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閃存及適當外圍以(yi)提供一種廉價解決方案的(de)飛利浦(pu) 80C51型MCU就是這樣一個例子(zi)。集成化閃存還能提供高效(xiao)及方便地(di)調試應用代碼并進行現場軟件升級(ji)(如果需要)的(de)能力。

       由于設計界(jie)不僅熟悉而且廣泛(fan)接受8位MCU，故軟硬件開發可(ke)快速進(jin)行。由眾多廠(chang)商(shang)提供的各種功(gong)能強大且并不昂貴(gui)的應用開發工具，也是這種方(fang)法(fa)的另一項優勢。

       利用這種(zhong)方(fang)法，設(she)計(ji)團隊(dui)不僅(jin)能(neng)極大地縮(suo)短(duan)設(she)計(ji)周期，而且還能(neng)進行更為復雜的設(she)計(ji)，并使項目的整體材料費(BOM)不超出可接(jie)受的范圍。

外圍電路集成
       譬如，當MCU集成(cheng)有(you)內(nei)部振(zhen)蕩器(qi)(qi)時(shi)，離線鋰電池充電器(qi)(qi)設計(ji)可(ke)從(cong)以下兩(liang)方面(mian)獲益。首先，可(ke)省掉外部振(zhen)蕩器(qi)(qi)，從(cong)而(er)節(jie)省成(cheng)本及PCB占(zhan)位(wei)；其(qi)次，內(nei)部振(zhen)蕩器(qi)(qi)可(ke)提高系(xi)統啟動時(shi)的(de)穩定性(xing)。

       四通(tong)道A/D轉換器是設(she)計工程師們(men)應該(gai)尋求集(ji)成到芯片(pian)中的另一種有(you)價(jia)值的外圍電(dian)(dian)路。除(chu)能(neng)比使用(yong)外部A/D轉換器更節約成本外，還能(neng)用(yong)它來檢測充電(dian)(dian)電(dian)(dian)壓、電(dian)(dian)流(liu)及(ji)電(dian)(dian)池溫度--幾乎(hu)包括安全電(dian)(dian)池充電(dian)(dian)操作中的所有(you)重要參數。

       用來實現以下所介紹設(she)(she)計(ji)(ji)的MCU(P89LPC916)不僅集成了上述所有這些特性(xing)(xing)而且還擁有可同時在兩個時鐘上執行指令的高性(xing)(xing)能處理(li)器(qi)架構，從而將其性(xing)(xing)能提(ti)高至標準80C51器(qi)件的6倍。Time0(計(ji)(ji)時器(qi)0)很(hen)容(rong)易被(bei)配置成PWM輸出，故易于設(she)(she)置及使用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(shun)序分成以下(xia)三個階段：預充(chong)電(dian)(dian)階段、恒定電(dian)(dian)流充(chong)電(dian)(dian)階段及恒定電(dian)(dian)壓(ya)充(chong)電(dian)(dian)階段。

     當電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)只剩下(xia)很(hen)少的(de)(de)電(dian)(dian)(dian)(dian)(dian)量(liang)且(qie)因此而只能(neng)產(chan)生(sheng)(sheng)很(hen)低的(de)(de)輸出電(dian)(dian)(dian)(dian)(dian)壓時(shi)，就必須(xu)有預(yu)充電(dian)(dian)(dian)(dian)(dian)階段(duan)。在此情況下(xia)，必須(xu)采用低電(dian)(dian)(dian)(dian)(dian)流充電(dian)(dian)(dian)(dian)(dian)以(yi)保護電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)。但(dan)如果被充電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)可產(chan)生(sheng)(sheng)較高電(dian)(dian)(dian)(dian)(dian)壓(>3V)，則可省略掉(diao)預(yu)充電(dian)(dian)(dian)(dian)(dian)階段(duan)。當然，這是(shi)最(zui)普遍的(de)(de)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)入電(dian)(dian)池(chi)。電(dian)(dian)池(chi)的最大允(yun)許(xu)充(chong)電(dian)(dian)電(dian)(dian)流(liu)由該電(dian)(dian)池(chi)的額定(ding)容量決定(ding)。對(dui)于快速(su)充(chong)電(dian)(dian)，例如額定(ding)700mAh的電(dian)(dian)池(chi)，可(ke)用350-400mA電(dian)(dian)流(liu)來充(chong)電(dian)(dian)。

