通用串行總線(USB)端口是一種帶有電源和地的雙向數據端口。USB可以連接所有類型的外圍設備，包括外部驅動器、存儲設備、鍵盤、鼠標、無線接口、攝像機和照相機、MP3播放器以及數不盡的各種電子設備。這些設備有許多采用電池供電，其中一些帶有內置電池。對于電池充電設計來說，應用廣泛的USB既帶來了機遇，也帶來了挑戰。本文闡述了如何將一個簡單的電池充電器與(yu)USB電(dian)(dian)(dian)(dian)(dian)源進行接(jie)口(kou)。文章(zhang)回顧了(le)(le)USB電(dian)(dian)(dian)(dian)(dian)源總線的特性，包括(kuo)電(dian)(dian)(dian)(dian)(dian)壓、電(dian)(dian)(dian)(dian)(dian)流限制、浪(lang)涌(yong)電(dian)(dian)(dian)(dian)(dian)流、連接(jie)器以及(ji)電(dian)(dian)(dian)(dian)(dian)纜連接(jie)問(wen)題。同時(shi)介(jie)紹了(le)(le)鎳氫電(dian)(dian)(dian)(dian)(dian)池(chi)(NiMH)和鋰電(dian)(dian)(dian)(dian)(dian)池(chi)技術(shu)、充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)方法以及(ji)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)終止技術(shu)。給出(chu)了(le)(le)一個完整的示例電(dian)(dian)(dian)(dian)(dian)路，用于實現USB端(duan)口(kou)對NiMH電(dian)(dian)(dian)(dian)(dian)池(chi)智(zhi)能充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)，并給出(chu)了(le)(le)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)數(shu)據。

　　USB特性

　　USB總(zong)線能夠為低功耗電子設備提(ti)供電源。總(zong)線電源與(yu)電網(wang)隔離，并且具有很好的穩定性(xing)。但(dan)是，可用電流(liu)有限(xian)，同時(shi)負載(zai)和主(zhu)機或(huo)電源之間存在(zai)潛(qian)在(zai)的互操作(zuo)問(wen)題。

　　USB端(duan)口由(you)(you)90?雙向差分屏蔽(bi)雙絞線、VBUS (+5V電源)和(he)(he)地組成。這4條線由(you)(you)鋁(lv)箔內屏蔽(bi)層(ceng)和(he)(he)編織網(wang)外屏蔽(bi)層(ceng)進(jin)行屏蔽(bi)。最(zui)新的(de)USB規(gui)范(fan)(fan)標準(zhun)是2.0版，可以從USB組織免(mian)費(fei)獲得。要做到完全符合該規(gui)范(fan)(fan)標準(zhun)，需要通過(guo)一個(ge)功能(neng)控制器來實現(xian)設(she)備(bei)和(he)(he)主機(ji)間的(de)雙向通信。規(gui)范(fan)(fan)定(ding)義了1個(ge)單位負載為100mA (最(zui)大)。任何設(she)備(bei)允許吸取的(de)最(zui)大電流為5個(ge)單位負載。

　　USB端(duan)口(kou)(kou)可(ke)分(fen)為低功(gong)率端(duan)口(kou)(kou)和大功(gong)率端(duan)口(kou)(kou)兩類，低功(gong)率端(duan)口(kou)(kou)可(ke)提供1個(ge)單位(wei)負載的(de)電流(liu)，大功(gong)率端(duan)口(kou)(kou)可(ke)最多提供5個(ge)單位(wei)負載的(de)電流(liu)。當設(she)備剛連接到USB端(duan)口(kou)(kou)時，枚舉(ju)過(guo)程(cheng)對器(qi)件(jian)(jian)進行識別，并確定其負載要求。在此(ci)過(guo)程(cheng)中，只允許設(she)備從主機(ji)吸取(qu)最多1個(ge)單位(wei)負載的(de)電流(liu)。枚舉(ju)過(guo)程(cheng)完成后，如(ru)果主機(ji)的(de)電源管理軟(ruan)件(jian)(jian)允許，則(ze)大功(gong)率設(she)備可(ke)以吸取(qu)更大的(de)電流(liu)。

