高效、低成本及可靠的電池充電器設計可用各種方法來實現，但采用8位閃速MCU不僅能縮短設計時間、降低成本及提供安全可靠的產品，而且還能使設計人員以最少的工作量來進行現場升級。
     考慮到電池安全充電的成本、設計效率及重要性，基于MCU的解決方案可為設計者們提供諸多優勢。通過選擇帶適當外圍與閃存的8位MCU，工程師們能充分利用其優勢來設計一種離線鋰電池充電器。帶(dai)2KB閃(shan)存及適(shi)當(dang)外圍以提(ti)供(gong)一種廉價解(jie)決(jue)方案的飛(fei)利(li)浦 80C51型(xing)MCU就(jiu)是這(zhe)樣一個(ge)例子。集成(cheng)化閃(shan)存還能(neng)提(ti)供(gong)高效及方便地調試應(ying)用代碼并(bing)進行(xing)現場軟件升(sheng)級(如果需要)的能(neng)力。

       由(you)于設計界不(bu)僅熟悉(xi)而且(qie)廣泛接(jie)受8位MCU，故(gu)軟(ruan)硬件開發可(ke)快速進行(xing)。由(you)眾多(duo)廠商提供(gong)的各種功能強大且(qie)并不(bu)昂貴的應用(yong)開發工具，也(ye)是這種方法(fa)的另一項優勢。

       利用這種方法，設計團隊不僅能(neng)極大地縮短設計周期，而且還能(neng)進行更為復雜的(de)設計，并使項目的(de)整(zheng)體材料費(BOM)不超出可接受(shou)的(de)范(fan)圍。

外圍電路集成
       譬如，當MCU集(ji)成有(you)內(nei)(nei)部振蕩器時，離(li)線鋰電池充電器設計可(ke)(ke)從以下兩(liang)方面(mian)獲益。首先，可(ke)(ke)省掉外部振蕩器，從而節(jie)省成本及PCB占位；其次，內(nei)(nei)部振蕩器可(ke)(ke)提高系統啟動時的穩定(ding)性。

       四(si)通道(dao)A/D轉換(huan)器是設計工程師們應該尋求集成(cheng)到(dao)芯片(pian)中的(de)另一種有價值的(de)外(wai)(wai)圍電(dian)路。除能比使用(yong)外(wai)(wai)部(bu)A/D轉換(huan)器更節(jie)約成(cheng)本外(wai)(wai)，還(huan)能用(yong)它來檢測充(chong)電(dian)電(dian)壓、電(dian)流(liu)及電(dian)池溫度(du)--幾乎包(bao)括安(an)全(quan)電(dian)池充(chong)電(dian)操作中的(de)所有重要參數。

       用來實(shi)現(xian)以下所(suo)介(jie)紹設計(ji)的MCU(P89LPC916)不僅(jin)集成了上述所(suo)有這些特性而且還擁有可(ke)同時在兩個時鐘(zhong)上執行(xing)指令的高性能(neng)(neng)處理(li)器架構，從而將其性能(neng)(neng)提高至標(biao)準80C51器件(jian)的6倍。Time0(計(ji)時器0)很(hen)容易(yi)被配置(zhi)(zhi)成PWM輸出，故易(yi)于(yu)設置(zhi)(zhi)及使用PWM功能(neng)(neng)。

基本電池充電標準
  

    本設計為專門針對額定700-750mAh、3.6V放電電壓及4.2V電壓極限的鋰電池充電器解決方案。
     充電(dian)順序分成以下三(san)個階段(duan)：預充電(dian)階段(duan)、恒定電(dian)流(liu)充電(dian)階段(duan)及(ji)恒定電(dian)壓充電(dian)階段(duan)。

     當電(dian)(dian)池只剩下很少的(de)電(dian)(dian)量且因此(ci)而(er)只能產生(sheng)(sheng)很低(di)(di)的(de)輸出電(dian)(dian)壓(ya)時，就必須有(you)預(yu)(yu)充(chong)電(dian)(dian)階段。在此(ci)情(qing)況下，必須采用低(di)(di)電(dian)(dian)流充(chong)電(dian)(dian)以保護(hu)電(dian)(dian)池。但如果被充(chong)電(dian)(dian)電(dian)(dian)池可(ke)產生(sheng)(sheng)較高(gao)電(dian)(dian)壓(ya)(>3V)，則可(ke)省略掉預(yu)(yu)充(chong)電(dian)(dian)階段。當然，這是(shi)最普遍的(de)情(qing)況。

       大部分電能是在恒定電流及恒定電壓充電階段從充電器流(liu)入電(dian)(dian)池(chi)。電(dian)(dian)池(chi)的最(zui)大允(yun)許(xu)充(chong)電(dian)(dian)電(dian)(dian)流(liu)由該電(dian)(dian)池(chi)的額(e)定容(rong)量決定。對于快速充(chong)電(dian)(dian)，例如額(e)定700mAh的電(dian)(dian)池(chi)，可用350-400mA電(dian)(dian)流(liu)來充(chong)電(dian)(dian)。

