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)源(yuan)進行接(jie)口。文章回顧了USB電(dian)(dian)源(yuan)總線的特性，包(bao)括(kuo)電(dian)(dian)壓(ya)、電(dian)(dian)流(liu)限制(zhi)、浪(lang)涌電(dian)(dian)流(liu)、連(lian)接(jie)器以(yi)及電(dian)(dian)纜連(lian)接(jie)問題。同時介紹了鎳氫電(dian)(dian)池(chi)(NiMH)和鋰電(dian)(dian)池(chi)技術、充(chong)(chong)電(dian)(dian)方(fang)法以(yi)及充(chong)(chong)電(dian)(dian)終止技術。給出(chu)了一(yi)個完整的示例電(dian)(dian)路，用(yong)于(yu)實現USB端口對NiMH電(dian)(dian)池(chi)智(zhi)能充(chong)(chong)電(dian)(dian)，并(bing)給出(chu)了充(chong)(chong)電(dian)(dian)數據(ju)。

　　USB特性

　　USB總線能夠為低功耗(hao)電(dian)子設備提供電(dian)源(yuan)。總線電(dian)源(yuan)與電(dian)網隔(ge)離，并且具有很(hen)好的穩定性。但是，可用電(dian)流有限，同時負載和主(zhu)機或(huo)電(dian)源(yuan)之間存(cun)在(zai)潛在(zai)的互(hu)操作問(wen)題。

　　USB端口由90?雙(shuang)向差分屏(ping)蔽(bi)雙(shuang)絞線、VBUS (+5V電源)和地組(zu)成。這4條(tiao)線由鋁箔內(nei)屏(ping)蔽(bi)層和編織(zhi)網(wang)外屏(ping)蔽(bi)層進行屏(ping)蔽(bi)。最新的USB規范標(biao)準是2.0版，可以從USB組(zu)織(zhi)免費獲得。要(yao)做到完全符合(he)該規范標(biao)準，需要(yao)通過一個功能控(kong)制(zhi)器來實現設(she)備和主機間(jian)的雙(shuang)向通信。規范定義了1個單位負載(zai)為(wei)100mA (最大)。任何設(she)備允許(xu)吸取的最大電流為(wei)5個單位負載(zai)。

　　USB端(duan)口(kou)(kou)可分為(wei)低(di)功率(lv)端(duan)口(kou)(kou)和大(da)功率(lv)端(duan)口(kou)(kou)兩(liang)類，低(di)功率(lv)端(duan)口(kou)(kou)可提供(gong)1個(ge)(ge)單位(wei)負(fu)載的(de)電流(liu)(liu)，大(da)功率(lv)端(duan)口(kou)(kou)可最多提供(gong)5個(ge)(ge)單位(wei)負(fu)載的(de)電流(liu)(liu)。當設備(bei)剛(gang)連接到(dao)USB端(duan)口(kou)(kou)時，枚(mei)舉過程對器件(jian)進行識(shi)別，并確定其(qi)負(fu)載要求。在此過程中，只允許(xu)設備(bei)從主機(ji)吸(xi)取最多1個(ge)(ge)單位(wei)負(fu)載的(de)電流(liu)(liu)。枚(mei)舉過程完(wan)成后，如果主機(ji)的(de)電源(yuan)管理(li)軟件(jian)允許(xu)，則大(da)功率(lv)設備(bei)可以吸(xi)取更大(da)的(de)電流(liu)(liu)。

