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)源進行接(jie)(jie)口(kou)(kou)。文(wen)章回(hui)顧(gu)了(le)(le)(le)USB電(dian)(dian)源總(zong)線(xian)的特性，包(bao)括電(dian)(dian)壓、電(dian)(dian)流限制、浪涌電(dian)(dian)流、連(lian)接(jie)(jie)器以(yi)及電(dian)(dian)纜連(lian)接(jie)(jie)問題。同時介紹(shao)了(le)(le)(le)鎳氫(qing)電(dian)(dian)池(chi)(NiMH)和(he)鋰電(dian)(dian)池(chi)技術、充(chong)(chong)電(dian)(dian)方(fang)法以(yi)及充(chong)(chong)電(dian)(dian)終止(zhi)技術。給出了(le)(le)(le)一個完(wan)整的示例電(dian)(dian)路，用于實(shi)現USB端口(kou)(kou)對NiMH電(dian)(dian)池(chi)智能充(chong)(chong)電(dian)(dian)，并給出了(le)(le)(le)充(chong)(chong)電(dian)(dian)數據。

　　USB特性

　　USB總線能(neng)夠為低功耗電(dian)子設備(bei)提供電(dian)源。總線電(dian)源與電(dian)網隔離，并且具有(you)很好的穩定性。但是，可用電(dian)流有(you)限，同時負載和主機或電(dian)源之(zhi)間存在潛在的互操作問題。

　　USB端口由(you)90?雙向(xiang)差分屏(ping)蔽雙絞線、VBUS (+5V電源)和(he)地組成。這(zhe)4條線由(you)鋁箔內屏(ping)蔽層(ceng)和(he)編織網外屏(ping)蔽層(ceng)進行(xing)屏(ping)蔽。最(zui)(zui)新的USB規范(fan)標(biao)準(zhun)是2.0版，可(ke)以從USB組織免費獲得。要(yao)做到完全符(fu)合該規范(fan)標(biao)準(zhun)，需要(yao)通過一個(ge)功能控制(zhi)器來實現設(she)備(bei)和(he)主機(ji)間(jian)的雙向(xiang)通信(xin)。規范(fan)定義了1個(ge)單位(wei)負載為100mA (最(zui)(zui)大)。任何設(she)備(bei)允許(xu)吸取的最(zui)(zui)大電流為5個(ge)單位(wei)負載。

　　USB端口可(ke)分(fen)為低(di)功率(lv)端口和大(da)(da)功率(lv)端口兩類，低(di)功率(lv)端口可(ke)提(ti)供(gong)1個單(dan)(dan)位負載(zai)的(de)(de)電(dian)(dian)流(liu)，大(da)(da)功率(lv)端口可(ke)最多提(ti)供(gong)5個單(dan)(dan)位負載(zai)的(de)(de)電(dian)(dian)流(liu)。當設備剛連接到USB端口時，枚舉過(guo)程對器件進行識別，并確定(ding)其負載(zai)要求。在此過(guo)程中，只允許設備從主機(ji)吸取最多1個單(dan)(dan)位負載(zai)的(de)(de)電(dian)(dian)流(liu)。枚舉過(guo)程完成后，如(ru)果(guo)主機(ji)的(de)(de)電(dian)(dian)源管理(li)軟件允許，則大(da)(da)功率(lv)設備可(ke)以吸取更大(da)(da)的(de)(de)電(dian)(dian)流(liu)。

　　某些主(zhu)(zhu)機系統(包括下游USB集線(xian)器(qi))通過保險(xian)絲或者有(you)源電(dian)流(liu)檢(jian)測器(qi)提供(gong)限流(liu)功能。如果(guo)USB設備未經過枚舉(ju)過程便從USB端(duan)口(kou)吸(xi)(xi)取大電(dian)流(liu)(超(chao)過1個(ge)(ge)單位負載(zai))，則主(zhu)(zhu)機會(hui)檢(jian)測到過流(liu)狀態，并會(hui)關閉(bi)正(zheng)在使用的(de)一(yi)個(ge)(ge)或多個(ge)(ge)USB端(duan)口(kou)。市場上供(gong)應(ying)的(de)許多USB設備，包括獨立(li)電(dian)池(chi)充電(dian)器(qi)，都(dou)沒有(you)功能控(kong)制器(qi)來處理枚舉(ju)過程，但吸(xi)(xi)取的(de)電(dian)流(liu)卻超(chao)過了100mA。在這種不(bu)恰(qia)當的(de)條件(jian)下，這些設備可能導致(zhi)主(zhu)(zhu)機出現問題(ti)。例如，如果(guo)一(yi)個(ge)(ge)吸(xi)(xi)取500mA電(dian)流(liu)的(de)設備插入總線(xian)供(gong)電(dian)的(de)USB集線(xian)器(qi)，而(er)且(qie)未進(jin)行正(zheng)確的(de)枚舉(ju)過程，則可能導致(zhi)集線(xian)器(qi)端(duan)口(kou)和主(zhu)(zhu)機端(duan)口(kou)同時過載(zai)。

