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2KB閃(shan)存(cun)及適當外(wai)圍以提(ti)供一(yi)種廉價(jia)解決(jue)方案的飛利浦 80C51型MCU就是(shi)這樣一(yi)個例子(zi)。集成化閃(shan)存(cun)還(huan)能提(ti)供高效及方便地調試應(ying)用代碼并進行現場軟件升(sheng)級(如果需要)的能力(li)。

       由(you)于設(she)計界(jie)不(bu)僅(jin)熟悉而且廣泛接受(shou)8位MCU，故(gu)軟硬件開(kai)發可快速進行。由(you)眾多廠商提供的(de)各種(zhong)功能強大(da)且并不(bu)昂(ang)貴(gui)的(de)應用開(kai)發工具，也是這種(zhong)方法的(de)另一項優勢(shi)。

       利(li)用這種方法，設計(ji)團隊不僅(jin)能(neng)極大地(di)縮短(duan)設計(ji)周期，而且還(huan)能(neng)進行更為復雜的設計(ji)，并使(shi)項目的整體材料(liao)費(fei)(BOM)不超(chao)出可接(jie)受的范圍。

外圍電路集成
       譬如(ru)，當MCU集(ji)成有內部振蕩器時，離線鋰電(dian)池充(chong)電(dian)器設(she)計可從以下兩方(fang)面獲益(yi)。首先，可省掉外部振蕩器，從而節省成本及PCB占位；其次，內部振蕩器可提高系統啟動時的穩定(ding)性。

       四通道A/D轉換器是設計工程師們(men)應(ying)該尋(xun)求集(ji)成到芯片中的另一種有價值(zhi)的外圍電(dian)(dian)路。除(chu)能比使用外部A/D轉換器更節約成本外，還能用它來檢測充電(dian)(dian)電(dian)(dian)壓、電(dian)(dian)流及電(dian)(dian)池溫(wen)度--幾乎包括安全(quan)電(dian)(dian)池充電(dian)(dian)操(cao)作中的所有重要(yao)參數(shu)。

       用來實現以下所介紹設計的MCU(P89LPC916)不僅集(ji)成了上(shang)述所有這(zhe)些特性而且還(huan)擁有可同時(shi)在兩個時(shi)鐘上(shang)執行指令的高性能處理器(qi)架構，從而將其性能提高至(zhi)標準80C51器(qi)件的6倍。Time0(計時(shi)器(qi)0)很容易被配置(zhi)成PWM輸出，故易于設置(zhi)及使(shi)用PWM功能。

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(chong)電順序(xu)分成以(yi)下三個階段：預充(chong)電階段、恒定電流充(chong)電階段及(ji)恒定電壓充(chong)電階段。

     當電(dian)(dian)(dian)(dian)池(chi)只剩下(xia)很少的電(dian)(dian)(dian)(dian)量且因(yin)此(ci)而只能(neng)產(chan)生很低(di)的輸出電(dian)(dian)(dian)(dian)壓時，就必(bi)須(xu)有預(yu)充電(dian)(dian)(dian)(dian)階段。在此(ci)情況下(xia)，必(bi)須(xu)采用低(di)電(dian)(dian)(dian)(dian)流充電(dian)(dian)(dian)(dian)以保護電(dian)(dian)(dian)(dian)池(chi)。但如果被充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)池(chi)可(ke)產(chan)生較高電(dian)(dian)(dian)(dian)壓(>3V)，則可(ke)省略(lve)掉預(yu)充電(dian)(dian)(dian)(dian)階段。當然，這是最普遍(bian)的情況。

       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)(liu)入電(dian)池。電(dian)池的最大允許充電(dian)電(dian)流(liu)(liu)由該電(dian)池的額定(ding)容量決定(ding)。對(dui)于快速充電(dian)，例如額定(ding)700mAh的電(dian)池，可用(yong)350-400mA電(dian)流(liu)(liu)來充電(dian)。

       在鋰電池情況下，MCU必須在保持電池正常充(chong)電電壓的(de)同時(shi)(shi)還(huan)監(jian)視(shi)充(chong)電電流，以(yi)在電池充(chong)滿時(shi)(shi)能終止充(chong)電過程。

