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USB電(dian)(dian)(dian)源(yuan)進(jin)行接(jie)口。文章回顧了USB電(dian)(dian)(dian)源(yuan)總線的特性，包括電(dian)(dian)(dian)壓、電(dian)(dian)(dian)流(liu)限(xian)制、浪涌電(dian)(dian)(dian)流(liu)、連接(jie)器(qi)以(yi)及(ji)電(dian)(dian)(dian)纜連接(jie)問題(ti)。同時介紹了鎳氫電(dian)(dian)(dian)池(NiMH)和鋰電(dian)(dian)(dian)池技術、充電(dian)(dian)(dian)方法以(yi)及(ji)充電(dian)(dian)(dian)終止(zhi)技術。給出了一個完整(zheng)的示(shi)例電(dian)(dian)(dian)路(lu)，用于實現USB端口對NiMH電(dian)(dian)(dian)池智能(neng)充電(dian)(dian)(dian)，并給出了充電(dian)(dian)(dian)數據(ju)。

　　USB特性

　　USB總(zong)線(xian)能夠(gou)為低功耗電(dian)子(zi)設(she)備提供電(dian)源(yuan)。總(zong)線(xian)電(dian)源(yuan)與電(dian)網(wang)隔離(li)，并且具有很好的穩定(ding)性。但是，可用電(dian)流有限，同時(shi)負載和主機或電(dian)源(yuan)之間存在(zai)(zai)潛在(zai)(zai)的互操作問題。

　　USB端口(kou)由90?雙向差(cha)分屏(ping)蔽(bi)(bi)雙絞(jiao)線(xian)、VBUS (+5V電(dian)源)和(he)(he)地組(zu)(zu)成。這(zhe)4條線(xian)由鋁箔內屏(ping)蔽(bi)(bi)層和(he)(he)編(bian)織網外屏(ping)蔽(bi)(bi)層進行屏(ping)蔽(bi)(bi)。最(zui)新的USB規范標準是(shi)2.0版，可以從USB組(zu)(zu)織免費獲(huo)得。要做(zuo)到完全符合該規范標準，需要通過(guo)一個(ge)功能控制器(qi)來實現(xian)設備和(he)(he)主機(ji)間的雙向通信。規范定義了1個(ge)單(dan)位負(fu)載為100mA (最(zui)大(da))。任何設備允許吸(xi)取(qu)的最(zui)大(da)電(dian)流為5個(ge)單(dan)位負(fu)載。

　　USB端(duan)(duan)口可(ke)分為(wei)低功(gong)率(lv)(lv)端(duan)(duan)口和大(da)功(gong)率(lv)(lv)端(duan)(duan)口兩類，低功(gong)率(lv)(lv)端(duan)(duan)口可(ke)提(ti)供(gong)1個單(dan)位負(fu)載的(de)(de)電(dian)流(liu)，大(da)功(gong)率(lv)(lv)端(duan)(duan)口可(ke)最(zui)多(duo)提(ti)供(gong)5個單(dan)位負(fu)載的(de)(de)電(dian)流(liu)。當設備剛連接到USB端(duan)(duan)口時，枚舉(ju)過程對器(qi)件進行識別，并確定其負(fu)載要求。在(zai)此過程中，只(zhi)允許設備從(cong)主機吸取最(zui)多(duo)1個單(dan)位負(fu)載的(de)(de)電(dian)流(liu)。枚舉(ju)過程完成后(hou)，如果主機的(de)(de)電(dian)源管理軟件允許，則大(da)功(gong)率(lv)(lv)設備可(ke)以(yi)吸取更大(da)的(de)(de)電(dian)流(liu)。