　　某(mou)些(xie)(xie)主(zhu)機(ji)系統(tong)(包括(kuo)下游(you)USB集(ji)線(xian)器(qi)(qi))通(tong)過(guo)(guo)保險絲或(huo)者有源(yuan)電(dian)(dian)流檢測器(qi)(qi)提供(gong)限流功能(neng)。如果USB設(she)備(bei)(bei)未經(jing)過(guo)(guo)枚(mei)舉過(guo)(guo)程便從USB端(duan)口吸(xi)取大(da)電(dian)(dian)流(超過(guo)(guo)1個(ge)(ge)(ge)單位負載)，則主(zhu)機(ji)會檢測到(dao)過(guo)(guo)流狀態，并會關閉正在使用的(de)(de)一個(ge)(ge)(ge)或(huo)多(duo)個(ge)(ge)(ge)USB端(duan)口。市場上供(gong)應的(de)(de)許多(duo)USB設(she)備(bei)(bei)，包括(kuo)獨立電(dian)(dian)池充電(dian)(dian)器(qi)(qi)，都(dou)沒有功能(neng)控(kong)制器(qi)(qi)來處理(li)枚(mei)舉過(guo)(guo)程，但吸(xi)取的(de)(de)電(dian)(dian)流卻超過(guo)(guo)了100mA。在這(zhe)(zhe)種不恰(qia)當的(de)(de)條件下，這(zhe)(zhe)些(xie)(xie)設(she)備(bei)(bei)可能(neng)導致(zhi)主(zhu)機(ji)出現問題。例(li)如，如果一個(ge)(ge)(ge)吸(xi)取500mA電(dian)(dian)流的(de)(de)設(she)備(bei)(bei)插入(ru)總(zong)線(xian)供(gong)電(dian)(dian)的(de)(de)USB集(ji)線(xian)器(qi)(qi)，而且未進(jin)行正確的(de)(de)枚(mei)舉過(guo)(guo)程，則可能(neng)導致(zhi)集(ji)線(xian)器(qi)(qi)端(duan)口和主(zhu)機(ji)端(duan)口同時過(guo)(guo)載。

　　主(zhu)機(ji)操作系(xi)統采用高級(ji)電源管理時情況會更加(jia)復雜，特別是筆記本電腦，它總(zong)是希望(wang)端口電流盡可能低(di)。在(zai)某些(xie)節(jie)電模式下，計(ji)算機(ji)會向USB設(she)備發出掛起(qi)命令(ling)，而后(hou)則認為設(she)備進(jin)入(ru)了低(di)功(gong)耗模式。設(she)備中包含一個能與主(zhu)機(ji)進(jin)行通信(xin)的功(gong)能控制器(qi)始終(zhong)是一個比較好的做法，即(ji)使(shi)對(dui)于(yu)低(di)功(gong)耗設(she)備來(lai)說也是如此。

　　USB 2.0規(gui)范非常全面，規(gui)定(ding)了電(dian)(dian)(dian)源(yuan)的(de)(de)(de)質(zhi)量、連接(jie)器(qi)構造、電(dian)(dian)(dian)纜(lan)材質(zhi)、容(rong)許的(de)(de)(de)電(dian)(dian)(dian)壓(ya)跌(die)(die)落以及浪涌電(dian)(dian)(dian)流(liu)(liu)(liu)等。低(di)(di)電(dian)(dian)(dian)流(liu)(liu)(liu)和(he)(he)(he)大(da)電(dian)(dian)(dian)流(liu)(liu)(liu)端口具(ju)有不同的(de)(de)(de)電(dian)(dian)(dian)源(yuan)指標(biao)。這主要是由(you)主機和(he)(he)(he)負(fu)載間的(de)(de)(de)連接(jie)器(qi)和(he)(he)(he)電(dian)(dian)(dian)纜(lan)上的(de)(de)(de)電(dian)(dian)(dian)壓(ya)跌(die)(die)落決定(ding)的(de)(de)(de)，并包括(kuo)由(you)USB供電(dian)(dian)(dian)的(de)(de)(de)集線(xian)(xian)器(qi)上產生的(de)(de)(de)電(dian)(dian)(dian)壓(ya)跌(die)(die)落。包括(kuo)計算機或者自供電(dian)(dian)(dian)USB集線(xian)(xian)器(qi)在內(nei)的(de)(de)(de)主機，都具(ju)有大(da)電(dian)(dian)(dian)流(liu)(liu)(liu)端口，可提供最大(da)500mA的(de)(de)(de)電(dian)(dian)(dian)流(liu)(liu)(liu)。無源(yuan)、總線(xian)(xian)供電(dian)(dian)(dian)的(de)(de)(de)USB集線(xian)(xian)器(qi)具(ju)有低(di)(di)電(dian)(dian)(dian)流(liu)(liu)(liu)端口。表1列出了USB大(da)電(dian)(dian)(dian)流(liu)(liu)(liu)和(he)(he)(he)低(di)(di)電(dian)(dian)(dian)流(liu)(liu)(liu)端口上游端(電(dian)(dian)(dian)源(yuan))引腳允許的(de)(de)(de)電(dian)(dian)(dian)壓(ya)容(rong)限。

