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(cun)及適當外圍以提供一種廉(lian)價解(jie)決方案的飛利浦 80C51型MCU就是這樣一個例子(zi)。集成化閃存(cun)還能(neng)提供高(gao)效及方便地(di)調試應用代碼并進行現場軟(ruan)件升級(如果需要)的能(neng)力(li)。

       由(you)于設計(ji)界不僅熟悉(xi)而且廣泛接受8位MCU，故(gu)軟硬件開發(fa)可(ke)快(kuai)速(su)進行。由(you)眾(zhong)多廠商提(ti)供(gong)的各種(zhong)功能強大且并(bing)不昂貴的應用開發(fa)工具，也是這種(zhong)方法的另一項優勢。

       利用這種方法，設計(ji)團隊不(bu)僅能極大地縮短設計(ji)周期，而且還能進行更為復雜的(de)設計(ji)，并(bing)使項目的(de)整體材料費(BOM)不(bu)超出可接受的(de)范圍。

外圍電路集成
       譬(pi)如，當MCU集成有內(nei)部(bu)(bu)振蕩(dang)器(qi)(qi)時(shi)，離線鋰電池充電器(qi)(qi)設計可(ke)從以(yi)下兩(liang)方面獲益(yi)。首先，可(ke)省(sheng)掉外部(bu)(bu)振蕩(dang)器(qi)(qi)，從而節省(sheng)成本(ben)及PCB占位；其(qi)次，內(nei)部(bu)(bu)振蕩(dang)器(qi)(qi)可(ke)提高系統啟動時(shi)的穩定性。

 ;      四通道(dao)A/D轉換(huan)器是設(she)計工(gong)程師們應(ying)該尋求集成到(dao)芯片中的(de)(de)另一種有價值的(de)(de)外圍電路。除能(neng)比使用(yong)外部A/D轉換(huan)器更(geng)節約成本外，還(huan)能(neng)用(yong)它來(lai)檢測充電電壓、電流及電池溫度(du)--幾乎包括安全電池充電操作(zuo)中的(de)(de)所有重(zhong)要(yao)參數。

       用來實(shi)現以下所介紹(shao)設(she)計的(de)MCU(P89LPC916)不僅集成(cheng)了上(shang)述(shu)所有這些特性而且(qie)還擁有可(ke)同時在兩(liang)個(ge)時鐘上(shang)執行(xing)指(zhi)令的(de)高(gao)性能(neng)(neng)處理(li)器(qi)架構(gou)，從而將其性能(neng)(neng)提高(gao)至(zhi)標準80C51器(qi)件的(de)6倍。Time0(計時器(qi)0)很容易被(bei)配(pei)置成(cheng)PWM輸出，故易于設(she)置及使用PWM功(gong)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)(dian)順序(xu)分成以下三個階段：預充電(dian)(dian)階段、恒(heng)定電(dian)(dian)流充電(dian)(dian)階段及(ji)恒(heng)定電(dian)(dian)壓充電(dian)(dian)階段。

     當電(dian)(dian)(dian)(dian)池只(zhi)剩(sheng)下很(hen)少的(de)(de)(de)電(dian)(dian)(dian)(dian)量且因此而只(zhi)能(neng)產生(sheng)很(hen)低(di)的(de)(de)(de)輸出(chu)電(dian)(dian)(dian)(dian)壓時(shi)，就必須有預充電(dian)(dian)(dian)(dian)階段。在此情況下，必須采用低(di)電(dian)(dian)(dian)(dian)流充電(dian)(dian)(dian)(dian)以保護(hu)電(dian)(dian)(dian)(dian)池。但如果被充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)池可(ke)產生(sheng)較高電(dian)(dian)(dian)(dian)壓(>3V)，則可(ke)省略(lve)掉預充電(dian)(dian)(dian)(dian)階段。當然(ran)，這是最普(pu)遍(bian)的(de)(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入電池(chi)。電池(chi)的最大允許充電電流由該電池(chi)的額定容量決定。對于快速充電，例如額定700mAh的電池(chi)，可用350-400mA電流來充電。

