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)進行接(jie)口。文章回(hui)顧(gu)了(le)USB電(dian)(dian)(dian)源(yuan)總線(xian)的特性，包括電(dian)(dian)(dian)壓、電(dian)(dian)(dian)流限制、浪涌電(dian)(dian)(dian)流、連接(jie)器以及電(dian)(dian)(dian)纜連接(jie)問題。同時介紹(shao)了(le)鎳氫電(dian)(dian)(dian)池(chi)(NiMH)和鋰電(dian)(dian)(dian)池(chi)技(ji)術、充(chong)電(dian)(dian)(dian)方法以及充(chong)電(dian)(dian)(dian)終止技(ji)術。給出了(le)一個完整的示例電(dian)(dian)(dian)路，用(yong)于實(shi)現USB端口對NiMH電(dian)(dian)(dian)池(chi)智(zhi)能充(chong)電(dian)(dian)(dian)，并給出了(le)充(chong)電(dian)(dian)(dian)數據(ju)。

　　USB特性

　　USB總(zong)線能夠為低(di)功耗電(dian)子設備提(ti)供電(dian)源(yuan)(yuan)。總(zong)線電(dian)源(yuan)(yuan)與(yu)電(dian)網隔離，并且(qie)具(ju)有(you)很好的(de)(de)穩定性。但是，可用電(dian)流有(you)限，同時負載和主機(ji)或電(dian)源(yuan)(yuan)之間存在潛在的(de)(de)互操作問題(ti)。

　　USB端口由(you)90?雙(shuang)向(xiang)差分屏(ping)(ping)蔽雙(shuang)絞(jiao)線、VBUS (+5V電源)和地(di)組(zu)成。這(zhe)4條線由(you)鋁(lv)箔內屏(ping)(ping)蔽層和編織(zhi)(zhi)網外(wai)屏(ping)(ping)蔽層進行屏(ping)(ping)蔽。最新(xin)的(de)USB規范標準(zhun)是2.0版，可以(yi)從USB組(zu)織(zhi)(zhi)免費獲得。要做(zuo)到(dao)完(wan)全符合該規范標準(zhun)，需要通過一個功能控制器來實現設備(bei)和主(zhu)機間的(de)雙(shuang)向(xiang)通信。規范定義(yi)了1個單(dan)位(wei)(wei)負載(zai)為100mA (最大(da))。任何(he)設備(bei)允許吸(xi)取的(de)最大(da)電流為5個單(dan)位(wei)(wei)負載(zai)。

　　USB端(duan)口可(ke)(ke)分為(wei)低功(gong)(gong)率(lv)端(duan)口和大(da)功(gong)(gong)率(lv)端(duan)口兩類，低功(gong)(gong)率(lv)端(duan)口可(ke)(ke)提供1個(ge)單位負載的(de)電(dian)(dian)流，大(da)功(gong)(gong)率(lv)端(duan)口可(ke)(ke)最多提供5個(ge)單位負載的(de)電(dian)(dian)流。當設(she)備剛連接到USB端(duan)口時，枚舉(ju)過程(cheng)對(dui)器件(jian)進(jin)行識別，并確定其負載要求。在此過程(cheng)中，只允許設(she)備從主機吸(xi)取最多1個(ge)單位負載的(de)電(dian)(dian)流。枚舉(ju)過程(cheng)完成后，如果(guo)主機的(de)電(dian)(dian)源管理軟(ruan)件(jian)允許，則大(da)功(gong)(gong)率(lv)設(she)備可(ke)(ke)以(yi)吸(xi)取更大(da)的(de)電(dian)(dian)流。

　　某些主(zhu)機(ji)系統(tong)(包括(kuo)下游USB集線(xian)器(qi))通過(guo)(guo)保險絲或者有源電(dian)(dian)(dian)流(liu)(liu)檢測器(qi)提(ti)供(gong)限流(liu)(liu)功能(neng)(neng)。如果USB設備未經過(guo)(guo)枚(mei)舉過(guo)(guo)程便從USB端(duan)口(kou)吸(xi)(xi)取(qu)大電(dian)(dian)(dian)流(liu)(liu)(超過(guo)(guo)1個(ge)單位負載)，則主(zhu)機(ji)會檢測到過(guo)(guo)流(liu)(liu)狀態，并會關閉正在使用的(de)(de)(de)一(yi)個(ge)或多個(ge)USB端(duan)口(kou)。市場上(shang)供(gong)應的(de)(de)(de)許(xu)多USB設備，包括(kuo)獨立電(dian)(dian)(dian)池充電(dian)(dian)(dian)器(qi)，都沒有功能(neng)(neng)控(kong)制器(qi)來處理枚(mei)舉過(guo)(guo)程，但吸(xi)(xi)取(qu)的(de)(de)(de)電(dian)(dian)(dian)流(liu)(liu)卻超過(guo)(guo)了100mA。在這種(zhong)不恰當的(de)(de)(de)條(tiao)件下，這些設備可能(neng)(neng)導(dao)致(zhi)主(zhu)機(ji)出現(xian)問題。例如，如果一(yi)個(ge)吸(xi)(xi)取(qu)500mA電(dian)(dian)(dian)流(liu)(liu)的(de)(de)(de)設備插入(ru)總線(xian)供(gong)電(dian)(dian)(dian)的(de)(de)(de)USB集線(xian)器(qi)，而(er)且未進行正確的(de)(de)(de)枚(mei)舉過(guo)(guo)程，則可能(neng)(neng)導(dao)致(zhi)集線(xian)器(qi)端(duan)口(kou)和(he)主(zhu)機(ji)端(duan)口(kou)同時過(guo)(guo)載。