　　表1. USB 2.0規(gui)范電(dian)源質量標(biao)準

Parameter Requirement DC voltage, high-power port* 4.75V to 5.25V DC voltage, low-power port* 4.40V to 5.25V Maximum quiescent current (low power, suspend mode) 500μA Maximum quiescent current (high power, suspend mode) 2500μA Maximum allowable Input capacitance (load side) 10μF Minimum required output capacitance (host side) 120μF ±20% Maximum allowable inrush charge Into load 50μC

       *這些指(zhi)標適用于上游端(duan)主機或集線器端(duan)口的連(lian)接器引腳。電纜和連(lian)接器上的I x R跌落需另外考慮(lv)。

       在符合USB 2.0規范的(de)(de)主機中，大(da)功率端口的(de)(de)上游(you)端具有(you)120μF、低ESR電(dian)容(rong)。所(suo)連接(jie)的(de)(de)USB設備的(de)(de)輸入電(dian)容(rong)限制(zhi)(zhi)在10μF以(yi)內，在最初的(de)(de)負載連接(jie)階(jie)段，允許負載從主機(或自供(gong)電(dian)集線(xian)器)吸取的(de)(de)最大(da)電(dian)荷(he)數為(wei)50μC。這樣一來，當新設備連接(jie)至(zhi)USB端口時(shi)，上游(you)端口的(de)(de)瞬態電(dian)壓跌落小于0.5V。如果負載正常工(gong)作時(shi)需要更大(da)的(de)(de)輸入電(dian)容(rong)，則必須提(ti)供(gong)浪(lang)涌(yong)電(dian)流限制(zhi)(zhi)器，以(yi)保(bao)證對更大(da)的(de)(de)電(dian)容(rong)充電(dian)時(shi)電(dian)流不會超過100mA。

　　當USB端口帶有一個總線(xian)(xian)(xian)供(gong)電(dian)(dian)的(de)(de)USB集(ji)(ji)線(xian)(xian)(xian)器，集(ji)(ji)線(xian)(xian)(xian)器上接(jie)了(le)低(di)功耗設備時，USB口上允許的(de)(de)直流電(dian)(dian)壓(ya)跌落如圖(tu)1所示(shi)。大功率(lv)負載與總線(xian)(xian)(xian)供(gong)電(dian)(dian)的(de)(de)集(ji)(ji)線(xian)(xian)(xian)器連(lian)接(jie)時，電(dian)(dian)壓(ya)跌落將超過圖(tu)1給出(chu)的(de)(de)指標，并會引起(qi)總線(xian)(xian)(xian)過載。

　　
圖1. 主機至低功率(lv)負載(zai)的(de)電(dian)壓跌落大于圖中給出的(de)允許直流電(dian)壓跌落時，會引(yin)起總線過載(zai)

　　電池充電要求

　　單節鋰離子和(he)鋰聚合物(wu)電池(chi)

　　如今的(de)鋰電(dian)(dian)池(chi)充電(dian)(dian)至最大額定容(rong)(rong)量后，其(qi)電(dian)(dian)壓通常為(wei)4.1V至4.2V之間。當前市場上正在出售的(de)、更(geng)新的(de)、容(rong)(rong)量更(geng)大的(de)電(dian)(dian)池(chi)，其(qi)電(dian)(dian)壓范圍在4.3V至4.4V之間。典型的(de)棱柱形鋰離子(zi)(Li+)和鋰聚(ju)合物(Li-Poly)電(dian)(dian)池(chi)容(rong)(rong)量為(wei)600mAh至1400mAh。

