch, notice the ESL of 3 kinds of all capacitor please.