　　主(zhu)機操(cao)作系統采用高級(ji)電源管(guan)理時(shi)情況會更加復雜，特別是筆記本電腦，它總是希望端口電流盡可能(neng)低。在某(mou)些節電模(mo)式(shi)下(xia)，計算機會向(xiang)USB設(she)(she)備發出(chu)掛起(qi)命令(ling)，而后則認為(wei)設(she)(she)備進入了(le)低功(gong)耗(hao)模(mo)式(shi)。設(she)(she)備中包含(han)一(yi)個能(neng)與(yu)主(zhu)機進行通信的(de)功(gong)能(neng)控制器始終是一(yi)個比較(jiao)好的(de)做(zuo)法(fa)，即使對于低功(gong)耗(hao)設(she)(she)備來說也是如(ru)此。

　　USB 2.0規范非常全面(mian)，規定了電(dian)(dian)源的(de)(de)(de)(de)(de)(de)質量、連接器構造、電(dian)(dian)纜(lan)材質、容許的(de)(de)(de)(de)(de)(de)電(dian)(dian)壓(ya)跌落(luo)以及浪涌電(dian)(dian)流(liu)等。低電(dian)(dian)流(liu)和大電(dian)(dian)流(liu)端(duan)口(kou)(kou)(kou)具有(you)不同的(de)(de)(de)(de)(de)(de)電(dian)(dian)源指標。這主要(yao)是由(you)主機(ji)(ji)和負(fu)載間(jian)的(de)(de)(de)(de)(de)(de)連接器和電(dian)(dian)纜(lan)上的(de)(de)(de)(de)(de)(de)電(dian)(dian)壓(ya)跌落(luo)決定的(de)(de)(de)(de)(de)(de)，并包括(kuo)由(you)USB供電(dian)(dian)的(de)(de)(de)(de)(de)(de)集(ji)線器上產生(sheng)的(de)(de)(de)(de)(de)(de)電(dian)(dian)壓(ya)跌落(luo)。包括(kuo)計算機(ji)(ji)或者自供電(dian)(dian)USB集(ji)線器在內的(de)(de)(de)(de)(de)(de)主機(ji)(ji)，都具有(you)大電(dian)(dian)流(liu)端(duan)口(kou)(kou)(kou)，可提(ti)供最大500mA的(de)(de)(de)(de)(de)(de)電(dian)(dian)流(liu)。無源、總線供電(dian)(dian)的(de)(de)(de)(de)(de)(de)USB集(ji)線器具有(you)低電(dian)(dian)流(liu)端(duan)口(kou)(kou)(kou)。表1列出(chu)了USB大電(dian)(dian)流(liu)和低電(dian)(dian)流(liu)端(duan)口(kou)(kou)(kou)上游端(duan)(電(dian)(dian)源)引(yin)腳(jiao)允許的(de)(de)(de)(de)(de)(de)電(dian)(dian)壓(ya)容限(xian)。

　　表(biao)1. USB 2.0規(gui)范電源(yuan)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

       *這些指標(biao)適用于上(shang)(shang)游端(duan)主機(ji)或集線器端(duan)口的連接(jie)(jie)器引腳。電纜(lan)和連接(jie)(jie)器上(shang)(shang)的I x R跌落需(xu)另外(wai)考慮。

