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)(dian)源進行(xing)接口。文章回顧了USB電(dian)(dian)(dian)(dian)源總線的特性，包括電(dian)(dian)(dian)(dian)壓、電(dian)(dian)(dian)(dian)流(liu)限(xian)制(zhi)、浪涌電(dian)(dian)(dian)(dian)流(liu)、連接器以及電(dian)(dian)(dian)(dian)纜連接問題。同時(shi)介(jie)紹了鎳氫電(dian)(dian)(dian)(dian)池(chi)(NiMH)和(he)鋰電(dian)(dian)(dian)(dian)池(chi)技術、充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)方法以及充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)終止(zhi)技術。給(gei)出了一個(ge)完(wan)整的示例電(dian)(dian)(dian)(dian)路，用于實現(xian)USB端口對NiMH電(dian)(dian)(dian)(dian)池(chi)智(zhi)能(neng)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)，并給(gei)出了充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)數據。

　　USB特性

　　USB總線能(neng)夠為(wei)低功耗電(dian)(dian)子設備提供電(dian)(dian)源。總線電(dian)(dian)源與電(dian)(dian)網隔離，并且具(ju)有(you)(you)很好的(de)穩定(ding)性。但是，可(ke)用(yong)電(dian)(dian)流(liu)有(you)(you)限(xian)，同時(shi)負載和主機或(huo)電(dian)(dian)源之間存在潛在的(de)互(hu)操作問題(ti)。

　　USB端口由(you)90?雙(shuang)向(xiang)差分(fen)屏蔽雙(shuang)絞線(xian)、VBUS (+5V電源)和(he)地(di)組成。這4條線(xian)由(you)鋁箔內(nei)屏蔽層和(he)編織網外屏蔽層進行屏蔽。最(zui)新的USB規范(fan)標準是2.0版，可以(yi)從USB組織免費(fei)獲得。要做到完(wan)全符合(he)該規范(fan)標準，需要通(tong)過一個功能控制器(qi)來實(shi)現設備和(he)主機間的雙(shuang)向(xiang)通(tong)信。規范(fan)定義了(le)1個單位負載為(wei)100mA (最(zui)大)。任何設備允許(xu)吸取的最(zui)大電流(liu)為(wei)5個單位負載。

　　USB端(duan)(duan)口(kou)可(ke)(ke)分為低功率(lv)端(duan)(duan)口(kou)和大(da)(da)功率(lv)端(duan)(duan)口(kou)兩類，低功率(lv)端(duan)(duan)口(kou)可(ke)(ke)提(ti)供(gong)1個(ge)(ge)單(dan)(dan)位(wei)負(fu)(fu)載的(de)電流(liu)，大(da)(da)功率(lv)端(duan)(duan)口(kou)可(ke)(ke)最多(duo)提(ti)供(gong)5個(ge)(ge)單(dan)(dan)位(wei)負(fu)(fu)載的(de)電流(liu)。當設備(bei)剛(gang)連(lian)接(jie)到USB端(duan)(duan)口(kou)時，枚舉過(guo)程對器(qi)件(jian)進行識別，并確(que)定其負(fu)(fu)載要求。在(zai)此過(guo)程中，只允許(xu)設備(bei)從主機吸取最多(duo)1個(ge)(ge)單(dan)(dan)位(wei)負(fu)(fu)載的(de)電流(liu)。枚舉過(guo)程完成(cheng)后(hou)，如果(guo)主機的(de)電源管理軟件(jian)允許(xu)，則(ze)大(da)(da)功率(lv)設備(bei)可(ke)(ke)以吸取更大(da)(da)的(de)電流(liu)。