　　某(mou)些(xie)主(zhu)機系統(包括(kuo)下(xia)游(you)USB集(ji)線器(qi))通過(guo)(guo)(guo)保(bao)險絲或者有源(yuan)電(dian)(dian)流檢測器(qi)提供限流功(gong)能(neng)(neng)。如果USB設備(bei)未經(jing)過(guo)(guo)(guo)枚舉過(guo)(guo)(guo)程便從USB端口(kou)吸取大電(dian)(dian)流(超過(guo)(guo)(guo)1個(ge)單位(wei)負載)，則(ze)主(zhu)機會檢測到過(guo)(guo)(guo)流狀態，并會關(guan)閉正在(zai)使用的一個(ge)或多個(ge)USB端口(kou)。市場(chang)上供應的許多USB設備(bei)，包括(kuo)獨立電(dian)(dian)池充電(dian)(dian)器(qi)，都沒有功(gong)能(neng)(neng)控制器(qi)來處理枚舉過(guo)(guo)(guo)程，但吸取的電(dian)(dian)流卻超過(guo)(guo)(guo)了100mA。在(zai)這種不(bu)恰當的條件(jian)下(xia)，這些(xie)設備(bei)可(ke)能(neng)(neng)導致主(zhu)機出(chu)現問題。例如，如果一個(ge)吸取500mA電(dian)(dian)流的設備(bei)插入(ru)總線供電(dian)(dian)的USB集(ji)線器(qi)，而且未進行正確的枚舉過(guo)(guo)(guo)程，則(ze)可(ke)能(neng)(neng)導致集(ji)線器(qi)端口(kou)和主(zhu)機端口(kou)同時過(guo)(guo)(guo)載。

　　主機操作系統采用高(gao)級電源管理時情況會更加復(fu)雜，特別(bie)是(shi)(shi)筆記本(ben)電腦，它總是(shi)(shi)希望端口電流盡(jin)可能(neng)低(di)。在某些節電模式下，計(ji)算機會向USB設備發出掛起命令，而后則認為(wei)設備進(jin)入了(le)低(di)功耗模式。設備中包(bao)含一(yi)個(ge)能(neng)與主機進(jin)行通信的功能(neng)控制器始終(zhong)是(shi)(shi)一(yi)個(ge)比較好的做(zuo)法，即使對于(yu)低(di)功耗設備來說(shuo)也是(shi)(shi)如此。

　　USB 2.0規(gui)范非常(chang)全面，規(gui)定了(le)電(dian)(dian)(dian)(dian)(dian)源(yuan)的質量、連接器(qi)(qi)構造、電(dian)(dian)(dian)(dian)(dian)纜(lan)材(cai)質、容(rong)許的電(dian)(dian)(dian)(dian)(dian)壓跌(die)(die)落(luo)以及浪(lang)涌(yong)電(dian)(dian)(dian)(dian)(dian)流(liu)等(deng)。低電(dian)(dian)(dian)(dian)(dian)流(liu)和(he)大(da)電(dian)(dian)(dian)(dian)(dian)流(liu)端(duan)口具有(you)不(bu)同的電(dian)(dian)(dian)(dian)(dian)源(yuan)指標。這(zhe)主要是由主機(ji)和(he)負載間的連接器(qi)(qi)和(he)電(dian)(dian)(dian)(dian)(dian)纜(lan)上的電(dian)(dian)(dian)(dian)(dian)壓跌(die)(die)落(luo)決定的，并包(bao)括(kuo)由USB供(gong)(gong)電(dian)(dian)(dian)(dian)(dian)的集線(xian)(xian)器(qi)(qi)上產生的電(dian)(dian)(dian)(dian)(dian)壓跌(die)(die)落(luo)。包(bao)括(kuo)計算(suan)機(ji)或者自供(gong)(gong)電(dian)(dian)(dian)(dian)(dian)USB集線(xian)(xian)器(qi)(qi)在內的主機(ji)，都具有(you)大(da)電(dian)(dian)(dian)(dian)(dian)流(liu)端(duan)口，可提(ti)供(gong)(gong)最大(da)500mA的電(dian)(dian)(dian)(dian)(dian)流(liu)。無(wu)源(yuan)、總線(xian)(xian)供(gong)(gong)電(dian)(dian)(dian)(dian)(dian)的USB集線(xian)(xian)器(qi)(qi)具有(you)低電(dian)(dian)(dian)(dian)(dian)流(liu)端(duan)口。表1列(lie)出了(le)USB大(da)電(dian)(dian)(dian)(dian)(dian)流(liu)和(he)低電(dian)(dian)(dian)(dian)(dian)流(liu)端(duan)口上游端(duan)(電(dian)(dian)(dian)(dian)(dian)源(yuan))引腳允許的電(dian)(dian)(dian)(dian)(dian)壓容(rong)限。