       在(zai)符合(he)USB 2.0規范的(de)(de)(de)主(zhu)機中，大(da)(da)功率(lv)端口的(de)(de)(de)上游端具有120μF、低ESR電(dian)容(rong)。所(suo)連接的(de)(de)(de)USB設(she)備(bei)的(de)(de)(de)輸(shu)入電(dian)容(rong)限(xian)制(zhi)在(zai)10μF以內(nei)，在(zai)最(zui)初(chu)的(de)(de)(de)負載連接階段(duan)，允(yun)許負載從主(zhu)機(或自供電(dian)集(ji)線器)吸取的(de)(de)(de)最(zui)大(da)(da)電(dian)荷數為(wei)50μC。這樣一來，當新設(she)備(bei)連接至USB端口時(shi)(shi)，上游端口的(de)(de)(de)瞬(shun)態電(dian)壓(ya)跌落小于0.5V。如果負載正常工作時(shi)(shi)需(xu)要更大(da)(da)的(de)(de)(de)輸(shu)入電(dian)容(rong)，則必(bi)須提供浪涌電(dian)流(liu)限(xian)制(zhi)器，以保證對(dui)更大(da)(da)的(de)(de)(de)電(dian)容(rong)充電(dian)時(shi)(shi)電(dian)流(liu)不會(hui)超過100mA。

　　當USB端口帶(dai)有(you)一個總線(xian)(xian)(xian)供(gong)電的USB集線(xian)(xian)(xian)器(qi)，集線(xian)(xian)(xian)器(qi)上接了低功耗設備時(shi)，USB口上允(yun)許(xu)的直流電壓跌(die)落(luo)如圖(tu)1所(suo)示。大功率負載與總線(xian)(xian)(xian)供(gong)電的集線(xian)(xian)(xian)器(qi)連接時(shi)，電壓跌(die)落(luo)將(jiang)超過圖(tu)1給(gei)出的指(zhi)標，并會引起總線(xian)(xian)(xian)過載。

　　
圖1. 主機(ji)至低(di)功率負載的電壓(ya)跌落大于(yu)圖中給出的允許直流電壓(ya)跌落時(shi)，會引起總線過載

　　電池充電要求

　　單節(jie)鋰(li)離子和鋰(li)聚合(he)物電池

　　如今的(de)(de)(de)鋰(li)電(dian)池(chi)充電(dian)至(zhi)最(zui)大(da)額(e)定容(rong)量后，其(qi)電(dian)壓通常為(wei)4.1V至(zhi)4.2V之間。當(dang)前市場上正在出售的(de)(de)(de)、更新的(de)(de)(de)、容(rong)量更大(da)的(de)(de)(de)電(dian)池(chi)，其(qi)電(dian)壓范圍在4.3V至(zhi)4.4V之間。典型的(de)(de)(de)棱柱形鋰(li)離子(Li+)和(he)鋰(li)聚合物(Li-Poly)電(dian)池(chi)容(rong)量為(wei)600mAh至(zhi)1400mAh。

　　對Li+和(he)Li-Poly電(dian)(dian)池來說(shuo)，首(shou)選的(de)(de)充(chong)(chong)(chong)(chong)電(dian)(dian)曲線是從恒流(liu)(liu)充(chong)(chong)(chong)(chong)電(dian)(dian)開(kai)(kai)始，一直持續到(dao)電(dian)(dian)池電(dian)(dian)壓(ya)(ya)達到(dao)額(e)定(ding)電(dian)(dian)壓(ya)(ya)。然(ran)后，充(chong)(chong)(chong)(chong)電(dian)(dian)器(qi)對電(dian)(dian)池兩端的(de)(de)電(dian)(dian)壓(ya)(ya)進(jin)行調節。這(zhe)兩種(zhong)調節方式(shi)構成了恒流(liu)(liu)(CC)恒壓(ya)(ya)(CV)充(chong)(chong)(chong)(chong)電(dian)(dian)方式(shi)。因(yin)此，這(zhe)種(zhong)類型(xing)(xing)的(de)(de)充(chong)(chong)(chong)(chong)電(dian)(dian)器(qi)通(tong)常稱為CCCV充(chong)(chong)(chong)(chong)電(dian)(dian)器(qi)。CCCV充(chong)(chong)(chong)(chong)電(dian)(dian)器(qi)進(jin)入CV模(mo)式(shi)后，電(dian)(dian)池的(de)(de)充(chong)(chong)(chong)(chong)電(dian)(dian)電(dian)(dian)流(liu)(liu)開(kai)(kai)始下降。若采用0.5C至1.5C的(de)(de)典(dian)型(xing)(xing)充(chong)(chong)(chong)(chong)電(dian)(dian)速率充(chong)(chong)(chong)(chong)電(dian)(dian)，則當(dang)電(dian)(dian)池達到(dao)其充(chong)(chong)(chong)(chong)滿容量的(de)(de)80%至90%時，充(chong)(chong)(chong)(chong)電(dian)(dian)器(qi)由CC模(mo)式(shi)轉(zhuan)換為CV模(mo)式(shi)。充(chong)(chong)(chong)(chong)電(dian)(dian)器(qi)一旦進(jin)入CV充(chong)(chong)(chong)(chong)電(dian)(dian)模(mo)式(shi)，則對電(dian)(dian)池電(dian)(dian)流(liu)(liu)進(jin)行監(jian)視；當(dang)電(dian)(dian)流(liu)(liu)達到(dao)最低(di)門限(幾毫安(an)或者幾十毫安(an))時，充(chong)(chong)(chong)(chong)電(dian)(dian)器(qi)終(zhong)止(zhi)充(chong)(chong)(chong)(chong)電(dian)(dian)。鋰電(dian)(dian)池的(de)(de)典(dian)型(xing)(xing)充(chong)(chong)(chong)(chong)電(dian)(dian)曲線如圖2所(suo)示(shi)。