　　某些主機(ji)(ji)系(xi)統(包括下游USB集線器)通過(guo)保險(xian)絲(si)或者有源電(dian)(dian)(dian)流(liu)(liu)檢(jian)測器提供限(xian)流(liu)(liu)功能(neng)。如果USB設(she)備(bei)未經過(guo)枚舉過(guo)程便(bian)從USB端口(kou)吸(xi)取(qu)大電(dian)(dian)(dian)流(liu)(liu)(超過(guo)1個單位(wei)負(fu)載(zai))，則主機(ji)(ji)會檢(jian)測到過(guo)流(liu)(liu)狀(zhuang)態，并會關閉正(zheng)在(zai)使用的(de)一個或多(duo)個USB端口(kou)。市(shi)場上供應的(de)許多(duo)USB設(she)備(bei)，包括獨(du)立電(dian)(dian)(dian)池充電(dian)(dian)(dian)器，都沒有功能(neng)控(kong)制器來處理(li)枚舉過(guo)程，但吸(xi)取(qu)的(de)電(dian)(dian)(dian)流(liu)(liu)卻超過(guo)了100mA。在(zai)這種不恰當(dang)的(de)條(tiao)件(jian)下，這些設(she)備(bei)可能(neng)導致主機(ji)(ji)出(chu)現問題(ti)。例如，如果一個吸(xi)取(qu)500mA電(dian)(dian)(dian)流(liu)(liu)的(de)設(she)備(bei)插入總線供電(dian)(dian)(dian)的(de)USB集線器，而(er)且(qie)未進行正(zheng)確的(de)枚舉過(guo)程，則可能(neng)導致集線器端口(kou)和主機(ji)(ji)端口(kou)同時過(guo)載(zai)。

　　主機(ji)操(cao)作系統采用高級電(dian)(dian)源管理時情況(kuang)會更加復(fu)雜，特(te)別是(shi)(shi)筆記本電(dian)(dian)腦(nao)，它總(zong)是(shi)(shi)希望端口電(dian)(dian)流盡可能低(di)。在某些節電(dian)(dian)模(mo)式(shi)下，計算機(ji)會向(xiang)USB設(she)備發(fa)出(chu)掛起命令，而后則認為(wei)設(she)備進入了低(di)功(gong)耗(hao)模(mo)式(shi)。設(she)備中包含一個能與(yu)主機(ji)進行通信的功(gong)能控制器始終是(shi)(shi)一個比較好(hao)的做法(fa)，即使對于低(di)功(gong)耗(hao)設(she)備來說也(ye)是(shi)(shi)如此。

　　USB 2.0規(gui)(gui)范(fan)非常全面，規(gui)(gui)定(ding)了電(dian)(dian)(dian)(dian)源的(de)(de)質量、連接(jie)器(qi)構造、電(dian)(dian)(dian)(dian)纜材質、容(rong)許的(de)(de)電(dian)(dian)(dian)(dian)壓跌落以及浪涌電(dian)(dian)(dian)(dian)流(liu)等。低(di)電(dian)(dian)(dian)(dian)流(liu)和(he)大電(dian)(dian)(dian)(dian)流(liu)端(duan)(duan)口具(ju)有(you)不同的(de)(de)電(dian)(dian)(dian)(dian)源指標。這主要是由主機和(he)負載(zai)間的(de)(de)連接(jie)器(qi)和(he)電(dian)(dian)(dian)(dian)纜上的(de)(de)電(dian)(dian)(dian)(dian)壓跌落決定(ding)的(de)(de)，并包括由USB供(gong)電(dian)(dian)(dian)(dian)的(de)(de)集(ji)線器(qi)上產生的(de)(de)電(dian)(dian)(dian)(dian)壓跌落。包括計算機或者自供(gong)電(dian)(dian)(dian)(dian)USB集(ji)線器(qi)在內的(de)(de)主機，都具(ju)有(you)大電(dian)(dian)(dian)(dian)流(liu)端(duan)(duan)口，可提供(gong)最大500mA的(de)(de)電(dian)(dian)(dian)(dian)流(liu)。無(wu)源、總線供(gong)電(dian)(dian)(dian)(dian)的(de)(de)USB集(ji)線器(qi)具(ju)有(you)低(di)電(dian)(dian)(dian)(dian)流(liu)端(duan)(duan)口。表1列出了USB大電(dian)(dian)(dian)(dian)流(liu)和(he)低(di)電(dian)(dian)(dian)(dian)流(liu)端(duan)(duan)口上游端(duan)(duan)(電(dian)(dian)(dian)(dian)源)引(yin)腳允許的(de)(de)電(dian)(dian)(dian)(dian)壓容(rong)限。