       溫度監視可(ke)用來確保執(zhi)行安全的(de)(de)充(chong)電步驟，因為隨著(zhu)電池充(chong)滿，任何額外的(de)(de)電能都(dou)將被轉換(huan)成熱量(liang)。盡(jin)管MCU必須為其完成的(de)(de)功(gong)能增加溫度監視，但當今市(shi)場(chang)上的(de)(de)大多(duo)數鋰(li)電池都(dou)帶有內置過(guo)充(chong)電保護，故(gu)溫度監視盡(jin)管需要但卻很(hen)少(shao)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)制充電(dian)(dian)(dian)開關。當開關閉合時(圖(tu)1a)，電(dian)(dian)(dian)流(liu)由于充電(dian)(dian)(dian)器提供的電(dian)(dian)(dian)壓(充電(dian)(dian)(dian)器Vin)而流(liu)過(guo)電(dian)(dian)(dian)路(lu)，此(ci)時電(dian)(dian)(dian)容通過(guo)電(dian)(dian)(dian)感(gan)充電(dian)(dian)(dian)。

       當開(kai)關打(da)開(kai)時(如1b所示)，電(dian)(dian)(dian)感試圖通過感應(ying)電(dian)(dian)(dian)壓(ya)來保(bao)持電(dian)(dian)(dian)流(liu)流(liu)動，但它(ta)不能立刻(ke)充電(dian)(dian)(dian)。然后(hou)電(dian)(dian)(dian)流(liu)流(liu)過肖特基二極(ji)管并給(gei)電(dian)(dian)(dian)容充電(dian)(dian)(dian)。此過程循環往(wang)復。

       當(dang)(dang)通過(guo)減少PWM占空比(bi)來縮短開關“通”時(shi)(shi)間時(shi)(shi)，平(ping)均電壓減少。相(xiang)反，當(dang)(dang)通過(guo)增(zeng)(zeng)加PWM占空比(bi)來延(yan)長開關“斷”時(shi)(shi)間時(shi)(shi)，平(ping)均電壓增(zeng)(zeng)加。故通過(guo)控(kong)制PWM占空比(bi)來使MCU調整充電電壓(或電流(liu))可(ke)達到(dao)所需(xu)的輸出值。

在討(tao)論設計細(xi)節以前，需(xu)先討(tao)論與電感及電容有關的兩個要點：

     1．電(dian)感大小

       不難看出，確定降壓轉換器電(dian)感的大(da)小(xiao)(xiao)是達到(dao)合(he)適充電(dian)電(dian)壓及電(dian)流的關鍵(jian)。電(dian)感大(da)小(xiao)(xiao)也與(yu)成本有關。電(dian)感容量可(ke)用公式1來計算(suan)：

      公(gong)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)。
       公式1顯示PWM的(de)開關頻率(lv)越(yue)高(亦即開關周期T越(yue)小)，則所需的(de)電感(gan)越(yue)小，這有助于減少器件成本。

       2．電容大小

       還(huan)需注意(yi)的是，此電路中的電容完全(quan)是用(yong)來減少(shao)紋波電流(liu)，故越大越好，因為紋波與電容值(zhi)成反比(bi)。

設計要點

       本設(she)計基(ji)于(yu)飛利浦(pu)P89LPC916型MCU，其(qi)整體設(she)計思(si)想是，通過先用恒定(ding)電(dian)流充電(dian)、然(ran)后再用恒定(ding)電(dian)壓充電(dian)來實現盡可能(neng)快的充電(dian)。MCU還控(kong)制用于(yu)指示充電(dian)器工作(zuo)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輸出解決(jue)方案(an)

