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(ji)適當(dang)外圍以提供一種廉價解決方(fang)案的(de)飛利浦 80C51型MCU就是這(zhe)樣一個例子。集成化閃存還能提供高效及(ji)方(fang)便地調試應用代(dai)碼(ma)并進行現場軟件升級(如果(guo)需要)的(de)能力。

       由于設計界(jie)不僅熟悉而且(qie)廣泛接受(shou)8位(wei)MCU，故軟硬(ying)件開發(fa)可(ke)快速進行。由眾多廠商提(ti)供的各種功能(neng)強大(da)且(qie)并不昂貴的應用開發(fa)工具，也是這(zhe)種方(fang)法的另一項優勢。

       利用這種方法，設(she)計(ji)團隊不僅能極(ji)大(da)地縮短設(she)計(ji)周期(qi)，而(er)且還能進行更為復雜的(de)設(she)計(ji)，并(bing)使(shi)項目的(de)整體(ti)材料費(BOM)不超出(chu)可接受的(de)范(fan)圍。

外圍電路集成
       譬如(ru)，當MCU集成有內部振蕩器(qi)時(shi)，離線鋰電(dian)池充(chong)電(dian)器(qi)設計可(ke)(ke)從以下(xia)兩方面(mian)獲益。首先，可(ke)(ke)省掉外部振蕩器(qi)，從而節省成本及PCB占(zhan)位；其次，內部振蕩器(qi)可(ke)(ke)提高系統啟動時(shi)的穩定性。

       四通道A/D轉換(huan)器是設(she)計工程師們(men)應該尋求集成到(dao)芯(xin)片中的(de)另一種有價值的(de)外(wai)圍(wei)電(dian)路(lu)。除能比使用外(wai)部(bu)A/D轉換(huan)器更(geng)節(jie)約(yue)成本外(wai)，還能用它來(lai)檢測充電(dian)電(dian)壓、電(dian)流(liu)及電(dian)池溫度(du)--幾乎包括安全電(dian)池充電(dian)操作(zuo)中的(de)所有重要(yao)參數。

       用(yong)(yong)來實現以下所介紹設(she)計的MCU(P89LPC916)不僅集成了上述所有這些特性而且還擁有可同時在兩(liang)個時鐘(zhong)上執行指令的高性能處理器(qi)(qi)架(jia)構(gou)，從而將(jiang)其性能提高至標準80C51器(qi)(qi)件的6倍。Time0(計時器(qi)(qi)0)很容易被(bei)配置成PWM輸出，故易于設(she)置及(ji)使用(yong)(yong)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)序分成以下三個階(jie)段(duan)：預充(chong)電(dian)(dian)階(jie)段(duan)、恒定(ding)電(dian)(dian)流充(chong)電(dian)(dian)階(jie)段(duan)及恒定(ding)電(dian)(dian)壓(ya)充(chong)電(dian)(dian)階(jie)段(duan)。

     當電(dian)(dian)(dian)池(chi)只剩(sheng)下(xia)很少的電(dian)(dian)(dian)量且因(yin)此而只能產(chan)生很低的輸(shu)出電(dian)(dian)(dian)壓(ya)時，就必(bi)(bi)須有(you)預充(chong)(chong)電(dian)(dian)(dian)階(jie)段。在此情(qing)況下(xia)，必(bi)(bi)須采用(yong)低電(dian)(dian)(dian)流充(chong)(chong)電(dian)(dian)(dian)以保(bao)護電(dian)(dian)(dian)池(chi)。但如果(guo)被充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)池(chi)可產(chan)生較高電(dian)(dian)(dian)壓(ya)(>3V)，則(ze)可省略掉預充(chong)(chong)電(dian)(dian)(dian)階(jie)段。當然，這是最(zui)普遍的情(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入電(dian)(dian)(dian)池(chi)(chi)。電(dian)(dian)(dian)池(chi)(chi)的最大允許充電(dian)(dian)(dian)電(dian)(dian)(dian)流由該電(dian)(dian)(dian)池(chi)(chi)的額定容量(liang)決(jue)定。對于快(kuai)速充電(dian)(dian)(dian)，例(li)如(ru)額定700mAh的電(dian)(dian)(dian)池(chi)(chi)，可用350-400mA電(dian)(dian)(dian)流來充電(dian)(dian)(dian)。