　　主機操作系統采(cai)用高級電(dian)源(yuan)管理時情況會(hui)更加復(fu)雜，特別是筆記本電(dian)腦，它總是希望端(duan)口(kou)電(dian)流盡可能(neng)低。在某些節電(dian)模(mo)式下，計算機會(hui)向USB設(she)(she)備發出掛起命令，而后則認為設(she)(she)備進入了低功耗模(mo)式。設(she)(she)備中包含(han)一(yi)個能(neng)與主機進行通信的(de)功能(neng)控(kong)制器(qi)始終是一(yi)個比較好的(de)做法(fa)，即使(shi)對于低功耗設(she)(she)備來說(shuo)也是如(ru)此(ci)。

　　USB 2.0規范非常全面，規定(ding)了(le)電源(yuan)(yuan)的質量(liang)、連(lian)接器(qi)構造、電纜材質、容許(xu)的電壓跌落以及浪涌電流等(deng)。低電流和大(da)(da)電流端口具(ju)(ju)有(you)不同的電源(yuan)(yuan)指標。這(zhe)主要是由主機和負載間的連(lian)接器(qi)和電纜上的電壓跌落決定(ding)的，并包括(kuo)由USB供電的集線(xian)器(qi)上產生的電壓跌落。包括(kuo)計算機或者自(zi)供電USB集線(xian)器(qi)在內的主機，都具(ju)(ju)有(you)大(da)(da)電流端口，可(ke)提(ti)供最大(da)(da)500mA的電流。無源(yuan)(yuan)、總線(xian)供電的USB集線(xian)器(qi)具(ju)(ju)有(you)低電流端口。表(biao)1列出了(le)USB大(da)(da)電流和低電流端口上游端(電源(yuan)(yuan))引腳允許(xu)的電壓容限。

　　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

       *這些指標適用于上游端主機或集線器(qi)端口的(de)連接(jie)器(qi)引腳。電纜和連接(jie)器(qi)上的(de)I x R跌(die)落(luo)需另外考慮。

       在(zai)符合USB 2.0規范的(de)(de)主機(ji)中，大功(gong)率端口的(de)(de)上游(you)端具(ju)有120μF、低ESR電容(rong)。所連(lian)接(jie)的(de)(de)USB設(she)備的(de)(de)輸入電容(rong)限(xian)制(zhi)在(zai)10μF以內，在(zai)最(zui)初的(de)(de)負(fu)載(zai)連(lian)接(jie)階段(duan)，允許負(fu)載(zai)從主機(ji)(或自供(gong)電集線(xian)器)吸取的(de)(de)最(zui)大電荷數(shu)為50μC。這樣一來，當新設(she)備連(lian)接(jie)至USB端口時，上游(you)端口的(de)(de)瞬態電壓跌(die)落小于0.5V。如果(guo)負(fu)載(zai)正常工作時需要(yao)更大的(de)(de)輸入電容(rong)，則必須(xu)提供(gong)浪(lang)涌電流(liu)(liu)限(xian)制(zhi)器，以保(bao)證對(dui)更大的(de)(de)電容(rong)充電時電流(liu)(liu)不會超過100mA。

　　當USB端口(kou)帶有一個總線(xian)(xian)(xian)供(gong)電的(de)USB集(ji)線(xian)(xian)(xian)器(qi)，集(ji)線(xian)(xian)(xian)器(qi)上接(jie)了低功(gong)耗設備時(shi)，USB口(kou)上允許的(de)直流電壓跌(die)落如圖1所示。大功(gong)率負載與總線(xian)(xian)(xian)供(gong)電的(de)集(ji)線(xian)(xian)(xian)器(qi)連接(jie)時(shi)，電壓跌(die)落將(jiang)超過圖1給(gei)出(chu)的(de)指標，并(bing)會(hui)引(yin)起總線(xian)(xian)(xian)過載。