　　
圖2. 使用CCCV充電(dian)器對(dui)Li+電(dian)池(chi)充電(dian)時的典型曲線

　　從(cong)圖1所(suo)示的(de)USB電(dian)壓跌(die)落指標可以(yi)看出(chu)，端口(kou)供電(dian)集線器(qi)的(de)下游低功(gong)率端口(kou)電(dian)壓不具(ju)備足夠的(de)余量，很難將電(dian)池充(chong)至4.2V。充(chong)電(dian)通(tong)路上存在(zai)的(de)小量額外電(dian)阻會妨礙(ai)正常(chang)充(chong)電(dian)。

　　Li+和Li-Poly電(dian)(dian)(dian)池(chi)應(ying)在合適的(de)(de)溫(wen)度下進行充(chong)(chong)(chong)電(dian)(dian)(dian)。制造商推薦的(de)(de)最高充(chong)(chong)(chong)電(dian)(dian)(dian)溫(wen)度通(tong)常為+45°C至+55°C之間，允(yun)許的(de)(de)最大放電(dian)(dian)(dian)溫(wen)度可再高出10°C左(zuo)右。這些電(dian)(dian)(dian)池(chi)使用(yong)的(de)(de)材料，化學性質(zhi)非常活潑(po)，如果(guo)電(dian)(dian)(dian)池(chi)溫(wen)度超(chao)過(guo)+70°C，會發生(sheng)燃燒。鋰電(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)器應(ying)具備熱關斷電(dian)(dian)(dian)路，該電(dian)(dian)(dian)路監視電(dian)(dian)(dian)池(chi)溫(wen)度，如果(guo)電(dian)(dian)(dian)池(chi)溫(wen)度超(chao)過(guo)制造商推薦的(de)(de)最大充(chong)(chong)(chong)電(dian)(dian)(dian)溫(wen)度時，則終止(zhi)充(chong)(chong)(chong)電(dian)(dian)(dian)。

　　鎳氫電池（NiMH）

　　NiMH電(dian)池(chi)比鋰(li)電(dian)池(chi)要重一些，其能量密度也比鋰(li)電(dian)池(chi)低。一直以(yi)來(lai)，NiMH電(dian)池(chi)比鋰(li)電(dian)池(chi)要便宜(yi)，但是最(zui)近二者的價格差在縮小。NiMH電(dian)池(chi)具有標準尺寸，在大多數應用中可直接替換堿性電(dian)池(chi)。每節電(dian)池(chi)的標稱(cheng)電(dian)壓為1.2V，充滿后會達到1.5V。

　　通常(chang)采用恒流源對NiMH電(dian)(dian)池(chi)(chi)充(chong)(chong)電(dian)(dian)。當達到(dao)充(chong)(chong)滿狀態時，會發生放熱化(hua)學反應，并導致電(dian)(dian)池(chi)(chi)溫度(du)上(shang)升(sheng)，電(dian)(dian)池(chi)(chi)端電(dian)(dian)壓(ya)降低。可檢(jian)測(ce)電(dian)(dian)池(chi)(chi)溫度(du)上(shang)升(sheng)速率(lv)或者負向電(dian)(dian)壓(ya)變化(hua)率(lv)，并用來終止充(chong)(chong)電(dian)(dian)。這些充(chong)(chong)電(dian)(dian)終止方法分別稱(cheng)為dT/dt和-ΔV。充(chong)(chong)電(dian)(dian)速率(lv)非常(chang)低時，dT/dt和-ΔV不太明顯(xian)，很難(nan)精確檢(jian)測(ce)到(dao)。電(dian)(dian)池(chi)(chi)開始(shi)進(jin)入過充(chong)(chong)狀態時，dT/dt和-ΔV響應開始(shi)顯(xian)現。此時如果繼續充(chong)(chong)電(dian)(dian)，將(jiang)損壞電(dian)(dian)池(chi)(chi)。

