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)源進行(xing)接口。文章回顧(gu)了USB電(dian)(dian)(dian)(dian)源總線的(de)特性，包括(kuo)電(dian)(dian)(dian)(dian)壓(ya)、電(dian)(dian)(dian)(dian)流限制、浪涌電(dian)(dian)(dian)(dian)流、連(lian)接器以及電(dian)(dian)(dian)(dian)纜連(lian)接問(wen)題(ti)。同時介紹了鎳(nie)氫電(dian)(dian)(dian)(dian)池(chi)(NiMH)和鋰電(dian)(dian)(dian)(dian)池(chi)技術、充電(dian)(dian)(dian)(dian)方(fang)法以及充電(dian)(dian)(dian)(dian)終止技術。給出了一個完整的(de)示(shi)例電(dian)(dian)(dian)(dian)路，用于實現USB端口對NiMH電(dian)(dian)(dian)(dian)池(chi)智能充電(dian)(dian)(dian)(dian)，并給出了充電(dian)(dian)(dian)(dian)數據。

　　USB特性

　　USB總線(xian)能夠為低功耗電子設備提供電源(yuan)。總線(xian)電源(yuan)與電網隔離，并且具有很好(hao)的(de)穩定性。但是，可用電流有限，同時負載和(he)主機或電源(yuan)之間(jian)存在(zai)潛(qian)在(zai)的(de)互操(cao)作問題。

　　USB端(duan)口由90?雙(shuang)向差分屏(ping)(ping)蔽(bi)雙(shuang)絞線、VBUS (+5V電源)和(he)地(di)組成。這4條線由鋁箔內屏(ping)(ping)蔽(bi)層和(he)編織網外屏(ping)(ping)蔽(bi)層進行屏(ping)(ping)蔽(bi)。最(zui)(zui)新的USB規范標準(zhun)是2.0版，可以從USB組織免費獲(huo)得(de)。要做到完全符合該規范標準(zhun)，需要通過一個功能控制器來實(shi)現(xian)設備和(he)主(zhu)機間的雙(shuang)向通信。規范定義了(le)1個單(dan)位(wei)負載為100mA (最(zui)(zui)大(da))。任何設備允許吸取(qu)的最(zui)(zui)大(da)電流為5個單(dan)位(wei)負載。

　　USB端(duan)口可(ke)分為低(di)功率端(duan)口和大功率端(duan)口兩類，低(di)功率端(duan)口可(ke)提(ti)供(gong)1個(ge)單(dan)位負(fu)(fu)載(zai)的(de)電流(liu)，大功率端(duan)口可(ke)最多提(ti)供(gong)5個(ge)單(dan)位負(fu)(fu)載(zai)的(de)電流(liu)。當(dang)設(she)備剛連(lian)接到USB端(duan)口時(shi)，枚舉過程(cheng)對器件(jian)(jian)進(jin)行識別(bie)，并確(que)定其(qi)負(fu)(fu)載(zai)要求(qiu)。在此過程(cheng)中，只允許設(she)備從主機吸(xi)取最多1個(ge)單(dan)位負(fu)(fu)載(zai)的(de)電流(liu)。枚舉過程(cheng)完成(cheng)后，如果主機的(de)電源管理軟(ruan)件(jian)(jian)允許，則大功率設(she)備可(ke)以吸(xi)取更大的(de)電流(liu)。

