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(yi)般充電器(qi)過充電、充電不足(zu)、效率低的缺點，實現了(le)對(dui)鋰電池組的智能充電，達到了(le)預期效果。該方案設(she)計(ji)靈活(huo)，可滿(man)意多種型號的鋰電池充電需求，且ATtiny261集成化的閃(shan)存使其便于軟件調試與(yu)升級。

鋰電池充電特性
　　鋰(li)電(dian)(dian)(dian)(dian)(dian)池充(chong)電(dian)(dian)(dian)(dian)(dian)需要控制它的充(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)壓，限(xian)制其充(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流。鋰(li)電(dian)(dian)(dian)(dian)(dian)池通常都采用三段充(chong)電(dian)(dian)(dian)(dian)(dian)法，即(ji)預充(chong)電(dian)(dian)(dian)(dian)(dian)、恒流寬電(dian)(dian)(dian)(dian)(dian)和恒壓充(chong)電(dian)(dian)(dian)(dian)(dian)。鋰(li)電(dian)(dian)(dian)(dian)(dian)池的充(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流通常應限(xian)制在1C(C為鋰(li)電(dian)(dian)(dian)(dian)(dian)池的容量(liang))一下，單體充(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)壓一般為4．2V，否則可(ke)能由于電(dian)(dian)(dian)(dian)(dian)聰過(guo)高(gao)會造成鍵(jian)電(dian)(dian)(dian)(dian)(dian)池永(yong)久性(xing)損壞。

　　預充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)主要是完成對過放的(de)(de)鋰電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)進行(xing)修復，若電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)低(di)(di)于3V，則必須進行(xing)預充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)，否剛可(ke)省略該階(jie)段(duan)(duan)。這也是最普(pu)遍(bian)的(de)(de)情況。在(zai)恒流階(jie)段(duan)(duan)，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)先給電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)提供大的(de)(de)恒定電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流，同時電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)上升(sheng)，當魄池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)達(da)到(dao)飽和電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)對，則轉入憾壓(ya)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)波動應控(kong)制(zhi)在(zai)50mV以內(nei)，同時充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流降低(di)(di)，當電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流逐漸減小到(dao)規定的(de)(de)值時，可(ke)結束充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)過程(cheng)。電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)的(de)(de)大部分電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)能(neng)在(zai)慣流及恒壓(ya)階(jie)段(duan)(duan)從充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)流入電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)。曲上可(ke)知(zhi)，充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)實際上是一個精密(mi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源，其電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)都(dou)被限(xian)制(zhi)在(zai)所要求的(de)(de)范圍之內(nei)。

硬件電路設計
　　該系統在電(dian)路(lu)設計(ji)上主要由單片開關電(dian)源、控制電(dian)路(lu)及保(bao)護電(dian)路(lu)三部分(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(tu)1單片開(kai)關電源

　　該電(dian)(dian)(dian)源采用配(pei)穩(wen)壓(ya)(ya)管的(de)(de)光(guang)藕反饋電(dian)(dian)(dian)路(lu)實(shi)現15V的(de)(de)低壓(ya)(ya)直流輸(shu)出(chu)(chu)，當(dang)輸(shu)出(chu)(chu)電(dian)(dian)(dian)壓(ya)(ya)發(fa)生(sheng)(sheng)變化時，通過(guo)線性光(guang)藕PC817的(de)(de)發(fa)光(guang)管的(de)(de)電(dian)(dian)(dian)流發(fa)生(sheng)(sheng)相(xiang)應的(de)(de)變化，使得TNY268P的(de)(de)EN腳流出(chu)(chu)電(dian)(dian)(dian)流也發(fa)生(sheng)(sheng)變化，從而控(kong)制其片內(nei)功率MOSFET的(de)(de)斷、通、調(diao)節(jie)輸(shu)出(chu)(chu)電(dian)(dian)(dian)壓(ya)(ya)，使輸(shu)壓(ya)(ya)電(dian)(dian)(dian)壓(ya)(ya)穩(wen)定。詳細反饋原(yuan)理分(fen)析詳見后文(wen)脈寬調(diao)制(PWM)的(de)(de)控(kong)制。