　　
圖(tu)1. 主(zhu)機至低功(gong)率負(fu)載的電(dian)壓(ya)跌落(luo)大于圖(tu)中給出的允(yun)許直流電(dian)壓(ya)跌落(luo)時，會引起總(zong)線過載

　　電池充電要求

　　單節(jie)鋰離(li)子和鋰聚合物電池

　　如今的(de)(de)鋰電池(chi)(chi)(chi)充電至(zhi)最大(da)(da)額定容(rong)量后，其電壓通常為4.1V至(zhi)4.2V之間。當前市場上(shang)正在出售的(de)(de)、更(geng)新的(de)(de)、容(rong)量更(geng)大(da)(da)的(de)(de)電池(chi)(chi)(chi)，其電壓范(fan)圍(wei)在4.3V至(zhi)4.4V之間。典型的(de)(de)棱柱(zhu)形鋰離子(zi)(Li+)和鋰聚合物(Li-Poly)電池(chi)(chi)(chi)容(rong)量為600mAh至(zhi)1400mAh。

　　對(dui)Li+和(he)Li-Poly電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)來(lai)說，首選的充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)曲線是從恒(heng)流(liu)(liu)(liu)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)開始，一直(zhi)持續到(dao)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)達到(dao)額(e)定電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)。然后，充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器對(dui)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)兩端的電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)進行(xing)調(diao)節(jie)。這(zhe)兩種(zhong)調(diao)節(jie)方式(shi)(shi)構成了恒(heng)流(liu)(liu)(liu)(CC)恒(heng)壓(ya)(CV)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)方式(shi)(shi)。因此(ci)，這(zhe)種(zhong)類型的充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器通(tong)常稱為CCCV充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器。CCCV充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器進入(ru)CV模式(shi)(shi)后，電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)開始下降(jiang)。若采用0.5C至1.5C的典型充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)速率充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)，則當電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)達到(dao)其充(chong)(chong)(chong)滿(man)容量的80%至90%時，充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器由CC模式(shi)(shi)轉(zhuan)換為CV模式(shi)(shi)。充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器一旦進入(ru)CV充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)模式(shi)(shi)，則對(dui)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)進行(xing)監視；當電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)達到(dao)最低門(men)限(幾(ji)毫安(an)或者幾(ji)十毫安(an))時，充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器終止充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)。鋰電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的典型充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)曲線如圖2所示。

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

　　從(cong)圖1所(suo)示的USB電(dian)壓跌(die)落指(zhi)標可以看出，端口供電(dian)集線器(qi)的下游低功率端口電(dian)壓不具備足夠的余量(liang)，很難將電(dian)池(chi)充(chong)至4.2V。充(chong)電(dian)通(tong)路上存在的小量(liang)額外電(dian)阻會妨礙正(zheng)常充(chong)電(dian)。

　　Li+和(he)Li-Poly電(dian)(dian)(dian)池應(ying)在合(he)適的(de)溫(wen)(wen)(wen)度(du)下進行充(chong)(chong)(chong)電(dian)(dian)(dian)。制造(zao)商推薦的(de)最高充(chong)(chong)(chong)電(dian)(dian)(dian)溫(wen)(wen)(wen)度(du)通(tong)常為+45°C至+55°C之間，允許的(de)最大放電(dian)(dian)(dian)溫(wen)(wen)(wen)度(du)可再高出10°C左右。這些電(dian)(dian)(dian)池使用的(de)材料(liao)，化學性質非常活潑，如果電(dian)(dian)(dian)池溫(wen)(wen)(wen)度(du)超過+70°C，會發生燃(ran)燒。鋰電(dian)(dian)(dian)池充(chong)(chong)(chong)電(dian)(dian)(dian)器(qi)應(ying)具備熱(re)關斷電(dian)(dian)(dian)路，該電(dian)(dian)(dian)路監視電(dian)(dian)(dian)池溫(wen)(wen)(wen)度(du)，如果電(dian)(dian)(dian)池溫(wen)(wen)(wen)度(du)超過制造(zao)商推薦的(de)最大充(chong)(chong)(chong)電(dian)(dian)(dian)溫(wen)(wen)(wen)度(du)時，則終止充(chong)(chong)(chong)電(dian)(dian)(dian)。