　　終止檢測(ce)在(zai)充(chong)(chong)電(dian)(dian)(dian)速率(lv)大于(yu)C/3時(shi)要比低充(chong)(chong)電(dian)(dian)(dian)速率(lv)時(shi)容(rong)易得多(duo)。溫度上升速率(lv)大約(yue)為1°C/分(fen)鐘，-ΔV響應也(ye)比低充(chong)(chong)電(dian)(dian)(dian)速率(lv)時(shi)更明顯(xian)。快充(chong)(chong)結束(shu)后，建議(yi)以(yi)更小的(de)(de)電(dian)(dian)(dian)流(liu)再充(chong)(chong)一(yi)段時(shi)間，以(yi)徹(che)底(di)充(chong)(chong)足電(dian)(dian)(dian)池(chi)(補(bu)足充(chong)(chong)電(dian)(dian)(dian))。補(bu)足充(chong)(chong)電(dian)(dian)(dian)階段結束(shu)后，采(cai)(cai)用(yong)C/20或者C/30的(de)(de)涓充(chong)(chong)電(dian)(dian)(dian)流(liu)來(lai)補(bu)償自放電(dian)(dian)(dian)效應，使電(dian)(dian)(dian)池(chi)維持在(zai)充(chong)(chong)滿狀態。圖3所示為采(cai)(cai)用(yong)DS2712 NiMH充(chong)(chong)電(dian)(dian)(dian)器(qi)對NiMH電(dian)(dian)(dian)池(chi)(事先已充(chong)(chong)了一(yi)部分(fen)電(dian)(dian)(dian))進行充(chong)(chong)電(dian)(dian)(dian)的(de)(de)電(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)壓曲(qu)線(xian)(xian)。在(zai)該(gai)圖中，上面(mian)一(yi)條(tiao)(tiao)曲(qu)線(xian)(xian)的(de)(de)數(shu)據在(zai)充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)正在(zai)灌(guan)入電(dian)(dian)(dian)池(chi)時(shi)獲得，下(xia)面(mian)那條(tiao)(tiao)曲(qu)線(xian)(xian)的(de)(de)數(shu)據在(zai)切斷(duan)電(dian)(dian)(dian)流(liu)時(shi)測(ce)得。在(zai)DS2712中，該(gai)電(dian)(dian)(dian)壓差被用(yong)來(lai)區(qu)分(fen)NiMH電(dian)(dian)(dian)池(chi)和堿性(xing)電(dian)(dian)(dian)池(chi)。如果檢測(ce)到堿性(xing)電(dian)(dian)(dian)池(chi)，則DS2712不會對它進行充(chong)(chong)電(dian)(dian)(dian)。

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

　　開關與線性

　　USB 2.0規(gui)范(fan)允許(xu)低功率(lv)端口(kou)提供最大(da)100mA電(dian)(dian)流，大(da)功率(lv)端口(kou)提供最大(da)500mA電(dian)(dian)流。如果采用線性調(diao)整(zheng)器(qi)件(jian)來調(diao)節電(dian)(dian)池充電(dian)(dian)電(dian)(dian)流，這(zhe)也(ye)就是(shi)最大(da)可提供的充電(dian)(dian)電(dian)(dian)流。線性調(diao)整(zheng)器(qi)件(jian)(圖4)的功耗(hao)為P = VQ x IBATT。這(zhe)會造成調(diao)整(zheng)管發熱(re)，可能需(xu)要安裝(zhuang)散(san)熱(re)器(qi)，以防止(zhi)過熱(re)。

　　
圖(tu)4. 功耗等(deng)于電(dian)(dian)池充電(dian)(dian)電(dian)(dian)流乘以調整管兩(liang)端的電(dian)(dian)壓

　　對(dui)應5V標稱(cheng)輸入(ru)電壓，調整器件(jian)消(xiao)耗(hao)的功率與電池(chi)類型、數(shu)量和(he)電池(chi)電壓有關。

　　
圖5. 采(cai)用(yong)5.0V電(dian)壓(ya)的(de)USB端口對NiMH電(dian)池(chi)充電(dian)時，線(xian)性調整(zheng)器(qi)件的(de)功(gong)耗

