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)(yuan)進行接(jie)(jie)口。文章回顧了USB電(dian)(dian)(dian)源(yuan)(yuan)總線的特性，包括電(dian)(dian)(dian)壓(ya)、電(dian)(dian)(dian)流(liu)限制、浪涌(yong)電(dian)(dian)(dian)流(liu)、連(lian)接(jie)(jie)器以(yi)及電(dian)(dian)(dian)纜連(lian)接(jie)(jie)問題。同時(shi)介紹了鎳(nie)氫電(dian)(dian)(dian)池(chi)(chi)(NiMH)和鋰電(dian)(dian)(dian)池(chi)(chi)技術(shu)、充電(dian)(dian)(dian)方法以(yi)及充電(dian)(dian)(dian)終止技術(shu)。給出了一個完整的示例電(dian)(dian)(dian)路，用于實現USB端口對NiMH電(dian)(dian)(dian)池(chi)(chi)智(zhi)能充電(dian)(dian)(dian)，并給出了充電(dian)(dian)(dian)數據。

　　USB特性

　　USB總線能夠為(wei)低功耗電(dian)子設備提供電(dian)源(yuan)。總線電(dian)源(yuan)與電(dian)網隔離，并(bing)且(qie)具有很好的(de)穩定性。但是，可用電(dian)流有限，同時負載和主機或電(dian)源(yuan)之間存在潛(qian)在的(de)互操作(zuo)問題。

　　USB端口(kou)由90?雙(shuang)向差分屏蔽(bi)(bi)(bi)雙(shuang)絞線、VBUS (+5V電源)和地組成。這(zhe)4條線由鋁箔內屏蔽(bi)(bi)(bi)層(ceng)和編(bian)織(zhi)網外屏蔽(bi)(bi)(bi)層(ceng)進行屏蔽(bi)(bi)(bi)。最新的(de)USB規范標準是2.0版(ban)，可以從USB組織(zhi)免(mian)費獲得(de)。要做到(dao)完全符(fu)合該規范標準，需(xu)要通(tong)過(guo)一個功能控制(zhi)器(qi)來實(shi)現設備和主機間的(de)雙(shuang)向通(tong)信。規范定義了1個單位(wei)負(fu)載為(wei)100mA (最大)。任何設備允許吸(xi)取的(de)最大電流(liu)為(wei)5個單位(wei)負(fu)載。

　　USB端(duan)(duan)口可分為低功(gong)率(lv)端(duan)(duan)口和大功(gong)率(lv)端(duan)(duan)口兩類，低功(gong)率(lv)端(duan)(duan)口可提供1個單(dan)位(wei)負(fu)載(zai)的電流(liu)，大功(gong)率(lv)端(duan)(duan)口可最多(duo)(duo)提供5個單(dan)位(wei)負(fu)載(zai)的電流(liu)。當(dang)設(she)備(bei)剛連接到USB端(duan)(duan)口時(shi)，枚舉過程(cheng)對器件進行識別，并確定其(qi)負(fu)載(zai)要求。在(zai)此(ci)過程(cheng)中，只允許(xu)(xu)設(she)備(bei)從主機吸取(qu)最多(duo)(duo)1個單(dan)位(wei)負(fu)載(zai)的電流(liu)。枚舉過程(cheng)完成后(hou)，如(ru)果主機的電源管理軟件允許(xu)(xu)，則大功(gong)率(lv)設(she)備(bei)可以吸取(qu)更(geng)大的電流(liu)。