　　某(mou)些主(zhu)機(ji)系統(包括下游USB集線(xian)器(qi))通過保險絲或者有(you)源電(dian)流檢測(ce)器(qi)提供(gong)限流功能。如(ru)(ru)果USB設備未經(jing)過枚舉(ju)過程便從(cong)USB端(duan)(duan)口(kou)吸取(qu)大(da)電(dian)流(超(chao)過1個(ge)(ge)單位負載)，則主(zhu)機(ji)會檢測(ce)到(dao)過流狀態，并會關閉正在使用的(de)一個(ge)(ge)或多(duo)(duo)個(ge)(ge)USB端(duan)(duan)口(kou)。市場上(shang)供(gong)應的(de)許多(duo)(duo)USB設備，包括獨立(li)電(dian)池充電(dian)器(qi)，都沒有(you)功能控制器(qi)來(lai)處理枚舉(ju)過程，但吸取(qu)的(de)電(dian)流卻超(chao)過了100mA。在這(zhe)種不恰(qia)當的(de)條件下，這(zhe)些設備可能導(dao)致主(zhu)機(ji)出(chu)現問題(ti)。例如(ru)(ru)，如(ru)(ru)果一個(ge)(ge)吸取(qu)500mA電(dian)流的(de)設備插入(ru)總線(xian)供(gong)電(dian)的(de)USB集線(xian)器(qi)，而且未進(jin)行(xing)正確的(de)枚舉(ju)過程，則可能導(dao)致集線(xian)器(qi)端(duan)(duan)口(kou)和主(zhu)機(ji)端(duan)(duan)口(kou)同時過載。

　　主(zhu)機(ji)(ji)操作系統采用高級電(dian)(dian)源管(guan)理時(shi)情(qing)況會(hui)更加(jia)復雜，特別是筆(bi)記(ji)本電(dian)(dian)腦(nao)，它總是希望端口電(dian)(dian)流(liu)盡可能低(di)。在某些(xie)節電(dian)(dian)模式下，計算(suan)機(ji)(ji)會(hui)向USB設(she)(she)備(bei)發出掛起命令，而后則認為設(she)(she)備(bei)進入了低(di)功耗模式。設(she)(she)備(bei)中包含(han)一個(ge)能與主(zhu)機(ji)(ji)進行通信的(de)功能控制器(qi)始終是一個(ge)比較好的(de)做法，即(ji)使(shi)對于(yu)低(di)功耗設(she)(she)備(bei)來說也是如此。

　　USB 2.0規(gui)范(fan)非(fei)常全面，規(gui)定了電(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)的(de)(de)(de)質(zhi)量、連接(jie)器構(gou)造、電(dian)(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)材質(zhi)、容許的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)跌(die)落(luo)以及(ji)浪涌電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)等。低電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)和(he)大電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)端(duan)口(kou)具(ju)有(you)不同的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)指標。這主要是由(you)主機和(he)負載間的(de)(de)(de)連接(jie)器和(he)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)上的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)跌(die)落(luo)決定的(de)(de)(de)，并包括由(you)USB供(gong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)的(de)(de)(de)集線(xian)器上產生的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)跌(die)落(luo)。包括計(ji)算機或者(zhe)自供(gong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)USB集線(xian)器在(zai)內的(de)(de)(de)主機，都具(ju)有(you)大電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)端(duan)口(kou)，可提供(gong)最大500mA的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)。無源(yuan)、總線(xian)供(gong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)的(de)(de)(de)USB集線(xian)器具(ju)有(you)低電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)端(duan)口(kou)。表1列(lie)出了USB大電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)和(he)低電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)端(duan)口(kou)上游端(duan)(電(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan))引腳允許的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)容限。

　　表(biao)1. USB 2.0規范電源質量標(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(xie)指標適用(yong)于上游端主機或集線器端口的(de)連接器引腳。電纜和連接器上的(de)I x R跌落需另外考慮。