　　表1. USB 2.0規范電(dian)源質量標準

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

       *這些指標適用于上(shang)游端主機或(huo)集(ji)線(xian)器(qi)端口的連接(jie)器(qi)引腳。電(dian)纜和連接(jie)器(qi)上(shang)的I x R跌落(luo)需另外考慮。

       在符合USB 2.0規范的主機中，大功率端口(kou)(kou)的上游(you)(you)端具(ju)有120μF、低ESR電(dian)(dian)(dian)容。所連接的USB設(she)備(bei)的輸(shu)入(ru)電(dian)(dian)(dian)容限制在10μF以內，在最初的負(fu)載連接階(jie)段，允許負(fu)載從(cong)主機(或自供電(dian)(dian)(dian)集(ji)線器)吸取的最大電(dian)(dian)(dian)荷數為50μC。這樣一來，當新設(she)備(bei)連接至USB端口(kou)(kou)時，上游(you)(you)端口(kou)(kou)的瞬態電(dian)(dian)(dian)壓跌落小于0.5V。如果負(fu)載正常工作(zuo)時需要更(geng)大的輸(shu)入(ru)電(dian)(dian)(dian)容，則必須提供浪涌電(dian)(dian)(dian)流(liu)限制器，以保(bao)證對更(geng)大的電(dian)(dian)(dian)容充電(dian)(dian)(dian)時電(dian)(dian)(dian)流(liu)不會超過100mA。

　　當(dang)USB端口帶有一(yi)個總(zong)線(xian)(xian)供電的(de)(de)USB集(ji)線(xian)(xian)器(qi)，集(ji)線(xian)(xian)器(qi)上接(jie)了低功耗設備(bei)時(shi)，USB口上允許的(de)(de)直(zhi)流電壓(ya)跌落如圖(tu)1所示。大功率負載(zai)與總(zong)線(xian)(xian)供電的(de)(de)集(ji)線(xian)(xian)器(qi)連接(jie)時(shi)，電壓(ya)跌落將(jiang)超過(guo)圖(tu)1給出(chu)的(de)(de)指標，并會引(yin)起總(zong)線(xian)(xian)過(guo)載(zai)。

　　
圖(tu)1. 主機至低(di)功率負載的(de)電(dian)壓跌落(luo)大于圖(tu)中(zhong)給出的(de)允許直(zhi)流電(dian)壓跌落(luo)時，會引起總線過載

　　電池充電要求

　　單(dan)節(jie)鋰(li)離子和鋰(li)聚(ju)合物電池

　　如今的鋰電(dian)池充(chong)電(dian)至最大(da)額定容量(liang)后，其電(dian)壓(ya)通(tong)常為(wei)4.1V至4.2V之(zhi)間(jian)。當前市場上正在(zai)出(chu)售的、更(geng)新的、容量(liang)更(geng)大(da)的電(dian)池，其電(dian)壓(ya)范圍在(zai)4.3V至4.4V之(zhi)間(jian)。典型的棱柱(zhu)形鋰離子(Li+)和鋰聚(ju)合物(Li-Poly)電(dian)池容量(liang)為(wei)600mAh至1400mAh。

　　對(dui)(dui)Li+和Li-Poly電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)來說(shuo)，首選的(de)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)曲線(xian)是從恒流(liu)(liu)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)開始(shi)，一直持續到(dao)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)達(da)(da)到(dao)額定電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)。然后，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器對(dui)(dui)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)兩端的(de)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)進(jin)行調節。這兩種調節方式構成了恒流(liu)(liu)(CC)恒壓(ya)(ya)(CV)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)方式。因此，這種類型(xing)的(de)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器通常(chang)稱為CCCV充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器。CCCV充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器進(jin)入CV模(mo)式后，電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)的(de)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)開始(shi)下降(jiang)。若(ruo)采用0.5C至1.5C的(de)典型(xing)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)速率(lv)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)，則當電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)達(da)(da)到(dao)其充(chong)(chong)(chong)(chong)滿容(rong)量的(de)80%至90%時(shi)，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器由CC模(mo)式轉換為CV模(mo)式。充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器一旦進(jin)入CV充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)模(mo)式，則對(dui)(dui)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)進(jin)行監視；當電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)達(da)(da)到(dao)最低(di)門限(幾(ji)(ji)毫安(an)或者幾(ji)(ji)十(shi)毫安(an))時(shi)，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器終止充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)。鋰(li)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)的(de)典型(xing)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)曲線(xian)如圖2所示。

