一次性電池不能夠充電，完全放電后的電池即為報廢電池。二次電池可以多次充電，但不同的化學特性對充電的要求也不同，特別是“快充”控制。目前，最常用的可充電電池是Li+、鎳氫(NiMH)、鎳鎘(Nicd)和鉛酸電池，其它類型的電池還很多，但沒有大規模投入使用。因為本文僅討論利用交流適配器供電的充電器設計，沒有考慮汽車電源供電的SLI鉛酸電池或更大容量電池的充電器。

不同化學類型的電池對充電器的要求  

通常快速充電比涓流充電更受歡迎，但快速充電對電路的要求也復雜，鎳氫或鎳鎘電池的快速充電技術與其它電池也是不同的。鎳鎘和鎳氫電池通常在恒流狀態下快充，而不是恒壓狀態下。快充終止檢測基于電壓和溫度的變化率，充電器還應具備過壓保護和過熱保護功能。Li+和鉛酸電池充電器必須保證作用到電池上的電壓和電流最大值在規定的范圍內，當電池電壓維持在最大單節電池電壓，充電電流下降到一個較低值時，充電器終止充電。Li+和鉛酸電池充電器的區別在于所允許的充電電流、最大電池電壓，另外鉛酸電池充電器一般具有浮充能力(浮充是為了補充鉛酸電池的自放電，而對一個已經完全充滿的電池繼續輸入充電電流的技術)，Li+電池的自放(fang)電非常低，所以不(bu)需要浮充。

Li+電池

為便攜(xie)式產品選擇電(dian)(dian)池(chi)時(shi)，最(zui)(zui)重(zhong)(zhong)要(yao)的是高(gao)(gao)能量密(mi)度(du)(du)、尺寸、重(zhong)(zhong)量和價格。迄今為止，Li+電(dian)(dian)池(chi)在最(zui)(zui)為流行(xing)的電(dian)(dian)池(chi)中能夠提供最(zui)(zui)高(gao)(gao)的體積和能量密(mi)度(du)(du)，但其每(mei)瓦時(shi)的成(cheng)本(ben)(ben)也較高(gao)(gao)。鉛酸電(dian)(dian)池(chi)的每(mei)瓦時(shi)成(cheng)本(ben)(ben)最(zui)(zui)低，但比較笨(ben)重(zhong)(zhong)。本(ben)(ben)文以Li+電(dian)(dian)池(chi)為例討論充電(dian)(dian)器的設(she)計，它在便攜(xie)式產品中已經被(bei)普(pu)遍使用。我們選擇780mA的充電(dian)(dian)電(dian)(dian)流和4.2V的終(zhong)止電(dian)(dian)壓。

充電器無需微處理器干預，輸入電源可以采用全球范圍的任何交流電源，省去了大體積60Hz變壓器，而且可提供單節Li+電池所要求的精確的充電電壓和電流。充電器IC(U4)集成了全面的保(bao)(bao)護功能(電壓(ya)、電流和自保(bao)(bao)護)，確保(bao)(bao)Li+電池的安全充電。

假設Li+電(dian)(dian)(dian)池帶有過(guo)(guo)流(liu)、過(guo)(guo)壓(ya)和(he)過(guo)(guo)熱保(bao)護，這些保(bao)護特(te)性(xing)(xing)在(zai)電(dian)(dian)(dian)池包里(li)是必需的，因為充(chong)電(dian)(dian)(dian)期間一(yi)旦電(dian)(dian)(dian)路出現故障(zhang)，Li+電(dian)(dian)(dian)池將存(cun)在(zai)很大的危(wei)險性(xing)(xing)，過(guo)(guo)流(liu)、過(guo)(guo)壓(ya)或溫度過(guo)(guo)高(gao)都有可(ke)能造成(cheng)Li+電(dian)(dian)(dian)池爆炸(zha)。