　　鎳氫電池（NiMH）

　　NiMH電(dian)(dian)池(chi)(chi)比鋰(li)(li)電(dian)(dian)池(chi)(chi)要重一些，其能量密度也(ye)比鋰(li)(li)電(dian)(dian)池(chi)(chi)低。一直以來(lai)，NiMH電(dian)(dian)池(chi)(chi)比鋰(li)(li)電(dian)(dian)池(chi)(chi)要便宜，但是最近二者的價格(ge)差在縮小。NiMH電(dian)(dian)池(chi)(chi)具(ju)有標準尺寸，在大多數(shu)應(ying)用中(zhong)可直接替(ti)換堿(jian)性電(dian)(dian)池(chi)(chi)。每節電(dian)(dian)池(chi)(chi)的標稱電(dian)(dian)壓(ya)為1.2V，充滿后會達到1.5V。

　　通(tong)常(chang)采(cai)用恒流源對NiMH電(dian)池(chi)(chi)(chi)充(chong)(chong)電(dian)。當達到(dao)(dao)充(chong)(chong)滿(man)狀態時，會發生放熱(re)化學反應(ying)(ying)，并導致電(dian)池(chi)(chi)(chi)溫(wen)(wen)度(du)(du)上升，電(dian)池(chi)(chi)(chi)端電(dian)壓(ya)降低。可檢測電(dian)池(chi)(chi)(chi)溫(wen)(wen)度(du)(du)上升速(su)率(lv)或(huo)者負向(xiang)電(dian)壓(ya)變化率(lv)，并用來終止(zhi)充(chong)(chong)電(dian)。這些充(chong)(chong)電(dian)終止(zhi)方法分別稱為(wei)dT/dt和(he)(he)-ΔV。充(chong)(chong)電(dian)速(su)率(lv)非常(chang)低時，dT/dt和(he)(he)-ΔV不太(tai)明顯，很難精確檢測到(dao)(dao)。電(dian)池(chi)(chi)(chi)開始進入過充(chong)(chong)狀態時，dT/dt和(he)(he)-ΔV響應(ying)(ying)開始顯現。此時如果(guo)繼續(xu)充(chong)(chong)電(dian)，將損壞電(dian)池(chi)(chi)(chi)。

　　終止(zhi)檢(jian)測在(zai)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)速(su)率大于C/3時(shi)要比(bi)低(di)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)速(su)率時(shi)容易得多。溫度(du)上(shang)升速(su)率大約為1°C/分(fen)鐘，-ΔV響(xiang)應(ying)也比(bi)低(di)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)速(su)率時(shi)更(geng)明顯。快(kuai)充(chong)(chong)(chong)結束(shu)后，建議以(yi)更(geng)小(xiao)的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)再充(chong)(chong)(chong)一段時(shi)間，以(yi)徹底(di)充(chong)(chong)(chong)足(zu)(zu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(補足(zu)(zu)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian))。補足(zu)(zu)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)階(jie)段結束(shu)后，采(cai)用C/20或者(zhe)C/30的(de)涓充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)來補償自放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)效應(ying)，使電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)維持在(zai)充(chong)(chong)(chong)滿狀態。圖(tu)3所示為采(cai)用DS2712 NiMH充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)對NiMH電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(事先已(yi)充(chong)(chong)(chong)了一部分(fen)電(dian)(dian)(dian)(dian)(dian)(dian)(dian))進(jin)行(xing)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)曲(qu)線。在(zai)該(gai)(gai)圖(tu)中(zhong)，上(shang)面一條曲(qu)線的(de)數據(ju)在(zai)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)正在(zai)灌入(ru)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)時(shi)獲得，下面那條曲(qu)線的(de)數據(ju)在(zai)切斷電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)時(shi)測得。在(zai)DS2712中(zhong)，該(gai)(gai)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)差被用來區分(fen)NiMH電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)和堿性(xing)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)。如果檢(jian)測到堿性(xing)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)，則DS2712不會對它進(jin)行(xing)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)。

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

　　開關與線性

　　USB 2.0規范允許(xu)低功(gong)率(lv)端口(kou)提(ti)供最(zui)大(da)(da)100mA電流(liu)，大(da)(da)功(gong)率(lv)端口(kou)提(ti)供最(zui)大(da)(da)500mA電流(liu)。如果(guo)采用線(xian)性調(diao)整器件(jian)來調(diao)節電池充電電流(liu)，這也就是(shi)最(zui)大(da)(da)可提(ti)供的充電電流(liu)。線(xian)性調(diao)整器件(jian)(圖4)的功(gong)耗為P = VQ x IBATT。這會造成調(diao)整管發熱(re)(re)，可能需要安裝散熱(re)(re)器，以(yi)防止過熱(re)(re)。

