陶瓷電容器濾除DC/DC變換器的輸入

　　最近的趨勢(shì)在便攜式設(shè)備的設(shè)計(jì)已經(jīng)用陶瓷電容器來(lái)濾除DC/DC變換器的輸入。陶瓷電容器往往選擇，因?yàn)樗麄冃〕叽?，低等效串?lián)電阻（ESR）和高有效電流容量。此外，最近，設(shè)計(jì)師有一直在尋找陶瓷電容器由于短缺鉭電容器。

　　不幸的是，使用陶瓷電容器輸入濾波會(huì)引起問(wèn)題。向陶瓷施加電壓臺(tái)階電容導(dǎo)致大電流浪涌，存儲(chǔ)能量在電感的電源引線。一個(gè)大的電壓當(dāng)存儲(chǔ)的能量被轉(zhuǎn)移時(shí)產(chǎn)生尖峰從這些電感到陶瓷電容器。這些電壓尖峰可以很容易的兩倍的振幅輸入電壓階躍。

　　插入墻壁適配器在您自己的風(fēng)險(xiǎn)輸入電壓瞬態(tài)問(wèn)題的能力有關(guān)序列。如果墻壁適配器插入交流插座和供電第一，堵墻適配器輸出到便攜式設(shè)備可能會(huì)導(dǎo)致輸入電壓瞬變可能會(huì)損壞內(nèi)部器件DC /直流轉(zhuǎn)換器。

　　Building the Test Circuit To illustrate the problem， a typical 24V wall adapter used in notebook computer applications was connected to the input of a typical notebook computer DC/DC converter. The DC/DC converter used was a synchronous buck converter that generates 3.3V from a 24V input.

　　The block diagram of the test setup is shown in Figure 1. The inductor LOUT represents the lumped equivalent inductance of the lead inductance and the output EMI filter inductor found in some wall adapters. The output capacitor in the wall adapter is usually on the order of 1000μF; for our purposes， we can assume that it has low ESR—in the 10mΩ to 30mΩ range. The equivalent circuit of the wall adapter and DC/DC converter interface is actually a series resonant tank， with the dominant components being LOUT， CIN and the lumped ESR （the lumped ESR must include the ESR of CIN， the lead resistance and the resistance of LOUT）。

　　The input capacitor， CIN， must be a low ESR device， capable of carrying the input ripple current. In a typical notebook computer application， this capacitor is in the range of 10μF