     標稱輸入電(dian)壓(ya)為(wei)(wei)5.0V時(shi)，線(xian)性(xing)USB充(chong)電(dian)器(qi)對(dui)NiMH電(dian)池(chi)充(chong)電(dian)的功(gong)耗(hao)(hao)計算結(jie)果如圖5所(suo)示(shi)。對(dui)單節電(dian)池(chi)充(chong)電(dian)時(shi)，線(xian)性(xing)充(chong)電(dian)器(qi)的效(xiao)率僅為(wei)(wei)30%；對(dui)兩節電(dian)池(chi)充(chong)電(dian)時(shi)，效(xiao)率為(wei)(wei)60%。用(yong)500mA電(dian)流對(dui)單節電(dian)池(chi)充(chong)電(dian)時(shi)，功(gong)耗(hao)(hao)會高達2W。這樣的功(gong)耗(hao)(hao)通常需(xu)要加(jia)散熱器(qi)。功(gong)耗(hao)(hao)為(wei)(wei)2W時(shi)，熱阻(zu)為(wei)(wei)+20°C/W的散熱器(qi)在(zai)+25°C環境溫度(du)下會被(bei)加(jia)熱至大約+65°C，要得到(dao)滿(man)額性(xing)能，還需(xu)要有流動空(kong)(kong)氣來協助其散熱。處于空(kong)(kong)氣靜止(zhi)的封閉空(kong)(kong)間內，溫度(du)會更(geng)高。

　　采用(yong)基(ji)于開關調(diao)節器的(de)充電器可解決多個問(wen)題。首先，與線性充電器相比，能夠以更(geng)快(kuai)的(de)速率(lv)、更(geng)大的(de)電流對電池(chi)進行充電(圖(tu)6)。由于功耗(hao)較低、發熱(re)較少(shao)，熱(re)管理方面的(de)問(wen)題也減少(shao)了。同時，由于運行溫度降(jiang)低，充電器更(geng)加可靠。

　　
圖6. 對單節NiMH電(dian)(dian)池充(chong)電(dian)(dian)時，線性充(chong)電(dian)(dian)器和開關充(chong)電(dian)(dian)器的充(chong)電(dian)(dian)時間不同

　　圖6中的(de)計(ji)算結(jie)果基于以(yi)下(xia)條件(jian)和假設得到：采(cai)用高功率USB口最大允許電流(liu)(500mA)的(de)大約90%充電；開關調節器(qi)(qi)采(cai)用非同(tong)步整流(liu)的(de)buck轉換(huan)器(qi)(qi)，具有77%效率。

　　電路實例

　　圖7所(suo)示電(dian)(dian)路是用于(yu)單節NiMH電(dian)(dian)池(chi)(chi)(chi)充(chong)電(dian)(dian)的(de)開(kai)關(guan)模式降(jiang)壓型調(diao)(diao)節器。它采用DS2712充(chong)電(dian)(dian)控(kong)制(zhi)(zhi)器調(diao)(diao)節充(chong)電(dian)(dian)電(dian)(dian)流和終(zhong)止(zhi)充(chong)電(dian)(dian)。充(chong)電(dian)(dian)控(kong)制(zhi)(zhi)器監(jian)視(shi)溫(wen)度、電(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)壓和電(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)流。如果(guo)溫(wen)度超過+45°C或者低于(yu)0°C，控(kong)制(zhi)(zhi)器不(bu)會對電(dian)(dian)池(chi)(chi)(chi)充(chong)電(dian)(dian)。

　　
圖7. USB端(duan)口對單節NiMH電池(chi)快速充電的原理(li)圖

　　如圖7所(suo)示，Q1是(shi)降壓型充電(dian)(dian)(dian)(dian)(dian)器(qi)的(de)(de)開(kai)關功率(lv)晶體管(guan)(guan)；L1是(shi)濾波(bo)電(dian)(dian)(dian)(dian)(dian)感；D1是(shi)續流或整流二極(ji)管(guan)(guan)。輸入(ru)電(dian)(dian)(dian)(dian)(dian)容(rong)(rong)C1為10μF、超低(di)ESR的(de)(de)陶瓷濾波(bo)電(dian)(dian)(dian)(dian)(dian)容(rong)(rong)。用鉭電(dian)(dian)(dian)(dian)(dian)容(rong)(rong)或者(zhe)其它(ta)電(dian)(dian)(dian)(dian)(dian)解電(dian)(dian)(dian)(dian)(dian)容(rong)(rong)替代C1會使(shi)充電(dian)(dian)(dian)(dian)(dian)器(qi)的(de)(de)性能降低(di)。R7是(shi)電(dian)(dian)(dian)(dian)(dian)流調節器(qi)檢測放大器(qi)的(de)(de)檢流電(dian)(dian)(dian)(dian)(dian)阻。DS2712的(de)(de)基準(zhun)電(dian)(dian)(dian)(dian)(dian)壓為0.125V，并具有24mV滯回。通過CSOUT提供閉(bi)環(huan)、開(kai)關模(mo)式電(dian)(dian)(dian)(dian)(dian)流控制。充電(dian)(dian)(dian)(dian)(dian)控制引腳CC1將Q2的(de)(de)柵(zha)(zha)極(ji)拉(la)低(di)時，使(shi)能Q1的(de)(de)柵(zha)(zha)極(ji)驅(qu)動(dong)。Q1和(he)Q2均為低(di)Vt (柵(zha)(zha)-源(yuan)門(men)限電(dian)(dian)(dian)(dian)(dian)壓)的(de)(de)pMOSFET。CC1和(he)CSOUT均為低(di)電(dian)(dian)(dian)(dian)(dian)平時，Q2的(de)(de)漏-源(yuan)電(dian)(dian)(dian)(dian)(dian)壓將稍大于Vt。該電(dian)(dian)(dian)(dian)(dian)壓以及CSOUT的(de)(de)正向(xiang)壓降構成(cheng)了Q1的(de)(de)柵(zha)(zha)極(ji)開(kai)關電(dian)(dian)(dian)(dian)(dian)壓。