　　
圖2. 使用(yong)CCCV充電(dian)(dian)器對Li+電(dian)(dian)池充電(dian)(dian)時(shi)的典型(xing)曲線

　　從圖1所示的USB電(dian)壓跌落(luo)指標可(ke)以看出，端(duan)口(kou)供電(dian)集線(xian)器的下(xia)游(you)低(di)功率端(duan)口(kou)電(dian)壓不(bu)具備足夠的余量，很難將電(dian)池充至(zhi)4.2V。充電(dian)通(tong)路上存在的小量額(e)外電(dian)阻會妨礙(ai)正常充電(dian)。

　　Li+和Li-Poly電(dian)(dian)池(chi)應在合適的(de)溫(wen)(wen)度(du)下進(jin)行充(chong)(chong)電(dian)(dian)。制造商(shang)推薦(jian)的(de)最高充(chong)(chong)電(dian)(dian)溫(wen)(wen)度(du)通常為+45°C至(zhi)+55°C之(zhi)間(jian)，允(yun)許(xu)的(de)最大放(fang)電(dian)(dian)溫(wen)(wen)度(du)可再(zai)高出10°C左右。這些電(dian)(dian)池(chi)使用的(de)材料，化學性(xing)質非常活潑，如(ru)果(guo)電(dian)(dian)池(chi)溫(wen)(wen)度(du)超過+70°C，會發生燃燒。鋰電(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)器應具備(bei)熱關斷電(dian)(dian)路，該電(dian)(dian)路監視電(dian)(dian)池(chi)溫(wen)(wen)度(du)，如(ru)果(guo)電(dian)(dian)池(chi)溫(wen)(wen)度(du)超過制造商(shang)推薦(jian)的(de)最大充(chong)(chong)電(dian)(dian)溫(wen)(wen)度(du)時，則終止充(chong)(chong)電(dian)(dian)。

　　鎳氫電(dian)池（NiMH）

　　NiMH電池(chi)(chi)比鋰電池(chi)(chi)要(yao)重一(yi)些(xie)，其能量密度(du)也比鋰電池(chi)(chi)低。一(yi)直以來，NiMH電池(chi)(chi)比鋰電池(chi)(chi)要(yao)便宜，但是最近二者的價格(ge)差在縮小(xiao)。NiMH電池(chi)(chi)具有(you)標(biao)準尺寸，在大多(duo)數應用中(zhong)可(ke)直接替換堿(jian)性電池(chi)(chi)。每節(jie)電池(chi)(chi)的標(biao)稱電壓(ya)為1.2V，充滿后會達到1.5V。

　　通(tong)常采用(yong)恒流源對(dui)NiMH電(dian)(dian)池(chi)(chi)(chi)充電(dian)(dian)。當達(da)到(dao)(dao)充滿狀態時(shi)，會發生放熱化(hua)學反應，并導致電(dian)(dian)池(chi)(chi)(chi)溫度上升，電(dian)(dian)池(chi)(chi)(chi)端電(dian)(dian)壓降低。可檢(jian)測電(dian)(dian)池(chi)(chi)(chi)溫度上升速率或(huo)者負(fu)向電(dian)(dian)壓變化(hua)率，并用(yong)來終止充電(dian)(dian)。這些充電(dian)(dian)終止方法分(fen)別(bie)稱(cheng)為dT/dt和(he)-ΔV。充電(dian)(dian)速率非常低時(shi)，dT/dt和(he)-ΔV不太明顯，很難精確檢(jian)測到(dao)(dao)。電(dian)(dian)池(chi)(chi)(chi)開始進入過充狀態時(shi)，dT/dt和(he)-ΔV響應開始顯現。此時(shi)如果(guo)繼(ji)續(xu)充電(dian)(dian)，將(jiang)損壞電(dian)(dian)池(chi)(chi)(chi)。