包括兩顆(ke)主器(qi)(qi)件(U1和U4)、精密基準(U3)和光耦(ou)(U2)。U1是離(li)線式開關(guan)控制(zhi)器(qi)(qi)，配合隔離(li)變壓(ya)器(qi)(qi)、光耦(ou)和基準產生(sheng)隔離(li)的(de)穩壓(ya)直流輸(shu)出。直流電(dian)壓(ya)供給電(dian)池(chi)(chi)充電(dian)器(qi)(qi)(U4)，U4提供電(dian)流限制(zhi)、電(dian)壓(ya)調節、電(dian)池(chi)(chi)充電(dian)和充電(dian)指示等功能(neng)。本方案中，U4是一款簡單的(de)線性充電(dian)器(qi)(qi)，內(nei)置(zhi)調整管，結合其(qi)熱控制(zhi)環路提供快速(su)充電(dian)。

MAX5022的特性

U1(MAX5022)在寬(kuan)輸入(ru)(ru)(ru)范(fan)圍(wei)的(de)應(ying)用中很容易設(she)計，本文所提供的(de)方案要(yao)求能(neng)夠滿足全球各地的(de)交流電(dian)(dian)(dian)源規格，這里(li)假設(she)通用的(de)輸入(ru)(ru)(ru)電(dian)(dian)(dian)源電(dian)(dian)(dian)壓范(fan)圍(wei)為85VAC至(zhi)265VAC。MAX5022的(de)輸入(ru)(ru)(ru)范(fan)圍(wei)指標(biao)以及評(ping)估板(ban)電(dian)(dian)(dian)路的(de)輸入(ru)(ru)(ru)范(fan)圍(wei)恰好滿足這一(yi)需求。對于圖1所示充電(dian)(dian)(dian)器的(de)電(dian)(dian)(dian)源端(duan)MAX5022評(ping)估板(ban)不需要(yao)作任何修(xiu)改(gai)，隔離端(duan)只需對評(ping)估板(ban)的(de)電(dian)(dian)(dian)阻(zu)值(zhi)稍作修(xiu)改(gai)。

對于(yu)(yu)更(geng)大(da)功率的(de)應(ying)用(yong)，只需選取功率較大(da)的(de)開關管和能(neng)夠處(chu)理(li)更(geng)大(da)功率的(de)變(bian)壓(ya)器即(ji)可。圖1所示(shi)電(dian)(dian)路要(yao)求(qiu)充電(dian)(dian)電(dian)(dian)流為780mA，最(zui)大(da)電(dian)(dian)池(chi)電(dian)(dian)壓(ya)為4.2V。對充電(dian)(dian)電(dian)(dian)源的(de)要(yao)求(qiu)與MAX5022評估(gu)板所提供的(de)輸出相吻合：5V@1A，因(yin)此，不(bu)需要(yao)更(geng)換評估(gu)板的(de)晶(jing)體管和變(bian)壓(ya)器。U4采用(yong)SOT23封(feng)裝(zhuang)，所以，充電(dian)(dian)器的(de)尺寸(cun)(cun)在很大(da)程度上(shang)取決于(yu)(yu)應(ying)用(yong)對充電(dian)(dian)電(dian)(dian)源的(de)需求(qiu)。變(bian)壓(ya)器是電(dian)(dian)路中尺寸(cun)(cun)最(zui)大(da)的(de)元件，由于(yu)(yu)開關控制器的(de)工(gong)作頻率大(da)于(yu)(yu)250kHz，允(yun)許系統選用(yong)小尺寸(cun)(cun)的(de)變(bian)壓(ya)器。