　　某些(xie)主(zhu)(zhu)機(ji)系統(包括下(xia)游(you)USB集(ji)線器(qi)(qi))通過保(bao)險絲或(huo)者(zhe)有(you)源電(dian)流(liu)(liu)檢測(ce)器(qi)(qi)提供限(xian)流(liu)(liu)功(gong)能。如(ru)果USB設備(bei)未(wei)(wei)經過枚舉過程便從USB端(duan)(duan)口吸取(qu)(qu)大電(dian)流(liu)(liu)(超過1個(ge)單位(wei)負(fu)載(zai))，則主(zhu)(zhu)機(ji)會檢測(ce)到過流(liu)(liu)狀(zhuang)態，并會關閉正(zheng)在使用(yong)的(de)(de)一(yi)個(ge)或(huo)多個(ge)USB端(duan)(duan)口。市(shi)場上供應的(de)(de)許多USB設備(bei)，包括獨立電(dian)池(chi)充(chong)電(dian)器(qi)(qi)，都沒(mei)有(you)功(gong)能控制器(qi)(qi)來處理枚舉過程，但(dan)吸取(qu)(qu)的(de)(de)電(dian)流(liu)(liu)卻超過了100mA。在這種不恰當的(de)(de)條件下(xia)，這些(xie)設備(bei)可(ke)能導致主(zhu)(zhu)機(ji)出現問題(ti)。例如(ru)，如(ru)果一(yi)個(ge)吸取(qu)(qu)500mA電(dian)流(liu)(liu)的(de)(de)設備(bei)插入(ru)總線供電(dian)的(de)(de)USB集(ji)線器(qi)(qi)，而且未(wei)(wei)進行正(zheng)確(que)的(de)(de)枚舉過程，則可(ke)能導致集(ji)線器(qi)(qi)端(duan)(duan)口和主(zhu)(zhu)機(ji)端(duan)(duan)口同時(shi)過載(zai)。

　　主機(ji)操作(zuo)系統采(cai)用高級電源(yuan)管理時情況會更加復雜，特別是(shi)筆(bi)記本電腦，它總是(shi)希望端口電流(liu)盡可能低。在(zai)某些節電模(mo)式下，計(ji)算機(ji)會向USB設備(bei)發出掛起命令，而后則認為設備(bei)進入了(le)低功耗模(mo)式。設備(bei)中(zhong)包含一個能與主機(ji)進行(xing)通信的功能控制器始(shi)終(zhong)是(shi)一個比較好的做法，即使對于低功耗設備(bei)來(lai)說也是(shi)如此。

　　USB 2.0規(gui)范非(fei)常全(quan)面，規(gui)定(ding)了(le)電(dian)(dian)(dian)(dian)源的(de)(de)質(zhi)量、連接(jie)器(qi)構(gou)造、電(dian)(dian)(dian)(dian)纜(lan)材質(zhi)、容許的(de)(de)電(dian)(dian)(dian)(dian)壓跌落(luo)(luo)以及浪涌電(dian)(dian)(dian)(dian)流(liu)(liu)等。低電(dian)(dian)(dian)(dian)流(liu)(liu)和(he)大電(dian)(dian)(dian)(dian)流(liu)(liu)端口(kou)具(ju)有(you)(you)(you)不(bu)同的(de)(de)電(dian)(dian)(dian)(dian)源指標。這主(zhu)要是由主(zhu)機(ji)和(he)負載間的(de)(de)連接(jie)器(qi)和(he)電(dian)(dian)(dian)(dian)纜(lan)上的(de)(de)電(dian)(dian)(dian)(dian)壓跌落(luo)(luo)決定(ding)的(de)(de)，并(bing)包括(kuo)由USB供(gong)電(dian)(dian)(dian)(dian)的(de)(de)集線器(qi)上產生的(de)(de)電(dian)(dian)(dian)(dian)壓跌落(luo)(luo)。包括(kuo)計算機(ji)或者自供(gong)電(dian)(dian)(dian)(dian)USB集線器(qi)在內的(de)(de)主(zhu)機(ji)，都具(ju)有(you)(you)(you)大電(dian)(dian)(dian)(dian)流(liu)(liu)端口(kou)，可提供(gong)最大500mA的(de)(de)電(dian)(dian)(dian)(dian)流(liu)(liu)。無源、總線供(gong)電(dian)(dian)(dian)(dian)的(de)(de)USB集線器(qi)具(ju)有(you)(you)(you)低電(dian)(dian)(dian)(dian)流(liu)(liu)端口(kou)。表1列出了(le)USB大電(dian)(dian)(dian)(dian)流(liu)(liu)和(he)低電(dian)(dian)(dian)(dian)流(liu)(liu)端口(kou)上游端(電(dian)(dian)(dian)(dian)源)引腳允許的(de)(de)電(dian)(dian)(dian)(dian)壓容限。