　　終止(zhi)檢測(ce)(ce)在(zai)(zai)(zai)充(chong)(chong)電(dian)(dian)(dian)(dian)速(su)率大(da)于C/3時(shi)要比低(di)充(chong)(chong)電(dian)(dian)(dian)(dian)速(su)率時(shi)容(rong)易得多。溫(wen)度上升速(su)率大(da)約為(wei)1°C/分(fen)鐘，-ΔV響應也(ye)比低(di)充(chong)(chong)電(dian)(dian)(dian)(dian)速(su)率時(shi)更明(ming)顯。快(kuai)充(chong)(chong)結束后(hou)，建議以更小的電(dian)(dian)(dian)(dian)流再充(chong)(chong)一(yi)段時(shi)間，以徹(che)底充(chong)(chong)足(zu)電(dian)(dian)(dian)(dian)池(chi)(chi)(補(bu)足(zu)充(chong)(chong)電(dian)(dian)(dian)(dian))。補(bu)足(zu)充(chong)(chong)電(dian)(dian)(dian)(dian)階(jie)段結束后(hou)，采用C/20或者C/30的涓充(chong)(chong)電(dian)(dian)(dian)(dian)流來補(bu)償自放電(dian)(dian)(dian)(dian)效應，使電(dian)(dian)(dian)(dian)池(chi)(chi)維持(chi)在(zai)(zai)(zai)充(chong)(chong)滿狀(zhuang)態。圖3所示為(wei)采用DS2712 NiMH充(chong)(chong)電(dian)(dian)(dian)(dian)器對NiMH電(dian)(dian)(dian)(dian)池(chi)(chi)(事先已充(chong)(chong)了一(yi)部分(fen)電(dian)(dian)(dian)(dian))進(jin)行(xing)充(chong)(chong)電(dian)(dian)(dian)(dian)的電(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)壓曲線。在(zai)(zai)(zai)該圖中，上面一(yi)條曲線的數據在(zai)(zai)(zai)充(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流正在(zai)(zai)(zai)灌入電(dian)(dian)(dian)(dian)池(chi)(chi)時(shi)獲得，下面那條曲線的數據在(zai)(zai)(zai)切斷電(dian)(dian)(dian)(dian)流時(shi)測(ce)(ce)得。在(zai)(zai)(zai)DS2712中，該電(dian)(dian)(dian)(dian)壓差被用來區分(fen)NiMH電(dian)(dian)(dian)(dian)池(chi)(chi)和堿(jian)性電(dian)(dian)(dian)(dian)池(chi)(chi)。如(ru)果檢測(ce)(ce)到堿(jian)性電(dian)(dian)(dian)(dian)池(chi)(chi)，則DS2712不會對它(ta)進(jin)行(xing)充(chong)(chong)電(dian)(dian)(dian)(dian)。

　　
圖3. 采用(yong)DS2712充電(dian)控制器(qi)對(dui)NiMH電(dian)池充電(dian)

　　開關與線性

　　USB 2.0規范允許低功率(lv)端口提供最(zui)大100mA電流，大功率(lv)端口提供最(zui)大500mA電流。如(ru)果(guo)采用線性調(diao)整器(qi)件來調(diao)節(jie)電池充(chong)電電流，這也就(jiu)是最(zui)大可(ke)提供的充(chong)電電流。線性調(diao)整器(qi)件(圖4)的功耗為(wei)P = VQ x IBATT。這會造(zao)成調(diao)整管發(fa)熱(re)(re)，可(ke)能需(xu)要安裝(zhuang)散熱(re)(re)器(qi)，以防止過熱(re)(re)。

　　
圖(tu)4. 功耗(hao)等(deng)于電池充電電流乘以調(diao)整管兩端的電壓

　　對應5V標(biao)稱輸入電(dian)壓，調整器件(jian)消耗(hao)的功率與電(dian)池(chi)類型(xing)、數量和電(dian)池(chi)電(dian)壓有(you)關(guan)。