上電(dian)(dian)(dian)(dian)(dian)時，在開關控制器工作(zuo)之前(qian)，U1調節器直接通過泄漏(lou)電(dian)(dian)(dian)(dian)(dian)阻(zu)R1、R2從整流(liu)通路吸(xi)取少量的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流(liu)，兩個(ge)電(dian)(dian)(dian)(dian)(dian)阻(zu)阻(zu)值保證每(mei)個(ge)電(dian)(dian)(dian)(dian)(dian)阻(zu)上的(de)(de)(de)壓(ya)降不(bu)(bu)會超(chao)出(chu)每(mei)個(ge)電(dian)(dian)(dian)(dian)(dian)阻(zu)的(de)(de)(de)容限(xian)，Vin引腳電(dian)(dian)(dian)(dian)(dian)壓(ya)不(bu)(bu)會超(chao)出(chu)其(qi)28V額定值(30V為絕對最大值)，U1在輸入(ru)電(dian)(dian)(dian)(dian)(dian)壓(ya)達到(dao)24V時開始開關操作(zuo)，并吸(xi)取更大的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流(liu)(高(gao)于R1、R2所能提(ti)供的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流(liu))，在變壓(ya)器副邊線圈能夠提(ti)供足夠的(de)(de)(de)能量之前(qian)，額外的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流(liu)由Vin引腳的(de)(de)(de)旁路電(dian)(dian)(dian)(dian)(dian)容供給(gei)。這種(zhong)處理方式能夠適合較(jiao)寬(kuan)的(de)(de)(de)輸入(ru)電(dian)(dian)(dian)(dian)(dian)壓(ya)范圍，對效率的(de)(de)(de)影響也很小(xiao)，這是因為在啟動(dong)過程中只需極小(xiao)的(de)(de)(de)供電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)(典型值為50uA)。  

為了(le)防止U1的(de)(de)Vin引腳出現(xian)過(guo)壓，可以(yi)在Vin與地之間接(jie)一個26V至30V的(de)(de)齊納(na)管，電(dian)壓值(zhi)需要高于(yu)欠壓鎖存電(dian)壓的(de)(de)最大值(zhi)，低于(yu)該(gai)引腳所允(yun)許(xu)的(de)(de)最大絕對電(dian)壓。如果器件沒有啟動(dong)正常(chang)的(de)(de)開關操作(zuo)，則可認為電(dian)路發生(sheng)了(le)某些問(wen)題，所以(yi)，這一保護(hu)措施(shi)對于(yu)U1來說并不(bu)是必需的(de)(de)。

U1是一(yi)款(kuan)電(dian)(dian)(dian)(dian)流模式PWM控制器，具有(you)逐周期限流電(dian)(dian)(dian)(dian)路(lu)，必要時其占(zhan)空比(bi)可以提高到(dao)75%，非常適合圖1所示的反激拓撲。原(yuan)邊開關電(dian)(dian)(dian)(dian)流檢(jian)測采用尺寸為1206的1.78Ω電(dian)(dian)(dian)(dian)阻(R7)，該電(dian)(dian)(dian)(dian)阻限制了變壓器原(yuan)邊的最(zui)(zui)大(da)電(dian)(dian)(dian)(dian)流。固(gu)定(ding)開關頻(pin)率(lv)和最(zui)(zui)大(da)占(zhan)空比(bi)限制使得最(zui)(zui)大(da)限流具有(you)輸出(chu)短路(lu)保護(hu)功能(neng)。  

一旦短路(lu)輸出造成電路(lu)達到(dao)了上(shang)述限制條件，第三組線圈將無(wu)法提供10V以(yi)上(shang)的電壓(UVLO的低電壓門限)，U1將再次進入(ru)啟動(dong)狀(zhuang)態(tai)，停止開關操(cao)作。短路(lu)故障解除后系統會(hui)自動(dong)恢復正常的工作狀(zhuang)態(tai)。  

電壓調節
 
變壓(ya)(ya)(ya)(ya)器副(fu)邊的(de)(de)電(dian)壓(ya)(ya)(ya)(ya)經過整流、濾(lv)波(bo)，然后由(you)R4、R5分壓(ya)(ya)(ya)(ya)在并聯型穩壓(ya)(ya)(ya)(ya)器U3的(de)(de)調節點(dian)產生1.24V(對應于(yu)(yu)4.5V輸(shu)(shu)出)的(de)(de)反饋電(dian)壓(ya)(ya)(ya)(ya)。為保證(zheng)電(dian)池(chi)充(chong)電(dian)器正(zheng)常(chang)工(gong)作，4.5V是最(zui)低電(dian)壓(ya)(ya)(ya)(ya)(包括容差)。因為充(chong)電(dian)器采(cai)用的(de)(de)是線(xian)性結構，充(chong)電(dian)效率與輸(shu)(shu)入、輸(shu)(shu)出電(dian)壓(ya)(ya)(ya)(ya)的(de)(de)差值成反比(bi)，用最(zui)低電(dian)壓(ya)(ya)(ya)(ya)為充(chong)電(dian)器供電(dian)有助于(yu)(yu)降低功率損耗(hao)。  