　　
圖4. 功耗等于電(dian)池充電(dian)電(dian)流乘以調整管兩端的電(dian)壓

　　對應5V標稱輸入電(dian)壓，調(diao)整器(qi)件消耗的(de)功率與電(dian)池類型、數量和(he)電(dian)池電(dian)壓有關。

　　
圖5. 采用5.0V電壓的USB端口對NiMH電池(chi)充(chong)電時，線性調(diao)整器(qi)件(jian)的功耗

     標稱輸入電(dian)(dian)(dian)壓為(wei)5.0V時，線性USB充電(dian)(dian)(dian)器(qi)(qi)對NiMH電(dian)(dian)(dian)池充電(dian)(dian)(dian)的(de)(de)功(gong)耗(hao)(hao)計算結果如(ru)圖5所示。對單節(jie)電(dian)(dian)(dian)池充電(dian)(dian)(dian)時，線性充電(dian)(dian)(dian)器(qi)(qi)的(de)(de)效率(lv)(lv)僅為(wei)30%；對兩節(jie)電(dian)(dian)(dian)池充電(dian)(dian)(dian)時，效率(lv)(lv)為(wei)60%。用(yong)500mA電(dian)(dian)(dian)流對單節(jie)電(dian)(dian)(dian)池充電(dian)(dian)(dian)時，功(gong)耗(hao)(hao)會(hui)高(gao)達2W。這樣的(de)(de)功(gong)耗(hao)(hao)通常(chang)需(xu)要(yao)加散熱(re)器(qi)(qi)。功(gong)耗(hao)(hao)為(wei)2W時，熱(re)阻(zu)為(wei)+20°C/W的(de)(de)散熱(re)器(qi)(qi)在+25°C環(huan)境溫度(du)下會(hui)被加熱(re)至(zhi)大(da)約(yue)+65°C，要(yao)得到(dao)滿額(e)性能(neng)，還需(xu)要(yao)有流動空(kong)氣來(lai)協助其散熱(re)。處(chu)于空(kong)氣靜止的(de)(de)封(feng)閉空(kong)間(jian)內(nei)，溫度(du)會(hui)更(geng)高(gao)。

　　采(cai)用基于(yu)(yu)開關調節器的(de)充電(dian)器可解決多個問題。首先，與線性充電(dian)器相比，能夠(gou)以更(geng)(geng)快的(de)速率(lv)、更(geng)(geng)大(da)的(de)電(dian)流對電(dian)池進行充電(dian)(圖6)。由于(yu)(yu)功耗較低、發(fa)熱(re)(re)較少，熱(re)(re)管理方面的(de)問題也減少了。同時(shi)，由于(yu)(yu)運(yun)行溫度降低，充電(dian)器更(geng)(geng)加可靠。

　　
圖6. 對單節NiMH電(dian)池充(chong)電(dian)時(shi)，線性充(chong)電(dian)器和開(kai)關充(chong)電(dian)器的充(chong)電(dian)時(shi)間不(bu)同

　　圖6中的計算結果基(ji)于以下(xia)條件和假設得(de)到：采用高(gao)功率USB口最大(da)允許電流(500mA)的大(da)約90%充電；開關調節器(qi)采用非同步整流的buck轉換(huan)器(qi)，具有77%效率。

　　電路實例

　　圖7所示電(dian)(dian)(dian)(dian)路是用(yong)于單節NiMH電(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)的開關模式降壓型調(diao)節器(qi)。它采用(yong)DS2712充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)控制器(qi)調(diao)節充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)和終止充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)。充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)控制器(qi)監視溫(wen)度、電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓和電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)流(liu)。如(ru)果溫(wen)度超過+45°C或(huo)者低于0°C，控制器(qi)不(bu)會對電(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)。

　　
圖(tu)7. USB端口對單節NiMH電池快速充電的原(yuan)理圖(tu)

