高集成度、低成本電池充電器
高集成度、低成本電池充電器工作原理
1 鋰電池充電方式概述
以目前在便攜式數碼產品中最為常見的鋰電池為例，鋰電池具有較高的能量密度，體積小，重量輕，壽命長，無記憶效應以及不污染環境等諸多優點，因此廣泛應用于數碼設備中，經濟實用，而且靈活方便。
常用的鋰電池充電器其核心都是恒流/恒壓調節器。一般通過檢測充電電池的電壓來判斷電池是否充滿，由于鋰電池的大量儲能是在端電壓接近其最高允許電壓時的充電期間建立的，鋰電池充電器設計往往需要較高的電壓檢測精度(精度高于l％)，試圖使空載端電壓接近容許的最高電壓。當電池的電壓較低時，典型的充電周期開始時恒流充電方式。當電池電壓上升到指定限度時，充電器轉換為恒壓調節，該方式一直持續到充電電流減小為零，這時電池充電完畢。在恒壓充電階段，電流按指數規律下降，該指數與電池電阻以及和該電池相串聯的所有電阻有關(和通過一電阻對電容充電過(guo)程很類似)。由于充電電流按指數規律下降，所以完成充電需要(yao)相當長的時間。
鋰電池充電器根據工(gong)(gong)作(zuo)原理不同，可以分為線性調(diao)節方(fang)式(shi)，開關方(fang)式(shi)以及脈沖方(fang)式(shi)三種工(gong)(gong)作(zuo)方(fang)式(shi)。
線性調節方式，例如MAXl898，MAX846A等，線性調節方式的充電器的(de)(de)充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)壓或電(dian)(dian)(dian)流來自于(yu)交流適(shi)配器的(de)(de)直(zhi)流輸出，通過(guo)控制外部PNP調(diao)整(zheng)管(guan)，調(diao)節(jie)電(dian)(dian)(dian)池的(de)(de)充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流以(yi)及(ji)充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)壓，具(ju)有結構簡單，外圍元件少等優(you)點，但(dan)外部的(de)(de)調(diao)整(zheng)管(guan)工作在(zai)線性方式下，具(ju)有較(jiao)高的(de)(de)功耗，因此該充(chong)(chong)電(dian)(dian)(dian)方式的(de)(de)效率較(jiao)低，發熱相(xiang)對(dui)較(jiao)大，用于(yu)小(xiao)型便(bian)攜式設備(bei)，例如手機、個人數字助理(PDA)等,會造成較(jiao)大的(de)(de)溫升。
開關方式(shi)(shi)充(chong)電(dian)器，以MAX745，MAXl757為(wei)例，相比(bi)線(xian)性(xing)調節(jie)方式(shi)(shi)，開關方式(shi)(shi)控制外部MOSFET對源電(dian)壓進行(xing)斬波(bo)，然后(hou)將斬波(bo)后(hou)的(de)電(dian)壓進行(xing)濾波(bo)，產生所需(xu)要的(de)充(chong)電(dian)電(dian)流(liu)或電(dian)壓。MOSFET工作(zuo)與開關狀態，大(da)(da)大(da)(da)減小(xiao)了調整(zheng)管(guan)的(de)功耗，但開關方式(shi)(shi)充(chong)電(dian)器的(de)功耗降低(di)是以增加電(dian)路尺(chi)寸和復雜(za)度為(wei)代價。
而脈沖方(fang)(fang)(fang)式(shi)(shi)的充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)具(ju)有(you)線(xian)性充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)和開(kai)關(guan)(guan)模式(shi)(shi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)兩者共有(you)的優點(dian)。該(gai)方(fang)(fang)(fang)式(shi)(shi)配合限(xian)流交流適(shi)配器(qi)(qi)以及(ji)外(wai)部(bu)(bu)P溝道(dao)MOSFET，構成(cheng)了一個(ge)脈沖方(fang)(fang)(fang)式(shi)(shi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)。以MAXl879為例(li)，如圖3所示，此時(shi)，當電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)低于(yu)設定的電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)值時(shi)，充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)打(da)開(kai)外(wai)部(bu)(bu)P溝道(dao)MOSFET對電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)進行充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)，充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流受限(xian)于(yu)外(wai)部(bu)(bu)電(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(即交流適(shi)配器(qi)(qi))；當電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)達到設定電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)時(shi)，MOSFET關(guan)(guan)斷(duan)。