當調節點電(dian)(dian)(dian)壓(ya)高于(yu)1.24V時，并聯(lian)型穩(wen)(wen)壓(ya)器(qi)將(jiang)(jiang)吸入電(dian)(dian)(dian)流(liu)，導通(tong)光(guang)耦(ou)LED，原邊的(de)(de)(de)光(guang)敏晶(jing)體管將(jiang)(jiang)處于(yu)發光(guang)狀(zhuang)態(tai)。U1的(de)(de)(de)OPTO輸(shu)(shu)(shu)入引腳用于(yu)直接(jie)連接(jie)光(guang)敏晶(jing)體管，以極其簡單(dan)的(de)(de)(de)方式構成(cheng)閉合環路(lu)。光(guang)敏晶(jing)體管導通(tong)時，OPTO引腳被拉低，內部比(bi)較(jiao)器(qi)在(zai)它(ta)的(de)(de)(de)一個(ge)(ge)輸(shu)(shu)(shu)入端(duan)檢測到較(jiao)低的(de)(de)(de)電(dian)(dian)(dian)壓(ya)，比(bi)較(jiao)器(qi)的(de)(de)(de)另一輸(shu)(shu)(shu)入電(dian)(dian)(dian)流(liu)檢測引腳(CS)的(de)(de)(de)峰值(zhi)電(dian)(dian)(dian)壓(ya)，OPTO引腳的(de)(de)(de)低電(dian)(dian)(dian)壓(ya)將(jiang)(jiang)在(zai)每個(ge)(ge)周(zhou)期直接(jie)限制功率MOSFETN1的(de)(de)(de)峰值(zhi)電(dian)(dian)(dian)流(liu)。由于(yu)U1是一個(ge)(ge)PWM控制器(qi)，電(dian)(dian)(dian)流(liu)限制使得(de)電(dian)(dian)(dian)壓(ya)調節受作(zuo)用在(zai)N1上的(de)(de)(de)占空比(bi)(導通(tong)時間(jian)與斷開(kai)時間(jian)的(de)(de)(de)比(bi)值(zhi))的(de)(de)(de)影(ying)響，環路(lu)穩(wen)(wen)定(ding)性通(tong)過(guo)調節R11和(he)(he)C12來保證。R10和(he)(he)C5(連接(jie)在(zai)U3的(de)(de)(de)第3和(he)(he)第4引腳)可(ke)消(xiao)除(chu)穩(wen)(wen)定(ding)工作(zuo)情況(kuang)下的(de)(de)(de)噪聲干擾。