       在(zai)鋰電(dian)池(chi)情況下，MCU必須在(zai)保(bao)持電(dian)池(chi)正常充電(dian)電(dian)壓的同時(shi)還監視充電(dian)電(dian)流，以(yi)在(zai)電(dian)池(chi)充滿時(shi)能終(zhong)止充電(dian)過程(cheng)。

       溫(wen)度監視可用來(lai)確保執(zhi)行安(an)全的(de)(de)充電(dian)步驟(zou)，因為隨著電(dian)池(chi)充滿，任何額外(wai)的(de)(de)電(dian)能(neng)都將被(bei)轉(zhuan)換(huan)成熱量(liang)。盡(jin)管(guan)MCU必須(xu)為其完成的(de)(de)功能(neng)增加溫(wen)度監視，但當今市場上的(de)(de)大多數鋰電(dian)池(chi)都帶有(you)內(nei)置過充電(dian)保護(hu)，故溫(wen)度監視盡(jin)管(guan)需要但卻很少使用。

降壓轉換器設計

       若要設計一種帶錐形端接特性的充電器，最有效及最經濟的方法是采用降壓轉換器來作為開關調整器。降壓轉換器使用電感來儲存電能。圖1a及1b分別為開關處于通/斷位置時的降壓轉換器工作示意圖。
       來自PWM的信號控(kong)制(zhi)充電(dian)(dian)開(kai)關。當開(kai)關閉合時(shi)(圖1a)，電(dian)(dian)流(liu)由于充電(dian)(dian)器提(ti)供(gong)的電(dian)(dian)壓(充電(dian)(dian)器Vin)而流(liu)過電(dian)(dian)路，此時(shi)電(dian)(dian)容通(tong)過電(dian)(dian)感充電(dian)(dian)。

       當開(kai)關打(da)開(kai)時(如(ru)1b所示)，電(dian)感試圖(tu)通過感應電(dian)壓(ya)來(lai)保(bao)持電(dian)流(liu)流(liu)動，但(dan)它不能立刻充電(dian)。然后電(dian)流(liu)流(liu)過肖特基二極管并(bing)給電(dian)容充電(dian)。此過程(cheng)循(xun)環往復(fu)。

       當(dang)(dang)通(tong)過減少PWM占空(kong)比來縮(suo)短開關“通(tong)”時(shi)間時(shi)，平均電(dian)(dian)壓減少。相(xiang)反，當(dang)(dang)通(tong)過增(zeng)加(jia)PWM占空(kong)比來延(yan)長開關“斷”時(shi)間時(shi)，平均電(dian)(dian)壓增(zeng)加(jia)。故通(tong)過控制PWM占空(kong)比來使MCU調整充(chong)電(dian)(dian)電(dian)(dian)壓(或電(dian)(dian)流)可(ke)達到所需的輸出值。

在討論設(she)計細節以(yi)前，需先(xian)討論與電(dian)感(gan)及電(dian)容有關(guan)的(de)兩個要點：

     1．電(dian)感大小

       不(bu)難看出(chu)，確定(ding)降(jiang)壓轉(zhuan)換器電感(gan)(gan)的大小(xiao)是達到合適(shi)充電電壓及電流的關(guan)(guan)鍵。電感(gan)(gan)大小(xiao)也與成本有關(guan)(guan)。電感(gan)(gan)容(rong)量可用(yong)公式1來計(ji)算：

      公式1

       其中：Vi：輸入至開關的充電器電壓；
    

       Vsat：開關“通”時開關的電壓損失；
       Vo：電壓輸出；
       T：PWM周期；
       DutyCycle：PWM占空比；
       Io：電流輸出(亦即恒定電流充電)。
       公(gong)式1顯示PWM的開關頻率越高(gao)(亦(yi)即開關周(zhou)期T越小)，則所需(xu)的電感越小，這有助于減少(shao)器件(jian)成本。

       2．電容(rong)大小(xiao)

       還需注(zhu)意(yi)的是，此電(dian)路(lu)中的電(dian)容(rong)完全是用來減少紋波電(dian)流(liu)，故越大(da)越好(hao)，因為紋波與電(dian)容(rong)值成反比。

設計要點

       本設計基于(yu)飛利(li)浦(pu)P89LPC916型(xing)MCU，其整體設計思想是(shi)，通(tong)過先用(yong)恒定(ding)電(dian)流充電(dian)、然后再(zai)用(yong)恒定(ding)電(dian)壓充電(dian)來(lai)實現盡可能快的充電(dian)。MCU還控制用(yong)于(yu)指示充電(dian)器工作狀態的LED。

     1．精密電源
　

       VDD需采用精密電壓源，因為此電壓被用作DA-DA轉換器的電壓參考。低壓降(LDO)調整器為該電壓源的最佳選擇，且本設計采用3端LDO LM1117來為VDD提供精密3.31 V電源。
       2．PWM輸出(chu)解決方案