　　對(dui)Li+和Li-Poly電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)來說(shuo)，首選(xuan)的(de)(de)(de)(de)(de)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)曲線是從恒(heng)流充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)開(kai)始，一直持續到(dao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓達(da)到(dao)額定電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓。然后，充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)對(dui)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)兩端的(de)(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓進(jin)(jin)行(xing)調(diao)節。這兩種(zhong)調(diao)節方式(shi)構成(cheng)了(le)恒(heng)流(CC)恒(heng)壓(CV)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)方式(shi)。因此(ci)，這種(zhong)類型的(de)(de)(de)(de)(de)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)通(tong)常(chang)稱為CCCV充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)。CCCV充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)進(jin)(jin)入CV模(mo)式(shi)后，電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)的(de)(de)(de)(de)(de)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流開(kai)始下降。若采用0.5C至(zhi)(zhi)1.5C的(de)(de)(de)(de)(de)典(dian)型充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)速率充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)，則當(dang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)達(da)到(dao)其充(chong)(chong)(chong)(chong)(chong)(chong)滿容(rong)量的(de)(de)(de)(de)(de)80%至(zhi)(zhi)90%時(shi)，充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)由CC模(mo)式(shi)轉(zhuan)換為CV模(mo)式(shi)。充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)一旦進(jin)(jin)入CV充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)模(mo)式(shi)，則對(dui)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流進(jin)(jin)行(xing)監(jian)視；當(dang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流達(da)到(dao)最低門限(幾毫安(an)或者幾十(shi)毫安(an))時(shi)，充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)終止(zhi)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)。鋰(li)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)的(de)(de)(de)(de)(de)典(dian)型充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)曲線如圖2所示。

　　
圖2. 使用CCCV充電器對Li+電池充電時的(de)典型曲線

　　從圖1所示的(de)USB電(dian)壓(ya)(ya)跌落(luo)指標可以看(kan)出，端口供(gong)電(dian)集線器的(de)下游低功率端口電(dian)壓(ya)(ya)不具備足(zu)夠的(de)余量，很難將電(dian)池(chi)充(chong)至4.2V。充(chong)電(dian)通路上存在的(de)小量額(e)外電(dian)阻(zu)會(hui)妨礙正常充(chong)電(dian)。

　　Li+和Li-Poly電(dian)(dian)池(chi)(chi)應在合適的(de)溫(wen)度(du)下進行充電(dian)(dian)。制造商(shang)推薦的(de)最(zui)(zui)高充電(dian)(dian)溫(wen)度(du)通(tong)常為+45°C至+55°C之間，允許的(de)最(zui)(zui)大(da)(da)放電(dian)(dian)溫(wen)度(du)可再高出(chu)10°C左右。這些(xie)電(dian)(dian)池(chi)(chi)使用的(de)材料，化(hua)學性質非常活潑，如果(guo)電(dian)(dian)池(chi)(chi)溫(wen)度(du)超過+70°C，會(hui)發生(sheng)燃燒(shao)。鋰電(dian)(dian)池(chi)(chi)充電(dian)(dian)器應具備(bei)熱關斷(duan)電(dian)(dian)路，該(gai)電(dian)(dian)路監視電(dian)(dian)池(chi)(chi)溫(wen)度(du)，如果(guo)電(dian)(dian)池(chi)(chi)溫(wen)度(du)超過制造商(shang)推薦的(de)最(zui)(zui)大(da)(da)充電(dian)(dian)溫(wen)度(du)時(shi)，則終止充電(dian)(dian)。

　　鎳氫電池（NiMH）

　　NiMH電池(chi)比鋰(li)電池(chi)要重(zhong)一些(xie)，其能(neng)量密度也比鋰(li)電池(chi)低(di)。一直以來(lai)，NiMH電池(chi)比鋰(li)電池(chi)要便(bian)宜，但是(shi)最(zui)近二者的價格差在(zai)(zai)縮(suo)小(xiao)。NiMH電池(chi)具有標準(zhun)尺寸，在(zai)(zai)大多數應用中(zhong)可直接替換堿(jian)性電池(chi)。每節電池(chi)的標稱電壓(ya)為1.2V，充滿后會達到1.5V。