U4的工作特點

U4是一(yi)(yi)款用(yong)于單節Li+電(dian)(dian)(dian)(dian)(dian)池充(chong)電(dian)(dian)(dian)(dian)(dian)、功(gong)能完備的充(chong)電(dian)(dian)(dian)(dian)(dian)器，采用(yong)線(xian)性充(chong)電(dian)(dian)(dian)(dian)(dian)模式，內(nei)(nei)置(zhi)調整管(guan)和電(dian)(dian)(dian)(dian)(dian)流檢(jian)測電(dian)(dian)(dian)(dian)(dian)路(lu)。另外，它(ta)還集成了一(yi)(yi)個反向(xiang)抑制(zhi)二極(ji)管(guan)，在(zai)去掉輸入(ru)電(dian)(dian)(dian)(dian)(dian)源時可(ke)防止(zhi)電(dian)(dian)(dian)(dian)(dian)池方(fang)向(xiang)放電(dian)(dian)(dian)(dian)(dian)。接(jie)入(ru)一(yi)(yi)個深度(du)放電(dian)(dian)(dian)(dian)(dian)的電(dian)(dian)(dian)(dian)(dian)池時，U4能夠檢(jian)測到低(di)(di)電(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)壓(低(di)(di)于2.5V)，這樣，U4將首先啟動(dong)涓(juan)流充(chong)電(dian)(dian)(dian)(dian)(dian)過程，在(zai)快充(chong)周期的1/10時間(jian)段對(dui)電(dian)(dian)(dian)(dian)(dian)池進行涓(juan)流充(chong)電(dian)(dian)(dian)(dian)(dian)，直到電(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)壓上升到2.5V以(yi)上。在(zai)涓(juan)流充(chong)電(dian)(dian)(dian)(dian)(dian)到快速充(chong)電(dian)(dian)(dian)(dian)(dian)轉換時，軟啟動(dong)電(dian)(dian)(dian)(dian)(dian)路(lu)迫使充(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流在(zai)3ms時間(jian)內(nei)(nei)緩慢上升，避(bi)免較大(da)的浪涌電(dian)(dian)(dian)(dian)(dian)流拉低(di)(di)輸入(ru)電(dian)(dian)(dian)(dian)(dian)源。

   上(shang)述功能的實施不需要微(wei)控制器的監(jian)管(guan)，而且，U4還具有可編程(cheng)的溫度限制功能，我(wo)們會在(zai)下(xia)面就這(zhe)一點進(jin)行詳細討論(lun)。用一個電(dian)(dian)(dian)(dian)阻設(she)置最大充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)，電(dian)(dian)(dian)(dian)阻兩端的電(dian)(dian)(dian)(dian)壓與充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)成正比，通過監(jian)視該電(dian)(dian)(dian)(dian)壓即可連續跟蹤(zong)充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)，更為復雜的系統也(ye)可以采用ADC跟蹤(zong)監(jian)視充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)。

Maxim還提供有另一款與MAX1507非常相似(si)的充(chong)(chong)電(dian)器―MAX1555，這款IC能夠在沒有交流電(dian)源(yuan)的情況下從USB主機獲得充(chong)(chong)電(dian)電(dian)源(yuan)。  

電流調節

如果把Li+電池直接與一個低阻電壓源連接，充電電流將不受任何限制，這種狀況會對電池及其周圍環境造成災難性的損害，從Li+電池的安全性考慮，必須將其充電電流限制在額定范圍以內。U4根據連接在ISET(第5引腳)的電阻限制充電電流，圖1電(dian)路中(zhong)選用(yong)了一個1.87kΩ的電(dian)阻，將最大電(dian)流(liu)設置在780mA:

Ifast=1461V/Rset=1461/1870≈780mA

電池電壓(ya)(ya)最小時(shi)，由于輸(shu)入穩定(ding)在4.5V，U4輸(shu)入與輸(shu)出(chu)的(de)壓(ya)(ya)差最大(da)，如(ru)果(guo)此時(shi)電池是以最大(da)電流充電，U4功(gong)耗將達到(dao)最大(da)值。例如(ru)，假設電池電壓(ya)(ya)為2.5V，則對應(ying)的(de)功(gong)耗為：

