鋰電池閑其比能量高、自放電小等長處，成為便攜式電子設備的理想電源。近年來，隨著筆記本電腦、PDA，無繩電話等大功耗大容量便攜式電子產品的普及，其對電源系統的要求也日益提高。為此，研發性能穩定、安全可靠、高效經濟的鋰電池充電器顯得尤為重要。

　　本文在綜合考慮電池安全充電的成本、設計散率及重要性的基礎上，設計了一種基于ATtiny261單片機PWM控制的單片開關電源式鋰電池充電器，有效地克服了(le)一般充電(dian)(dian)(dian)器過充電(dian)(dian)(dian)、充電(dian)(dian)(dian)不足、效率低的(de)缺點，實現了(le)對鋰(li)電(dian)(dian)(dian)池組的(de)智能(neng)充電(dian)(dian)(dian)，達(da)到(dao)了(le)預(yu)期(qi)效果。該方(fang)案設(she)計靈活，可滿意多種型號的(de)鋰(li)電(dian)(dian)(dian)池充電(dian)(dian)(dian)需(xu)求，且ATtiny261集成化(hua)的(de)閃存使其便于軟件調試與升級(ji)。

鋰電池充電特性
　　鋰電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)需要(yao)控制它的(de)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)，限制其充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流。鋰電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)通常都采用(yong)三段(duan)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)法，即預充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)、恒(heng)流寬電(dian)(dian)(dian)(dian)(dian)(dian)和恒(heng)壓(ya)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)。鋰電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)的(de)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流通常應限制在1C(C為鋰電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)的(de)容量)一(yi)下，單體充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)一(yi)般為4．2V，否則(ze)可能由于電(dian)(dian)(dian)(dian)(dian)(dian)聰過高會造成鍵電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)永久性(xing)損壞。

　　預(yu)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)主要是(shi)完(wan)成對(dui)過放的鋰電(dian)(dian)(dian)(dian)(dian)池(chi)進行修(xiu)復，若電(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)壓低于3V，則(ze)必(bi)須進行預(yu)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)，否剛可省略該階段(duan)。這也(ye)是(shi)最普遍的情況。在恒(heng)(heng)流(liu)階段(duan)，充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)先給電(dian)(dian)(dian)(dian)(dian)池(chi)提供大(da)的恒(heng)(heng)定電(dian)(dian)(dian)(dian)(dian)流(liu)，同時電(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)壓上(shang)升，當魄池(chi)電(dian)(dian)(dian)(dian)(dian)壓達到飽和電(dian)(dian)(dian)(dian)(dian)壓對(dui)，則(ze)轉入憾壓充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)，充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)壓波動應控制(zhi)在50mV以內，同時充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)降(jiang)低，當電(dian)(dian)(dian)(dian)(dian)流(liu)逐漸減(jian)小(xiao)到規定的值(zhi)時，可結(jie)束(shu)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)過程。電(dian)(dian)(dian)(dian)(dian)池(chi)的大(da)部分電(dian)(dian)(dian)(dian)(dian)能在慣流(liu)及恒(heng)(heng)壓階段(duan)從充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)流(liu)入電(dian)(dian)(dian)(dian)(dian)池(chi)。曲上(shang)可知，充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)實際上(shang)是(shi)一(yi)個(ge)精密(mi)電(dian)(dian)(dian)(dian)(dian)源，其電(dian)(dian)(dian)(dian)(dian)流(liu)電(dian)(dian)(dian)(dian)(dian)壓都被限制(zhi)在所要求(qiu)的范圍之(zhi)內。

硬件電路設計
　　該系統在(zai)電(dian)(dian)路設計上(shang)主要由單片開關電(dian)(dian)源(yuan)、控制電(dian)(dian)路及保護電(dian)(dian)路三部(bu)分(fen)組成。

　　1.單片開關電源
　　單片開關電源負責將電能轉化為電池充電所需要的形式，構成了充電器的主要功率轉換方式。與傳統線性充電器大損耗、低效率的缺點相比，由美國Power Integrations公司的TNY268P構成的單片開關電源，其輸入電壓范圍寬(85265VAC)、體積小、重量輕、效率高，其有調壓、限流、過熱保護等功能，特殊適合于構成充電電源。其原理圖如圖1所示。
 

圖1單片開關電源

　　該電(dian)(dian)(dian)源采用配穩(wen)壓(ya)管的(de)光藕(ou)反(fan)(fan)饋(kui)電(dian)(dian)(dian)路實(shi)現15V的(de)低壓(ya)直流(liu)輸(shu)出(chu)(chu)，當輸(shu)出(chu)(chu)電(dian)(dian)(dian)壓(ya)發生(sheng)(sheng)變化時(shi)，通過(guo)線性(xing)光藕(ou)PC817的(de)發光管的(de)電(dian)(dian)(dian)流(liu)發生(sheng)(sheng)相(xiang)應的(de)變化，使得TNY268P的(de)EN腳流(liu)出(chu)(chu)電(dian)(dian)(dian)流(liu)也發生(sheng)(sheng)變化，從而(er)控制(zhi)其(qi)片內功(gong)率MOSFET的(de)斷、通、調節輸(shu)出(chu)(chu)電(dian)(dian)(dian)壓(ya)，使輸(shu)壓(ya)電(dian)(dian)(dian)壓(ya)穩(wen)定。詳細反(fan)(fan)饋(kui)原(yuan)理(li)分析詳見后文脈寬調制(zhi)(PWM)的(de)控制(zhi)。