　　通常(chang)采用(yong)恒流(liu)源對NiMH電(dian)池充(chong)(chong)(chong)電(dian)。當達(da)到(dao)充(chong)(chong)(chong)滿狀態時(shi)(shi)，會發生放熱化學反應，并導致電(dian)池溫度上升(sheng)，電(dian)池端電(dian)壓降(jiang)低(di)。可檢測(ce)(ce)電(dian)池溫度上升(sheng)速(su)率或者(zhe)負向電(dian)壓變化率，并用(yong)來終止充(chong)(chong)(chong)電(dian)。這些(xie)充(chong)(chong)(chong)電(dian)終止方(fang)法(fa)分別稱為dT/dt和(he)-ΔV。充(chong)(chong)(chong)電(dian)速(su)率非常(chang)低(di)時(shi)(shi)，dT/dt和(he)-ΔV不太明顯(xian)，很難精確檢測(ce)(ce)到(dao)。電(dian)池開始進入(ru)過充(chong)(chong)(chong)狀態時(shi)(shi)，dT/dt和(he)-ΔV響應開始顯(xian)現。此時(shi)(shi)如果繼續充(chong)(chong)(chong)電(dian)，將損壞電(dian)池。

　　終(zhong)止檢(jian)測在充(chong)(chong)(chong)電(dian)速(su)(su)率大于C/3時要比低充(chong)(chong)(chong)電(dian)速(su)(su)率時容(rong)易得(de)(de)多。溫度上(shang)升速(su)(su)率大約為(wei)1°C/分(fen)鐘，-ΔV響(xiang)應也(ye)比低充(chong)(chong)(chong)電(dian)速(su)(su)率時更(geng)(geng)明顯。快充(chong)(chong)(chong)結束(shu)后，建(jian)議(yi)以更(geng)(geng)小的電(dian)流(liu)再(zai)充(chong)(chong)(chong)一(yi)(yi)段時間，以徹底充(chong)(chong)(chong)足電(dian)池(補(bu)足充(chong)(chong)(chong)電(dian))。補(bu)足充(chong)(chong)(chong)電(dian)階段結束(shu)后，采(cai)用C/20或者C/30的涓充(chong)(chong)(chong)電(dian)流(liu)來(lai)補(bu)償自(zi)放(fang)電(dian)效應，使電(dian)池維持在充(chong)(chong)(chong)滿狀態。圖3所示(shi)為(wei)采(cai)用DS2712 NiMH充(chong)(chong)(chong)電(dian)器對NiMH電(dian)池(事先已充(chong)(chong)(chong)了一(yi)(yi)部分(fen)電(dian))進行充(chong)(chong)(chong)電(dian)的電(dian)池電(dian)壓(ya)曲(qu)線(xian)。在該圖中，上(shang)面一(yi)(yi)條曲(qu)線(xian)的數據在充(chong)(chong)(chong)電(dian)電(dian)流(liu)正在灌(guan)入電(dian)池時獲得(de)(de)，下面那(nei)條曲(qu)線(xian)的數據在切斷電(dian)流(liu)時測得(de)(de)。在DS2712中，該電(dian)壓(ya)差(cha)被用來(lai)區(qu)分(fen)NiMH電(dian)池和堿性(xing)電(dian)池。如果檢(jian)測到堿性(xing)電(dian)池，則(ze)DS2712不會對它進行充(chong)(chong)(chong)電(dian)。

　　
圖3. 采用DS2712充電(dian)控制器對NiMH電(dian)池充電(dian)

　　開關與線性

　　USB 2.0規范允許低功(gong)率(lv)端口提供(gong)最(zui)大(da)100mA電流，大(da)功(gong)率(lv)端口提供(gong)最(zui)大(da)500mA電流。如果采用線(xian)性(xing)調整器件來調節電池充電電流，這(zhe)也就是最(zui)大(da)可(ke)提供(gong)的充電電流。線(xian)性(xing)調整器件(圖4)的功(gong)耗為(wei)P = VQ x IBATT。這(zhe)會造成調整管發熱，可(ke)能需要安裝散熱器，以防止過熱。

　　
圖4. 功耗等于電池充(chong)電電流乘以調整管兩(liang)端(duan)的(de)電壓

　　對應(ying)5V標稱輸入電(dian)(dian)壓，調整器件消耗的(de)功(gong)率與電(dian)(dian)池(chi)類型、數量和電(dian)(dian)池(chi)電(dian)(dian)壓有關。