　　表1. USB 2.0規(gui)范電源(yuan)質量標準(zhun)

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(yu)上游端(duan)主機或(huo)集線器(qi)端(duan)口的連(lian)接器(qi)引腳。電纜和(he)連(lian)接器(qi)上的I x R跌落需(xu)另(ling)外考慮。

       在符合USB 2.0規范的(de)(de)主機(ji)(ji)中，大(da)功(gong)率端(duan)口的(de)(de)上游(you)端(duan)具有120μF、低ESR電(dian)(dian)(dian)(dian)容。所連接的(de)(de)USB設(she)備的(de)(de)輸入電(dian)(dian)(dian)(dian)容限制在10μF以內(nei)，在最初的(de)(de)負載連接階(jie)段，允許負載從(cong)主機(ji)(ji)(或自供(gong)電(dian)(dian)(dian)(dian)集線器)吸取的(de)(de)最大(da)電(dian)(dian)(dian)(dian)荷數(shu)為(wei)50μC。這樣一來，當新設(she)備連接至USB端(duan)口時(shi)，上游(you)端(duan)口的(de)(de)瞬態電(dian)(dian)(dian)(dian)壓跌落小于0.5V。如果負載正常工作時(shi)需要更大(da)的(de)(de)輸入電(dian)(dian)(dian)(dian)容，則(ze)必(bi)須提供(gong)浪涌電(dian)(dian)(dian)(dian)流限制器，以保證(zheng)對更大(da)的(de)(de)電(dian)(dian)(dian)(dian)容充電(dian)(dian)(dian)(dian)時(shi)電(dian)(dian)(dian)(dian)流不會超過(guo)100mA。

　　當USB端口帶有一個總(zong)(zong)線(xian)供電(dian)的USB集(ji)線(xian)器(qi)(qi)，集(ji)線(xian)器(qi)(qi)上(shang)接(jie)了(le)低功耗設備時(shi)，USB口上(shang)允(yun)許(xu)的直流(liu)電(dian)壓(ya)跌(die)落如圖1所示。大功率(lv)負(fu)載與總(zong)(zong)線(xian)供電(dian)的集(ji)線(xian)器(qi)(qi)連接(jie)時(shi)，電(dian)壓(ya)跌(die)落將超過(guo)圖1給(gei)出的指標，并會引起總(zong)(zong)線(xian)過(guo)載。

　　
圖(tu)1. 主機至(zhi)低功(gong)率負載的電(dian)壓跌落(luo)大于(yu)圖(tu)中(zhong)給出的允許直流電(dian)壓跌落(luo)時(shi)，會(hui)引起總線過載

　　電池充電要求

　　單節鋰離子(zi)和鋰聚合物電(dian)池

　　如今的(de)(de)鋰電(dian)池充(chong)電(dian)至(zhi)最大額定容量(liang)后，其電(dian)壓(ya)通常為4.1V至(zhi)4.2V之間。當前市(shi)場上正在出售的(de)(de)、更新(xin)的(de)(de)、容量(liang)更大的(de)(de)電(dian)池，其電(dian)壓(ya)范圍在4.3V至(zhi)4.4V之間。典型的(de)(de)棱柱形(xing)鋰離子(Li+)和鋰聚合物(Li-Poly)電(dian)池容量(liang)為600mAh至(zhi)1400mAh。