　　CC1為(wei)(wei)低(di)電(dian)(dian)(dian)(dian)平(ping)時(shi)，啟動電(dian)(dian)(dian)(dian)流(liu)閉環控制。圖8所示為(wei)(wei)啟動開(kai)(kai)(kai)關時(shi)的(de)波(bo)(bo)形。上方(fang)波(bo)(bo)形是0.125? (檢流(liu)電(dian)(dian)(dian)(dian)阻(zu)(zu)兩(liang)端(duan)的(de)電(dian)(dian)(dian)(dian)壓，下方(fang)波(bo)(bo)形是Q1漏極至GND的(de)電(dian)(dian)(dian)(dian)壓。開(kai)(kai)(kai)始(shi)時(shi)，當(dang)(dang)Q1打(da)開(kai)(kai)(kai)(CC1和CSOUT均(jun)為(wei)(wei)低(di)電(dian)(dian)(dian)(dian)平(ping))時(shi)，電(dian)(dian)(dian)(dian)感(gan)電(dian)(dian)(dian)(dian)流(liu)向上爬升(sheng)。當(dang)(dang)電(dian)(dian)(dian)(dian)流(liu)增大到(dao)(dao)使檢流(liu)電(dian)(dian)(dian)(dian)阻(zu)(zu)兩(liang)端(duan)的(de)電(dian)(dian)(dian)(dian)壓達(da)到(dao)(dao)0.125V時(shi)，CSOUT變(bian)為(wei)(wei)高電(dian)(dian)(dian)(dian)平(ping)，開(kai)(kai)(kai)關關斷。此后，電(dian)(dian)(dian)(dian)感(gan)電(dian)(dian)(dian)(dian)流(liu)開(kai)(kai)(kai)始(shi)下降，直到(dao)(dao)檢流(liu)電(dian)(dian)(dian)(dian)阻(zu)(zu)兩(liang)端(duan)的(de)電(dian)(dian)(dian)(dian)壓達(da)到(dao)(dao)約0.1V，CSOUT又變(bian)為(wei)(wei)低(di)電(dian)(dian)(dian)(dian)平(ping)。只要CC1為(wei)(wei)低(di)電(dian)(dian)(dian)(dian)平(ping)，該過程(cheng)將(jiang)一直持續。