　　某些主(zhu)機系(xi)統(tong)(包括(kuo)下游USB集線器)通(tong)過(guo)(guo)保險絲(si)或者(zhe)有源電流(liu)檢測器提供限流(liu)功(gong)能(neng)。如果(guo)USB設備未經過(guo)(guo)枚舉過(guo)(guo)程(cheng)便從(cong)USB端口(kou)吸(xi)取(qu)大(da)電流(liu)(超(chao)過(guo)(guo)1個單位負載(zai))，則(ze)主(zhu)機會檢測到過(guo)(guo)流(liu)狀態，并會關(guan)閉(bi)正(zheng)在使用的一(yi)個或多個USB端口(kou)。市場上供應的許多USB設備，包括(kuo)獨立電池充電器，都沒(mei)有功(gong)能(neng)控制器來(lai)處理枚舉過(guo)(guo)程(cheng)，但吸(xi)取(qu)的電流(liu)卻超(chao)過(guo)(guo)了100mA。在這種(zhong)不恰當的條件下，這些設備可能(neng)導致主(zhu)機出現問題(ti)。例如，如果(guo)一(yi)個吸(xi)取(qu)500mA電流(liu)的設備插入(ru)總(zong)線供電的USB集線器，而且(qie)未進行正(zheng)確的枚舉過(guo)(guo)程(cheng)，則(ze)可能(neng)導致集線器端口(kou)和主(zhu)機端口(kou)同(tong)時過(guo)(guo)載(zai)。

　　主機(ji)操作系(xi)統采用高級電(dian)(dian)(dian)源(yuan)管(guan)理時情況(kuang)會(hui)更加復雜(za)，特別(bie)是(shi)筆記本電(dian)(dian)(dian)腦，它總是(shi)希望端口電(dian)(dian)(dian)流盡(jin)可能低。在某些節電(dian)(dian)(dian)模(mo)式下，計算機(ji)會(hui)向(xiang)USB設備發出(chu)掛起命令，而后則認為設備進入了低功耗(hao)模(mo)式。設備中(zhong)包含一(yi)個(ge)能與主機(ji)進行(xing)通信的功能控制器(qi)始終是(shi)一(yi)個(ge)比較(jiao)好的做法，即使對于低功耗(hao)設備來說也是(shi)如此。

　　USB 2.0規范非常全面(mian)，規定(ding)了電(dian)(dian)(dian)源(yuan)的(de)質量、連(lian)接(jie)器構造、電(dian)(dian)(dian)纜材質、容許的(de)電(dian)(dian)(dian)壓跌(die)落以及浪(lang)涌電(dian)(dian)(dian)流(liu)(liu)(liu)等。低(di)電(dian)(dian)(dian)流(liu)(liu)(liu)和大(da)電(dian)(dian)(dian)流(liu)(liu)(liu)端(duan)(duan)口具(ju)有不同的(de)電(dian)(dian)(dian)源(yuan)指標。這主要是(shi)由(you)主機(ji)(ji)和負(fu)載間的(de)連(lian)接(jie)器和電(dian)(dian)(dian)纜上的(de)電(dian)(dian)(dian)壓跌(die)落決定(ding)的(de)，并包(bao)括由(you)USB供電(dian)(dian)(dian)的(de)集(ji)線(xian)器上產生(sheng)的(de)電(dian)(dian)(dian)壓跌(die)落。包(bao)括計算機(ji)(ji)或者自供電(dian)(dian)(dian)USB集(ji)線(xian)器在內的(de)主機(ji)(ji)，都具(ju)有大(da)電(dian)(dian)(dian)流(liu)(liu)(liu)端(duan)(duan)口，可提供最大(da)500mA的(de)電(dian)(dian)(dian)流(liu)(liu)(liu)。無(wu)源(yuan)、總(zong)線(xian)供電(dian)(dian)(dian)的(de)USB集(ji)線(xian)器具(ju)有低(di)電(dian)(dian)(dian)流(liu)(liu)(liu)端(duan)(duan)口。表1列出了USB大(da)電(dian)(dian)(dian)流(liu)(liu)(liu)和低(di)電(dian)(dian)(dian)流(liu)(liu)(liu)端(duan)(duan)口上游端(duan)(duan)(電(dian)(dian)(dian)源(yuan))引腳允許的(de)電(dian)(dian)(dian)壓容限。