       在符合(he)USB 2.0規范的主(zhu)機(ji)中(zhong)，大(da)(da)功(gong)率(lv)端(duan)(duan)(duan)口(kou)的上游(you)端(duan)(duan)(duan)具(ju)有120μF、低ESR電(dian)(dian)(dian)(dian)容。所連(lian)接的USB設(she)備(bei)的輸入(ru)電(dian)(dian)(dian)(dian)容限制在10μF以內，在最(zui)(zui)初的負載連(lian)接階段(duan)，允許負載從主(zhu)機(ji)(或自供電(dian)(dian)(dian)(dian)集(ji)線器)吸(xi)取的最(zui)(zui)大(da)(da)電(dian)(dian)(dian)(dian)荷數為50μC。這樣一來(lai)，當新設(she)備(bei)連(lian)接至(zhi)USB端(duan)(duan)(duan)口(kou)時(shi)(shi)，上游(you)端(duan)(duan)(duan)口(kou)的瞬(shun)態(tai)電(dian)(dian)(dian)(dian)壓跌(die)落小于0.5V。如果負載正常(chang)工(gong)作時(shi)(shi)需要更大(da)(da)的輸入(ru)電(dian)(dian)(dian)(dian)容，則必須提供浪涌電(dian)(dian)(dian)(dian)流限制器，以保證對(dui)更大(da)(da)的電(dian)(dian)(dian)(dian)容充(chong)電(dian)(dian)(dian)(dian)時(shi)(shi)電(dian)(dian)(dian)(dian)流不會超過100mA。

　　當USB端口帶(dai)有(you)一個總(zong)(zong)線(xian)供(gong)電(dian)的USB集線(xian)器(qi)(qi)，集線(xian)器(qi)(qi)上(shang)接(jie)了低功(gong)耗設備時，USB口上(shang)允許的直流電(dian)壓跌落如圖1所(suo)示。大功(gong)率負(fu)載(zai)與總(zong)(zong)線(xian)供(gong)電(dian)的集線(xian)器(qi)(qi)連接(jie)時，電(dian)壓跌落將超(chao)過圖1給出(chu)的指(zhi)標，并會引起(qi)總(zong)(zong)線(xian)過載(zai)。

　　
圖1. 主(zhu)機至低功率負載的電(dian)壓跌落大于圖中給出(chu)的允(yun)許直流電(dian)壓跌落時，會(hui)引起總線過載

　　電池充電要求

　　單節鋰(li)(li)離子和鋰(li)(li)聚合物電(dian)池

　　如今的鋰(li)電(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)至最大額定(ding)容(rong)量(liang)后，其電(dian)(dian)(dian)壓通(tong)常為4.1V至4.2V之間。當(dang)前市場上正在(zai)出售的、更(geng)(geng)新(xin)的、容(rong)量(liang)更(geng)(geng)大的電(dian)(dian)(dian)池(chi)，其電(dian)(dian)(dian)壓范圍在(zai)4.3V至4.4V之間。典型的棱(leng)柱形鋰(li)離子(Li+)和鋰(li)聚(ju)合物(Li-Poly)電(dian)(dian)(dian)池(chi)容(rong)量(liang)為600mAh至1400mAh。

　　對Li+和(he)Li-Poly電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)來說，首選的(de)(de)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)曲線(xian)是從(cong)恒流充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)開(kai)始，一(yi)直持續到(dao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)達到(dao)額(e)定電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)。然后(hou)，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)對電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)兩端的(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)進(jin)(jin)行(xing)調節。這兩種調節方式(shi)構成了恒流(CC)恒壓(ya)(CV)充(chong)(chong)(chong)(chong)電(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)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)通常稱為(wei)CCCV充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)。CCCV充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)進(jin)(jin)入(ru)CV模式(shi)后(hou)，電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)的(de)(de)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流開(kai)始下降。若采用0.5C至1.5C的(de)(de)典(dian)型充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)速率(lv)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)，則當電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)達到(dao)其(qi)充(chong)(chong)(chong)(chong)滿(man)容(rong)量的(de)(de)80%至90%時(shi)，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)由CC模式(shi)轉換為(wei)CV模式(shi)。充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)一(yi)旦進(jin)(jin)入(ru)CV充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)模式(shi)，則對電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流進(jin)(jin)行(xing)監視；當電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流達到(dao)最低門(men)限(幾(ji)毫(hao)安或者(zhe)幾(ji)十毫(hao)安)時(shi)，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)終止充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)。鋰(li)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)的(de)(de)典(dian)型充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)曲線(xian)如圖2所示。