　　表1. USB 2.0規范電(dian)源質(zhi)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(zhi)標適用(yong)于上游端(duan)主機或集線器(qi)端(duan)口的(de)連接器(qi)引腳。電纜和連接器(qi)上的(de)I x R跌落需(xu)另外(wai)考(kao)慮。

       在符合USB 2.0規范的(de)(de)(de)(de)主機中，大(da)功(gong)率端口(kou)的(de)(de)(de)(de)上(shang)游端具有(you)120μF、低(di)ESR電(dian)(dian)(dian)容。所連(lian)(lian)接(jie)(jie)的(de)(de)(de)(de)USB設(she)備的(de)(de)(de)(de)輸(shu)入電(dian)(dian)(dian)容限制在10μF以(yi)內，在最(zui)初的(de)(de)(de)(de)負載(zai)連(lian)(lian)接(jie)(jie)階段，允(yun)許負載(zai)從主機(或自(zi)供電(dian)(dian)(dian)集線(xian)器(qi))吸取的(de)(de)(de)(de)最(zui)大(da)電(dian)(dian)(dian)荷(he)數為50μC。這樣一(yi)來(lai)，當(dang)新(xin)設(she)備連(lian)(lian)接(jie)(jie)至USB端口(kou)時(shi)，上(shang)游端口(kou)的(de)(de)(de)(de)瞬態電(dian)(dian)(dian)壓跌落(luo)小于0.5V。如果負載(zai)正常工作(zuo)時(shi)需要更(geng)大(da)的(de)(de)(de)(de)輸(shu)入電(dian)(dian)(dian)容，則必須提供浪涌電(dian)(dian)(dian)流(liu)限制器(qi)，以(yi)保證對更(geng)大(da)的(de)(de)(de)(de)電(dian)(dian)(dian)容充電(dian)(dian)(dian)時(shi)電(dian)(dian)(dian)流(liu)不(bu)會(hui)超過100mA。

　　當USB端口(kou)帶有一個總線(xian)供電(dian)(dian)的USB集(ji)線(xian)器，集(ji)線(xian)器上接(jie)了低功耗設備時，USB口(kou)上允許(xu)的直流電(dian)(dian)壓跌(die)落如圖1所示(shi)。大功率(lv)負(fu)載與總線(xian)供電(dian)(dian)的集(ji)線(xian)器連接(jie)時，電(dian)(dian)壓跌(die)落將超過圖1給出的指標(biao)，并會引(yin)起(qi)總線(xian)過載。

　　
圖1. 主機至(zhi)低功率負載的(de)電壓跌落(luo)大于圖中給出的(de)允許直流(liu)電壓跌落(luo)時(shi)，會(hui)引起(qi)總線過載

　　電池充電要求

　　單節鋰(li)離子和鋰(li)聚合物電(dian)池

　　如(ru)今的鋰(li)電池充電至(zhi)最大額定容(rong)(rong)量(liang)后，其電壓通常為4.1V至(zhi)4.2V之間。當前(qian)市場上(shang)正在(zai)出(chu)售的、更新的、容(rong)(rong)量(liang)更大的電池，其電壓范(fan)圍在(zai)4.3V至(zhi)4.4V之間。典(dian)型的棱(leng)柱形(xing)鋰(li)離子(zi)(Li+)和鋰(li)聚(ju)合物(Li-Poly)電池容(rong)(rong)量(liang)為600mAh至(zhi)1400mAh。

　　對Li+和(he)Li-Poly電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)來說(shuo)，首選的(de)(de)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)曲(qu)線(xian)是從恒(heng)流(liu)(liu)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)開始(shi)，一(yi)直持續到(dao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)達到(dao)額定電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)。然后，充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)對電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)兩端的(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)進行調節。這兩種調節方式(shi)構成了恒(heng)流(liu)(liu)(CC)恒(heng)壓(ya)(CV)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)方式(shi)。因此，這種類型的(de)(de)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)通常(chang)稱(cheng)為(wei)CCCV充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)。CCCV充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)進入CV模(mo)式(shi)后，電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)的(de)(de)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)開始(shi)下降。若(ruo)采用0.5C至1.5C的(de)(de)典型充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)速率(lv)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)，則當(dang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)達到(dao)其充(chong)(chong)(chong)滿容(rong)量的(de)(de)80%至90%時(shi)(shi)，充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)由CC模(mo)式(shi)轉換(huan)為(wei)CV模(mo)式(shi)。充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)一(yi)旦(dan)進入CV充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)模(mo)式(shi)，則對電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)進行監(jian)視；當(dang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)達到(dao)最低門(men)限(幾(ji)毫(hao)安或者幾(ji)十毫(hao)安)時(shi)(shi)，充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)終止充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)。鋰電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)的(de)(de)典型充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)曲(qu)線(xian)如圖2所(suo)示。