　　對Li+和Li-Poly電(dian)(dian)(dian)(dian)(dian)(dian)池來說(shuo)，首選的充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)曲線(xian)是從(cong)恒(heng)流(liu)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)開(kai)始，一直持(chi)續到(dao)(dao)電(dian)(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)達到(dao)(dao)額定(ding)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(qi)(qi)對電(dian)(dian)(dian)(dian)(dian)(dian)池兩端的電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)進(jin)行調(diao)節。這兩種調(diao)節方(fang)式(shi)(shi)構成(cheng)了恒(heng)流(liu)(CC)恒(heng)壓(ya)(ya)(CV)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)方(fang)式(shi)(shi)。因此，這種類型的充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)通(tong)常稱為(wei)CCCV充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)。CCCV充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)進(jin)入CV模(mo)(mo)式(shi)(shi)后，電(dian)(dian)(dian)(dian)(dian)(dian)池的充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)開(kai)始下降。若采用(yong)0.5C至1.5C的典型充(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(dang)電(dian)(dian)(dian)(dian)(dian)(dian)池達到(dao)(dao)其(qi)充(chong)(chong)(chong)(chong)滿容量(liang)的80%至90%時，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)由(you)CC模(mo)(mo)式(shi)(shi)轉換(huan)為(wei)CV模(mo)(mo)式(shi)(shi)。充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)一旦進(jin)入CV充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)模(mo)(mo)式(shi)(shi)，則對電(dian)(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)進(jin)行監(jian)視；當(dang)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)達到(dao)(dao)最低門限(幾毫(hao)安或者幾十毫(hao)安)時，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)終止充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)。鋰電(dian)(dian)(dian)(dian)(dian)(dian)池的典型充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)曲線(xian)如圖2所示。

　　
圖2. 使用CCCV充(chong)電(dian)器對Li+電(dian)池充(chong)電(dian)時的典型曲線

　　從(cong)圖(tu)1所示(shi)的(de)(de)USB電壓(ya)跌落指標可以看出，端口(kou)(kou)供電集線(xian)器的(de)(de)下游低功率(lv)端口(kou)(kou)電壓(ya)不具(ju)備足夠的(de)(de)余量(liang)，很難(nan)將電池充至4.2V。充電通路上存(cun)在的(de)(de)小量(liang)額(e)外電阻會(hui)妨礙正常充電。

　　Li+和Li-Poly電池(chi)應在合適的(de)溫(wen)(wen)度(du)(du)(du)下進行充(chong)(chong)電。制造商(shang)推薦(jian)的(de)最(zui)(zui)高充(chong)(chong)電溫(wen)(wen)度(du)(du)(du)通常為+45°C至+55°C之間(jian)，允許的(de)最(zui)(zui)大放電溫(wen)(wen)度(du)(du)(du)可(ke)再高出10°C左右。這些電池(chi)使(shi)用的(de)材(cai)料，化學(xue)性質非常活(huo)潑，如果電池(chi)溫(wen)(wen)度(du)(du)(du)超過+70°C，會(hui)發(fa)生燃燒。鋰電池(chi)充(chong)(chong)電器應具備熱關斷電路，該電路監視電池(chi)溫(wen)(wen)度(du)(du)(du)，如果電池(chi)溫(wen)(wen)度(du)(du)(du)超過制造商(shang)推薦(jian)的(de)最(zui)(zui)大充(chong)(chong)電溫(wen)(wen)度(du)(du)(du)時，則(ze)終(zhong)止(zhi)充(chong)(chong)電。

　　鎳(nie)氫電池（NiMH）

　　NiMH電(dian)(dian)(dian)(dian)(dian)池(chi)比鋰電(dian)(dian)(dian)(dian)(dian)池(chi)要(yao)重一些，其能量密度也比鋰電(dian)(dian)(dian)(dian)(dian)池(chi)低。一直以來，NiMH電(dian)(dian)(dian)(dian)(dian)池(chi)比鋰電(dian)(dian)(dian)(dian)(dian)池(chi)要(yao)便宜，但是(shi)最近(jin)二者的(de)價格差在縮小。NiMH電(dian)(dian)(dian)(dian)(dian)池(chi)具(ju)有標準尺寸，在大多數應用中可直接替換堿性(xing)電(dian)(dian)(dian)(dian)(dian)池(chi)。每節(jie)電(dian)(dian)(dian)(dian)(dian)池(chi)的(de)標稱電(dian)(dian)(dian)(dian)(dian)壓為1.2V，充(chong)滿后會達到1.5V。