       在鋰電(dian)(dian)(dian)池(chi)情況下(xia)，MCU必須在保持電(dian)(dian)(dian)池(chi)正常充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)壓的同(tong)時(shi)還監視充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流，以在電(dian)(dian)(dian)池(chi)充(chong)(chong)滿(man)時(shi)能終止充(chong)(chong)電(dian)(dian)(dian)過(guo)程。

       溫(wen)度(du)監(jian)視(shi)可用來(lai)確保執(zhi)行安全(quan)的(de)充電(dian)(dian)步(bu)驟，因為(wei)隨著電(dian)(dian)池(chi)(chi)充滿，任何額外的(de)電(dian)(dian)能(neng)都(dou)將被轉換(huan)成熱量(liang)。盡(jin)管MCU必須為(wei)其完成的(de)功能(neng)增加溫(wen)度(du)監(jian)視(shi)，但當(dang)今市場上的(de)大多數鋰(li)電(dian)(dian)池(chi)(chi)都(dou)帶有內置過(guo)充電(dian)(dian)保護，故溫(wen)度(du)監(jian)視(shi)盡(jin)管需要但卻很少使(shi)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(xin)號控制(zhi)充(chong)電(dian)(dian)開(kai)關。當開(kai)關閉合(he)時(shi)(圖1a)，電(dian)(dian)流由(you)于充(chong)電(dian)(dian)器(qi)提供(gong)的電(dian)(dian)壓(充(chong)電(dian)(dian)器(qi)Vin)而流過電(dian)(dian)路，此時(shi)電(dian)(dian)容(rong)通過電(dian)(dian)感充(chong)電(dian)(dian)。

       當開(kai)關(guan)打開(kai)時(如1b所(suo)示)，電感試圖通過(guo)感應電壓來保持電流流動，但它(ta)不(bu)能立刻充電。然后電流流過(guo)肖特基二極管并給電容充電。此過(guo)程循環(huan)往(wang)復。

       當通(tong)過減少(shao)(shao)PWM占空(kong)(kong)比來縮短開(kai)(kai)關“通(tong)”時(shi)(shi)間時(shi)(shi)，平均電壓減少(shao)(shao)。相反，當通(tong)過增加(jia)PWM占空(kong)(kong)比來延(yan)長(chang)開(kai)(kai)關“斷”時(shi)(shi)間時(shi)(shi)，平均電壓增加(jia)。故通(tong)過控制PWM占空(kong)(kong)比來使MCU調整充電電壓(或電流)可達到所需(xu)的輸(shu)出值。

在(zai)討論設計細節以前，需先(xian)討論與電(dian)感及電(dian)容有關(guan)的(de)兩(liang)個要點(dian)：

     1．電(dian)感(gan)大(da)小(xiao)

       不難看出，確定降壓轉(zhuan)換(huan)器電(dian)(dian)感(gan)的大小是達到合適充電(dian)(dian)電(dian)(dian)壓及電(dian)(dian)流的關(guan)鍵(jian)。電(dian)(dian)感(gan)大小也與成本有(you)關(guan)。電(dian)(dian)感(gan)容量(liang)可(ke)用公式(shi)1來(lai)計算：

      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(shi)1顯示PWM的(de)開關頻(pin)率越高(亦即開關周期T越小)，則所需的(de)電感越小，這有助于減(jian)少器件成本(ben)。

       2．電容(rong)大小

       還需注(zhu)意的是(shi)，此(ci)電(dian)(dian)(dian)路中的電(dian)(dian)(dian)容完(wan)全是(shi)用來減少(shao)紋(wen)波電(dian)(dian)(dian)流(liu)，故越(yue)大越(yue)好，因為紋(wen)波與電(dian)(dian)(dian)容值成反比。

設計要點

       本(ben)設計基(ji)于飛利(li)浦(pu)P89LPC916型MCU，其整體(ti)設計思想是，通(tong)過先用(yong)恒定電(dian)(dian)流(liu)充電(dian)(dian)、然后(hou)再用(yong)恒定電(dian)(dian)壓充電(dian)(dian)來(lai)實現盡(jin)可能快的充電(dian)(dian)。MCU還(huan)控制用(yong)于指示充電(dian)(dian)器工作狀(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(shu)出解決方案