　　
圖8. USB NiMH充(chong)電器的啟(qi)動波形

　　DS2712的內(nei)部(bu)狀(zhuang)態(tai)機(ji)控制著(zhu)CC1的工作。充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)開始時(shi)(shi)(shi)，DS2712先對(dui)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)進(jin)(jin)(jin)行(xing)狀(zhuang)態(tai)測(ce)試，以(yi)確保電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)在(zai)1.0V至1.65V之(zhi)(zhi)間(jian)，并確認溫度(du)在(zai)0°C至+45°C之(zhi)(zhi)間(jian)。如(ru)果(guo)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)低于(yu)1.0V，DS2712將(jiang)以(yi)0.125的占(zhan)空(kong)(kong)比(bi)拉低CC1，對(dui)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)緩(huan)慢充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)，以(yi)防損(sun)壞電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)。一旦電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)超過(guo)1.0V后(hou)(hou)(hou)(hou)，狀(zhuang)態(tai)機(ji)轉為(wei)快(kuai)充(chong)(chong)(chong)(chong)模(mo)式(shi)(shi)。快(kuai)充(chong)(chong)(chong)(chong)時(shi)(shi)(shi)占(zhan)空(kong)(kong)比(bi)為(wei)31/32，即大約97%。“跳過(guo)”的間(jian)隙(xi)內(nei)進(jin)(jin)(jin)行(xing)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)阻(zu)(zu)抗測(ce)試，以(yi)確保不(bu)會對(dui)錯誤放入(ru)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器的高阻(zu)(zu)抗電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(例如(ru)堿性(xing)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi))進(jin)(jin)(jin)行(xing)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)。檢測(ce)到-2mV的-ΔV后(hou)(hou)(hou)(hou)，快(kuai)充(chong)(chong)(chong)(chong)結(jie)束。如(ru)果(guo)未檢測(ce)到-ΔV，將(jiang)持(chi)續快(kuai)充(chong)(chong)(chong)(chong)，直到快(kuai)充(chong)(chong)(chong)(chong)定(ding)(ding)時(shi)(shi)(shi)器超時(shi)(shi)(shi)，或(huo)(huo)檢測(ce)到過(guo)溫或(huo)(huo)者過(guo)壓(ya)(ya)故障狀(zhuang)態(tai)(包括阻(zu)(zu)抗不(bu)合格)為(wei)止。快(kuai)充(chong)(chong)(chong)(chong)完成(由(you)于(yu)-ΔV或(huo)(huo)快(kuai)充(chong)(chong)(chong)(chong)定(ding)(ding)時(shi)(shi)(shi)器超時(shi)(shi)(shi)) 后(hou)(hou)(hou)(hou)，DS2712進(jin)(jin)(jin)入(ru)定(ding)(ding)時(shi)(shi)(shi)補足(zu)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)模(mo)式(shi)(shi)，占(zhan)空(kong)(kong)比(bi)為(wei)12.5%，持(chi)續時(shi)(shi)(shi)間(jian)為(wei)所設(she)快(kuai)充(chong)(chong)(chong)(chong)定(ding)(ding)時(shi)(shi)(shi)的一半。補足(zu)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)完成后(hou)(hou)(hou)(hou)，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器進(jin)(jin)(jin)入(ru)維持(chi)模(mo)式(shi)(shi)，占(zhan)空(kong)(kong)比(bi)為(wei)1/64，直到電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)被拿走(zou)或(huo)(huo)重新上電(dian)(dian)(dian)(dian)(dian)(dian)(dian)。

　　采用圖7所示充電器和(he)大(da)功(gong)率USB端口對2100mAh NiMH電(dian)(dian)池充(chong)電(dian)(dian)時，快充(chong)時間為2小時多(duo)一點(dian)，大(da)約3個(ge)小時完成包括補(bu)足充(chong)電(dian)(dian)在內的全部充(chong)電(dian)(dian)過程。從端口吸取的電(dian)(dian)流(liu)為420mA。如(ru)果(guo)需(xu)要(yao)(yao)與主機進(jin)行枚(mei)舉過程，并需(xu)要(yao)(yao)大(da)電(dian)(dian)流(liu)使能操作(zuo)，可(ke)在R9和(he)地之間串聯一個(ge)開(kai)漏極(ji)nMOSFET。如(ru)果(guo)MOSFET關斷(duan)，則TMR浮空，DS2712進(jin)入掛起狀態。

　　總結

　　對于(yu)小型(xing)消費類電(dian)子設備(bei)的(de)(de)(de)(de)電(dian)池充電(dian)而言，USB端(duan)口(kou)是一個經(jing)濟、實用的(de)(de)(de)(de)電(dian)源。為完全符(fu)合(he)(he)USB 2.0規范(fan)，連接在(zai)USB端(duan)口(kou)上的(de)(de)(de)(de)負(fu)載(zai)(zai)必須能夠與主機進(jin)行雙向通信。負(fu)載(zai)(zai)也必須符(fu)合(he)(he)電(dian)源管理要求，包括(kuo)低功耗模式，以及(ji)便于(yu)主機確(que)定(ding)何時需要從端(duan)口(kou)吸(xi)取大(da)電(dian)流(liu)的(de)(de)(de)(de)手段。盡管部(bu)分兼(jian)容的(de)(de)(de)(de)系(xi)統能夠適應大(da)部(bu)分USB主機，但(dan)有時會(hui)出(chu)現意想不到(dao)的(de)(de)(de)(de)結果。只有很好地(di)理解USB規范(fan)要求和負(fu)載(zai)(zai)的(de)(de)(de)(de)期望(wang)，才能在(zai)對于(yu)規范(fan)的(de)(de)(de)(de)兼(jian)容性與負(fu)載(zai)(zai)復雜度(du)之(zhi)間取得較好的(de)(de)(de)(de)平衡。