　　在電路(lu)結構上，線(xian)性(xing)光藕(ou)PC817，不但可以起到反饋作(zuo)(zuo)用(yong)(yong)還可起到隔(ge)離(li)作(zuo)(zuo)用(yong)(yong)。由(you)PNP管Q2和(he)電阻R9、R1O及R12組成的限流電路(lu)，則(ze)從源頭(tou)上防止(zhi)了(le)過電流的問題。由(you)C6及R11構成的緩啟電路(lu)，則(ze)有效抑止(zhi)了(le)電源上電瞬間(jian)的產生的電壓尖峰(feng)。而二極管D9則(ze)防止(zhi)了(le)電池(chi)組的反向放電。此外(wai)，對整個(ge)充電系統(tong)而言，當因意外(wai)情況(kuang)系統(tong)失(shi)控時，開關(guan)電源所提供的15V直流低壓也(ye)在某(mou)種程度上起到了(le)限制其最(zui)高電壓的作(zuo)(zuo)用(yong)(yong)。

　　2.控制電路
　　單片機(ji)負責控(kong)制(zhi)整(zheng)個系統的(de)運行，包(bao)括充電(dian)(dian)(dian)電(dian)(dian)(dian)流電(dian)(dian)(dian)壓值(zhi)的(de)設定(ding)，電(dian)(dian)(dian)流電(dian)(dian)(dian)壓的(de)檢測與(yu)(yu)調整(zheng)，充放電(dian)(dian)(dian)狀態的(de)顯示等(deng)。與(yu)(yu)專用充電(dian)(dian)(dian)控(kong)制(zhi)芯片相比(bi)，單片機(ji)控(kong)制(zhi)系統不僅不受電(dian)(dian)(dian)池組容量大小的(de)阻將電(dian)(dian)(dian)流轉換為(wei)電(dian)(dian)(dian)壓進行的(de)，因(yin)此(ci)其(qi)PWM控(kong)制(zhi)調整(zheng)過程與(yu)(yu)恒限制(zhi)，還(huan)可通(tong)過軟硬件配合(he)實現更(geng)靈活的(de)綜(zong)合(he)控(kong)制(zhi)，也便于進一步的(de)后(hou)續開發。

　　系統控制(zhi)選用Atmel公司的AVRATtiny261來實現，控制(zhi)框圖見圖2。ATtiny261采用AVR RISC結構，其大部分指令執行時間僅為(wei)1個(ge)(ge)時鐘周(zhou)期．可達(da)到接近1MIPS/MHZ的性能；11路lObitADC。且15對(dui)具有可編程增益(yi)的ADC差(cha)分通道，精(jing)度高達(da)2.5mV的內(nei)置(zhi)2．56V基準源(yuan)，3個(ge)(ge)獨立PWM發生器(qi)，片上溫度傳(chuan)感器(qi)，足(zu)以滿足(zu)設計(ji)需求。

圖2系統控制結構框圖

　　　系(xi)統電壓(ya)采樣采用(yong)精(jing)密電阻分壓(ya)方(fang)法，將測(ce)量電壓(ya)范圍(wei)轉(zhuan)換成(cheng)0-2．56V，然(ran)后通過1倍的差分ADC通道轉(zhuan)換成(cheng)數(shu)字信號，在充電過程中將測(ce)得的電壓(ya)值與預先設定的值進行比較(jiao)，再控制調(diao)整PWM占空比完成(cheng)對充電電壓(ya)的控制與調(diao)節。

　  電流檢測
　　在系統電(dian)流(liu)的(de)榆測上，由于(yu)選用(yong)(yong)ATtiny261的(de)ADC差分(fen)(fen)通道(dao)，這就要求其(qi)正端輸(shu)(shu)入(ru)電(dian)壓必須大予負端輸(shu)(shu)入(ru)電(dian)壓。困(kun)此，在電(dian)路設(she)計上，通過串(chuan)聯在電(dian)流(liu)主(zhu)回路中(zhong)的(de)高精度(du)采(cai)樣電(dian)阻RsenseB和RsenseA，經ADC2-ADCl和ADCl-ADC0兩對32倍的(de)ADC差分(fen)(fen)通道(dao)(參見(jian)圖3)，分(fen)(fen)別(bie)完成對充(chong)、放(fang)電(dian)電(dian)流(liu)的(de)檢測。可見(jian)，差分(fen)(fen)ADC的(de)選用(yong)(yong)，既保證(zheng)了(le)(le)電(dian)流(liu)采(cai)樣的(de)精準，又避(bi)免了(le)(le)因電(dian)路中(zhong)引入(ru)差分(fen)(fen)遠放(fang)所帶來的(de)功率(lv)損(sun)耗問(wen)題，很(hen)好的(de)滿足(zu)了(le)(le)系統性能與(yu)功耗兩方面的(de)要求，充(chong)分(fen)(fen)體現了(le)(le)ATtiny261的(de)優勢。

  
圖3電池保護電路