       Timer0(定(ding)時(shi)器0)的(de)(de)一個通(tong)(tong)道(dao)用來(lai)產生控(kong)制降(jiang)壓轉換(huan)器開關的(de)(de)PWM信號。由于(yu)LPC916帶有(you)其自己的(de)(de)片(pian)上RC振蕩器，故充電更加穩定(ding)而(er)有(you)效--尤其在(zai)電壓控(kong)制工作(zuo)模式下。所需的(de)(de)PWM頻率(lv)僅大約為(wei)14kHz，故能很(hen)好地控(kong)制在(zai)片(pian)上振蕩器的(de)(de)頻率(lv)范圍內。可(ke)通(tong)(tong)過改變降(jiang)壓轉換(huan)器的(de)(de)“開”時(shi)間來(lai)調(diao)整PWM占空比(bi)。

系統設計

       圖2為鋰電池充電器系統組成框圖。其中PWM輸出控制充電開關，且其占空比可根據需要用充電電壓及電流的反饋來調整。LPC916的8位片上高速A/D轉換器提供了監視充電電壓所需的高精度。避免鋰離子應用中的過充電非常重要，因為將充電保持在其最大值以內可延長電池的使用壽命。表1為該電路的輸入/輸出參數規格。
　

       下一步是計算電感值，首先必須指出的是，公式1給出了占空比、輸出電流、PWM周期及其他變量之間的關系。電感值可通過假設Vi＝5.1V、所需輸出電壓Vsat＝0.5V(在Io=350mA上，Vo＝4.25V、所需輸出電流Io＝350mA、1/T＝14.7kHz以及占空比為50%來計算)。采用以上這些值，用公式1可計算出電感值不小于10μH。在本設計中，建議電感值為33-10μH。盡管可以采用大于5.1V的輸入電壓，但更高的輸入電壓要求采用更高頻率的PWM或更大的電感，從而使器件成本提高。
       鋰電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)應以(yi)三個獨立的(de)階(jie)(jie)段(duan)來充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)。如果電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)低于(yu)3V，則需要有預充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)階(jie)(jie)段(duan)且充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)應保持為65mA。一旦電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)達到(dao)3V+-1%，即開始進入快速(su)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)階(jie)(jie)段(duan)，并(bing)采用350mA的(de)恒定(ding)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)。通過調(diao)整控制脈(mo)沖(chong)可使充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)保持恒定(ding)。當電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)達到(dao)4V+-1%時，即開始接恒定(ding)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)階(jie)(jie)段(duan)。此(ci)時電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)被(bei)保持在4.23V，充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)處于(yu)監視下(xia)。

       在恒(heng)定電(dian)壓充(chong)電(dian)階(jie)段之后，電(dian)池被另外(wai)再充(chong)電(dian)50分鐘，同時(shi)保(bao)持充(chong)電(dian)電(dian)流小(xiao)于30mA。充(chong)電(dian)時(shi)間可(ke)用一(yi)個計時(shi)器(qi)來控(kong)制(zhi)，但監視充(chong)電(dian)終結的(de)方法(fa)有三種：檢(jian)測充(chong)電(dian)電(dian)流、使(shi)用計時(shi)器(qi)以及(ji)監視溫度(可(ke)選)。

       充電過程(cheng)如圖3所示。從一個(ge)階段進入到另一個(ge)階段的準確標志(zhi)如下(xia)：

       預充(chong)電(dian)(dian)階段(duan)(duan)(當(dang)需要時(shi))：如果Vbat<3.0(1%，則設置(zhi)Iout=10%；Ireg=65mA；快速(su)充(chong)電(dian)(dian)階段(duan)(duan)(恒定電(dian)(dian)流充(chong)電(dian)(dian))：當(dang)Vbat<=4.00+-1%V時(shi)，設置(zhi)Iout=Ireg=350mA；計時(shi)器控(kong)制充(chong)電(dian)(dian)階段(duan)(duan)(恒定電(dian)(dian)壓充(chong)電(dian)(dian))：當(dang)Ibat<60mA時(shi)，設置(zhi)Vout=Vreg=4.23V(50分鐘)以保證電(dian)(dian)池充(chong)分充(chong)電(dian)(dian)，但使充(chong)電(dian)(dian)電(dian)(dian)流小于30mA。

       充電在4小時內完成。

       考慮到最終用(yong)(yong)戶(hu)，設計中采用(yong)(yong)了LED狀態(tai)指示燈，以提供有關充電序列狀態(tai)的(de)信息。

       設計方(fang)案的測試

       可用(yong)來在充(chong)電過程中測試該設(she)計(ji)的電路框圖如圖4所示(shi)。用(yong)兩塊萬用(yong)表來測量Vout及(ji)Vsense_res讀數。

       Vout=Vbat+Vsense_res，充電(dian)電(dian)流可用公式(shi)Iout=Vsense res/0.75來計算。

       當充(chong)電(dian)開始(shi)時，每(mei)15秒(miao)記錄(lu)一(yi)次(ci)數(shu)據，但當電(dian)流及電(dian)壓穩定后，記錄(lu)周期可縮短為每(mei)5秒(miao)記錄(lu)一(yi)次(ci)。

       結(jie)果可能會隨不(bu)同電(dian)池(chi)(chi)的化學特(te)征而(er)變化，而(er)且(qie)電(dian)池(chi)(chi)的起始電(dian)壓(ya)也對結(jie)果有影響(xiang)。