       在鋰電(dian)(dian)(dian)池情況下，MCU必須在保(bao)持電(dian)(dian)(dian)池正常充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)壓的同時(shi)還(huan)監視充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流，以(yi)在電(dian)(dian)(dian)池充(chong)(chong)滿時(shi)能終止充(chong)(chong)電(dian)(dian)(dian)過程。

       溫度(du)監(jian)(jian)視可用(yong)來確保執行安(an)全的(de)充(chong)電步驟(zou)，因為隨著(zhu)電池充(chong)滿，任何額外的(de)電能(neng)都(dou)將被(bei)轉換(huan)成熱量。盡(jin)(jin)管MCU必須為其(qi)完(wan)成的(de)功(gong)能(neng)增加溫度(du)監(jian)(jian)視，但當今市場上的(de)大多(duo)數鋰電池都(dou)帶有內置過充(chong)電保護，故溫度(du)監(jian)(jian)視盡(jin)(jin)管需(xu)要但卻(que)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(hao)控制(zhi)充電開(kai)關(guan)。當(dang)開(kai)關(guan)閉合時(圖1a)，電流由于充電器提供的電壓(充電器Vin)而流過(guo)電路(lu)，此時電容(rong)通過(guo)電感充電。

       當開關打開時(如1b所示)，電(dian)(dian)感(gan)試圖(tu)通過(guo)感(gan)應(ying)電(dian)(dian)壓來保持(chi)電(dian)(dian)流(liu)流(liu)動，但它不能立刻充電(dian)(dian)。然后電(dian)(dian)流(liu)流(liu)過(guo)肖特基二極管并給電(dian)(dian)容(rong)充電(dian)(dian)。此過(guo)程(cheng)循環(huan)往(wang)復。

       當通(tong)(tong)(tong)過減少(shao)PWM占空(kong)比來(lai)縮短開關“通(tong)(tong)(tong)”時間時，平(ping)均電壓(ya)減少(shao)。相反，當通(tong)(tong)(tong)過增(zeng)加PWM占空(kong)比來(lai)延長(chang)開關“斷”時間時，平(ping)均電壓(ya)增(zeng)加。故通(tong)(tong)(tong)過控制PWM占空(kong)比來(lai)使MCU調整充電電壓(ya)(或電流(liu))可達(da)到(dao)所需的輸出值(zhi)。

在討論設(she)計細節以(yi)前(qian)，需先討論與(yu)電(dian)感(gan)及電(dian)容有關(guan)的兩個要點：

     1．電感大小

   ;    不難看出(chu)，確定降壓(ya)轉換器(qi)電(dian)(dian)感的(de)大小是達到合適(shi)充電(dian)(dian)電(dian)(dian)壓(ya)及電(dian)(dian)流的(de)關(guan)鍵。電(dian)(dian)感大小也與成(cheng)本(ben)有關(guan)。電(dian)(dian)感容量(liang)可用(yong)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(xian)示PWM的(de)開(kai)關頻率(lv)越高(亦即(ji)開(kai)關周期T越小(xiao))，則(ze)所(suo)需(xu)的(de)電感越小(xiao)，這有助于減少(shao)器件成本(ben)。

       2．電容大小

       還需(xu)注意的(de)(de)是(shi)，此電(dian)路中的(de)(de)電(dian)容完全是(shi)用來減少紋波(bo)電(dian)流，故越(yue)大越(yue)好(hao)，因(yin)為(wei)紋波(bo)與電(dian)容值(zhi)成反(fan)比。

設計要點

       本(ben)設(she)計(ji)基于飛(fei)利(li)浦P89LPC916型(xing)MCU，其整體(ti)設(she)計(ji)思想(xiang)是，通過先用(yong)恒(heng)定(ding)電(dian)流充(chong)電(dian)、然后(hou)再用(yong)恒(heng)定(ding)電(dian)壓充(chong)電(dian)來實(shi)現盡(jin)可能(neng)快的(de)充(chong)電(dian)。MCU還控制用(yong)于指示充(chong)電(dian)器工作狀態(tai)的(de)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(fang)案

