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)存及(ji)適當(dang)外圍以提供一(yi)種廉價解決方(fang)案的飛利浦(pu) 80C51型MCU就是這樣一(yi)個例子。集(ji)成(cheng)化閃(shan)存還(huan)能(neng)提供高效及(ji)方(fang)便(bian)地(di)調試應用(yong)代碼并進行(xing)現場軟(ruan)件升級(ji)(如果需要(yao))的能(neng)力(li)。

       由于設計界不僅(jin)熟(shu)悉而(er)且廣泛接受8位MCU，故軟硬(ying)件開(kai)發可快速進行。由眾(zhong)多廠(chang)商提供的(de)(de)各種(zhong)功能(neng)強大且并不昂貴的(de)(de)應用開(kai)發工具，也是這種(zhong)方(fang)法(fa)的(de)(de)另一項(xiang)優勢。

       利用這種方法，設(she)計團隊不僅能極大地縮短設(she)計周期，而且還能進行更為復雜的設(she)計，并(bing)使項目的整體材料費(fei)(BOM)不超出可接受的范(fan)圍。

外圍電路集成
       譬如，當MCU集成(cheng)有內部振蕩器(qi)(qi)時，離線鋰電(dian)池(chi)充電(dian)器(qi)(qi)設計可從以下兩(liang)方面(mian)獲(huo)益(yi)。首先，可省(sheng)掉(diao)外部振蕩器(qi)(qi)，從而節省(sheng)成(cheng)本及PCB占位；其次，內部振蕩器(qi)(qi)可提高系(xi)統啟動時的穩定性。

       四通道(dao)A/D轉(zhuan)換器(qi)是設計(ji)工程師(shi)們(men)應(ying)該尋求集成(cheng)到芯片中(zhong)的(de)另一種有(you)價(jia)值的(de)外圍(wei)電(dian)路(lu)。除能比使(shi)用外部A/D轉(zhuan)換器(qi)更節(jie)約成(cheng)本外，還能用它來檢測(ce)充電(dian)電(dian)壓(ya)、電(dian)流及電(dian)池(chi)溫度(du)--幾乎包括安全電(dian)池(chi)充電(dian)操作中(zhong)的(de)所有(you)重要參數。

       用來(lai)實現(xian)以下所介紹設(she)計的MCU(P89LPC916)不僅集成了上(shang)述(shu)所有這些特性(xing)而且還擁有可(ke)同時(shi)在(zai)兩個時(shi)鐘上(shang)執行指令的高(gao)性(xing)能處理器(qi)架構(gou)，從而將其性(xing)能提高(gao)至標準(zhun)80C51器(qi)件的6倍(bei)。Time0(計時(shi)器(qi)0)很容易被配(pei)置(zhi)成PWM輸(shu)出，故易于設(she)置(zhi)及使用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(dian)(dian)順序分成以下三(san)個(ge)階(jie)(jie)段(duan)(duan)(duan)：預充(chong)電(dian)(dian)階(jie)(jie)段(duan)(duan)(duan)、恒定(ding)電(dian)(dian)流充(chong)電(dian)(dian)階(jie)(jie)段(duan)(duan)(duan)及恒定(ding)電(dian)(dian)壓充(chong)電(dian)(dian)階(jie)(jie)段(duan)(duan)(duan)。

     當電(dian)(dian)(dian)池(chi)只剩下(xia)很(hen)少的電(dian)(dian)(dian)量且因此而只能(neng)產(chan)生(sheng)很(hen)低的輸出電(dian)(dian)(dian)壓(ya)時，就必須(xu)有預充(chong)電(dian)(dian)(dian)階段。在此情況(kuang)下(xia)，必須(xu)采用低電(dian)(dian)(dian)流充(chong)電(dian)(dian)(dian)以保護電(dian)(dian)(dian)池(chi)。但如果被(bei)充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)池(chi)可(ke)產(chan)生(sheng)較高電(dian)(dian)(dian)壓(ya)(>3V)，則可(ke)省(sheng)略掉預充(chong)電(dian)(dian)(dian)階段。當然，這是最普遍(bian)的情況(kuang)。

       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)(dian)池(chi)(chi)。電(dian)(dian)池(chi)(chi)的(de)(de)最大允許充電(dian)(dian)電(dian)(dian)流(liu)(liu)由(you)該電(dian)(dian)池(chi)(chi)的(de)(de)額定容量決(jue)定。對于快速(su)充電(dian)(dian)，例如額定700mAh的(de)(de)電(dian)(dian)池(chi)(chi)，可用350-400mA電(dian)(dian)流(liu)(liu)來充電(dian)(dian)。