　　表1. USB 2.0規范(fan)電源質量(liang)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(yong)于上游端(duan)主(zhu)機或集線器(qi)(qi)端(duan)口的連(lian)接(jie)器(qi)(qi)引腳。電纜和連(lian)接(jie)器(qi)(qi)上的I x R跌落需另(ling)外考慮。

       在符合USB 2.0規(gui)范的主機中，大功率端(duan)口的上游(you)端(duan)具(ju)有120μF、低ESR電(dian)容(rong)(rong)(rong)。所連接(jie)(jie)的USB設(she)備的輸入(ru)電(dian)容(rong)(rong)(rong)限制(zhi)在10μF以內，在最(zui)初的負(fu)載(zai)連接(jie)(jie)階段，允許負(fu)載(zai)從主機(或自供電(dian)集線(xian)器(qi))吸取(qu)的最(zui)大電(dian)荷數為50μC。這(zhe)樣一來，當新設(she)備連接(jie)(jie)至USB端(duan)口時，上游(you)端(duan)口的瞬態電(dian)壓跌落小于0.5V。如(ru)果(guo)負(fu)載(zai)正常工作時需(xu)要更大的輸入(ru)電(dian)容(rong)(rong)(rong)，則必須提(ti)供浪涌(yong)電(dian)流限制(zhi)器(qi)，以保證對更大的電(dian)容(rong)(rong)(rong)充電(dian)時電(dian)流不會超過100mA。

　　當USB端口(kou)帶有一個總線供電(dian)(dian)的(de)USB集(ji)線器，集(ji)線器上(shang)接了低功耗設備時，USB口(kou)上(shang)允許的(de)直流電(dian)(dian)壓(ya)跌(die)落(luo)如圖1所示。大功率負載與總線供電(dian)(dian)的(de)集(ji)線器連接時，電(dian)(dian)壓(ya)跌(die)落(luo)將(jiang)超過(guo)圖1給(gei)出(chu)的(de)指標，并會引起(qi)總線過(guo)載。

　　
圖1. 主機(ji)至低功(gong)率(lv)負載的電壓(ya)跌落(luo)大于(yu)圖中給出的允許直流(liu)電壓(ya)跌落(luo)時(shi)，會引起(qi)總線過載

　　電池充電要求

　　單節(jie)鋰(li)離子和鋰(li)聚(ju)合(he)物電池

　　如今的(de)鋰電(dian)池(chi)(chi)充電(dian)至(zhi)(zhi)最大額定容(rong)量后，其電(dian)壓(ya)通常為(wei)4.1V至(zhi)(zhi)4.2V之間。當(dang)前市場上(shang)正在出(chu)售(shou)的(de)、更新的(de)、容(rong)量更大的(de)電(dian)池(chi)(chi)，其電(dian)壓(ya)范圍在4.3V至(zhi)(zhi)4.4V之間。典型的(de)棱柱(zhu)形鋰離子(Li+)和鋰聚(ju)合物(wu)(Li-Poly)電(dian)池(chi)(chi)容(rong)量為(wei)600mAh至(zhi)(zhi)1400mAh。

　　對(dui)Li+和Li-Poly電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)來說，首選的(de)(de)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)曲(qu)線是從(cong)恒流充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)開始(shi)，一直持續(xu)到電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓達到額(e)定電(dian)(dian)(dian)(dian)(dian)(dian)壓。然后(hou)，充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)對(dui)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)兩端(duan)的(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)壓進行調(diao)節(jie)。這(zhe)兩種調(diao)節(jie)方式(shi)構成了恒流(CC)恒壓(CV)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)方式(shi)。因此，這(zhe)種類型(xing)(xing)的(de)(de)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)通(tong)常(chang)稱為CCCV充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)。CCCV充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)進入CV模式(shi)后(hou)，電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)的(de)(de)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流開始(shi)下降。若采用0.5C至1.5C的(de)(de)典(dian)型(xing)(xing)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)速(su)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)(chi)達到其充(chong)(chong)(chong)滿容量的(de)(de)80%至90%時，充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)由CC模式(shi)轉換為CV模式(shi)。充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)一旦進入CV充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)模式(shi)，則對(dui)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)流進行監視(shi)；當電(dian)(dian)(dian)(dian)(dian)(dian)流達到最低門限(幾毫(hao)安或者幾十毫(hao)安)時，充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)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)(chi)的(de)(de)典(dian)型(xing)(xing)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)曲(qu)線如(ru)圖2所示。