       Timer0(定(ding)時(shi)器(qi)0)的(de)(de)一(yi)個通道用來(lai)產(chan)生控(kong)制(zhi)(zhi)(zhi)降(jiang)壓(ya)(ya)轉(zhuan)換器(qi)開關的(de)(de)PWM信號。由于LPC916帶有其(qi)自(zi)己(ji)的(de)(de)片上RC振蕩器(qi)，故(gu)充電更(geng)加穩定(ding)而有效--尤其(qi)在電壓(ya)(ya)控(kong)制(zhi)(zhi)(zhi)工作模式下。所需的(de)(de)PWM頻率僅大約為14kHz，故(gu)能很好地控(kong)制(zhi)(zhi)(zhi)在片上振蕩器(qi)的(de)(de)頻率范圍內。可通過改變(bian)降(jiang)壓(ya)(ya)轉(zhuan)換器(qi)的(de)(de)“開”時(shi)間來(lai)調(diao)整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)應以三個獨立的階段來充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)。如果(guo)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓低于(yu)3V，則(ze)需要有預(yu)充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)階段且充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流應保持(chi)(chi)為65mA。一旦電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓達到3V+-1%，即開始進入快(kuai)速充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)階段，并采用350mA的恒(heng)定充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流。通過調(diao)整控制脈沖可使充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流保持(chi)(chi)恒(heng)定。當電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓達到4V+-1%時，即開始接恒(heng)定電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)階段。此時電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓被保持(chi)(chi)在4.23V，充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流處于(yu)監(jian)視下(xia)。

       在恒定電(dian)(dian)(dian)(dian)壓充電(dian)(dian)(dian)(dian)階(jie)段之后，電(dian)(dian)(dian)(dian)池被另外再充電(dian)(dian)(dian)(dian)50分鐘，同時(shi)保持充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流小于30mA。充電(dian)(dian)(dian)(dian)時(shi)間可用一個(ge)計時(shi)器來控制，但監視(shi)(shi)充電(dian)(dian)(dian)(dian)終結的方法(fa)有三種：檢測充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流、使用計時(shi)器以及監視(shi)(shi)溫(wen)度(可選)。

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

       預充(chong)(chong)電(dian)階(jie)段(當(dang)需要時(shi))：如果Vbat<3.0(1%，則設(she)置Iout=10%；Ireg=65mA；快速充(chong)(chong)電(dian)階(jie)段(恒定電(dian)流充(chong)(chong)電(dian))：當(dang)Vbat<=4.00+-1%V時(shi)，設(she)置Iout=Ireg=350mA；計時(shi)器(qi)控制(zhi)充(chong)(chong)電(dian)階(jie)段(恒定電(dian)壓充(chong)(chong)電(dian))：當(dang)Ibat<60mA時(shi)，設(she)置Vout=Vreg=4.23V(50分(fen)鐘)以保證電(dian)池(chi)充(chong)(chong)分(fen)充(chong)(chong)電(dian)，但使充(chong)(chong)電(dian)電(dian)流小(xiao)于(yu)30mA。

   &nbsp;   充電在4小時內完成(cheng)。

&nbsp;      考慮到(dao)最終用戶，設(she)計中采用了LED狀(zhuang)態指示燈(deng)，以提供有關充電序列狀(zhuang)態的信息。

       設計(ji)方案的測(ce)試

&nbsp;      可用來(lai)在充電(dian)過程中測(ce)試(shi)該(gai)設計的電(dian)路框圖(tu)如圖(tu)4所示。用兩塊萬用表(biao)來(lai)測(ce)量Vout及Vsense_res讀(du)數。

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

       當充電(dian)開(kai)始時，每15秒記(ji)錄一(yi)次數(shu)據，但當電(dian)流及(ji)電(dian)壓穩定后，記(ji)錄周期可縮短為每5秒記(ji)錄一(yi)次。

  ;   &nbsp; 結果可能(neng)會隨(sui)不同電(dian)池的化學特征(zheng)而(er)變化，而(er)且電(dian)池的起始(shi)電(dian)壓也對結果有影(ying)響。