　　如圖7所示(shi)，Q1是降壓(ya)型(xing)充電(dian)(dian)(dian)(dian)器的(de)開關(guan)功率晶體管；L1是濾波(bo)電(dian)(dian)(dian)(dian)感；D1是續流(liu)或(huo)整流(liu)二極(ji)(ji)管。輸(shu)入電(dian)(dian)(dian)(dian)容(rong)(rong)C1為10μF、超(chao)低ESR的(de)陶瓷濾波(bo)電(dian)(dian)(dian)(dian)容(rong)(rong)。用鉭電(dian)(dian)(dian)(dian)容(rong)(rong)或(huo)者其(qi)它(ta)電(dian)(dian)(dian)(dian)解電(dian)(dian)(dian)(dian)容(rong)(rong)替(ti)代C1會使充電(dian)(dian)(dian)(dian)器的(de)性能降低。R7是電(dian)(dian)(dian)(dian)流(liu)調節器檢測放(fang)大器的(de)檢流(liu)電(dian)(dian)(dian)(dian)阻。DS2712的(de)基(ji)準電(dian)(dian)(dian)(dian)壓(ya)為0.125V，并(bing)具(ju)有24mV滯回。通過CSOUT提供閉(bi)環、開關(guan)模式(shi)電(dian)(dian)(dian)(dian)流(liu)控(kong)制(zhi)。充電(dian)(dian)(dian)(dian)控(kong)制(zhi)引腳(jiao)CC1將Q2的(de)柵極(ji)(ji)拉低時，使能Q1的(de)柵極(ji)(ji)驅動。Q1和Q2均(jun)為低Vt (柵-源(yuan)門限電(dian)(dian)(dian)(dian)壓(ya))的(de)pMOSFET。CC1和CSOUT均(jun)為低電(dian)(dian)(dian)(dian)平時，Q2的(de)漏-源(yuan)電(dian)(dian)(dian)(dian)壓(ya)將稍大于Vt。該(gai)電(dian)(dian)(dian)(dian)壓(ya)以及(ji)CSOUT的(de)正向壓(ya)降構成(cheng)了Q1的(de)柵極(ji)(ji)開關(guan)電(dian)(dian)(dian)(dian)壓(ya)。

　　CC1為(wei)低(di)電(dian)(dian)(dian)平時，啟動電(dian)(dian)(dian)流閉環控(kong)制(zhi)。圖8所示為(wei)啟動開(kai)(kai)關時的波形(xing)。上(shang)(shang)方波形(xing)是(shi)0.125? (檢(jian)流電(dian)(dian)(dian)阻(zu)(zu)兩端的電(dian)(dian)(dian)壓(ya)，下方波形(xing)是(shi)Q1漏極至(zhi)GND的電(dian)(dian)(dian)壓(ya)。開(kai)(kai)始時，當Q1打(da)開(kai)(kai)(CC1和CSOUT均(jun)為(wei)低(di)電(dian)(dian)(dian)平)時，電(dian)(dian)(dian)感電(dian)(dian)(dian)流向上(shang)(shang)爬升。當電(dian)(dian)(dian)流增(zeng)大到(dao)(dao)使檢(jian)流電(dian)(dian)(dian)阻(zu)(zu)兩端的電(dian)(dian)(dian)壓(ya)達到(dao)(dao)0.125V時，CSOUT變(bian)為(wei)高電(dian)(dian)(dian)平，開(kai)(kai)關關斷。此后，電(dian)(dian)(dian)感電(dian)(dian)(dian)流開(kai)(kai)始下降，直(zhi)到(dao)(dao)檢(jian)流電(dian)(dian)(dian)阻(zu)(zu)兩端的電(dian)(dian)(dian)壓(ya)達到(dao)(dao)約0.1V，CSOUT又變(bian)為(wei)低(di)電(dian)(dian)(dian)平。只要CC1為(wei)低(di)電(dian)(dian)(dian)平，該過(guo)程將一直(zhi)持續。

　　
圖8. USB NiMH充電器(qi)的啟動波形(xing)