　　
圖2. 使用CCCV充電器對Li+電池充電時的典型(xing)曲線

　　從圖1所(suo)示的(de)(de)(de)USB電(dian)壓跌落指標可以看出，端口(kou)供電(dian)集線器(qi)的(de)(de)(de)下游低功率端口(kou)電(dian)壓不(bu)具備足夠的(de)(de)(de)余量(liang)，很難(nan)將電(dian)池充至4.2V。充電(dian)通(tong)路(lu)上存在的(de)(de)(de)小量(liang)額外電(dian)阻(zu)會(hui)妨礙正(zheng)常充電(dian)。

　　Li+和Li-Poly電(dian)(dian)池(chi)應在合適的(de)(de)溫(wen)度(du)(du)(du)下進行充(chong)電(dian)(dian)。制造(zao)商(shang)推薦(jian)的(de)(de)最(zui)(zui)(zui)高充(chong)電(dian)(dian)溫(wen)度(du)(du)(du)通常(chang)為(wei)+45°C至(zhi)+55°C之間(jian)，允許的(de)(de)最(zui)(zui)(zui)大放電(dian)(dian)溫(wen)度(du)(du)(du)可再高出10°C左(zuo)右。這些電(dian)(dian)池(chi)使用(yong)的(de)(de)材料，化學性質非常(chang)活(huo)潑，如(ru)果(guo)電(dian)(dian)池(chi)溫(wen)度(du)(du)(du)超過(guo)+70°C，會發生(sheng)燃燒。鋰電(dian)(dian)池(chi)充(chong)電(dian)(dian)器應具備熱關斷電(dian)(dian)路(lu)(lu)，該電(dian)(dian)路(lu)(lu)監視電(dian)(dian)池(chi)溫(wen)度(du)(du)(du)，如(ru)果(guo)電(dian)(dian)池(chi)溫(wen)度(du)(du)(du)超過(guo)制造(zao)商(shang)推薦(jian)的(de)(de)最(zui)(zui)(zui)大充(chong)電(dian)(dian)溫(wen)度(du)(du)(du)時，則終止充(chong)電(dian)(dian)。

　　鎳氫電池(chi)（NiMH）

　　NiMH電(dian)(dian)池(chi)比鋰電(dian)(dian)池(chi)要重一些，其能量密度也比鋰電(dian)(dian)池(chi)低(di)。一直(zhi)以來，NiMH電(dian)(dian)池(chi)比鋰電(dian)(dian)池(chi)要便宜，但是(shi)最近二者的(de)(de)價格差(cha)在縮(suo)小。NiMH電(dian)(dian)池(chi)具有標準尺(chi)寸(cun)，在大多數應(ying)用中可直(zhi)接替換堿性電(dian)(dian)池(chi)。每(mei)節電(dian)(dian)池(chi)的(de)(de)標稱電(dian)(dian)壓為1.2V，充滿后會達(da)到1.5V。

　　通常采用恒流源對NiMH電(dian)(dian)池充(chong)電(dian)(dian)。當達到充(chong)滿狀態(tai)時(shi)，會(hui)發生放熱化(hua)學(xue)反(fan)應，并導致(zhi)電(dian)(dian)池溫(wen)度(du)上(shang)升，電(dian)(dian)池端電(dian)(dian)壓(ya)降低。可檢(jian)測(ce)電(dian)(dian)池溫(wen)度(du)上(shang)升速(su)率(lv)或者負(fu)向電(dian)(dian)壓(ya)變(bian)化(hua)率(lv)，并用來終(zhong)(zhong)止充(chong)電(dian)(dian)。這些(xie)充(chong)電(dian)(dian)終(zhong)(zhong)止方法分別(bie)稱為(wei)dT/dt和(he)-ΔV。充(chong)電(dian)(dian)速(su)率(lv)非(fei)常低時(shi)，dT/dt和(he)-ΔV不太明顯，很(hen)難精確檢(jian)測(ce)到。電(dian)(dian)池開(kai)始(shi)進入(ru)過充(chong)狀態(tai)時(shi)，dT/dt和(he)-ΔV響(xiang)應開(kai)始(shi)顯現。此(ci)時(shi)如(ru)果繼續充(chong)電(dian)(dian)，將損壞電(dian)(dian)池。