　　
圖(tu)2. 使(shi)用CCCV充電器(qi)對Li+電池充電時(shi)的典型曲線

　　從圖1所示的(de)USB電(dian)壓跌落指標(biao)可以看出，端(duan)口供電(dian)集線器的(de)下游低功率端(duan)口電(dian)壓不具備足夠的(de)余量，很難將電(dian)池(chi)充(chong)至4.2V。充(chong)電(dian)通路(lu)上存在的(de)小量額外電(dian)阻會妨礙正常(chang)充(chong)電(dian)。

　　Li+和Li-Poly電(dian)(dian)池(chi)應(ying)在(zai)合適(shi)的溫度(du)(du)(du)下進行充(chong)(chong)電(dian)(dian)。制(zhi)造商(shang)推薦的最高充(chong)(chong)電(dian)(dian)溫度(du)(du)(du)通(tong)常為+45°C至+55°C之間，允許(xu)的最大放(fang)電(dian)(dian)溫度(du)(du)(du)可再高出10°C左右。這(zhe)些電(dian)(dian)池(chi)使用(yong)的材料，化學性質非常活潑，如果電(dian)(dian)池(chi)溫度(du)(du)(du)超(chao)過+70°C，會(hui)發(fa)生(sheng)燃(ran)燒。鋰電(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)器應(ying)具備熱關(guan)斷(duan)電(dian)(dian)路，該電(dian)(dian)路監視電(dian)(dian)池(chi)溫度(du)(du)(du)，如果電(dian)(dian)池(chi)溫度(du)(du)(du)超(chao)過制(zhi)造商(shang)推薦的最大充(chong)(chong)電(dian)(dian)溫度(du)(du)(du)時，則終(zhong)止充(chong)(chong)電(dian)(dian)。

　　鎳氫電池（NiMH）

　　NiMH電(dian)(dian)池(chi)比鋰(li)電(dian)(dian)池(chi)要(yao)重一(yi)些，其能量密度也比鋰(li)電(dian)(dian)池(chi)低。一(yi)直以來，NiMH電(dian)(dian)池(chi)比鋰(li)電(dian)(dian)池(chi)要(yao)便宜，但是最近(jin)二者的價格(ge)差在縮小(xiao)。NiMH電(dian)(dian)池(chi)具有標準(zhun)尺寸，在大多數應(ying)用中(zhong)可直接替換堿性電(dian)(dian)池(chi)。每節電(dian)(dian)池(chi)的標稱電(dian)(dian)壓為1.2V，充滿后(hou)會達到1.5V。

　　通(tong)常采用恒流源(yuan)對NiMH電(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)。當(dang)達到充(chong)(chong)(chong)滿狀態(tai)時，會發生放(fang)熱化學反應，并導致電(dian)(dian)(dian)池(chi)溫度上(shang)升，電(dian)(dian)(dian)池(chi)端電(dian)(dian)(dian)壓降(jiang)低(di)。可檢測電(dian)(dian)(dian)池(chi)溫度上(shang)升速(su)率或者(zhe)負向電(dian)(dian)(dian)壓變化率，并用來終止充(chong)(chong)(chong)電(dian)(dian)(dian)。這些充(chong)(chong)(chong)電(dian)(dian)(dian)終止方法分別稱為dT/dt和-ΔV。充(chong)(chong)(chong)電(dian)(dian)(dian)速(su)率非常低(di)時，dT/dt和-ΔV不太明顯，很難精確檢測到。電(dian)(dian)(dian)池(chi)開始進入過充(chong)(chong)(chong)狀態(tai)時，dT/dt和-ΔV響應開始顯現。此時如果繼續充(chong)(chong)(chong)電(dian)(dian)(dian)，將(jiang)損壞電(dian)(dian)(dian)池(chi)。