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

　　從圖(tu)1所示的USB電(dian)(dian)(dian)壓(ya)跌落指(zhi)標可以看出，端口(kou)供電(dian)(dian)(dian)集線器的下游低功率端口(kou)電(dian)(dian)(dian)壓(ya)不具備足夠的余量(liang)，很(hen)難將(jiang)電(dian)(dian)(dian)池充(chong)(chong)至4.2V。充(chong)(chong)電(dian)(dian)(dian)通路上(shang)存在的小量(liang)額外電(dian)(dian)(dian)阻(zu)會妨礙正(zheng)常充(chong)(chong)電(dian)(dian)(dian)。

　　Li+和Li-Poly電(dian)池(chi)(chi)(chi)(chi)(chi)應在合適的(de)溫度(du)(du)(du)下進行充(chong)電(dian)。制造商推薦(jian)的(de)最高(gao)充(chong)電(dian)溫度(du)(du)(du)通常為(wei)+45°C至(zhi)+55°C之間，允許的(de)最大(da)放電(dian)溫度(du)(du)(du)可再高(gao)出10°C左右。這(zhe)些電(dian)池(chi)(chi)(chi)(chi)(chi)使(shi)用的(de)材料，化(hua)學(xue)性質非常活潑，如果電(dian)池(chi)(chi)(chi)(chi)(chi)溫度(du)(du)(du)超(chao)過(guo)+70°C，會發生(sheng)燃燒。鋰電(dian)池(chi)(chi)(chi)(chi)(chi)充(chong)電(dian)器應具備熱關斷電(dian)路，該電(dian)路監視(shi)電(dian)池(chi)(chi)(chi)(chi)(chi)溫度(du)(du)(du)，如果電(dian)池(chi)(chi)(chi)(chi)(chi)溫度(du)(du)(du)超(chao)過(guo)制造商推薦(jian)的(de)最大(da)充(chong)電(dian)溫度(du)(du)(du)時，則(ze)終(zhong)止充(chong)電(dian)。

　　鎳(nie)氫電池（NiMH）

　　NiMH電池(chi)比(bi)鋰(li)電池(chi)要(yao)重一些，其能量密度也比(bi)鋰(li)電池(chi)低。一直以(yi)來(lai)，NiMH電池(chi)比(bi)鋰(li)電池(chi)要(yao)便(bian)宜(yi)，但是(shi)最近二者(zhe)的價格差在縮小。NiMH電池(chi)具有標(biao)準(zhun)尺寸，在大多數應用(yong)中可直接替(ti)換堿性電池(chi)。每節電池(chi)的標(biao)稱電壓為1.2V，充滿后會達到1.5V。

　　通常(chang)采用(yong)恒流源對NiMH電(dian)(dian)(dian)池充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)。當(dang)達(da)到(dao)充(chong)(chong)(chong)(chong)滿狀態(tai)時，會發(fa)生放熱(re)化(hua)學反應，并(bing)(bing)導(dao)致(zhi)電(dian)(dian)(dian)池溫(wen)度上升，電(dian)(dian)(dian)池端電(dian)(dian)(dian)壓(ya)降(jiang)低。可(ke)檢測電(dian)(dian)(dian)池溫(wen)度上升速率(lv)或者負向電(dian)(dian)(dian)壓(ya)變化(hua)率(lv)，并(bing)(bing)用(yong)來終止充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)。這些(xie)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)終止方法(fa)分別稱為(wei)dT/dt和-ΔV。充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)速率(lv)非(fei)常(chang)低時，dT/dt和-ΔV不太明顯，很難精確(que)檢測到(dao)。電(dian)(dian)(dian)池開始進入過(guo)充(chong)(chong)(chong)(chong)狀態(tai)時，dT/dt和-ΔV響應開始顯現。此時如果(guo)繼續充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)，將損壞電(dian)(dian)(dian)池。