　　終(zhong)止(zhi)檢測(ce)(ce)在(zai)(zai)(zai)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)速(su)(su)率(lv)(lv)大于(yu)C/3時(shi)(shi)要(yao)比(bi)低(di)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)速(su)(su)率(lv)(lv)時(shi)(shi)容易得多。溫度上升速(su)(su)率(lv)(lv)大約為1°C/分鐘，-ΔV響(xiang)應也比(bi)低(di)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)速(su)(su)率(lv)(lv)時(shi)(shi)更明顯。快充(chong)(chong)(chong)(chong)結束后(hou)，建(jian)議以更小的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)再充(chong)(chong)(chong)(chong)一段(duan)(duan)時(shi)(shi)間，以徹底充(chong)(chong)(chong)(chong)足(zu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(補(bu)足(zu)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian))。補(bu)足(zu)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)階段(duan)(duan)結束后(hou)，采(cai)用C/20或(huo)者C/30的(de)涓充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)來補(bu)償(chang)自放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)效(xiao)應，使電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池維(wei)持(chi)在(zai)(zai)(zai)充(chong)(chong)(chong)(chong)滿狀(zhuang)態。圖3所示為采(cai)用DS2712 NiMH充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器對NiMH電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(事先已充(chong)(chong)(chong)(chong)了一部分電(dian)(dian)(dian)(dian)(dian)(dian)(dian))進(jin)行充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓曲線(xian)。在(zai)(zai)(zai)該圖中，上面一條曲線(xian)的(de)數據在(zai)(zai)(zai)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)正在(zai)(zai)(zai)灌入電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池時(shi)(shi)獲得，下(xia)面那條曲線(xian)的(de)數據在(zai)(zai)(zai)切斷電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)時(shi)(shi)測(ce)(ce)得。在(zai)(zai)(zai)DS2712中，該電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓差被用來區(qu)分NiMH電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池和堿性(xing)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池。如(ru)果(guo)檢測(ce)(ce)到堿性(xing)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池，則(ze)DS2712不會(hui)對它進(jin)行充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)。

　　
圖(tu)3. 采(cai)用DS2712充(chong)電控制器(qi)對NiMH電池充(chong)電

　　開關與線性

　　USB 2.0規范允許低(di)功(gong)率(lv)端(duan)口提供(gong)最(zui)大(da)100mA電(dian)流(liu)(liu)，大(da)功(gong)率(lv)端(duan)口提供(gong)最(zui)大(da)500mA電(dian)流(liu)(liu)。如果采(cai)用線性(xing)調整器(qi)件來調節電(dian)池充電(dian)電(dian)流(liu)(liu)，這也(ye)就是(shi)最(zui)大(da)可提供(gong)的充電(dian)電(dian)流(liu)(liu)。線性(xing)調整器(qi)件(圖4)的功(gong)耗為P = VQ x IBATT。這會造成調整管發熱，可能需(xu)要安裝散熱器(qi)，以防止過熱。

　　
圖4. 功耗等(deng)于(yu)電(dian)池充(chong)電(dian)電(dian)流乘以調整管兩端(duan)的電(dian)壓

　　對應5V標稱輸(shu)入(ru)電(dian)壓(ya)，調整器件消耗的(de)功率與電(dian)池類(lei)型、數量和電(dian)池電(dian)壓(ya)有關。

　　
圖5. 采用5.0V電(dian)壓(ya)的USB端口對NiMH電(dian)池充電(dian)時(shi)，線性調整器件(jian)的功耗(hao)