　　終(zhong)止檢測(ce)在(zai)(zai)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)速率(lv)大于(yu)C/3時(shi)要比(bi)低充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)速率(lv)時(shi)容易得多。溫度上升速率(lv)大約為(wei)(wei)1°C/分(fen)鐘，-ΔV響(xiang)應(ying)(ying)也比(bi)低充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)速率(lv)時(shi)更(geng)明顯。快充(chong)(chong)(chong)結(jie)束(shu)后，建議以更(geng)小的(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流再充(chong)(chong)(chong)一(yi)段(duan)時(shi)間，以徹底充(chong)(chong)(chong)足(zu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(補足(zu)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian))。補足(zu)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)階段(duan)結(jie)束(shu)后，采(cai)用(yong)C/20或者(zhe)C/30的(de)(de)涓充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流來補償自放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)效應(ying)(ying)，使電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)維(wei)持(chi)在(zai)(zai)充(chong)(chong)(chong)滿狀(zhuang)態。圖(tu)3所示為(wei)(wei)采(cai)用(yong)DS2712 NiMH充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)對(dui)NiMH電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(事先已充(chong)(chong)(chong)了一(yi)部分(fen)電(dian)(dian)(dian)(dian)(dian)(dian)(dian))進(jin)(jin)行充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)的(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)曲(qu)(qu)線。在(zai)(zai)該圖(tu)中，上面一(yi)條曲(qu)(qu)線的(de)(de)數據(ju)在(zai)(zai)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流正在(zai)(zai)灌入電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)時(shi)獲(huo)得，下面那(nei)條曲(qu)(qu)線的(de)(de)數據(ju)在(zai)(zai)切(qie)斷(duan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流時(shi)測(ce)得。在(zai)(zai)DS2712中，該電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)差被用(yong)來區(qu)分(fen)NiMH電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)和堿性電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)。如果檢測(ce)到堿性電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)，則DS2712不會對(dui)它進(jin)(jin)行充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)。

　　
圖(tu)3. 采用DS2712充電控制器對NiMH電池充電

　　開關與線性

　　USB 2.0規范允許(xu)低功(gong)率(lv)(lv)端口提(ti)供最大100mA電(dian)(dian)流(liu)(liu)，大功(gong)率(lv)(lv)端口提(ti)供最大500mA電(dian)(dian)流(liu)(liu)。如果(guo)采用線性調(diao)整(zheng)器件(jian)來調(diao)節電(dian)(dian)池(chi)充(chong)電(dian)(dian)電(dian)(dian)流(liu)(liu)，這也就是最大可提(ti)供的充(chong)電(dian)(dian)電(dian)(dian)流(liu)(liu)。線性調(diao)整(zheng)器件(jian)(圖4)的功(gong)耗(hao)為P = VQ x IBATT。這會造成(cheng)調(diao)整(zheng)管發熱(re)，可能(neng)需要安裝散熱(re)器，以防止過(guo)熱(re)。

　　
圖4. 功耗等于(yu)電池充電電流乘以調整管兩端的電壓

　　對應5V標(biao)稱(cheng)輸(shu)入電(dian)(dian)壓，調(diao)整器件消耗的(de)功(gong)率(lv)與電(dian)(dian)池(chi)類(lei)型、數(shu)量和電(dian)(dian)池(chi)電(dian)(dian)壓有關。