　　
圖(tu)5. 采用5.0V電壓的USB端口對(dui)NiMH電池充(chong)電時，線性調整器件的功耗

     標稱輸入電(dian)壓為5.0V時(shi)(shi)，線性USB充(chong)(chong)電(dian)器(qi)對(dui)NiMH電(dian)池(chi)(chi)充(chong)(chong)電(dian)的(de)功(gong)耗(hao)計算結(jie)果如圖5所示(shi)。對(dui)單節電(dian)池(chi)(chi)充(chong)(chong)電(dian)時(shi)(shi)，線性充(chong)(chong)電(dian)器(qi)的(de)效率僅(jin)為30%；對(dui)兩(liang)節電(dian)池(chi)(chi)充(chong)(chong)電(dian)時(shi)(shi)，效率為60%。用500mA電(dian)流(liu)對(dui)單節電(dian)池(chi)(chi)充(chong)(chong)電(dian)時(shi)(shi)，功(gong)耗(hao)會(hui)高達2W。這樣的(de)功(gong)耗(hao)通常需(xu)要(yao)加散熱(re)器(qi)。功(gong)耗(hao)為2W時(shi)(shi)，熱(re)阻為+20°C/W的(de)散熱(re)器(qi)在+25°C環境溫度下會(hui)被加熱(re)至(zhi)大約(yue)+65°C，要(yao)得到滿額性能，還需(xu)要(yao)有流(liu)動空(kong)氣來協助其散熱(re)。處于空(kong)氣靜(jing)止的(de)封(feng)閉空(kong)間(jian)內，溫度會(hui)更(geng)高。

　　采用基于開關調節(jie)器(qi)的(de)(de)充(chong)電(dian)(dian)器(qi)可解決多個問題。首先，與線(xian)性充(chong)電(dian)(dian)器(qi)相(xiang)比，能夠以(yi)更(geng)快的(de)(de)速率、更(geng)大(da)的(de)(de)電(dian)(dian)流對電(dian)(dian)池進行(xing)充(chong)電(dian)(dian)(圖6)。由于功耗(hao)較低(di)、發熱(re)(re)較少(shao)，熱(re)(re)管理方(fang)面的(de)(de)問題也(ye)減少(shao)了。同時，由于運行(xing)溫(wen)度(du)降低(di)，充(chong)電(dian)(dian)器(qi)更(geng)加可靠。

　　
圖6. 對(dui)單節NiMH電(dian)(dian)(dian)(dian)池充(chong)(chong)電(dian)(dian)(dian)(dian)時(shi)(shi)，線性充(chong)(chong)電(dian)(dian)(dian)(dian)器和開(kai)關充(chong)(chong)電(dian)(dian)(dian)(dian)器的充(chong)(chong)電(dian)(dian)(dian)(dian)時(shi)(shi)間不同

　　圖6中的計算結果基于以下(xia)條件和假設得到：采(cai)用(yong)高功(gong)率USB口最大允(yun)許(xu)電(dian)流(500mA)的大約90%充電(dian)；開關調(diao)節器(qi)采(cai)用(yong)非同步整流的buck轉換器(qi)，具有77%效率。

　　電路實例

　　圖7所(suo)示電(dian)(dian)路是用于單節(jie)NiMH電(dian)(dian)池充電(dian)(dian)的開關模式(shi)降壓型調節(jie)器(qi)。它采用DS2712充電(dian)(dian)控(kong)制(zhi)器(qi)調節(jie)充電(dian)(dian)電(dian)(dian)流和(he)終止充電(dian)(dian)。充電(dian)(dian)控(kong)制(zhi)器(qi)監視溫度、電(dian)(dian)池電(dian)(dian)壓和(he)電(dian)(dian)池電(dian)(dian)流。如果溫度超過+45°C或者低于0°C，控(kong)制(zhi)器(qi)不(bu)會對電(dian)(dian)池充電(dian)(dian)。

