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(xiao)地克服了(le)一般充電(dian)器過充電(dian)、充電(dian)不足、效(xiao)率低的(de)(de)缺點，實現了(le)對鋰電(dian)池組的(de)(de)智能充電(dian)，達到了(le)預期效(xiao)果(guo)。該方案(an)設計靈活，可滿意多種型號(hao)的(de)(de)鋰電(dian)池充電(dian)需求，且ATtiny261集(ji)成化的(de)(de)閃存使其(qi)便于軟件(jian)調試與(yu)升級。

鋰電池充電特性
　　鋰(li)(li)電(dian)(dian)(dian)池(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)需(xu)要控制它(ta)的充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)壓，限(xian)制其充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流。鋰(li)(li)電(dian)(dian)(dian)池(chi)(chi)通常都采(cai)用三段充(chong)(chong)電(dian)(dian)(dian)法(fa)，即預充(chong)(chong)電(dian)(dian)(dian)、恒(heng)流寬電(dian)(dian)(dian)和恒(heng)壓充(chong)(chong)電(dian)(dian)(dian)。鋰(li)(li)電(dian)(dian)(dian)池(chi)(chi)的充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流通常應限(xian)制在1C(C為鋰(li)(li)電(dian)(dian)(dian)池(chi)(chi)的容量)一下，單(dan)體充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)壓一般(ban)為4．2V，否(fou)則可(ke)能(neng)由于電(dian)(dian)(dian)聰過高會造成鍵電(dian)(dian)(dian)池(chi)(chi)永久性損壞。

　　預充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)主要是(shi)完成對過(guo)放(fang)的鋰(li)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)進行修復(fu)，若電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)低(di)于3V，則必須進行預充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)，否剛可省略該階段(duan)。這也是(shi)最普遍的情況(kuang)。在恒流階段(duan)，充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)先給電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)提供大的恒定(ding)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流，同時電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)上(shang)(shang)升，當魄池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)達(da)到飽和(he)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)對，則轉入憾壓(ya)充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)，充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)波動應控制在50mV以內，同時充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流降低(di)，當電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流逐漸減(jian)小到規定(ding)的值時，可結束(shu)充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)過(guo)程。電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的大部(bu)分電(dian)(dian)(dian)(dian)(dian)(dian)(dian)能(neng)在慣流及恒壓(ya)階段(duan)從充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)流入電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)。曲上(shang)(shang)可知(zhi)，充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)實際上(shang)(shang)是(shi)一個精(jing)密電(dian)(dian)(dian)(dian)(dian)(dian)(dian)源，其電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)都(dou)被限(xian)制在所要求的范圍之(zhi)內。

硬件電路設計
　　該(gai)系統(tong)在電路設計(ji)上主要由單片開關電源、控制電路及保護電路三(san)部分組成。

　　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(dan)片開關電源(yuan)

　　該電(dian)源采用配穩壓(ya)管(guan)(guan)的(de)(de)(de)光(guang)藕反(fan)饋電(dian)路(lu)實現15V的(de)(de)(de)低壓(ya)直流輸出(chu)，當(dang)輸出(chu)電(dian)壓(ya)發生(sheng)(sheng)變化時，通過線性光(guang)藕PC817的(de)(de)(de)發光(guang)管(guan)(guan)的(de)(de)(de)電(dian)流發生(sheng)(sheng)相(xiang)應(ying)的(de)(de)(de)變化，使(shi)得TNY268P的(de)(de)(de)EN腳(jiao)流出(chu)電(dian)流也發生(sheng)(sheng)變化，從而控(kong)(kong)制其片內(nei)功(gong)率MOSFET的(de)(de)(de)斷、通、調節輸出(chu)電(dian)壓(ya)，使(shi)輸壓(ya)電(dian)壓(ya)穩定。詳(xiang)細(xi)反(fan)饋原(yuan)理分析詳(xiang)見后文脈寬調制(PWM)的(de)(de)(de)控(kong)(kong)制。

　　在電(dian)(dian)(dian)路(lu)結構上(shang)，線(xian)性光藕PC817，不但可以起(qi)(qi)到反饋(kui)作用還可起(qi)(qi)到隔(ge)離作用。由PNP管(guan)Q2和電(dian)(dian)(dian)阻R9、R1O及R12組成的限流電(dian)(dian)(dian)路(lu)，則(ze)從源頭上(shang)防止(zhi)了(le)過電(dian)(dian)(dian)流的問題。由C6及R11構成的緩啟電(dian)(dian)(dian)路(lu)，則(ze)有效抑止(zhi)了(le)電(dian)(dian)(dian)源上(shang)電(dian)(dian)(dian)瞬(shun)間的產生的電(dian)(dian)(dian)壓(ya)尖峰。而二(er)極管(guan)D9則(ze)防止(zhi)了(le)電(dian)(dian)(dian)池組的反向放(fang)電(dian)(dian)(dian)。此(ci)外(wai)，對整個充(chong)電(dian)(dian)(dian)系統(tong)而言，當因意外(wai)情況系統(tong)失(shi)控時，開關電(dian)(dian)(dian)源所提供的15V直流低壓(ya)也在某種程度(du)上(shang)起(qi)(qi)到了(le)限制其最高電(dian)(dian)(dian)壓(ya)的作用。