　　
圖5. 采用5.0V電壓的(de)USB端口對NiMH電池(chi)充電時，線性(xing)調(diao)整器件的(de)功耗

     標稱輸(shu)入電(dian)(dian)(dian)壓為(wei)(wei)(wei)5.0V時，線性USB充電(dian)(dian)(dian)器對NiMH電(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)的(de)(de)功耗(hao)(hao)計算結(jie)果如圖5所示。對單節(jie)電(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)時，線性充電(dian)(dian)(dian)器的(de)(de)效率僅為(wei)(wei)(wei)30%；對兩節(jie)電(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)時，效率為(wei)(wei)(wei)60%。用500mA電(dian)(dian)(dian)流對單節(jie)電(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)時，功耗(hao)(hao)會高達2W。這樣的(de)(de)功耗(hao)(hao)通常(chang)需(xu)要(yao)加散熱器。功耗(hao)(hao)為(wei)(wei)(wei)2W時，熱阻為(wei)(wei)(wei)+20°C/W的(de)(de)散熱器在+25°C環(huan)境溫(wen)度下會被加熱至大約(yue)+65°C，要(yao)得到滿額(e)性能，還需(xu)要(yao)有流動空(kong)(kong)氣來協助(zhu)其散熱。處于(yu)空(kong)(kong)氣靜止的(de)(de)封(feng)閉空(kong)(kong)間內，溫(wen)度會更高。

　　采用基(ji)于(yu)開關調節器(qi)(qi)的充(chong)(chong)(chong)電(dian)器(qi)(qi)可解決多個問題(ti)(ti)。首先，與線性充(chong)(chong)(chong)電(dian)器(qi)(qi)相比，能夠以更(geng)快的速(su)率、更(geng)大的電(dian)流對電(dian)池進行充(chong)(chong)(chong)電(dian)(圖6)。由(you)于(yu)功耗較低、發熱(re)較少，熱(re)管理方(fang)面的問題(ti)(ti)也減少了。同時，由(you)于(yu)運行溫度降低，充(chong)(chong)(chong)電(dian)器(qi)(qi)更(geng)加可靠。

　　
圖6. 對單(dan)節NiMH電(dian)池充(chong)電(dian)時，線性充(chong)電(dian)器和開(kai)關充(chong)電(dian)器的充(chong)電(dian)時間不同(tong)

　　圖6中的(de)計算結(jie)果基(ji)于以下條件和假設得到(dao)：采(cai)用高功率USB口最大允許(xu)電(dian)流(liu)(liu)(500mA)的(de)大約(yue)90%充電(dian)；開關調節器采(cai)用非(fei)同步整流(liu)(liu)的(de)buck轉換器，具有77%效率。

　　電路實例

　　圖7所示電路是用于單節NiMH電池充(chong)電的(de)開關(guan)模式降(jiang)壓(ya)型調節器(qi)(qi)。它采用DS2712充(chong)電控制(zhi)器(qi)(qi)調節充(chong)電電流和終止充(chong)電。充(chong)電控制(zhi)器(qi)(qi)監(jian)視溫(wen)度、電池電壓(ya)和電池電流。如果(guo)溫(wen)度超(chao)過+45°C或者低于0°C，控制(zhi)器(qi)(qi)不會對電池充(chong)電。