　　
圖7. USB端口對單節NiMH電池快(kuai)速(su)充電的原理圖

　　如(ru)圖7所示，Q1是降壓(ya)(ya)(ya)型(xing)充電(dian)器的開關功率晶體管(guan)；L1是濾波電(dian)感；D1是續流(liu)(liu)(liu)或整(zheng)流(liu)(liu)(liu)二(er)極管(guan)。輸入電(dian)容(rong)C1為(wei)10μF、超(chao)低(di)(di)ESR的陶瓷(ci)濾波電(dian)容(rong)。用鉭電(dian)容(rong)或者其它電(dian)解(jie)電(dian)容(rong)替(ti)代C1會使充電(dian)器的性能(neng)降低(di)(di)。R7是電(dian)流(liu)(liu)(liu)調節器檢(jian)測放大器的檢(jian)流(liu)(liu)(liu)電(dian)阻。DS2712的基(ji)準電(dian)壓(ya)(ya)(ya)為(wei)0.125V，并具有(you)24mV滯回(hui)。通過(guo)CSOUT提供閉環、開關模式電(dian)流(liu)(liu)(liu)控制。充電(dian)控制引腳CC1將(jiang)Q2的柵(zha)(zha)極拉低(di)(di)時，使能(neng)Q1的柵(zha)(zha)極驅動(dong)。Q1和(he)Q2均(jun)為(wei)低(di)(di)Vt (柵(zha)(zha)-源(yuan)門限(xian)電(dian)壓(ya)(ya)(ya))的pMOSFET。CC1和(he)CSOUT均(jun)為(wei)低(di)(di)電(dian)平(ping)時，Q2的漏(lou)-源(yuan)電(dian)壓(ya)(ya)(ya)將(jiang)稍大于Vt。該電(dian)壓(ya)(ya)(ya)以及CSOUT的正向壓(ya)(ya)(ya)降構成了Q1的柵(zha)(zha)極開關電(dian)壓(ya)(ya)(ya)。

　　CC1為(wei)低電(dian)(dian)(dian)(dian)(dian)(dian)平時(shi)(shi)(shi)，啟(qi)動電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)閉環控制(zhi)。圖(tu)8所示為(wei)啟(qi)動開(kai)(kai)(kai)關(guan)時(shi)(shi)(shi)的波(bo)形(xing)。上(shang)方波(bo)形(xing)是(shi)0.125? (檢流(liu)(liu)電(dian)(dian)(dian)(dian)(dian)(dian)阻(zu)兩端(duan)的電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)，下方波(bo)形(xing)是(shi)Q1漏極至GND的電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)。開(kai)(kai)(kai)始時(shi)(shi)(shi)，當Q1打開(kai)(kai)(kai)(CC1和(he)CSOUT均為(wei)低電(dian)(dian)(dian)(dian)(dian)(dian)平)時(shi)(shi)(shi)，電(dian)(dian)(dian)(dian)(dian)(dian)感電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)向上(shang)爬(pa)升。當電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)增大到使檢流(liu)(liu)電(dian)(dian)(dian)(dian)(dian)(dian)阻(zu)兩端(duan)的電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)達到0.125V時(shi)(shi)(shi)，CSOUT變(bian)(bian)為(wei)高電(dian)(dian)(dian)(dian)(dian)(dian)平，開(kai)(kai)(kai)關(guan)關(guan)斷(duan)。此后，電(dian)(dian)(dian)(dian)(dian)(dian)感電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)開(kai)(kai)(kai)始下降，直(zhi)到檢流(liu)(liu)電(dian)(dian)(dian)(dian)(dian)(dian)阻(zu)兩端(duan)的電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)達到約0.1V，CSOUT又變(bian)(bian)為(wei)低電(dian)(dian)(dian)(dian)(dian)(dian)平。只要CC1為(wei)低電(dian)(dian)(dian)(dian)(dian)(dian)平，該(gai)過程將一直(zhi)持續。