　　DS2712的內(nei)部狀(zhuang)態機控制著CC1的工作。充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)開(kai)始時(shi)，DS2712先對電(dian)(dian)(dian)(dian)池(chi)(chi)進行狀(zhuang)態測試(shi)，以(yi)確(que)保(bao)電(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)壓在1.0V至(zhi)1.65V之間(jian)(jian)，并確(que)認溫(wen)度在0°C至(zhi)+45°C之間(jian)(jian)。如(ru)果電(dian)(dian)(dian)(dian)壓低于(yu)1.0V，DS2712將(jiang)(jiang)以(yi)0.125的占空比拉低CC1，對電(dian)(dian)(dian)(dian)池(chi)(chi)緩慢充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)，以(yi)防損壞(huai)電(dian)(dian)(dian)(dian)池(chi)(chi)。一旦電(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)壓超(chao)(chao)過1.0V后(hou)，狀(zhuang)態機轉為(wei)(wei)快充(chong)(chong)(chong)模(mo)(mo)式(shi)(shi)。快充(chong)(chong)(chong)時(shi)占空比為(wei)(wei)31/32，即大約(yue)97%。“跳過”的間(jian)(jian)隙內(nei)進行電(dian)(dian)(dian)(dian)池(chi)(chi)阻(zu)(zu)抗測試(shi)，以(yi)確(que)保(bao)不(bu)會(hui)對錯誤放入(ru)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)(qi)的高阻(zu)(zu)抗電(dian)(dian)(dian)(dian)池(chi)(chi)(例如(ru)堿性(xing)電(dian)(dian)(dian)(dian)池(chi)(chi))進行充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)。檢(jian)測到(dao)-2mV的-ΔV后(hou)，快充(chong)(chong)(chong)結束。如(ru)果未檢(jian)測到(dao)-ΔV，將(jiang)(jiang)持(chi)續快充(chong)(chong)(chong)，直到(dao)快充(chong)(chong)(chong)定(ding)(ding)時(shi)器(qi)(qi)超(chao)(chao)時(shi)，或檢(jian)測到(dao)過溫(wen)或者過壓故障狀(zhuang)態(包括阻(zu)(zu)抗不(bu)合格)為(wei)(wei)止。快充(chong)(chong)(chong)完(wan)成(由于(yu)-ΔV或快充(chong)(chong)(chong)定(ding)(ding)時(shi)器(qi)(qi)超(chao)(chao)時(shi)) 后(hou)，DS2712進入(ru)定(ding)(ding)時(shi)補(bu)足充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)模(mo)(mo)式(shi)(shi)，占空比為(wei)(wei)12.5%，持(chi)續時(shi)間(jian)(jian)為(wei)(wei)所設快充(chong)(chong)(chong)定(ding)(ding)時(shi)的一半(ban)。補(bu)足充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)完(wan)成后(hou)，充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)(qi)進入(ru)維持(chi)模(mo)(mo)式(shi)(shi)，占空比為(wei)(wei)1/64，直到(dao)電(dian)(dian)(dian)(dian)池(chi)(chi)被拿走(zou)或重新上電(dian)(dian)(dian)(dian)。

　　采用圖7所示充電器和大功率USB端(duan)口(kou)對2100mAh NiMH電池充電時(shi)，快(kuai)充時(shi)間為(wei)2小時(shi)多一點，大約(yue)3個小時(shi)完成包括補足充電在內的全部充電過(guo)程(cheng)。從(cong)端(duan)口(kou)吸(xi)取的電流(liu)為(wei)420mA。如果需要與主(zhu)機進行枚舉過(guo)程(cheng)，并需要大電流(liu)使能操(cao)作，可在R9和地之間串(chuan)聯一個開漏極(ji)nMOSFET。如果MOSFET關斷，則(ze)TMR浮空，DS2712進入掛起(qi)狀(zhuang)態。

　　總結

　　對于小(xiao)型消費類電(dian)子(zi)設備的(de)(de)電(dian)池充(chong)電(dian)而言，USB端口(kou)(kou)是一個經濟、實用的(de)(de)電(dian)源。為完全符(fu)合USB 2.0規(gui)(gui)范，連接在USB端口(kou)(kou)上的(de)(de)負載(zai)(zai)必須能(neng)夠(gou)與主機進行雙向通信。負載(zai)(zai)也必須符(fu)合電(dian)源管理要求(qiu)，包括低功耗(hao)模式，以(yi)及便于主機確定何時需要從端口(kou)(kou)吸取大(da)電(dian)流的(de)(de)手段。盡(jin)管部(bu)分兼容的(de)(de)系統(tong)能(neng)夠(gou)適應大(da)部(bu)分USB主機，但有時會(hui)出(chu)現意想(xiang)不到的(de)(de)結果。只(zhi)有很好(hao)地理解(jie)USB規(gui)(gui)范要求(qiu)和負載(zai)(zai)的(de)(de)期望，才(cai)能(neng)在對于規(gui)(gui)范的(de)(de)兼容性與負載(zai)(zai)復雜度之間(jian)取得(de)較好(hao)的(de)(de)平(ping)衡。