　　終止檢測在(zai)(zai)充(chong)(chong)電(dian)(dian)(dian)速率(lv)大于(yu)C/3時(shi)要比低(di)(di)充(chong)(chong)電(dian)(dian)(dian)速率(lv)時(shi)容易得多。溫度上升速率(lv)大約為1°C/分(fen)(fen)鐘(zhong)，-ΔV響(xiang)應也比低(di)(di)充(chong)(chong)電(dian)(dian)(dian)速率(lv)時(shi)更(geng)明顯。快充(chong)(chong)結束(shu)后，建議以更(geng)小的(de)(de)電(dian)(dian)(dian)流(liu)再充(chong)(chong)一(yi)段時(shi)間(jian)，以徹底(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(jie)段結束(shu)后，采用C/20或(huo)者(zhe)C/30的(de)(de)涓充(chong)(chong)電(dian)(dian)(dian)流(liu)來(lai)補(bu)償自(zi)放(fang)電(dian)(dian)(dian)效應，使(shi)電(dian)(dian)(dian)池(chi)維持在(zai)(zai)充(chong)(chong)滿狀態。圖3所示為采用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)(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)壓曲線。在(zai)(zai)該圖中，上面(mian)一(yi)條(tiao)曲線的(de)(de)數據在(zai)(zai)充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)正在(zai)(zai)灌(guan)入電(dian)(dian)(dian)池(chi)時(shi)獲得，下面(mian)那(nei)條(tiao)曲線的(de)(de)數據在(zai)(zai)切斷電(dian)(dian)(dian)流(liu)時(shi)測得。在(zai)(zai)DS2712中，該電(dian)(dian)(dian)壓差被(bei)用來(lai)區分(fen)(fen)NiMH電(dian)(dian)(dian)池(chi)和堿性(xing)電(dian)(dian)(dian)池(chi)。如果檢測到堿性(xing)電(dian)(dian)(dian)池(chi)，則(ze)DS2712不會對它進行充(chong)(chong)電(dian)(dian)(dian)。

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

　　開關與線性

　　USB 2.0規范允(yun)許低(di)功率(lv)端(duan)口(kou)提(ti)供最大100mA電(dian)流(liu)(liu)，大功率(lv)端(duan)口(kou)提(ti)供最大500mA電(dian)流(liu)(liu)。如果采用線性(xing)調(diao)整(zheng)器件來調(diao)節電(dian)池充(chong)電(dian)電(dian)流(liu)(liu)，這(zhe)也(ye)就(jiu)是(shi)最大可提(ti)供的(de)充(chong)電(dian)電(dian)流(liu)(liu)。線性(xing)調(diao)整(zheng)器件(圖4)的(de)功耗為P = VQ x IBATT。這(zhe)會造成調(diao)整(zheng)管發熱(re)，可能需要安裝(zhuang)散(san)熱(re)器，以(yi)防止過熱(re)。

　　
圖4. 功耗等(deng)于(yu)電池充(chong)電電流乘以調(diao)整管兩端的電壓

　　對(dui)應5V標稱輸(shu)入(ru)電壓，調整器件消耗(hao)的功率與電池(chi)(chi)類型(xing)、數量(liang)和電池(chi)(chi)電壓有關。