　　
圖7. USB端口對單節NiMH電池快(kuai)速充電的原理(li)圖

　　如圖7所示，Q1是降(jiang)壓(ya)(ya)型充(chong)(chong)電(dian)(dian)(dian)器(qi)(qi)(qi)的(de)(de)(de)(de)(de)(de)開(kai)關功率晶體(ti)管；L1是濾波電(dian)(dian)(dian)感；D1是續流或整流二極管。輸(shu)入電(dian)(dian)(dian)容(rong)C1為10μF、超低(di)ESR的(de)(de)(de)(de)(de)(de)陶瓷濾波電(dian)(dian)(dian)容(rong)。用鉭電(dian)(dian)(dian)容(rong)或者其它(ta)電(dian)(dian)(dian)解電(dian)(dian)(dian)容(rong)替代C1會(hui)使充(chong)(chong)電(dian)(dian)(dian)器(qi)(qi)(qi)的(de)(de)(de)(de)(de)(de)性能降(jiang)低(di)。R7是電(dian)(dian)(dian)流調節器(qi)(qi)(qi)檢測放大(da)(da)器(qi)(qi)(qi)的(de)(de)(de)(de)(de)(de)檢流電(dian)(dian)(dian)阻。DS2712的(de)(de)(de)(de)(de)(de)基(ji)準電(dian)(dian)(dian)壓(ya)(ya)為0.125V，并具有24mV滯回。通過CSOUT提供(gong)閉環、開(kai)關模式電(dian)(dian)(dian)流控制(zhi)。充(chong)(chong)電(dian)(dian)(dian)控制(zhi)引腳(jiao)CC1將(jiang)Q2的(de)(de)(de)(de)(de)(de)柵(zha)(zha)極拉低(di)時，使能Q1的(de)(de)(de)(de)(de)(de)柵(zha)(zha)極驅動。Q1和(he)Q2均(jun)為低(di)Vt (柵(zha)(zha)-源門(men)限電(dian)(dian)(dian)壓(ya)(ya))的(de)(de)(de)(de)(de)(de)pMOSFET。CC1和(he)CSOUT均(jun)為低(di)電(dian)(dian)(dian)平時，Q2的(de)(de)(de)(de)(de)(de)漏-源電(dian)(dian)(dian)壓(ya)(ya)將(jiang)稍大(da)(da)于(yu)Vt。該電(dian)(dian)(dian)壓(ya)(ya)以及CSOUT的(de)(de)(de)(de)(de)(de)正向(xiang)壓(ya)(ya)降(jiang)構成(cheng)了Q1的(de)(de)(de)(de)(de)(de)柵(zha)(zha)極開(kai)關電(dian)(dian)(dian)壓(ya)(ya)。

　　CC1為(wei)(wei)(wei)低(di)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平(ping)時(shi)(shi)，啟動電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)閉環控制(zhi)。圖8所(suo)示為(wei)(wei)(wei)啟動開關時(shi)(shi)的波(bo)(bo)形(xing)。上方波(bo)(bo)形(xing)是0.125? (檢流(liu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻(zu)(zu)兩端的電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)，下(xia)方波(bo)(bo)形(xing)是Q1漏極至GND的電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)。開始(shi)時(shi)(shi)，當Q1打開(CC1和CSOUT均(jun)為(wei)(wei)(wei)低(di)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平(ping))時(shi)(shi)，電(dian)(dian)(dian)(dian)(dian)(dian)(dian)感(gan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)向上爬升。當電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)增大(da)到(dao)使(shi)檢流(liu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻(zu)(zu)兩端的電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)達到(dao)0.125V時(shi)(shi)，CSOUT變為(wei)(wei)(wei)高電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平(ping)，開關關斷。此后，電(dian)(dian)(dian)(dian)(dian)(dian)(dian)感(gan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)開始(shi)下(xia)降(jiang)，直到(dao)檢流(liu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻(zu)(zu)兩端的電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)達到(dao)約0.1V，CSOUT又變為(wei)(wei)(wei)低(di)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平(ping)。只要CC1為(wei)(wei)(wei)低(di)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平(ping)，該過程將一直持續(xu)。

　　
圖(tu)8. USB NiMH充電器的(de)啟動波形(xing)