　　通常采用恒流源對NiMH電(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)。當達到(dao)充(chong)滿狀(zhuang)態(tai)(tai)時(shi)，會發生放熱(re)化學反應，并導致電(dian)(dian)(dian)池(chi)(chi)溫度(du)上升，電(dian)(dian)(dian)池(chi)(chi)端(duan)電(dian)(dian)(dian)壓(ya)降低。可檢測(ce)電(dian)(dian)(dian)池(chi)(chi)溫度(du)上升速率或(huo)者負向(xiang)電(dian)(dian)(dian)壓(ya)變(bian)化率，并用來終止充(chong)電(dian)(dian)(dian)。這些(xie)充(chong)電(dian)(dian)(dian)終止方法分(fen)別稱為dT/dt和-ΔV。充(chong)電(dian)(dian)(dian)速率非(fei)常低時(shi)，dT/dt和-ΔV不太明顯(xian)(xian)，很難(nan)精確檢測(ce)到(dao)。電(dian)(dian)(dian)池(chi)(chi)開始進入過充(chong)狀(zhuang)態(tai)(tai)時(shi)，dT/dt和-ΔV響應開始顯(xian)(xian)現(xian)。此時(shi)如果繼續(xu)充(chong)電(dian)(dian)(dian)，將損(sun)壞(huai)電(dian)(dian)(dian)池(chi)(chi)。

　　終止檢測(ce)在(zai)(zai)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)速率(lv)大(da)于C/3時(shi)要比低充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)速率(lv)時(shi)容易得多(duo)。溫(wen)度(du)上(shang)(shang)升速率(lv)大(da)約(yue)為(wei)(wei)1°C/分鐘，-ΔV響應也(ye)比低充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)速率(lv)時(shi)更明顯。快充(chong)(chong)(chong)(chong)結束(shu)后，建議以(yi)更小的(de)電(dian)(dian)(dian)(dian)(dian)流再充(chong)(chong)(chong)(chong)一段時(shi)間，以(yi)徹底(di)充(chong)(chong)(chong)(chong)足電(dian)(dian)(dian)(dian)(dian)池(chi)(補(bu)足充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian))。補(bu)足充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)階段結束(shu)后，采(cai)用C/20或者(zhe)C/30的(de)涓充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)流來補(bu)償(chang)自放電(dian)(dian)(dian)(dian)(dian)效(xiao)應，使(shi)電(dian)(dian)(dian)(dian)(dian)池(chi)維持在(zai)(zai)充(chong)(chong)(chong)(chong)滿狀態。圖3所示為(wei)(wei)采(cai)用DS2712 NiMH充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)對(dui)NiMH電(dian)(dian)(dian)(dian)(dian)池(chi)(事先已充(chong)(chong)(chong)(chong)了一部分電(dian)(dian)(dian)(dian)(dian))進行充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)的(de)電(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)壓曲(qu)線(xian)。在(zai)(zai)該圖中，上(shang)(shang)面一條曲(qu)線(xian)的(de)數據在(zai)(zai)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流正在(zai)(zai)灌入電(dian)(dian)(dian)(dian)(dian)池(chi)時(shi)獲得，下面那條曲(qu)線(xian)的(de)數據在(zai)(zai)切斷電(dian)(dian)(dian)(dian)(dian)流時(shi)測(ce)得。在(zai)(zai)DS2712中，該電(dian)(dian)(dian)(dian)(dian)壓差(cha)被用來區分NiMH電(dian)(dian)(dian)(dian)(dian)池(chi)和堿(jian)性電(dian)(dian)(dian)(dian)(dian)池(chi)。如果檢測(ce)到堿(jian)性電(dian)(dian)(dian)(dian)(dian)池(chi)，則DS2712不會對(dui)它進行充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)。

　　
圖3. 采(cai)用DS2712充(chong)電控(kong)制器對NiMH電池充(chong)電

　　開關與線性

　　USB 2.0規范允許低功率端口提(ti)供(gong)最(zui)大100mA電(dian)(dian)(dian)流，大功率端口提(ti)供(gong)最(zui)大500mA電(dian)(dian)(dian)流。如果(guo)采(cai)用線(xian)性調整(zheng)器(qi)(qi)件來調節電(dian)(dian)(dian)池充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流，這(zhe)(zhe)也就是最(zui)大可提(ti)供(gong)的(de)充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流。線(xian)性調整(zheng)器(qi)(qi)件(圖4)的(de)功耗為P = VQ x IBATT。這(zhe)(zhe)會造成調整(zheng)管(guan)發熱，可能需要安裝散熱器(qi)(qi)，以防止過熱。