　　
圖5. 采用5.0V電(dian)(dian)壓的USB端口對NiMH電(dian)(dian)池充電(dian)(dian)時，線性調整器件的功耗

     標稱輸入電(dian)(dian)(dian)(dian)壓為(wei)5.0V時，線(xian)性USB充(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)對(dui)NiMH電(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)的功(gong)耗計算結果如圖5所(suo)示。對(dui)單節(jie)(jie)電(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)時，線(xian)性充(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)的效率僅為(wei)30%；對(dui)兩節(jie)(jie)電(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)時，效率為(wei)60%。用500mA電(dian)(dian)(dian)(dian)流對(dui)單節(jie)(jie)電(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)時，功(gong)耗會高達(da)2W。這樣的功(gong)耗通常(chang)需(xu)要(yao)加(jia)散熱(re)(re)器(qi)。功(gong)耗為(wei)2W時，熱(re)(re)阻為(wei)+20°C/W的散熱(re)(re)器(qi)在+25°C環(huan)境溫度下會被加(jia)熱(re)(re)至大(da)約+65°C，要(yao)得到滿額性能，還(huan)需(xu)要(yao)有流動空(kong)氣來協(xie)助其散熱(re)(re)。處于空(kong)氣靜止的封(feng)閉(bi)空(kong)間內，溫度會更高。

　　采(cai)用基于(yu)開關調節器(qi)(qi)的(de)(de)充(chong)電(dian)(dian)器(qi)(qi)可(ke)解(jie)決多個問題。首先，與線性(xing)充(chong)電(dian)(dian)器(qi)(qi)相比(bi)，能夠以更快的(de)(de)速率、更大的(de)(de)電(dian)(dian)流對電(dian)(dian)池進(jin)行(xing)充(chong)電(dian)(dian)(圖6)。由(you)(you)于(yu)功耗較低、發熱較少，熱管理方面的(de)(de)問題也(ye)減少了。同時，由(you)(you)于(yu)運行(xing)溫(wen)度降(jiang)低，充(chong)電(dian)(dian)器(qi)(qi)更加可(ke)靠。

　　
圖(tu)6. 對單節NiMH電(dian)池充(chong)電(dian)時(shi)，線(xian)性充(chong)電(dian)器和開關充(chong)電(dian)器的(de)充(chong)電(dian)時(shi)間不同(tong)

　　圖6中(zhong)的(de)(de)計算結(jie)果(guo)基(ji)于以下(xia)條件(jian)和假設得到：采用高功率USB口最大允(yun)許電流(500mA)的(de)(de)大約90%充電；開關(guan)調節器采用非同步整流的(de)(de)buck轉換(huan)器，具有77%效(xiao)率。

　　電路實例

　　圖(tu)7所示電(dian)(dian)(dian)(dian)(dian)路(lu)是(shi)用于單(dan)節(jie)(jie)NiMH電(dian)(dian)(dian)(dian)(dian)池充(chong)電(dian)(dian)(dian)(dian)(dian)的開關(guan)模式降壓(ya)型調節(jie)(jie)器(qi)。它采用DS2712充(chong)電(dian)(dian)(dian)(dian)(dian)控(kong)制(zhi)(zhi)(zhi)器(qi)調節(jie)(jie)充(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流和終止充(chong)電(dian)(dian)(dian)(dian)(dian)。充(chong)電(dian)(dian)(dian)(dian)(dian)控(kong)制(zhi)(zhi)(zhi)器(qi)監視溫度(du)、電(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)壓(ya)和電(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)流。如(ru)果(guo)溫度(du)超過+45°C或者(zhe)低于0°C，控(kong)制(zhi)(zhi)(zhi)器(qi)不會(hui)對(dui)電(dian)(dian)(dian)(dian)(dian)池充(chong)電(dian)(dian)(dian)(dian)(dian)。