　　
圖5. 采用5.0V電壓的USB端口(kou)對NiMH電池充電時，線性調整器件的功(gong)耗

     標稱輸入電(dian)(dian)(dian)壓為(wei)5.0V時(shi)，線性USB充(chong)(chong)電(dian)(dian)(dian)器對(dui)NiMH電(dian)(dian)(dian)池(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)的(de)(de)(de)功耗(hao)計(ji)算結果如圖5所示。對(dui)單節電(dian)(dian)(dian)池(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)時(shi)，線性充(chong)(chong)電(dian)(dian)(dian)器的(de)(de)(de)效(xiao)(xiao)率僅為(wei)30%；對(dui)兩節電(dian)(dian)(dian)池(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)時(shi)，效(xiao)(xiao)率為(wei)60%。用500mA電(dian)(dian)(dian)流對(dui)單節電(dian)(dian)(dian)池(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)時(shi)，功耗(hao)會高達2W。這樣(yang)的(de)(de)(de)功耗(hao)通常需(xu)(xu)要(yao)加(jia)散熱器。功耗(hao)為(wei)2W時(shi)，熱阻為(wei)+20°C/W的(de)(de)(de)散熱器在+25°C環境溫度(du)下(xia)會被加(jia)熱至大約+65°C，要(yao)得到滿額性能，還(huan)需(xu)(xu)要(yao)有(you)流動空氣(qi)來(lai)協助其散熱。處于空氣(qi)靜(jing)止的(de)(de)(de)封閉空間(jian)內，溫度(du)會更(geng)高。

　　采用(yong)基(ji)于開關調節器(qi)的(de)充(chong)電(dian)器(qi)可(ke)解決多(duo)個問(wen)題。首先，與(yu)線性充(chong)電(dian)器(qi)相比(bi)，能夠以更快的(de)速率(lv)、更大的(de)電(dian)流對電(dian)池進行充(chong)電(dian)(圖6)。由于功耗(hao)較(jiao)低(di)、發熱較(jiao)少，熱管理方面的(de)問(wen)題也減少了。同(tong)時，由于運(yun)行溫度降低(di)，充(chong)電(dian)器(qi)更加可(ke)靠(kao)。

　　
圖(tu)6. 對單(dan)節(jie)NiMH電(dian)池充(chong)電(dian)時，線(xian)性充(chong)電(dian)器和開關充(chong)電(dian)器的充(chong)電(dian)時間(jian)不同(tong)

　　圖6中的計算結(jie)果基于以下條(tiao)件和假(jia)設得(de)到：采(cai)用(yong)高功率USB口(kou)最(zui)大允許電流(500mA)的大約90%充電；開(kai)關調節器采(cai)用(yong)非同步整流的buck轉換(huan)器，具有77%效率。

　　電路實例

　　圖(tu)7所示電(dian)路是(shi)用于(yu)單(dan)節NiMH電(dian)池(chi)(chi)充(chong)(chong)電(dian)的(de)開關模式降壓(ya)型(xing)調(diao)(diao)節器(qi)(qi)。它采(cai)用DS2712充(chong)(chong)電(dian)控(kong)(kong)(kong)制器(qi)(qi)調(diao)(diao)節充(chong)(chong)電(dian)電(dian)流(liu)(liu)和(he)終止(zhi)充(chong)(chong)電(dian)。充(chong)(chong)電(dian)控(kong)(kong)(kong)制器(qi)(qi)監視溫(wen)度、電(dian)池(chi)(chi)電(dian)壓(ya)和(he)電(dian)池(chi)(chi)電(dian)流(liu)(liu)。如果(guo)溫(wen)度超過+45°C或者低(di)于(yu)0°C，控(kong)(kong)(kong)制器(qi)(qi)不會對電(dian)池(chi)(chi)充(chong)(chong)電(dian)。