       Timer0(定時器(qi)(qi)(qi)0)的一個通(tong)道用來產生控(kong)制(zhi)降(jiang)(jiang)壓轉換器(qi)(qi)(qi)開關的PWM信號。由于LPC916帶有(you)其自己(ji)的片上RC振(zhen)蕩(dang)器(qi)(qi)(qi)，故(gu)(gu)充電更加穩定而有(you)效(xiao)--尤其在電壓控(kong)制(zhi)工作模式下(xia)。所需的PWM頻率(lv)僅大(da)約為14kHz，故(gu)(gu)能很好地控(kong)制(zhi)在片上振(zhen)蕩(dang)器(qi)(qi)(qi)的頻率(lv)范(fan)圍內。可通(tong)過改變降(jiang)(jiang)壓轉換器(qi)(qi)(qi)的“開”時間來調整PWM占空(kong)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)池(chi)(chi)應(ying)以三個獨(du)立(li)的階段(duan)來(lai)充電(dian)(dian)(dian)(dian)(dian)(dian)。如果電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓低于(yu)3V，則需要有預充電(dian)(dian)(dian)(dian)(dian)(dian)階段(duan)且(qie)充電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)應(ying)保持(chi)為65mA。一旦電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓達到3V+-1%，即開始(shi)進入快速充電(dian)(dian)(dian)(dian)(dian)(dian)階段(duan)，并采用350mA的恒定(ding)充電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)。通(tong)過調整控制脈沖可使充電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)保持(chi)恒定(ding)。當電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓達到4V+-1%時，即開始(shi)接(jie)恒定(ding)電(dian)(dian)(dian)(dian)(dian)(dian)壓充電(dian)(dian)(dian)(dian)(dian)(dian)階段(duan)。此時電(dian)(dian)(dian)(dian)(dian)(dian)壓被(bei)保持(chi)在4.23V，充電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)處于(yu)監視下(xia)。

       在恒定(ding)電(dian)(dian)(dian)(dian)(dian)壓(ya)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)階段之(zhi)后，電(dian)(dian)(dian)(dian)(dian)池被另外再充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)50分鐘(zhong)，同(tong)時(shi)保持充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)小于30mA。充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)時(shi)間可用一個計(ji)時(shi)器(qi)來控制，但監(jian)視充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)終結(jie)的方法有三種：檢測充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)、使用計(ji)時(shi)器(qi)以(yi)及監(jian)視溫度(可選)。

       充電(dian)過程如圖(tu)3所示。從(cong)一(yi)個(ge)階段進入到另一(yi)個(ge)階段的準確標志(zhi)如下：

       預充(chong)電(dian)(dian)階(jie)段(duan)(當(dang)需要時(shi))：如果Vbat<3.0(1%，則設置(zhi)(zhi)Iout=10%；Ireg=65mA；快速充(chong)電(dian)(dian)階(jie)段(duan)(恒定電(dian)(dian)流(liu)充(chong)電(dian)(dian))：當(dang)Vbat<=4.00+-1%V時(shi)，設置(zhi)(zhi)Iout=Ireg=350mA；計時(shi)器(qi)控制充(chong)電(dian)(dian)階(jie)段(duan)(恒定電(dian)(dian)壓充(chong)電(dian)(dian))：當(dang)Ibat<60mA時(shi)，設置(zhi)(zhi)Vout=Vreg=4.23V(50分鐘(zhong))以保證電(dian)(dian)池(chi)充(chong)分充(chong)電(dian)(dian)，但使充(chong)電(dian)(dian)電(dian)(dian)流(liu)小于30mA。

       充電(dian)在4小時內完成。

       考慮(lv)到最終用戶，設計中采(cai)用了LED狀態指示燈，以提供有關充電序列狀態的信息。

       設計方案的測試

       可用來在(zai)充電過(guo)程(cheng)中測(ce)試(shi)該設計(ji)的電路(lu)框圖如圖4所示。用兩塊(kuai)萬用表來測(ce)量(liang)Vout及(ji)Vsense_res讀數。

       Vout=Vbat+Vsense_res，充電電流可用公式Iout=Vsense res/0.75來計算(suan)。

       當充電(dian)開始時(shi)，每15秒(miao)(miao)記(ji)錄(lu)一次數據，但(dan)當電(dian)流及電(dian)壓穩定(ding)后，記(ji)錄(lu)周(zhou)期可(ke)縮短(duan)為每5秒(miao)(miao)記(ji)錄(lu)一次。

       結果(guo)可能(neng)會隨不同電池的化(hua)學特征而變化(hua)，而且電池的起始(shi)電壓也(ye)對(dui)結果(guo)有影響。