　　在(zai)電(dian)(dian)(dian)路結構(gou)(gou)上(shang)，線性(xing)光藕PC817，不但可以起到反饋作(zuo)用(yong)還可起到隔離作(zuo)用(yong)。由PNP管Q2和電(dian)(dian)(dian)阻R9、R1O及R12組成(cheng)(cheng)的(de)限(xian)流(liu)電(dian)(dian)(dian)路，則從源頭上(shang)防止了(le)過(guo)電(dian)(dian)(dian)流(liu)的(de)問題。由C6及R11構(gou)(gou)成(cheng)(cheng)的(de)緩啟電(dian)(dian)(dian)路，則有效抑(yi)止了(le)電(dian)(dian)(dian)源上(shang)電(dian)(dian)(dian)瞬(shun)間的(de)產生的(de)電(dian)(dian)(dian)壓(ya)尖峰。而二極管D9則防止了(le)電(dian)(dian)(dian)池組的(de)反向(xiang)放電(dian)(dian)(dian)。此外，對整個充電(dian)(dian)(dian)系(xi)統而言，當因意外情況系(xi)統失控(kong)時，開關(guan)電(dian)(dian)(dian)源所(suo)提供的(de)15V直流(liu)低壓(ya)也(ye)在(zai)某(mou)種程度上(shang)起到了(le)限(xian)制(zhi)其最(zui)高電(dian)(dian)(dian)壓(ya)的(de)作(zuo)用(yong)。

　　2.控制電路
　　單(dan)片機負責控(kong)制整個系(xi)統的(de)(de)運行，包括充(chong)電(dian)(dian)電(dian)(dian)流(liu)(liu)電(dian)(dian)壓值的(de)(de)設(she)定，電(dian)(dian)流(liu)(liu)電(dian)(dian)壓的(de)(de)檢(jian)測與調整，充(chong)放電(dian)(dian)狀(zhuang)態(tai)的(de)(de)顯示等。與專用充(chong)電(dian)(dian)控(kong)制芯(xin)片相比(bi)，單(dan)片機控(kong)制系(xi)統不(bu)(bu)僅不(bu)(bu)受電(dian)(dian)池組(zu)容量大小(xiao)的(de)(de)阻將電(dian)(dian)流(liu)(liu)轉換為電(dian)(dian)壓進行的(de)(de)，因此其PWM控(kong)制調整過程與恒限(xian)制，還可通過軟硬(ying)件配合(he)實現更靈(ling)活的(de)(de)綜合(he)控(kong)制，也便于進一步的(de)(de)后續開發。

　　系統控制選用Atmel公司(si)的AVRATtiny261來實現，控制框圖見(jian)圖2。ATtiny261采用AVR RISC結(jie)構，其大部(bu)分(fen)指(zhi)令(ling)執行時間僅(jin)為1個時鐘周期．可達到(dao)接近1MIPS/MHZ的性能；11路(lu)lObitADC。且15對(dui)具有可編(bian)程增益的ADC差分(fen)通道，精度(du)高達2.5mV的內(nei)置2．56V基準(zhun)源，3個獨立PWM發(fa)生(sheng)器(qi)，片上溫(wen)度(du)傳感器(qi)，足以滿(man)足設計需(xu)求。

圖(tu)2系統控制結構框圖(tu)

　　　系統電(dian)(dian)(dian)壓(ya)采樣采用精(jing)密電(dian)(dian)(dian)阻分(fen)壓(ya)方(fang)法，將(jiang)測(ce)量(liang)電(dian)(dian)(dian)壓(ya)范(fan)圍轉換成0-2．56V，然后通過(guo)(guo)1倍的(de)差分(fen)ADC通道轉換成數(shu)字信號，在(zai)充(chong)電(dian)(dian)(dian)過(guo)(guo)程(cheng)中將(jiang)測(ce)得的(de)電(dian)(dian)(dian)壓(ya)值與(yu)預(yu)先設定的(de)值進行比較，再控制(zhi)調整PWM占(zhan)空(kong)比完成對充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)壓(ya)的(de)控制(zhi)與(yu)調節。

　  電流檢測
　　在系統電(dian)流(liu)的(de)榆測(ce)上(shang)，由于(yu)選用ATtiny261的(de)ADC差分通道(dao)，這就要(yao)求(qiu)其正端輸入(ru)(ru)電(dian)壓(ya)必(bi)須大予負端輸入(ru)(ru)電(dian)壓(ya)。困此，在電(dian)路設計上(shang)，通過串(chuan)聯在電(dian)流(liu)主回路中的(de)高精度(du)采樣(yang)電(dian)阻RsenseB和(he)RsenseA，經ADC2-ADCl和(he)ADCl-ADC0兩對32倍的(de)ADC差分通道(dao)(參見(jian)圖3)，分別完成(cheng)對充(chong)、放(fang)電(dian)電(dian)流(liu)的(de)檢(jian)測(ce)。可見(jian)，差分ADC的(de)選用，既保(bao)證了電(dian)流(liu)采樣(yang)的(de)精準，又避免了因電(dian)路中引入(ru)(ru)差分遠放(fang)所帶來的(de)功(gong)率損耗問題(ti)，很(hen)好的(de)滿足了系統性能與(yu)功(gong)耗兩方(fang)面的(de)要(yao)求(qiu)，充(chong)分體現了ATtiny261的(de)優勢(shi)。

  
圖3電池保護電路