     標稱輸入電(dian)壓為(wei)5.0V時，線(xian)性USB充(chong)電(dian)器對(dui)(dui)NiMH電(dian)池(chi)充(chong)電(dian)的功(gong)耗(hao)計算結果如圖5所(suo)示。對(dui)(dui)單節電(dian)池(chi)充(chong)電(dian)時，線(xian)性充(chong)電(dian)器的效率僅為(wei)30%；對(dui)(dui)兩節電(dian)池(chi)充(chong)電(dian)時，效率為(wei)60%。用500mA電(dian)流(liu)(liu)對(dui)(dui)單節電(dian)池(chi)充(chong)電(dian)時，功(gong)耗(hao)會(hui)高(gao)達(da)2W。這樣(yang)的功(gong)耗(hao)通(tong)常需(xu)要加(jia)散(san)熱(re)(re)器。功(gong)耗(hao)為(wei)2W時，熱(re)(re)阻(zu)為(wei)+20°C/W的散(san)熱(re)(re)器在+25°C環境(jing)溫(wen)度下會(hui)被加(jia)熱(re)(re)至大約+65°C，要得到滿額性能，還需(xu)要有流(liu)(liu)動空氣來協(xie)助其散(san)熱(re)(re)。處于空氣靜止的封閉空間內，溫(wen)度會(hui)更高(gao)。

　　采用基(ji)于(yu)(yu)開關調節器的充(chong)電(dian)器可解決(jue)多個問題。首先(xian)，與線性充(chong)電(dian)器相(xiang)比(bi)，能夠以更(geng)快的速(su)率(lv)、更(geng)大(da)的電(dian)流對電(dian)池進行充(chong)電(dian)(圖6)。由于(yu)(yu)功耗較低、發(fa)熱較少(shao)，熱管理方面的問題也減(jian)少(shao)了。同時，由于(yu)(yu)運行溫度降(jiang)低，充(chong)電(dian)器更(geng)加可靠。

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

　　圖6中的計算(suan)結果(guo)基于(yu)以下條件和假設(she)得到：采用高功率USB口最大(da)(da)允許電流(500mA)的大(da)(da)約(yue)90%充(chong)電；開關調(diao)節(jie)器采用非同步(bu)整流的buck轉換器，具有77%效率。

　　電路實例

　　圖7所(suo)示電(dian)路(lu)是用于單節NiMH電(dian)池(chi)充(chong)(chong)電(dian)的開關模式降(jiang)壓型(xing)調(diao)節器(qi)。它采用DS2712充(chong)(chong)電(dian)控(kong)(kong)制(zhi)器(qi)調(diao)節充(chong)(chong)電(dian)電(dian)流和終止充(chong)(chong)電(dian)。充(chong)(chong)電(dian)控(kong)(kong)制(zhi)器(qi)監視溫度、電(dian)池(chi)電(dian)壓和電(dian)池(chi)電(dian)流。如果溫度超過+45°C或者(zhe)低于0°C，控(kong)(kong)制(zhi)器(qi)不會對電(dian)池(chi)充(chong)(chong)電(dian)。

　　
圖7. USB端(duan)口對(dui)單節NiMH電(dian)池快速充電(dian)的原理(li)圖

　　如圖7所示，Q1是降壓型充(chong)電(dian)(dian)(dian)器的開(kai)關(guan)功率(lv)晶體管；L1是濾波電(dian)(dian)(dian)感(gan)；D1是續流(liu)或整流(liu)二極管。輸入(ru)電(dian)(dian)(dian)容C1為10μF、超低ESR的陶瓷(ci)濾波電(dian)(dian)(dian)容。用(yong)鉭電(dian)(dian)(dian)容或者其它電(dian)(dian)(dian)解電(dian)(dian)(dian)容替(ti)代(dai)C1會使(shi)充(chong)電(dian)(dian)(dian)器的性能(neng)降低。R7是電(dian)(dian)(dian)流(liu)調(diao)節器檢(jian)測放大器的檢(jian)流(liu)電(dian)(dian)(dian)阻。DS2712的基準電(dian)(dian)(dian)壓為0.125V，并具有24mV滯回。通過(guo)CSOUT提供閉(bi)環(huan)、開(kai)關(guan)模式電(dian)(dian)(dian)流(liu)控制。充(chong)電(dian)(dian)(dian)控制引腳CC1將Q2的柵極拉低時(shi)，使(shi)能(neng)Q1的柵極驅(qu)動(dong)。Q1和(he)Q2均(jun)為低Vt (柵-源門限電(dian)(dian)(dian)壓)的pMOSFET。CC1和(he)CSOUT均(jun)為低電(dian)(dian)(dian)平(ping)時(shi)，Q2的漏-源電(dian)(dian)(dian)壓將稍大于Vt。該電(dian)(dian)(dian)壓以及(ji)CSOUT的正(zheng)向(xiang)壓降構(gou)成(cheng)了Q1的柵極開(kai)關(guan)電(dian)(dian)(dian)壓。