　　
圖7. USB端(duan)口對單(dan)節(jie)NiMH電(dian)池快速充電(dian)的原理圖

　　如圖7所示，Q1是(shi)(shi)降(jiang)壓(ya)型充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)的開(kai)(kai)(kai)關(guan)功率晶(jing)體管；L1是(shi)(shi)濾波電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)感；D1是(shi)(shi)續流或整流二極管。輸入電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)C1為(wei)(wei)10μF、超低ESR的陶瓷濾波電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)。用鉭(tan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)或者(zhe)其它電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)解電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)替代C1會使充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)的性(xing)能降(jiang)低。R7是(shi)(shi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流調節器(qi)檢(jian)測放大器(qi)的檢(jian)流電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻。DS2712的基準電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)為(wei)(wei)0.125V，并具有(you)24mV滯回。通(tong)過(guo)CSOUT提供閉環、開(kai)(kai)(kai)關(guan)模式(shi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流控制。充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)控制引(yin)腳CC1將(jiang)Q2的柵(zha)(zha)極拉低時(shi)，使能Q1的柵(zha)(zha)極驅動(dong)。Q1和Q2均(jun)為(wei)(wei)低Vt (柵(zha)(zha)-源門限電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya))的pMOSFET。CC1和CSOUT均(jun)為(wei)(wei)低電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)平時(shi)，Q2的漏-源電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)將(jiang)稍大于Vt。該(gai)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)以及CSOUT的正(zheng)向壓(ya)降(jiang)構成(cheng)了Q1的柵(zha)(zha)極開(kai)(kai)(kai)關(guan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)。

　　CC1為低(di)電(dian)(dian)(dian)(dian)平(ping)時，啟動電(dian)(dian)(dian)(dian)流(liu)閉環控制(zhi)。圖8所示為啟動開(kai)(kai)關(guan)時的(de)波(bo)(bo)形(xing)(xing)。上(shang)方(fang)波(bo)(bo)形(xing)(xing)是0.125? (檢流(liu)電(dian)(dian)(dian)(dian)阻兩(liang)端(duan)的(de)電(dian)(dian)(dian)(dian)壓(ya)(ya)，下(xia)方(fang)波(bo)(bo)形(xing)(xing)是Q1漏極至GND的(de)電(dian)(dian)(dian)(dian)壓(ya)(ya)。開(kai)(kai)始(shi)時，當(dang)Q1打開(kai)(kai)(CC1和(he)CSOUT均為低(di)電(dian)(dian)(dian)(dian)平(ping))時，電(dian)(dian)(dian)(dian)感電(dian)(dian)(dian)(dian)流(liu)向上(shang)爬升。當(dang)電(dian)(dian)(dian)(dian)流(liu)增大到(dao)使檢流(liu)電(dian)(dian)(dian)(dian)阻兩(liang)端(duan)的(de)電(dian)(dian)(dian)(dian)壓(ya)(ya)達到(dao)0.125V時，CSOUT變為高電(dian)(dian)(dian)(dian)平(ping)，開(kai)(kai)關(guan)關(guan)斷。此后，電(dian)(dian)(dian)(dian)感電(dian)(dian)(dian)(dian)流(liu)開(kai)(kai)始(shi)下(xia)降，直(zhi)到(dao)檢流(liu)電(dian)(dian)(dian)(dian)阻兩(liang)端(duan)的(de)電(dian)(dian)(dian)(dian)壓(ya)(ya)達到(dao)約0.1V，CSOUT又變為低(di)電(dian)(dian)(dian)(dian)平(ping)。只要(yao)CC1為低(di)電(dian)(dian)(dian)(dian)平(ping)，該過程將一(yi)直(zhi)持續。