　　
圖7. USB端口對(dui)單節NiMH電池快速(su)充(chong)電的(de)原(yuan)理圖

　　如圖7所示，Q1是降壓(ya)(ya)型充電(dian)(dian)(dian)器(qi)的(de)(de)開(kai)關功率晶體管(guan)；L1是濾(lv)波(bo)電(dian)(dian)(dian)感；D1是續(xu)流(liu)或整流(liu)二(er)極管(guan)。輸入電(dian)(dian)(dian)容C1為10μF、超低ESR的(de)(de)陶(tao)瓷濾(lv)波(bo)電(dian)(dian)(dian)容。用鉭電(dian)(dian)(dian)容或者(zhe)其它(ta)電(dian)(dian)(dian)解電(dian)(dian)(dian)容替(ti)代C1會使充電(dian)(dian)(dian)器(qi)的(de)(de)性能降低。R7是電(dian)(dian)(dian)流(liu)調節器(qi)檢(jian)測放大器(qi)的(de)(de)檢(jian)流(liu)電(dian)(dian)(dian)阻。DS2712的(de)(de)基準(zhun)電(dian)(dian)(dian)壓(ya)(ya)為0.125V，并具有24mV滯回(hui)。通過CSOUT提供閉環、開(kai)關模(mo)式(shi)電(dian)(dian)(dian)流(liu)控制(zhi)。充電(dian)(dian)(dian)控制(zhi)引腳CC1將Q2的(de)(de)柵極拉低時，使能Q1的(de)(de)柵極驅動。Q1和(he)(he)Q2均為低Vt (柵-源門限電(dian)(dian)(dian)壓(ya)(ya))的(de)(de)pMOSFET。CC1和(he)(he)CSOUT均為低電(dian)(dian)(dian)平時，Q2的(de)(de)漏-源電(dian)(dian)(dian)壓(ya)(ya)將稍大于Vt。該電(dian)(dian)(dian)壓(ya)(ya)以及CSOUT的(de)(de)正(zheng)向壓(ya)(ya)降構成了(le)Q1的(de)(de)柵極開(kai)關電(dian)(dian)(dian)壓(ya)(ya)。

　　CC1為低電(dian)(dian)(dian)(dian)(dian)平(ping)(ping)時(shi)，啟(qi)動電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)閉環控制(zhi)。圖8所示為啟(qi)動開(kai)關(guan)時(shi)的(de)波形。上方波形是0.125? (檢(jian)流(liu)(liu)電(dian)(dian)(dian)(dian)(dian)阻(zu)兩(liang)端(duan)的(de)電(dian)(dian)(dian)(dian)(dian)壓，下方波形是Q1漏極至GND的(de)電(dian)(dian)(dian)(dian)(dian)壓。開(kai)始(shi)時(shi)，當Q1打開(kai)(CC1和CSOUT均為低電(dian)(dian)(dian)(dian)(dian)平(ping)(ping))時(shi)，電(dian)(dian)(dian)(dian)(dian)感(gan)(gan)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)向上爬升。當電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)增(zeng)大到(dao)使檢(jian)流(liu)(liu)電(dian)(dian)(dian)(dian)(dian)阻(zu)兩(liang)端(duan)的(de)電(dian)(dian)(dian)(dian)(dian)壓達到(dao)0.125V時(shi)，CSOUT變為高電(dian)(dian)(dian)(dian)(dian)平(ping)(ping)，開(kai)關(guan)關(guan)斷(duan)。此后，電(dian)(dian)(dian)(dian)(dian)感(gan)(gan)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)開(kai)始(shi)下降，直到(dao)檢(jian)流(liu)(liu)電(dian)(dian)(dian)(dian)(dian)阻(zu)兩(liang)端(duan)的(de)電(dian)(dian)(dian)(dian)(dian)壓達到(dao)約0.1V，CSOUT又變為低電(dian)(dian)(dian)(dian)(dian)平(ping)(ping)。只要CC1為低電(dian)(dian)(dian)(dian)(dian)平(ping)(ping)，該過程將(jiang)一直持續。