在這(zhe)種模式(shi)(shi)下，外(wai)部(bu)(bu)MOSFET工(gong)作在開(kai)關(guan)(guan)方(fang)(fang)(fang)式(shi)(shi)下，不(bu)對充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流進行調節(jie)，因(yin)此降低了功耗；同(tong)時(shi)，由(you)于(yu)無需輸出濾波(bo)器(qi)(qi)，所以同(tong)時(shi)具(ju)有(you)線(xian)性調節(jie)方(fang)(fang)(fang)式(shi)(shi)的結(jie)構簡單(dan)的優點(dian)。該(gai)控制方(fang)(fang)(fang)式(shi)(shi)同(tong)樣存在缺點(dian)，即交流適(shi)配器(qi)(qi)需要具(ju)備限(xian)流功能，因(yin)此增加(jia)了交流適(shi)配器(qi)(qi)的成(cheng)本(ben)。
2 高集成度、低成本鋰電池充電器
2.1 MAX8730的工作原理
MAX8730開關方式充電器集成了“Li+、NiMH和NiCd電池充電所必需的所有功能。利用一個高效率、降壓型DC-DC轉換器實現精確的恒流、恒壓充電器，選擇3節或4節電池模式。DC-DC轉換器驅動一個P溝道MOSFET，并且外接一個高速肖特基二極管。充電電流和輸入檢流放大器失調誤差較低，因此可采用較小的檢流電阻，降低功耗。
MAX8730的典型工作電路如圖4所示，該器件內置一個滯回比較器，監測交流適配器的連接狀態，并且自動選擇適當的電源。當適配器連接時(VASNS>VBATT-100 mV)，關斷P溝道MOSFET(P3)，將電池與系統負載斷開。適配器斷開時(VASNS
圖5為MAX8730的功能框圖，該器件內置一路電壓調節環(CCV)和兩路電流調節環(CCI和CCS)。三個環路獨立工作，MAX8730根據工作情況不同，控制輸入電流(CCS控制環)、充電電流(CCI控制環)或充電電壓(CCV控制環)。三個控制環――CCS、CCI和CCV內部連接到最低電壓箝位(LVC)放大器,LVC輸出CCV、CCI或CCS三者中的最小電壓,作為DC-DC控制器的反饋控制信號。CCV電壓調節環監控電池電壓，通過監測電池電壓來判定電池是否滿充;而CCI電池電流調節環監控流入電池的電流,確保充電電流不超過設定的限流值。當電池電壓低于設定值時，充電電流調節環起作用。而當電池電壓達到其設定值時，電壓調節環起作用，將電池電壓維持在設定值處。另一個電流調節環(CCS)控制充電電流，當適配器電流超過設定的輸入限流值時，通過降低充電電流優先為系統負載供電，從而降低了對適配器的要求。
3 穩定性討論
MAX8730的充電電壓調節環、充電電流調節環和輸入電流調節環分別在CCV、CCI和CCS引腳處進行補償。
如(ru)圖4所(suo)示(shi)，電(dian)(dian)(dian)(dian)壓調(diao)節環路(lu)的(de)補(bu)(bu)(bu)償(chang)(chang)需要引(yin)入(ru)(ru)一(yi)對(dui)零(ling)點(dian)(dian)-極(ji)(ji)(ji)點(dian)(dian)對(dui)，因此補(bu)(bu)(bu)償(chang)(chang)網絡由(you)(you)CCV和(he)RCV構成(cheng)。在低頻時，極(ji)(ji)(ji)點(dian)(dian)為電(dian)(dian)(dian)(dian)壓環響(xiang)應提供必要的(de)滾(gun)降。零(ling)點(dian)(dian)用(yong)于補(bu)(bu)(bu)償(chang)(chang)輸(shu)出電(dian)(dian)(dian)(dian)容和(he)負(fu)載所(suo)引(yin)入(ru)(ru)的(de)極(ji)(ji)(ji)點(dian)(dian)。由(you)(you)于輸(shu)出電(dian)(dian)(dian)(dian)容的(de)阻抗對(dui)充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)環路(lu)以及輸(shu)入(ru)(ru)電(dian)(dian)(dian)(dian)流(liu)環路(lu)響(xiang)應的(de)影響(xiang)極(ji)(ji)(ji)小，因此僅需要引(yin)入(ru)(ru)單個極(ji)(ji)(ji)點(dian)(dian)對(dui)兩(liang)個電(dian)(dian)(dian)(dian)流(liu)環路(lu)進行補(bu)(bu)(bu)償(chang)(chang)。CCV、CCI和(he)CCS環路(lu)補(bu)(bu)(bu)償(chang)(chang)的(de)詳細計算方法參見MAX8730數(shu)據(ju)資料，圖4給出了(le)補(bu)(bu)(bu)償(chang)(chang)電(dian)(dian)(dian)(dian)容和(he)電(dian)(dian)(dian)(dian)阻的(de)推薦(jian)值。