　　CC1為(wei)(wei)低(di)(di)(di)電(dian)(dian)(dian)平時(shi)(shi)，啟動電(dian)(dian)(dian)流閉環(huan)控制。圖8所示(shi)為(wei)(wei)啟動開(kai)(kai)關時(shi)(shi)的(de)波形(xing)(xing)。上方波形(xing)(xing)是(shi)0.125? (檢(jian)(jian)流電(dian)(dian)(dian)阻兩端的(de)電(dian)(dian)(dian)壓(ya)(ya)，下(xia)方波形(xing)(xing)是(shi)Q1漏(lou)極至GND的(de)電(dian)(dian)(dian)壓(ya)(ya)。開(kai)(kai)始時(shi)(shi)，當Q1打開(kai)(kai)(CC1和(he)CSOUT均為(wei)(wei)低(di)(di)(di)電(dian)(dian)(dian)平)時(shi)(shi)，電(dian)(dian)(dian)感電(dian)(dian)(dian)流向上爬升。當電(dian)(dian)(dian)流增(zeng)大到使檢(jian)(jian)流電(dian)(dian)(dian)阻兩端的(de)電(dian)(dian)(dian)壓(ya)(ya)達到0.125V時(shi)(shi)，CSOUT變為(wei)(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(xia)降，直到檢(jian)(jian)流電(dian)(dian)(dian)阻兩端的(de)電(dian)(dian)(dian)壓(ya)(ya)達到約0.1V，CSOUT又變為(wei)(wei)低(di)(di)(di)電(dian)(dian)(dian)平。只要CC1為(wei)(wei)低(di)(di)(di)電(dian)(dian)(dian)平，該過程將一直持續。