　　
圖(tu)8. USB NiMH充電(dian)器的啟(qi)動波(bo)形

　　DS2712的(de)內(nei)部(bu)狀態機控制著(zhu)CC1的(de)工(gong)作。充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)開始時，DS2712先(xian)對(dui)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)進行(xing)(xing)狀態測(ce)(ce)試，以(yi)(yi)確保(bao)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓在1.0V至1.65V之間，并確認溫度在0°C至+45°C之間。如果(guo)電(dian)(dian)(dian)(dian)(dian)(dian)壓低(di)于1.0V，DS2712將以(yi)(yi)0.125的(de)占(zhan)空(kong)(kong)比(bi)拉(la)低(di)CC1，對(dui)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)緩慢充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)，以(yi)(yi)防損壞電(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)壓超(chao)過(guo)(guo)1.0V后(hou)(hou)(hou)，狀態機轉為快充(chong)(chong)(chong)模(mo)式。快充(chong)(chong)(chong)時占(zhan)空(kong)(kong)比(bi)為31/32，即大約97%。“跳過(guo)(guo)”的(de)間隙內(nei)進行(xing)(xing)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)阻(zu)抗(kang)測(ce)(ce)試，以(yi)(yi)確保(bao)不(bu)會對(dui)錯誤放入(ru)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)的(de)高阻(zu)抗(kang)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(例如堿性(xing)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi))進行(xing)(xing)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)。檢(jian)測(ce)(ce)到-2mV的(de)-ΔV后(hou)(hou)(hou)，快充(chong)(chong)(chong)結(jie)束(shu)。如果(guo)未檢(jian)測(ce)(ce)到-ΔV，將持續(xu)快充(chong)(chong)(chong)，直(zhi)到快充(chong)(chong)(chong)定時器(qi)超(chao)時，或檢(jian)測(ce)(ce)到過(guo)(guo)溫或者(zhe)過(guo)(guo)壓故(gu)障(zhang)狀態(包括阻(zu)抗(kang)不(bu)合格)為止。快充(chong)(chong)(chong)完成(cheng)(由于-ΔV或快充(chong)(chong)(chong)定時器(qi)超(chao)時) 后(hou)(hou)(hou)，DS2712進入(ru)定時補(bu)足充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)模(mo)式，占(zhan)空(kong)(kong)比(bi)為12.5%，持續(xu)時間為所設快充(chong)(chong)(chong)定時的(de)一(yi)半。補(bu)足充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)完成(cheng)后(hou)(hou)(hou)，充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)進入(ru)維持模(mo)式，占(zhan)空(kong)(kong)比(bi)為1/64，直(zhi)到電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)被拿(na)走或重(zhong)新上電(dian)(dian)(dian)(dian)(dian)(dian)。

　　采用圖7所示充電器和大(da)功率USB端口對2100mAh NiMH電(dian)(dian)池充電(dian)(dian)時(shi)，快(kuai)充時(shi)間為(wei)2小(xiao)時(shi)多一點(dian)，大(da)約3個(ge)(ge)小(xiao)時(shi)完成包括補足(zu)充電(dian)(dian)在(zai)內的全(quan)部充電(dian)(dian)過程。從端口吸取(qu)的電(dian)(dian)流為(wei)420mA。如果需要(yao)與(yu)主機進行枚舉過程，并需要(yao)大(da)電(dian)(dian)流使能操作，可在(zai)R9和地之間串聯一個(ge)(ge)開漏(lou)極nMOSFET。如果MOSFET關斷，則TMR浮空(kong)，DS2712進入掛起狀態(tai)。

　　總結

　　對(dui)于(yu)(yu)小(xiao)型(xing)消費類電(dian)子設備的(de)(de)(de)(de)(de)電(dian)池充電(dian)而言，USB端口(kou)是一個經濟(ji)、實用的(de)(de)(de)(de)(de)電(dian)源(yuan)(yuan)。為(wei)完(wan)全符合USB 2.0規(gui)范(fan)，連接(jie)在(zai)USB端口(kou)上(shang)的(de)(de)(de)(de)(de)負(fu)載必須(xu)能夠與主(zhu)機(ji)進行雙向通信。負(fu)載也必須(xu)符合電(dian)源(yuan)(yuan)管(guan)理要求，包括低功耗模(mo)式，以及便于(yu)(yu)主(zhu)機(ji)確(que)定何時(shi)(shi)需要從端口(kou)吸取大電(dian)流的(de)(de)(de)(de)(de)手段(duan)。盡管(guan)部分兼(jian)容(rong)的(de)(de)(de)(de)(de)系統能夠適應大部分USB主(zhu)機(ji)，但有時(shi)(shi)會出現(xian)意想不到(dao)的(de)(de)(de)(de)(de)結果。只有很好(hao)地(di)理解USB規(gui)范(fan)要求和負(fu)載的(de)(de)(de)(de)(de)期望，才能在(zai)對(dui)于(yu)(yu)規(gui)范(fan)的(de)(de)(de)(de)(de)兼(jian)容(rong)性與負(fu)載復雜度(du)之間取得較(jiao)好(hao)的(de)(de)(de)(de)(de)平衡。