　　
圖4. 功耗等(deng)于電池(chi)充電電流乘(cheng)以調整管兩(liang)端的(de)電壓

　　對(dui)應5V標(biao)稱(cheng)輸入電(dian)(dian)壓，調整器件消耗的功率與(yu)電(dian)(dian)池類(lei)型、數量(liang)和電(dian)(dian)池電(dian)(dian)壓有關。

　　
圖5. 采(cai)用5.0V電壓的USB端(duan)口(kou)對NiMH電池充(chong)電時，線性調整器件(jian)的功耗

     標稱輸(shu)入(ru)電(dian)(dian)(dian)(dian)壓(ya)為(wei)5.0V時，線性USB充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)對(dui)NiMH電(dian)(dian)(dian)(dian)池充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)的(de)(de)功耗計算結果如圖5所示。對(dui)單節(jie)電(dian)(dian)(dian)(dian)池充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)時，線性充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)的(de)(de)效(xiao)(xiao)率(lv)僅為(wei)30%；對(dui)兩節(jie)電(dian)(dian)(dian)(dian)池充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)時，效(xiao)(xiao)率(lv)為(wei)60%。用500mA電(dian)(dian)(dian)(dian)流(liu)對(dui)單節(jie)電(dian)(dian)(dian)(dian)池充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)時，功耗會(hui)高(gao)達2W。這樣的(de)(de)功耗通常需要加(jia)散(san)(san)熱(re)器(qi)。功耗為(wei)2W時，熱(re)阻為(wei)+20°C/W的(de)(de)散(san)(san)熱(re)器(qi)在+25°C環境溫度(du)下(xia)會(hui)被加(jia)熱(re)至大(da)約(yue)+65°C，要得到滿(man)額性能，還需要有流(liu)動空氣來(lai)協助其散(san)(san)熱(re)。處(chu)于空氣靜止的(de)(de)封(feng)閉空間(jian)內，溫度(du)會(hui)更高(gao)。

　　采用基于開關調節器(qi)的(de)充(chong)電器(qi)可解(jie)決多個(ge)問(wen)題。首先，與線性充(chong)電器(qi)相比，能夠以更快的(de)速率、更大的(de)電流對電池進(jin)行(xing)(xing)充(chong)電(圖(tu)6)。由(you)(you)于功耗較低、發(fa)熱(re)較少，熱(re)管理方面的(de)問(wen)題也減少了。同時，由(you)(you)于運行(xing)(xing)溫(wen)度降(jiang)低，充(chong)電器(qi)更加可靠。

　　
圖6. 對單節NiMH電(dian)(dian)池充(chong)(chong)電(dian)(dian)時(shi)，線性(xing)充(chong)(chong)電(dian)(dian)器(qi)和(he)開關充(chong)(chong)電(dian)(dian)器(qi)的充(chong)(chong)電(dian)(dian)時(shi)間不同

　　圖6中(zhong)的計算結果基(ji)于以下(xia)條(tiao)件和假(jia)設(she)得(de)到：采(cai)用高功率(lv)USB口最(zui)大允(yun)許電流(liu)(500mA)的大約(yue)90%充電；開(kai)關調節器(qi)采(cai)用非同步整流(liu)的buck轉換器(qi)，具有77%效率(lv)。

　　電路實例

　　圖(tu)7所(suo)示(shi)電(dian)(dian)(dian)路是用于單節(jie)NiMH電(dian)(dian)(dian)池(chi)(chi)充電(dian)(dian)(dian)的(de)開關模式降壓(ya)型調(diao)節(jie)器。它(ta)采用DS2712充電(dian)(dian)(dian)控制器調(diao)節(jie)充電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)和(he)(he)終止充電(dian)(dian)(dian)。充電(dian)(dian)(dian)控制器監視溫度、電(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)壓(ya)和(he)(he)電(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)流(liu)。如果溫度超過+45°C或者(zhe)低于0°C，控制器不會(hui)對電(dian)(dian)(dian)池(chi)(chi)充電(dian)(dian)(dian)。