　　
圖8. USB NiMH充(chong)電器的啟(qi)動波形

　　DS2712的(de)內部狀態(tai)機(ji)控制著(zhu)CC1的(de)工作。充(chong)(chong)(chong)(chong)(chong)電(dian)開始時(shi)，DS2712先對(dui)(dui)電(dian)池(chi)(chi)進(jin)(jin)行(xing)狀態(tai)測(ce)試，以(yi)(yi)確(que)保電(dian)池(chi)(chi)電(dian)壓(ya)(ya)在1.0V至1.65V之間，并確(que)認溫度在0°C至+45°C之間。如(ru)果電(dian)壓(ya)(ya)低于(yu)1.0V，DS2712將(jiang)以(yi)(yi)0.125的(de)占(zhan)(zhan)(zhan)空比(bi)拉低CC1，對(dui)(dui)電(dian)池(chi)(chi)緩慢充(chong)(chong)(chong)(chong)(chong)電(dian)，以(yi)(yi)防損壞電(dian)池(chi)(chi)。一旦電(dian)池(chi)(chi)電(dian)壓(ya)(ya)超(chao)(chao)過(guo)1.0V后(hou)(hou)，狀態(tai)機(ji)轉為(wei)快(kuai)(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)模(mo)式(shi)。快(kuai)(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)時(shi)占(zhan)(zhan)(zhan)空比(bi)為(wei)31/32，即(ji)大約(yue)97%。“跳過(guo)”的(de)間隙(xi)內進(jin)(jin)行(xing)電(dian)池(chi)(chi)阻(zu)抗測(ce)試，以(yi)(yi)確(que)保不(bu)(bu)會對(dui)(dui)錯誤放入(ru)(ru)充(chong)(chong)(chong)(chong)(chong)電(dian)器(qi)(qi)的(de)高阻(zu)抗電(dian)池(chi)(chi)(例如(ru)堿性電(dian)池(chi)(chi))進(jin)(jin)行(xing)充(chong)(chong)(chong)(chong)(chong)電(dian)。檢(jian)(jian)測(ce)到(dao)-2mV的(de)-ΔV后(hou)(hou)，快(kuai)(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)結束。如(ru)果未檢(jian)(jian)測(ce)到(dao)-ΔV，將(jiang)持(chi)續(xu)快(kuai)(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)，直到(dao)快(kuai)(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)定時(shi)器(qi)(qi)超(chao)(chao)時(shi)，或檢(jian)(jian)測(ce)到(dao)過(guo)溫或者過(guo)壓(ya)(ya)故障狀態(tai)(包括阻(zu)抗不(bu)(bu)合(he)格)為(wei)止。快(kuai)(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)完成(由(you)于(yu)-ΔV或快(kuai)(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)定時(shi)器(qi)(qi)超(chao)(chao)時(shi)) 后(hou)(hou)，DS2712進(jin)(jin)入(ru)(ru)定時(shi)補足充(chong)(chong)(chong)(chong)(chong)電(dian)模(mo)式(shi)，占(zhan)(zhan)(zhan)空比(bi)為(wei)12.5%，持(chi)續(xu)時(shi)間為(wei)所設快(kuai)(kuai)(kuai)充(chong)(chong)(chong)(chong)(chong)定時(shi)的(de)一半。補足充(chong)(chong)(chong)(chong)(chong)電(dian)完成后(hou)(hou)，充(chong)(chong)(chong)(chong)(chong)電(dian)器(qi)(qi)進(jin)(jin)入(ru)(ru)維持(chi)模(mo)式(shi)，占(zhan)(zhan)(zhan)空比(bi)為(wei)1/64，直到(dao)電(dian)池(chi)(chi)被(bei)拿走或重(zhong)新上電(dian)。

　　采用圖7所示充電器和(he)大(da)(da)功(gong)率(lv)USB端口對2100mAh NiMH電(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)時(shi)，快充(chong)時(shi)間為2小(xiao)時(shi)多一(yi)點，大(da)(da)約3個小(xiao)時(shi)完(wan)成(cheng)包括(kuo)補足充(chong)電(dian)(dian)(dian)(dian)(dian)在內(nei)的(de)全部充(chong)電(dian)(dian)(dian)(dian)(dian)過(guo)程。從端口吸取的(de)電(dian)(dian)(dian)(dian)(dian)流為420mA。如果需要(yao)與(yu)主機進(jin)行枚舉(ju)過(guo)程，并需要(yao)大(da)(da)電(dian)(dian)(dian)(dian)(dian)流使能操作(zuo)，可在R9和(he)地之間串聯一(yi)個開漏極nMOSFET。如果MOSFET關斷，則TMR浮空，DS2712進(jin)入掛起狀態。

　　總結

　　對于(yu)小型消(xiao)費類(lei)電子設備(bei)的電池(chi)充電而言，USB端口是(shi)一個經(jing)濟、實用的電源。為完全(quan)符(fu)合(he)USB 2.0規(gui)范(fan)(fan)，連(lian)接在USB端口上的負載必須(xu)能(neng)(neng)夠(gou)與主(zhu)機進行雙(shuang)向(xiang)通信。負載也必須(xu)符(fu)合(he)電源管理(li)要求(qiu)，包括低功耗(hao)模式，以及便于(yu)主(zhu)機確定何時需要從端口吸(xi)取(qu)大(da)電流的手段。盡管部分兼容的系(xi)統能(neng)(neng)夠(gou)適應大(da)部分USB主(zhu)機，但有時會出現意想不(bu)到的結果。只有很好地理(li)解(jie)USB規(gui)范(fan)(fan)要求(qiu)和負載的期望(wang)，才能(neng)(neng)在對于(yu)規(gui)范(fan)(fan)的兼容性與負載復(fu)雜度之間(jian)取(qu)得較好的平衡(heng)。