P=IV=(0.78A)×(4.5V-2.5V)=1.56W

U4封裝(8引腳薄型DFN-EP，EP表示裸露焊盤)在溫度高達70℃時可以耗散的最大功率是1.951W，這樣，在整個電池電壓范圍內以780mA的電流充電應該沒有任何問題。但是，如果IC管芯由于較高的環境溫度或不良的散熱條件達到了70℃，U4內部的熱調節環路將自動降低充電電流，這一過程有別于其它充電IC中的“熱關斷”，“熱關斷”方式只是簡單地中止芯片中的所有電路工作，直到晶片溫度降至規定的數值。MAX1507所采用的方案能夠保證在芯片溫度不超出最大限制范圍的條件下提供最快的充電速率，隨著電池電壓的升高，充電電流在保持最高溫度的前提下逐漸升高，直到達到最大充電電流。在此之后，充電器在滿足芯片溫度限制的條件下將以最大充電電流對電池充電，直到電(dian)(dian)池(chi)電(dian)(dian)壓達(da)到內部設置的(de)4.2V±1%。電(dian)(dian)池(chi)電(dian)(dian)壓達(da)到門(men)限值后，U4將輸出(chu)電(dian)(dian)壓保持在4.2V，從而使充(chong)電(dian)(dian)電(dian)(dian)流(liu)開始下降。充(chong)電(dian)(dian)電(dian)(dian)流(liu)降低(di)到快(kuai)充(chong)電(dian)(dian)流(liu)的(de)10%時，充(chong)電(dian)(dian)指示(shi)(shi)燈關(guan)閉，表(biao)示(shi)(shi)充(chong)電(dian)(dian)周(zhou)期(qi)結束。

如(ru)果(guo)充(chong)電過程(cheng)中輸出負載增大(da)，例如(ru)，用(yong)戶(hu)在(zai)電池充(chong)電時啟(qi)動了(le)其它電路，U4允許返(fan)回到(dao)限流(liu)模式。

測試結果

圖1電(dian)(dian)(dian)(dian)路輸(shu)(shu)入為(wei)110V交流電(dian)(dian)(dian)(dian)源，輸(shu)(shu)出為(wei)4.14V、740mA負載(zai)，效率可達到(dao)46%。當(dang)負載(zai)電(dian)(dian)(dian)(dian)流增大使電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓(ya)降至2.7V時(shi)，電(dian)(dian)(dian)(dian)路將輸(shu)(shu)出電(dian)(dian)(dian)(dian)流限(xian)制在802mA，比設置(zhi)的(de)780mA限(xian)流門限(xian)提高不(bu)到(dao)3%，在規(gui)定的(de)參(can)數指標以(yi)內。MAX1507允許(xu)±8%的(de)變化。當(dang)電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓(ya)拉至2.5V時(shi)，輸(shu)(shu)出電(dian)(dian)(dian)(dian)流降至76.1mA，在78mA(780mA的(de)1/10)的(de)3%以(yi)內。U4數據手(shou)冊中(zhong)規(gui)定允許(xu)±5%的(de)范圍。輸(shu)(shu)出短路時(shi)電(dian)(dian)(dian)(dian)路還允許(xu)返回(hui)到(dao)該模式。

空載時，輸出穩定在4.2V左右，形成一個三角波，在U4輸出接一個負載電阻可以消除這個偏差，保持其連續性，但會降低系統效率，但對于交流電供電系統來說(shuo)，效率并(bing)不(bu)重要(yao)。

利用(yong)(yong)電(dian)(dian)(dian)流模(mo)式PWM控制器(U1)構成(cheng)隔離電(dian)(dian)(dian)源，結合Li+電(dian)(dian)(dian)池充電(dian)(dian)(dian)IC(U4)和其它元件可(ke)以構成(cheng)一個由通(tong)用(yong)(yong)的交(jiao)流電(dian)(dian)(dian)源供電(dian)(dian)(dian)的Li+電(dian)(dian)(dian)池充電(dian)(dian)(dian)器。該方案(an)不需(xu)要(yao)任何額外的控制電(dian)(dian)(dian)路(lu)(如(ru)微控制器的干預(yu))，也不需(xu)要(yao)軟件開發(fa)。硬(ying)件開發(fa)只需(xu)對Maxim提供的標準評估板稍作修改即(ji)可(ke)，具有(you)成(cheng)本(ben)低(di)、開發(fa)周期短等(deng)優勢。

由于系統沒有(you)60Hz的(de)隔離變壓器，具有(you)尺寸小、輕便等特(te)點，可用(yong)于手機或其它(ta)手持設(she)備(bei)(bei)的(de)充(chong)電(dian)(dian)。所有(you)熱量均(jun)集中在(zai)交流適配器內(nei)，與內(nei)置(zhi)充(chong)電(dian)(dian)器的(de)手持設(she)備(bei)(bei)相比，不需要過多考慮散(san)熱問題。