       在(zai)鋰電(dian)池情況下，MCU必須(xu)在(zai)保持電(dian)池正常充(chong)電(dian)電(dian)壓的同時還監(jian)視充(chong)電(dian)電(dian)流，以在(zai)電(dian)池充(chong)滿時能終(zhong)止充(chong)電(dian)過程。

       溫(wen)度監視可(ke)用來(lai)確保執行安全的(de)(de)充電(dian)步驟，因(yin)為隨著電(dian)池(chi)充滿(man)，任何額外的(de)(de)電(dian)能都將被轉換成(cheng)熱量(liang)。盡管(guan)(guan)MCU必(bi)須為其(qi)完成(cheng)的(de)(de)功能增(zeng)加溫(wen)度監視，但當(dang)今(jin)市場上(shang)的(de)(de)大多數(shu)鋰電(dian)池(chi)都帶(dai)有內置過(guo)充電(dian)保護，故溫(wen)度監視盡管(guan)(guan)需(xu)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(lai)自(zi)PWM的信(xin)號控(kong)制充(chong)電(dian)(dian)(dian)(dian)開(kai)關(guan)。當開(kai)關(guan)閉合(he)時(圖1a)，電(dian)(dian)(dian)(dian)流(liu)由于充(chong)電(dian)(dian)(dian)(dian)器提供的電(dian)(dian)(dian)(dian)壓(充(chong)電(dian)(dian)(dian)(dian)器Vin)而流(liu)過(guo)電(dian)(dian)(dian)(dian)路，此時電(dian)(dian)(dian)(dian)容通過(guo)電(dian)(dian)(dian)(dian)感(gan)充(chong)電(dian)(dian)(dian)(dian)。

       當(dang)開(kai)關打開(kai)時(如1b所示)，電感試圖通過(guo)(guo)感應電壓(ya)來保持電流流動，但它不能立刻充(chong)電。然后電流流過(guo)(guo)肖(xiao)特基二極(ji)管并給電容充(chong)電。此過(guo)(guo)程循環(huan)往(wang)復。

       當通(tong)過減(jian)少PWM占(zhan)空比來(lai)(lai)縮短開關“通(tong)”時(shi)間時(shi)，平(ping)均電(dian)壓減(jian)少。相反，當通(tong)過增加(jia)PWM占(zhan)空比來(lai)(lai)延(yan)長開關“斷”時(shi)間時(shi)，平(ping)均電(dian)壓增加(jia)。故通(tong)過控制PWM占(zhan)空比來(lai)(lai)使MCU調(diao)整(zheng)充電(dian)電(dian)壓(或電(dian)流)可達到所(suo)需的輸(shu)出值。

在討(tao)論(lun)(lun)設計細節以前，需先討(tao)論(lun)(lun)與電感(gan)及電容(rong)有關的(de)兩個要點：

     1．電(dian)感(gan)大小

       不難看出，確定降壓(ya)轉換器電(dian)(dian)(dian)感(gan)的大(da)小(xiao)是達到合(he)適充電(dian)(dian)(dian)電(dian)(dian)(dian)壓(ya)及(ji)電(dian)(dian)(dian)流的關鍵。電(dian)(dian)(dian)感(gan)大(da)小(xiao)也與成本有關。電(dian)(dian)(dian)感(gan)容量可(ke)用公式1來(lai)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1顯示PWM的開關頻率越(yue)(yue)高(亦即開關周期T越(yue)(yue)小(xiao))，則所(suo)需的電感越(yue)(yue)小(xiao)，這(zhe)有助于減(jian)少(shao)器件(jian)成本(ben)。

       2．電容(rong)大(da)小

       還需注意(yi)的是，此電路中的電容完(wan)全是用來(lai)減少(shao)紋波電流，故(gu)越大(da)越好(hao)，因為紋波與電容值成反比。

設計要點

       本設計基于飛利浦P89LPC916型MCU，其整體設計思想是，通過先用(yong)恒(heng)定(ding)電(dian)(dian)(dian)流(liu)充(chong)電(dian)(dian)(dian)、然后(hou)再用(yong)恒(heng)定(ding)電(dian)(dian)(dian)壓充(chong)電(dian)(dian)(dian)來實現盡(jin)可能快的充(chong)電(dian)(dian)(dian)。MCU還控制(zhi)用(yong)于指示充(chong)電(dian)(dian)(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(shu)出解決方案(an)