　　
圖(tu)(tu)7. USB端口對單(dan)節NiMH電池快速充電的原理圖(tu)(tu)

　　如圖7所示(shi)，Q1是降壓(ya)型充(chong)電(dian)器的開關功率晶體管；L1是濾(lv)波(bo)電(dian)感；D1是續流(liu)或整流(liu)二(er)極(ji)(ji)管。輸(shu)入電(dian)容(rong)C1為10μF、超(chao)低(di)ESR的陶瓷(ci)濾(lv)波(bo)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(shi)充(chong)電(dian)器的性能降低(di)。R7是電(dian)流(liu)調(diao)節器檢測放(fang)大(da)器的檢流(liu)電(dian)阻。DS2712的基準電(dian)壓(ya)為0.125V，并具有24mV滯回。通過CSOUT提供閉環(huan)、開關模(mo)式(shi)電(dian)流(liu)控(kong)(kong)制。充(chong)電(dian)控(kong)(kong)制引(yin)腳CC1將Q2的柵(zha)極(ji)(ji)拉(la)低(di)時，使(shi)能Q1的柵(zha)極(ji)(ji)驅動(dong)。Q1和(he)Q2均(jun)為低(di)Vt (柵(zha)-源門限電(dian)壓(ya))的pMOSFET。CC1和(he)CSOUT均(jun)為低(di)電(dian)平時，Q2的漏(lou)-源電(dian)壓(ya)將稍大(da)于Vt。該(gai)電(dian)壓(ya)以及CSOUT的正向壓(ya)降構成(cheng)了Q1的柵(zha)極(ji)(ji)開關電(dian)壓(ya)。

　　CC1為低(di)電(dian)平(ping)時(shi)(shi)，啟(qi)動(dong)電(dian)流(liu)(liu)(liu)閉環(huan)控制。圖8所示(shi)為啟(qi)動(dong)開(kai)關時(shi)(shi)的(de)波形。上方(fang)波形是0.125? (檢流(liu)(liu)(liu)電(dian)阻兩端(duan)的(de)電(dian)壓，下方(fang)波形是Q1漏極至GND的(de)電(dian)壓。開(kai)始時(shi)(shi)，當Q1打開(kai)(CC1和CSOUT均為低(di)電(dian)平(ping))時(shi)(shi)，電(dian)感(gan)電(dian)流(liu)(liu)(liu)向(xiang)上爬升。當電(dian)流(liu)(liu)(liu)增大到(dao)使檢流(liu)(liu)(liu)電(dian)阻兩端(duan)的(de)電(dian)壓達(da)到(dao)0.125V時(shi)(shi)，CSOUT變為高電(dian)平(ping)，開(kai)關關斷。此(ci)后，電(dian)感(gan)電(dian)流(liu)(liu)(liu)開(kai)始下降，直到(dao)檢流(liu)(liu)(liu)電(dian)阻兩端(duan)的(de)電(dian)壓達(da)到(dao)約0.1V，CSOUT又(you)變為低(di)電(dian)平(ping)。只(zhi)要CC1為低(di)電(dian)平(ping)，該過(guo)程將一(yi)直持續(xu)。

　　
圖8. USB NiMH充(chong)電(dian)器的啟(qi)動波形(xing)