　　DS2712的(de)內(nei)部(bu)狀態(tai)機控制著(zhu)CC1的(de)工(gong)作(zuo)。充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)開始時(shi)，DS2712先(xian)對電(dian)(dian)池(chi)(chi)(chi)進(jin)(jin)行狀態(tai)測試(shi)，以(yi)(yi)(yi)(yi)確(que)保(bao)(bao)電(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)壓(ya)(ya)在1.0V至1.65V之間(jian)，并確(que)認溫度在0°C至+45°C之間(jian)。如(ru)(ru)果電(dian)(dian)壓(ya)(ya)低于1.0V，DS2712將(jiang)以(yi)(yi)(yi)(yi)0.125的(de)占空比拉低CC1，對電(dian)(dian)池(chi)(chi)(chi)緩(huan)慢充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)，以(yi)(yi)(yi)(yi)防損壞電(dian)(dian)池(chi)(chi)(chi)。一(yi)旦電(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)壓(ya)(ya)超(chao)過(guo)1.0V后，狀態(tai)機轉為快(kuai)充(chong)(chong)(chong)(chong)(chong)(chong)模(mo)式。快(kuai)充(chong)(chong)(chong)(chong)(chong)(chong)時(shi)占空比為31/32，即(ji)大約97%。“跳(tiao)過(guo)”的(de)間(jian)隙內(nei)進(jin)(jin)行電(dian)(dian)池(chi)(chi)(chi)阻抗(kang)測試(shi)，以(yi)(yi)(yi)(yi)確(que)保(bao)(bao)不會(hui)對錯(cuo)誤(wu)放入(ru)(ru)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)器的(de)高阻抗(kang)電(dian)(dian)池(chi)(chi)(chi)(例如(ru)(ru)堿(jian)性電(dian)(dian)池(chi)(chi)(chi))進(jin)(jin)行充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)。檢測到(dao)-2mV的(de)-ΔV后，快(kuai)充(chong)(chong)(chong)(chong)(chong)(chong)結束。如(ru)(ru)果未檢測到(dao)-ΔV，將(jiang)持(chi)續快(kuai)充(chong)(chong)(chong)(chong)(chong)(chong)，直(zhi)(zhi)到(dao)快(kuai)充(chong)(chong)(chong)(chong)(chong)(chong)定(ding)(ding)時(shi)器超(chao)時(shi)，或檢測到(dao)過(guo)溫或者過(guo)壓(ya)(ya)故障狀態(tai)(包括阻抗(kang)不合(he)格(ge))為止。快(kuai)充(chong)(chong)(chong)(chong)(chong)(chong)完成(由于-ΔV或快(kuai)充(chong)(chong)(chong)(chong)(chong)(chong)定(ding)(ding)時(shi)器超(chao)時(shi)) 后，DS2712進(jin)(jin)入(ru)(ru)定(ding)(ding)時(shi)補(bu)足(zu)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)模(mo)式，占空比為12.5%，持(chi)續時(shi)間(jian)為所設(she)快(kuai)充(chong)(chong)(chong)(chong)(chong)(chong)定(ding)(ding)時(shi)的(de)一(yi)半。補(bu)足(zu)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)完成后，充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)器進(jin)(jin)入(ru)(ru)維持(chi)模(mo)式，占空比為1/64，直(zhi)(zhi)到(dao)電(dian)(dian)池(chi)(chi)(chi)被拿走(zou)或重新上電(dian)(dian)。

　　采用圖7所示充電器和(he)大功率(lv)USB端口對2100mAh NiMH電池充(chong)電時，快充(chong)時間為2小時多一(yi)點，大約3個小時完成包(bao)括補足充(chong)電在(zai)內的(de)全部充(chong)電過程(cheng)。從端口吸取的(de)電流為420mA。如果需要與(yu)主機進行枚舉過程(cheng)，并需要大電流使能操作，可(ke)在(zai)R9和(he)地之間串聯(lian)一(yi)個開(kai)漏極(ji)nMOSFET。如果MOSFET關斷，則TMR浮空，DS2712進入(ru)掛起(qi)狀(zhuang)態。

　　總結

　　對于(yu)小型消費類(lei)電子設備的(de)(de)電池(chi)充(chong)電而言，USB端(duan)口(kou)是一(yi)個經(jing)濟、實用的(de)(de)電源。為完(wan)全符合USB 2.0規(gui)范(fan)，連接在USB端(duan)口(kou)上(shang)的(de)(de)負載(zai)必須能夠(gou)與主(zhu)(zhu)機進行(xing)雙向(xiang)通信。負載(zai)也(ye)必須符合電源管理(li)(li)要(yao)求，包括低功(gong)耗模式，以(yi)及便于(yu)主(zhu)(zhu)機確定(ding)何時(shi)需要(yao)從端(duan)口(kou)吸取大電流(liu)的(de)(de)手段。盡管部分(fen)兼容的(de)(de)系(xi)統能夠(gou)適應大部分(fen)USB主(zhu)(zhu)機，但有時(shi)會出現意想不到的(de)(de)結果。只有很好地理(li)(li)解(jie)USB規(gui)范(fan)要(yao)求和(he)負載(zai)的(de)(de)期望(wang)，才能在對于(yu)規(gui)范(fan)的(de)(de)兼容性與負載(zai)復雜度(du)之間取得較好的(de)(de)平衡。