　　
圖8. USB NiMH充電器的啟動(dong)波形

　　DS2712的(de)(de)內(nei)部狀(zhuang)態機控制(zhi)著(zhu)CC1的(de)(de)工作。充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)開始(shi)時(shi)，DS2712先對電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)進行狀(zhuang)態測試，以確保電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)在1.0V至(zhi)1.65V之間，并確認溫度在0°C至(zhi)+45°C之間。如(ru)果(guo)(guo)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)低(di)于(yu)1.0V，DS2712將以0.125的(de)(de)占(zhan)空(kong)(kong)(kong)比(bi)拉低(di)CC1，對電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)緩(huan)慢(man)充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)，以防損(sun)壞電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)。一(yi)旦電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)超過(guo)1.0V后(hou)(hou)，狀(zhuang)態機轉為(wei)快(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)模(mo)(mo)式(shi)。快(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)時(shi)占(zhan)空(kong)(kong)(kong)比(bi)為(wei)31/32，即大約(yue)97%。“跳過(guo)”的(de)(de)間隙內(nei)進行電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)阻(zu)抗測試，以確保不(bu)會對錯誤放入(ru)充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)的(de)(de)高阻(zu)抗電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(例如(ru)堿性(xing)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi))進行充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)。檢(jian)測到(dao)(dao)-2mV的(de)(de)-ΔV后(hou)(hou)，快(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)結(jie)束。如(ru)果(guo)(guo)未檢(jian)測到(dao)(dao)-ΔV，將持續快(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)，直到(dao)(dao)快(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)定時(shi)器(qi)超時(shi)，或(huo)檢(jian)測到(dao)(dao)過(guo)溫或(huo)者過(guo)壓(ya)故(gu)障(zhang)狀(zhuang)態(包括阻(zu)抗不(bu)合格(ge))為(wei)止。快(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)完成(由于(yu)-ΔV或(huo)快(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)定時(shi)器(qi)超時(shi)) 后(hou)(hou)，DS2712進入(ru)定時(shi)補足充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)模(mo)(mo)式(shi)，占(zhan)空(kong)(kong)(kong)比(bi)為(wei)12.5%，持續時(shi)間為(wei)所設快(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)定時(shi)的(de)(de)一(yi)半。補足充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)完成后(hou)(hou)，充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)進入(ru)維持模(mo)(mo)式(shi)，占(zhan)空(kong)(kong)(kong)比(bi)為(wei)1/64，直到(dao)(dao)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)被拿走(zou)或(huo)重新上電(dian)(dian)(dian)(dian)(dian)(dian)。

　　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)，快(kuai)充時(shi)間為2小時(shi)多一點，大約3個(ge)小時(shi)完成包(bao)括補足充電(dian)在內(nei)的全部充電(dian)過程。從(cong)端口吸取的電(dian)流為420mA。如果需要與主機(ji)進(jin)(jin)行(xing)枚舉過程，并(bing)需要大電(dian)流使(shi)能操作(zuo)，可(ke)在R9和地之間串聯一個(ge)開(kai)漏極nMOSFET。如果MOSFET關(guan)斷(duan)，則TMR浮(fu)空，DS2712進(jin)(jin)入掛起狀態(tai)。

　　總結

　　對于(yu)小型消費類電子設備的(de)電池充電而言，USB端(duan)口是一個經(jing)濟、實用的(de)電源。為完全符(fu)合(he)USB 2.0規(gui)范(fan)，連接在USB端(duan)口上的(de)負載(zai)必(bi)(bi)須(xu)能夠(gou)與主機進行雙向(xiang)通(tong)信。負載(zai)也必(bi)(bi)須(xu)符(fu)合(he)電源管理要求，包括低(di)功(gong)耗(hao)模式，以及便于(yu)主機確定何時需要從端(duan)口吸取(qu)大(da)電流的(de)手段。盡管部(bu)分兼容(rong)的(de)系統能夠(gou)適(shi)應(ying)大(da)部(bu)分USB主機，但有(you)時會出(chu)現意想不到的(de)結果。只有(you)很(hen)好地理解USB規(gui)范(fan)要求和負載(zai)的(de)期望，才能在對于(yu)規(gui)范(fan)的(de)兼容(rong)性(xing)與負載(zai)復(fu)雜度(du)之(zhi)間取(qu)得較好的(de)平衡。