　　DS2712的(de)內部狀態機(ji)控制著(zhu)CC1的(de)工作。充(chong)(chong)電開始時(shi)(shi)(shi)，DS2712先對(dui)電池(chi)(chi)(chi)(chi)進(jin)行狀態測試(shi)，以確(que)保(bao)電池(chi)(chi)(chi)(chi)電壓(ya)在1.0V至1.65V之間，并確(que)認溫度在0°C至+45°C之間。如果(guo)電壓(ya)低于1.0V，DS2712將(jiang)以0.125的(de)占空比(bi)拉(la)低CC1，對(dui)電池(chi)(chi)(chi)(chi)緩慢充(chong)(chong)電，以防損壞電池(chi)(chi)(chi)(chi)。一(yi)旦電池(chi)(chi)(chi)(chi)電壓(ya)超過1.0V后，狀態機(ji)轉為快(kuai)(kuai)(kuai)充(chong)(chong)模(mo)式。快(kuai)(kuai)(kuai)充(chong)(chong)時(shi)(shi)(shi)占空比(bi)為31/32，即(ji)大約97%。“跳過”的(de)間隙內進(jin)行電池(chi)(chi)(chi)(chi)阻抗(kang)測試(shi)，以確(que)保(bao)不會對(dui)錯(cuo)誤放入充(chong)(chong)電器的(de)高阻抗(kang)電池(chi)(chi)(chi)(chi)(例如堿性電池(chi)(chi)(chi)(chi))進(jin)行充(chong)(chong)電。檢測到-2mV的(de)-ΔV后，快(kuai)(kuai)(kuai)充(chong)(chong)結束。如果(guo)未檢測到-ΔV，將(jiang)持續快(kuai)(kuai)(kuai)充(chong)(chong)，直(zhi)到快(kuai)(kuai)(kuai)充(chong)(chong)定時(shi)(shi)(shi)器超時(shi)(shi)(shi)，或檢測到過溫或者(zhe)過壓(ya)故(gu)障狀態(包括(kuo)阻抗(kang)不合格)為止。快(kuai)(kuai)(kuai)充(chong)(chong)完成(由于-ΔV或快(kuai)(kuai)(kuai)充(chong)(chong)定時(shi)(shi)(shi)器超時(shi)(shi)(shi)) 后，DS2712進(jin)入定時(shi)(shi)(shi)補足充(chong)(chong)電模(mo)式，占空比(bi)為12.5%，持續時(shi)(shi)(shi)間為所設快(kuai)(kuai)(kuai)充(chong)(chong)定時(shi)(shi)(shi)的(de)一(yi)半。補足充(chong)(chong)電完成后，充(chong)(chong)電器進(jin)入維持模(mo)式，占空比(bi)為1/64，直(zhi)到電池(chi)(chi)(chi)(chi)被拿走或重新上(shang)電。

　　采用圖7所示充電器和大功率USB端口對2100mAh NiMH電(dian)池充電(dian)時(shi)(shi)，快充時(shi)(shi)間(jian)為2小(xiao)時(shi)(shi)多一點，大約3個(ge)小(xiao)時(shi)(shi)完成包括補足充電(dian)在內的全部充電(dian)過(guo)程。從端口吸取的電(dian)流為420mA。如果(guo)需要與主(zhu)機(ji)進行枚舉過(guo)程，并需要大電(dian)流使能操作，可在R9和地之間(jian)串聯一個(ge)開漏極nMOSFET。如果(guo)MOSFET關(guan)斷，則TMR浮空，DS2712進入掛(gua)起狀態。

　　總結

　　對(dui)(dui)于小型(xing)消費(fei)類電子設(she)備的(de)(de)(de)(de)電池(chi)充電而言，USB端口(kou)是(shi)一(yi)個經濟、實用的(de)(de)(de)(de)電源(yuan)。為完全符合USB 2.0規范，連接在(zai)USB端口(kou)上的(de)(de)(de)(de)負(fu)(fu)(fu)載(zai)(zai)必(bi)須能夠(gou)與(yu)主機進行(xing)雙(shuang)向通信。負(fu)(fu)(fu)載(zai)(zai)也必(bi)須符合電源(yuan)管理(li)要求，包括低功耗模式，以(yi)及便于主機確定何時需要從(cong)端口(kou)吸取(qu)(qu)大電流的(de)(de)(de)(de)手(shou)段(duan)。盡(jin)管部分兼容的(de)(de)(de)(de)系統能夠(gou)適應大部分USB主機，但有時會出現意(yi)想不到的(de)(de)(de)(de)結(jie)果。只有很好(hao)地理(li)解USB規范要求和負(fu)(fu)(fu)載(zai)(zai)的(de)(de)(de)(de)期望(wang)，才能在(zai)對(dui)(dui)于規范的(de)(de)(de)(de)兼容性與(yu)負(fu)(fu)(fu)載(zai)(zai)復雜度之(zhi)間取(qu)(qu)得較好(hao)的(de)(de)(de)(de)平衡。