       Timer0(定時器(qi)(qi)0)的(de)(de)一個通(tong)道用來(lai)產生控(kong)制降壓(ya)轉(zhuan)換器(qi)(qi)開關的(de)(de)PWM信號。由于LPC916帶(dai)有其自己的(de)(de)片上RC振(zhen)蕩(dang)器(qi)(qi)，故(gu)充電更加穩定而有效(xiao)--尤其在(zai)電壓(ya)控(kong)制工作模式下。所(suo)需的(de)(de)PWM頻率僅大(da)約(yue)為14kHz，故(gu)能很(hen)好地控(kong)制在(zai)片上振(zhen)蕩(dang)器(qi)(qi)的(de)(de)頻率范圍內。可通(tong)過改變降壓(ya)轉(zhuan)換器(qi)(qi)的(de)(de)“開”時間(jian)來(lai)調(diao)整(zheng)PWM占空(kong)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。
       鋰電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)應以三個(ge)獨(du)立的階(jie)段(duan)來充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)。如果電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)低(di)于3V，則需要有(you)預充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)階(jie)段(duan)且充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流應保持為65mA。一旦電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)達(da)到3V+-1%，即(ji)開始進入快(kuai)速充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)階(jie)段(duan)，并采用350mA的恒(heng)定充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流。通過調整控(kong)制(zhi)脈沖可使(shi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流保持恒(heng)定。當電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)達(da)到4V+-1%時，即(ji)開始接恒(heng)定電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)階(jie)段(duan)。此時電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)被保持在4.23V，充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流處于監視下。

       在恒(heng)定(ding)電(dian)(dian)壓充(chong)電(dian)(dian)階段(duan)之后，電(dian)(dian)池被另外(wai)再(zai)充(chong)電(dian)(dian)50分鐘(zhong)，同時(shi)保(bao)持充(chong)電(dian)(dian)電(dian)(dian)流小于30mA。充(chong)電(dian)(dian)時(shi)間可用(yong)(yong)一個計(ji)時(shi)器(qi)來控制，但監視充(chong)電(dian)(dian)終結的方法有三(san)種(zhong)：檢測充(chong)電(dian)(dian)電(dian)(dian)流、使用(yong)(yong)計(ji)時(shi)器(qi)以及(ji)監視溫度(可選)。

       充電過程如圖3所示(shi)。從一個階(jie)段進(jin)入到另一個階(jie)段的準確標志如下：

       預充(chong)(chong)電(dian)(dian)(dian)階(jie)(jie)段(當需要時(shi))：如果Vbat<3.0(1%，則(ze)設置Iout=10%；Ireg=65mA；快(kuai)速充(chong)(chong)電(dian)(dian)(dian)階(jie)(jie)段(恒(heng)定(ding)電(dian)(dian)(dian)流充(chong)(chong)電(dian)(dian)(dian))：當Vbat<=4.00+-1%V時(shi)，設置Iout=Ireg=350mA；計時(shi)器控制充(chong)(chong)電(dian)(dian)(dian)階(jie)(jie)段(恒(heng)定(ding)電(dian)(dian)(dian)壓充(chong)(chong)電(dian)(dian)(dian))：當Ibat<60mA時(shi)，設置Vout=Vreg=4.23V(50分(fen)(fen)鐘)以保(bao)證(zheng)電(dian)(dian)(dian)池充(chong)(chong)分(fen)(fen)充(chong)(chong)電(dian)(dian)(dian)，但(dan)使充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流小于30mA。

       充電在4小時內(nei)完(wan)成。

       考慮到最終用戶，設計中采用了LED狀態(tai)指示燈，以提供有關(guan)充電序列(lie)狀態(tai)的信息(xi)。

       設計方案的測試(shi)

       可用來在充電(dian)(dian)過程中測試(shi)該設計(ji)的電(dian)(dian)路(lu)框圖如圖4所示(shi)。用兩塊萬用表來測量(liang)Vout及Vsense_res讀數。

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

       當充(chong)電(dian)(dian)開始時，每15秒(miao)(miao)記錄一次(ci)(ci)數(shu)據，但當電(dian)(dian)流(liu)及(ji)電(dian)(dian)壓穩定后，記錄周期可縮短為每5秒(miao)(miao)記錄一次(ci)(ci)。

       結(jie)果可能會(hui)隨不同電(dian)(dian)池的化學特(te)征而(er)變(bian)化，而(er)且(qie)電(dian)(dian)池的起始(shi)電(dian)(dian)壓也對結(jie)果有影響。