　　2.控制電路
　　單片(pian)機負責控制整(zheng)個(ge)系統(tong)的運行(xing)，包括充電(dian)(dian)電(dian)(dian)流(liu)電(dian)(dian)壓值(zhi)的設定，電(dian)(dian)流(liu)電(dian)(dian)壓的檢測與(yu)調(diao)整(zheng)，充放電(dian)(dian)狀態的顯示等。與(yu)專用充電(dian)(dian)控制芯片(pian)相比，單片(pian)機控制系統(tong)不僅不受電(dian)(dian)池(chi)組容(rong)量(liang)大(da)小的阻將電(dian)(dian)流(liu)轉換為電(dian)(dian)壓進行(xing)的，因此其PWM控制調(diao)整(zheng)過程與(yu)恒限制，還(huan)可通過軟硬件配合實現更(geng)靈活的綜合控制，也(ye)便于(yu)進一步的后續開(kai)發。

　　系統控制選(xuan)用Atmel公司的(de)AVRATtiny261來實現，控制框圖(tu)見圖(tu)2。ATtiny261采用AVR RISC結(jie)構，其大部(bu)分(fen)(fen)指令(ling)執行時(shi)(shi)間(jian)僅為1個(ge)(ge)時(shi)(shi)鐘周期(qi)．可達(da)(da)到(dao)接近1MIPS/MHZ的(de)性能；11路lObitADC。且15對(dui)具有可編程增益的(de)ADC差(cha)分(fen)(fen)通道，精度高達(da)(da)2.5mV的(de)內置2．56V基準源(yuan)，3個(ge)(ge)獨(du)立PWM發(fa)生器，片(pian)上溫度傳(chuan)感器，足(zu)以滿足(zu)設計需求。

圖(tu)2系統控制結構框圖(tu)

　　　系統電(dian)壓采樣采用精密電(dian)阻分壓方(fang)法，將測量電(dian)壓范(fan)圍轉(zhuan)換(huan)成0-2．56V，然后通過(guo)1倍的(de)(de)差分ADC通道轉(zhuan)換(huan)成數(shu)字信號，在充電(dian)過(guo)程中將測得(de)的(de)(de)電(dian)壓值(zhi)與預先(xian)設定的(de)(de)值(zhi)進行比較，再(zai)控制調整PWM占空比完(wan)成對充電(dian)電(dian)壓的(de)(de)控制與調節。

　  電流檢測
　　在(zai)系(xi)統電(dian)(dian)(dian)流的(de)(de)(de)榆測(ce)上，由于選用ATtiny261的(de)(de)(de)ADC差(cha)(cha)分(fen)(fen)通道，這就要求(qiu)其正端輸入(ru)電(dian)(dian)(dian)壓(ya)必須(xu)大予負端輸入(ru)電(dian)(dian)(dian)壓(ya)。困此，在(zai)電(dian)(dian)(dian)路設計上，通過串聯在(zai)電(dian)(dian)(dian)流主回(hui)路中的(de)(de)(de)高精度(du)采樣(yang)電(dian)(dian)(dian)阻RsenseB和RsenseA，經ADC2-ADCl和ADCl-ADC0兩對(dui)32倍的(de)(de)(de)ADC差(cha)(cha)分(fen)(fen)通道(參(can)見圖(tu)3)，分(fen)(fen)別完成對(dui)充、放電(dian)(dian)(dian)電(dian)(dian)(dian)流的(de)(de)(de)檢測(ce)。可見，差(cha)(cha)分(fen)(fen)ADC的(de)(de)(de)選用，既保證了(le)(le)(le)電(dian)(dian)(dian)流采樣(yang)的(de)(de)(de)精準，又避免了(le)(le)(le)因(yin)電(dian)(dian)(dian)路中引入(ru)差(cha)(cha)分(fen)(fen)遠放所帶來的(de)(de)(de)功率損耗問題，很好的(de)(de)(de)滿足了(le)(le)(le)系(xi)統性能與(yu)功耗兩方面的(de)(de)(de)要求(qiu)，充分(fen)(fen)體現了(le)(le)(le)ATtiny261的(de)(de)(de)優勢(shi)。

  
圖3電池保(bao)護(hu)電路