       Timer0(定(ding)時(shi)器(qi)0)的(de)(de)一個通(tong)道用來產生控制(zhi)(zhi)降壓轉換(huan)器(qi)開關(guan)的(de)(de)PWM信號(hao)。由(you)于LPC916帶有其自己的(de)(de)片上RC振(zhen)蕩(dang)器(qi)，故充電更加穩定(ding)而有效(xiao)--尤其在電壓控制(zhi)(zhi)工作模(mo)式下。所(suo)需的(de)(de)PWM頻(pin)率僅(jin)大約(yue)為14kHz，故能很好地控制(zhi)(zhi)在片上振(zhen)蕩(dang)器(qi)的(de)(de)頻(pin)率范(fan)圍內。可通(tong)過(guo)改變降壓轉換(huan)器(qi)的(de)(de)“開”時(shi)間來調整PWM占空比。

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。
       鋰電(dian)(dian)(dian)(dian)池(chi)應以三(san)個獨立的階段(duan)來充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)。如果電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓低于3V，則需要有預充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)階段(duan)且充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)應保持(chi)為(wei)65mA。一旦電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓達到(dao)3V+-1%，即(ji)開(kai)始(shi)進(jin)入(ru)快速充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)階段(duan)，并采用350mA的恒(heng)定(ding)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)。通過調整控(kong)制脈(mo)沖可使充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)保持(chi)恒(heng)定(ding)。當電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓達到(dao)4V+-1%時，即(ji)開(kai)始(shi)接恒(heng)定(ding)電(dian)(dian)(dian)(dian)壓充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)階段(duan)。此(ci)時電(dian)(dian)(dian)(dian)壓被保持(chi)在4.23V，充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)處(chu)于監(jian)視下。

       在恒定電(dian)(dian)(dian)壓充(chong)電(dian)(dian)(dian)階(jie)段之后(hou)，電(dian)(dian)(dian)池被(bei)另外再充(chong)電(dian)(dian)(dian)50分鐘(zhong)，同(tong)時保持充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流小于30mA。充(chong)電(dian)(dian)(dian)時間可用一(yi)個計時器來控制，但監視(shi)充(chong)電(dian)(dian)(dian)終結的方法有三種：檢(jian)測充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流、使(shi)用計時器以及監視(shi)溫度(可選)。

       充電(dian)過程如(ru)圖3所示。從一(yi)個階(jie)段進入到(dao)另一(yi)個階(jie)段的(de)準確標志如(ru)下：

       預(yu)充(chong)(chong)電(dian)(dian)(dian)階段(當(dang)需要時)：如(ru)果Vbat<3.0(1%，則設(she)(she)置Iout=10%；Ireg=65mA；快速充(chong)(chong)電(dian)(dian)(dian)階段(恒定電(dian)(dian)(dian)流(liu)充(chong)(chong)電(dian)(dian)(dian))：當(dang)Vbat<=4.00+-1%V時，設(she)(she)置Iout=Ireg=350mA；計時器控制充(chong)(chong)電(dian)(dian)(dian)階段(恒定電(dian)(dian)(dian)壓充(chong)(chong)電(dian)(dian)(dian))：當(dang)Ibat<60mA時，設(she)(she)置Vout=Vreg=4.23V(50分鐘)以保證電(dian)(dian)(dian)池充(chong)(chong)分充(chong)(chong)電(dian)(dian)(dian)，但使充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)小于30mA。

       充電(dian)在4小時內完成。

       考慮到最終用戶，設計中采(cai)用了LED狀(zhuang)態指示燈，以提(ti)供有關充電(dian)序列狀(zhuang)態的信息。

       設(she)計方案的(de)測試

       可(ke)用來在充電(dian)過(guo)程(cheng)中測試(shi)該設計的(de)電(dian)路框(kuang)圖如圖4所示。用兩(liang)塊(kuai)萬用表來測量Vout及Vsense_res讀數(shu)。

       Vout=Vbat+Vsense_res，充電(dian)電(dian)流可(ke)用(yong)公(gong)式Iout=Vsense res/0.75來計算。

       當(dang)充電開始(shi)時，每15秒記(ji)錄一次數據，但當(dang)電流及電壓(ya)穩定后，記(ji)錄周期(qi)可(ke)縮短為每5秒記(ji)錄一次。

       結(jie)果(guo)(guo)可能會隨不同電(dian)池的化(hua)學特征而變化(hua)，而且電(dian)池的起始電(dian)壓(ya)也對結(jie)果(guo)(guo)有影響(xiang)。