       在鋰電(dian)(dian)(dian)池情(qing)況下，MCU必須在保持電(dian)(dian)(dian)池正常充電(dian)(dian)(dian)電(dian)(dian)(dian)壓的同時(shi)還監(jian)視充電(dian)(dian)(dian)電(dian)(dian)(dian)流，以在電(dian)(dian)(dian)池充滿時(shi)能終(zhong)止充電(dian)(dian)(dian)過程。

       溫(wen)度監(jian)視可用(yong)來確保執行安全的充(chong)(chong)電步驟，因(yin)為隨著(zhu)電池充(chong)(chong)滿，任何額外的電能都將被轉換成(cheng)熱量。盡管MCU必須為其完(wan)成(cheng)的功能增加(jia)溫(wen)度監(jian)視，但(dan)當今市場(chang)上的大多數鋰(li)電池都帶有(you)內置(zhi)過充(chong)(chong)電保護，故溫(wen)度監(jian)視盡管需(xu)要但(dan)卻很少使用(yong)。

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的信號控制(zhi)充(chong)(chong)電(dian)(dian)(dian)(dian)開關(guan)(guan)。當開關(guan)(guan)閉合時(shi)(圖1a)，電(dian)(dian)(dian)(dian)流由于充(chong)(chong)電(dian)(dian)(dian)(dian)器提供的電(dian)(dian)(dian)(dian)壓(充(chong)(chong)電(dian)(dian)(dian)(dian)器Vin)而流過(guo)電(dian)(dian)(dian)(dian)路(lu)，此(ci)時(shi)電(dian)(dian)(dian)(dian)容(rong)通過(guo)電(dian)(dian)(dian)(dian)感充(chong)(chong)電(dian)(dian)(dian)(dian)。

       當開關打(da)開時(如(ru)1b所(suo)示)，電(dian)感(gan)試圖通過感(gan)應電(dian)壓來保持(chi)電(dian)流(liu)流(liu)動(dong)，但它不能(neng)立刻充(chong)電(dian)。然后電(dian)流(liu)流(liu)過肖特基(ji)二極(ji)管并給電(dian)容充(chong)電(dian)。此過程循環(huan)往復。

       當(dang)(dang)通(tong)(tong)(tong)過減少PWM占(zhan)空比來(lai)縮短開關(guan)“通(tong)(tong)(tong)”時(shi)間時(shi)，平均(jun)電(dian)壓(ya)減少。相反，當(dang)(dang)通(tong)(tong)(tong)過增加PWM占(zhan)空比來(lai)延長(chang)開關(guan)“斷”時(shi)間時(shi)，平均(jun)電(dian)壓(ya)增加。故通(tong)(tong)(tong)過控制PWM占(zhan)空比來(lai)使MCU調(diao)整充電(dian)電(dian)壓(ya)(或(huo)電(dian)流)可達到(dao)所(suo)需的輸出值。

在討論設計細節以前，需先(xian)討論與(yu)電感及電容有關的兩個要點(dian)：

     1．電(dian)感大小

       不(bu)難(nan)看出，確定降壓轉(zhuan)換器電感(gan)的大(da)小是(shi)達到(dao)合適(shi)充(chong)電電壓及(ji)電流的關(guan)鍵。電感(gan)大(da)小也(ye)與成本有關(guan)。電感(gan)容量可用公式1來計算：

      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)。
       公式(shi)1顯示PWM的開關(guan)頻率(lv)越(yue)高(亦(yi)即(ji)開關(guan)周(zhou)期(qi)T越(yue)小)，則所需的電感越(yue)小，這(zhe)有(you)助(zhu)于減(jian)少器(qi)件成本。

       2．電容大小

       還需(xu)注意(yi)的(de)是，此電(dian)(dian)路中(zhong)的(de)電(dian)(dian)容完(wan)全是用來減少紋波電(dian)(dian)流，故越大越好(hao)，因為紋波與電(dian)(dian)容值成(cheng)反比。

設計要點

       本設(she)計基(ji)于飛利(li)浦P89LPC916型MCU，其整體設(she)計思想是(shi)，通(tong)過先用恒(heng)定(ding)電(dian)流充電(dian)、然后再用恒(heng)定(ding)電(dian)壓充電(dian)來實現盡可能快的充電(dian)。MCU還控制(zhi)用于指示充電(dian)器工(gong)作(zuo)狀(zhuang)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輸出解決方案