　　
圖(tu)8. USB NiMH充(chong)電器(qi)的啟動波形

　　DS2712的(de)內部狀態(tai)(tai)機(ji)(ji)控制(zhi)著CC1的(de)工作。充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)開(kai)始時(shi)(shi)(shi)(shi)，DS2712先對(dui)(dui)電(dian)(dian)(dian)池(chi)(chi)進(jin)行(xing)狀態(tai)(tai)測(ce)試，以(yi)確保電(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)壓在(zai)1.0V至1.65V之間(jian)，并確認溫(wen)度在(zai)0°C至+45°C之間(jian)。如果電(dian)(dian)(dian)壓低于1.0V，DS2712將以(yi)0.125的(de)占(zhan)空比拉(la)低CC1，對(dui)(dui)電(dian)(dian)(dian)池(chi)(chi)緩慢(man)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)，以(yi)防損壞(huai)電(dian)(dian)(dian)池(chi)(chi)。一(yi)旦電(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)壓超過(guo)1.0V后，狀態(tai)(tai)機(ji)(ji)轉(zhuan)為快(kuai)(kuai)充(chong)(chong)(chong)(chong)模式(shi)。快(kuai)(kuai)充(chong)(chong)(chong)(chong)時(shi)(shi)(shi)(shi)占(zhan)空比為31/32，即大約97%。“跳過(guo)”的(de)間(jian)隙內進(jin)行(xing)電(dian)(dian)(dian)池(chi)(chi)阻抗測(ce)試，以(yi)確保不會(hui)對(dui)(dui)錯誤放(fang)入充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器(qi)的(de)高阻抗電(dian)(dian)(dian)池(chi)(chi)(例如堿(jian)性(xing)電(dian)(dian)(dian)池(chi)(chi))進(jin)行(xing)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)。檢(jian)(jian)測(ce)到-2mV的(de)-ΔV后，快(kuai)(kuai)充(chong)(chong)(chong)(chong)結束。如果未(wei)檢(jian)(jian)測(ce)到-ΔV，將持(chi)續(xu)快(kuai)(kuai)充(chong)(chong)(chong)(chong)，直(zhi)到快(kuai)(kuai)充(chong)(chong)(chong)(chong)定(ding)(ding)時(shi)(shi)(shi)(shi)器(qi)超時(shi)(shi)(shi)(shi)，或檢(jian)(jian)測(ce)到過(guo)溫(wen)或者過(guo)壓故障狀態(tai)(tai)(包括(kuo)阻抗不合格(ge))為止。快(kuai)(kuai)充(chong)(chong)(chong)(chong)完成(由于-ΔV或快(kuai)(kuai)充(chong)(chong)(chong)(chong)定(ding)(ding)時(shi)(shi)(shi)(shi)器(qi)超時(shi)(shi)(shi)(shi)) 后，DS2712進(jin)入定(ding)(ding)時(shi)(shi)(shi)(shi)補足(zu)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)模式(shi)，占(zhan)空比為12.5%，持(chi)續(xu)時(shi)(shi)(shi)(shi)間(jian)為所(suo)設快(kuai)(kuai)充(chong)(chong)(chong)(chong)定(ding)(ding)時(shi)(shi)(shi)(shi)的(de)一(yi)半。補足(zu)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)完成后，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器(qi)進(jin)入維(wei)持(chi)模式(shi)，占(zhan)空比為1/64，直(zhi)到電(dian)(dian)(dian)池(chi)(chi)被(bei)拿走或重新上(shang)電(dian)(dian)(dian)。

　　采用圖7所示充電器和(he)大(da)(da)功率USB端(duan)口對2100mAh NiMH電(dian)池充(chong)(chong)電(dian)時(shi)(shi)，快充(chong)(chong)時(shi)(shi)間為2小時(shi)(shi)多一點(dian)，大(da)(da)約3個小時(shi)(shi)完成包括補足充(chong)(chong)電(dian)在內的全部充(chong)(chong)電(dian)過(guo)程(cheng)。從(cong)端(duan)口吸取的電(dian)流(liu)為420mA。如(ru)果(guo)需要(yao)與主機(ji)進行枚舉過(guo)程(cheng)，并需要(yao)大(da)(da)電(dian)流(liu)使(shi)能操作，可(ke)在R9和(he)地之間串(chuan)聯(lian)一個開(kai)漏極nMOSFET。如(ru)果(guo)MOSFET關斷，則(ze)TMR浮(fu)空，DS2712進入掛起狀態。

　　總結

　　對于小型(xing)消(xiao)費(fei)類電(dian)子設(she)備的(de)(de)(de)(de)電(dian)池充電(dian)而言，USB端(duan)口(kou)是一個經(jing)濟(ji)、實用的(de)(de)(de)(de)電(dian)源(yuan)。為(wei)完全符(fu)合(he)USB 2.0規范，連接在(zai)USB端(duan)口(kou)上的(de)(de)(de)(de)負(fu)載必須能(neng)夠與主(zhu)機進行雙向通信(xin)。負(fu)載也必須符(fu)合(he)電(dian)源(yuan)管(guan)(guan)理要求，包括低(di)功耗模式，以及(ji)便于主(zhu)機確(que)定何時(shi)需要從端(duan)口(kou)吸取大電(dian)流的(de)(de)(de)(de)手段。盡管(guan)(guan)部(bu)分(fen)兼容的(de)(de)(de)(de)系統能(neng)夠適應(ying)大部(bu)分(fen)USB主(zhu)機，但有時(shi)會出現意想(xiang)不到的(de)(de)(de)(de)結(jie)果。只有很(hen)好地理解USB規范要求和(he)負(fu)載的(de)(de)(de)(de)期(qi)望，才能(neng)在(zai)對于規范的(de)(de)(de)(de)兼容性與負(fu)載復(fu)雜度之(zhi)間取得較好的(de)(de)(de)(de)平衡。