       Timer0(定(ding)時器(qi)0)的一(yi)個通(tong)道用(yong)來(lai)產生(sheng)控制(zhi)(zhi)降(jiang)壓(ya)轉換器(qi)開關(guan)的PWM信號。由于LPC916帶有(you)其自己的片上(shang)RC振蕩器(qi)，故(gu)充電(dian)更加穩定(ding)而(er)有(you)效--尤(you)其在(zai)電(dian)壓(ya)控制(zhi)(zhi)工作模式下。所(suo)需的PWM頻率僅大(da)約為14kHz，故(gu)能很(hen)好地控制(zhi)(zhi)在(zai)片上(shang)振蕩器(qi)的頻率范圍(wei)內。可通(tong)過改變降(jiang)壓(ya)轉換器(qi)的“開”時間來(lai)調整PWM占(zhan)空比。

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(li)電(dian)(dian)(dian)(dian)(dian)池(chi)應以(yi)三個獨(du)立的(de)階段來充電(dian)(dian)(dian)(dian)(dian)。如果電(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)壓(ya)低(di)于3V，則需(xu)要有預充電(dian)(dian)(dian)(dian)(dian)階段且充電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)應保(bao)持為(wei)65mA。一(yi)旦電(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)壓(ya)達到3V+-1%，即開始進入快速(su)充電(dian)(dian)(dian)(dian)(dian)階段，并采用350mA的(de)恒(heng)定(ding)充電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)。通(tong)過(guo)調(diao)整(zheng)控制脈沖可使充電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)保(bao)持恒(heng)定(ding)。當電(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)壓(ya)達到4V+-1%時，即開始接(jie)恒(heng)定(ding)電(dian)(dian)(dian)(dian)(dian)壓(ya)充電(dian)(dian)(dian)(dian)(dian)階段。此時電(dian)(dian)(dian)(dian)(dian)壓(ya)被保(bao)持在4.23V，充電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)處(chu)于監視下(xia)。

       在恒定電(dian)(dian)壓充(chong)(chong)電(dian)(dian)階段之后，電(dian)(dian)池被另(ling)外再充(chong)(chong)電(dian)(dian)50分鐘，同時(shi)保持充(chong)(chong)電(dian)(dian)電(dian)(dian)流小于30mA。充(chong)(chong)電(dian)(dian)時(shi)間可用(yong)一(yi)個計時(shi)器來控制(zhi)，但監視充(chong)(chong)電(dian)(dian)終結的(de)方法有三種：檢測(ce)充(chong)(chong)電(dian)(dian)電(dian)(dian)流、使用(yong)計時(shi)器以及監視溫度(可選)。

       充電(dian)過程如(ru)圖3所(suo)示(shi)。從一個階段(duan)(duan)進入到另一個階段(duan)(duan)的準(zhun)確(que)標志如(ru)下：

       預充(chong)電(dian)(dian)(dian)階段(當(dang)需要(yao)時)：如果Vbat<3.0(1%，則設(she)置Iout=10%；Ireg=65mA；快(kuai)速充(chong)電(dian)(dian)(dian)階段(恒定(ding)電(dian)(dian)(dian)流(liu)充(chong)電(dian)(dian)(dian))：當(dang)Vbat<=4.00+-1%V時，設(she)置Iout=Ireg=350mA；計時器控制充(chong)電(dian)(dian)(dian)階段(恒定(ding)電(dian)(dian)(dian)壓充(chong)電(dian)(dian)(dian))：當(dang)Ibat<60mA時，設(she)置Vout=Vreg=4.23V(50分鐘)以保證(zheng)電(dian)(dian)(dian)池充(chong)分充(chong)電(dian)(dian)(dian)，但使充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)小于30mA。

       充電(dian)在4小時內完成。

       考慮到最終(zhong)用(yong)戶(hu)，設計中采用(yong)了(le)LED狀態指示燈，以提供有關充電序列狀態的(de)信息。

       設(she)計方案的(de)測(ce)試

       可用(yong)來在充電過程中測試該設(she)計的電路(lu)框圖如(ru)圖4所示。用(yong)兩(liang)塊萬用(yong)表來測量Vout及Vsense_res讀數。

       Vout=Vbat+Vsense_res，充電電流可(ke)用公(gong)式Iout=Vsense res/0.75來(lai)計算。

       當(dang)充電(dian)開始時，每15秒記(ji)錄一次數據，但當(dang)電(dian)流(liu)及電(dian)壓穩定后，記(ji)錄周期可縮短為每5秒記(ji)錄一次。

       結(jie)果(guo)可能會隨不同電池的(de)化學(xue)特征而變化，而且電池的(de)起始(shi)電壓也對結(jie)果(guo)有(you)影響(xiang)。