本文介紹(shao)了(le)一個單(dan)節鋰離子(Li+)電(dian)池的充電(dian)方(fang)(fang)案,該方(fang)(fang)案基于(yu)MAX5022芯片,適(shi)用于(yu)全球的交流電(dian)源,而且無需軟(ruan)件控(kong)制或微處理器干預就(jiu)能實現(xian)所(suo)有的充電(dian)功能。   

    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(de)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(zheng)作(zuo)用(yong)到(dao)電(dian)池上的(de)(de)電(dian)壓(ya)和電(dian)流最大值(zhi)(zhi)在(zai)規定的(de)(de)范圍內,當電(dian)池電(dian)壓(ya)維持在(zai)最大單(dan)節電(dian)池電(dian)壓(ya),充(chong)(chong)(chong)(chong)電(dian)電(dian)流下降(jiang)到(dao)一個較低值(zhi)(zhi)時,充(chong)(chong)(chong)(chong)電(dian)器(qi)終止充(chong)(chong)(chong)(chong)電(dian)。Li+和鉛(qian)酸(suan)電(dian)池充(chong)(chong)(chong)(chong)電(dian)器(qi)的(de)(de)區別在(zai)于所(suo)允許的(de)(de)充(chong)(chong)(chong)(chong)電(dian)電(dian)流、最大電(dian)池電(dian)壓(ya),另外鉛(qian)酸(suan)電(dian)池充(chong)(chong)(chong)(chong)電(dian)器(qi)一般具有浮充(chong)(chong)(chong)(chong)能(neng)力(浮充(chong)(chong)(chong)(chong)是為了補充(chong)(chong)(chong)(chong)鉛(qian)酸(suan)電(dian)池的(de)(de)自放電(dian),而(er)對一個已經完全充(chong)(chong)(chong)(chong)滿的(de)(de)電(dian)池繼續輸入充(chong)(chong)(chong)(chong)電(dian)電(dian)流的(de)(de)技術(shu)),Li+電(dian)池的(de)(de)自放電(dian)非常(chang)低,所(suo)以不需要浮充(chong)(chong)(chong)(chong)。   

    Li+電(dian)池(chi)   

    為便攜(xie)式(shi)產品(pin)選(xuan)擇(ze)電(dian)(dian)(dian)(dian)池(chi)時(shi)(shi),最(zui)重(zhong)要的是高能(neng)量密度、尺寸、重(zhong)量和價格(ge)。迄今為止(zhi),Li+電(dian)(dian)(dian)(dian)池(chi)在最(zui)為流行的電(dian)(dian)(dian)(dian)池(chi)中能(neng)夠提供(gong)最(zui)高的體積和能(neng)量密度,但(dan)其每(mei)瓦時(shi)(shi)的成(cheng)本(ben)也較(jiao)高。鉛酸電(dian)(dian)(dian)(dian)池(chi)的每(mei)瓦時(shi)(shi)成(cheng)本(ben)最(zui)低,但(dan)比較(jiao)笨重(zhong)。本(ben)文以Li+電(dian)(dian)(dian)(dian)池(chi)為例(li)討論充(chong)(chong)電(dian)(dian)(dian)(dian)器的設計,它在便攜(xie)式(shi)產品(pin)中已經被普遍(bian)使用。我(wo)們選(xuan)擇(ze)780mA的充(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流和4.2V的終(zhong)止(zhi)電(dian)(dian)(dian)(dian)壓。

    圖1：通用輸入交流電壓的離線式Li+電池充電器。   

    圖1所示充電器無需微(wei)處理器干預(yu),輸入(ru)電(dian)(dian)(dian)源(yuan)可以采用全(quan)球范(fan)圍的(de)任(ren)何交流電(dian)(dian)(dian)源(yuan),省(sheng)去了大體(ti)積60Hz變(bian)壓(ya)器,而且可提(ti)供單節Li+電(dian)(dian)(dian)池所要求的(de)精確(que)的(de)充電(dian)(dian)(dian)電(dian)(dian)(dian)壓(ya)和電(dian)(dian)(dian)流。充電(dian)(dian)(dian)器IC(U4)集成(cheng)了全(quan)面的(de)保護(hu)功(gong)能(電(dian)(dian)(dian)壓(ya)、電(dian)(dian)(dian)流和自保護(hu)),確(que)保Li+電(dian)(dian)(dian)池的(de)安全(quan)充電(dian)(dian)(dian)。   

    假設(she)Li+電(dian)(dian)池(chi)帶(dai)有(you)過流(liu)、過壓和過熱保(bao)護,這些保(bao)護特性在電(dian)(dian)池(chi)包里是必需的(de),因為充電(dian)(dian)期間一旦電(dian)(dian)路出現故障,Li+電(dian)(dian)池(chi)將存(cun)在很大的(de)危險(xian)性,過流(liu)、過壓或溫度過高(gao)都有(you)可能造成(cheng)Li+電(dian)(dian)池(chi)爆炸。   

    電路描述    

    圖1包括兩顆主器件(U1和U4)、精密基準(U3)和光耦(U2)。U1是離線式開關控制器,配合隔離變壓器、光耦和基準產生隔離的穩壓直流輸出。直流電壓供給電池充電器(U4),U4提供電流限制、電壓調節、電池充電和充電指示等功能。本方案中,U4是一款簡單的線性充電器,內置調(diao)整(zheng)管,結合其(qi)熱控制環路(lu)提供快速(su)充電。   

    MAX5022的特性    

    U1(MAX5022)在寬輸入范圍的應用中很容易設計,本文所提供的方案要求能夠滿足全球各地的交流電源規格,這里假設通用的輸入電源電壓范圍為85VAC至265VAC。MAX5022的輸入范圍指標以及評估板電路的輸入范圍恰好滿足這一需求。對于圖1所示充電器的(de)電(dian)源端MAX5022評(ping)估板不需要作任何修改(gai),隔離端只需對(dui)評(ping)估板的(de)電(dian)阻(zu)值(zhi)稍(shao)作修改(gai)。   

    對于(yu)更(geng)(geng)大(da)(da)功(gong)率(lv)的(de)(de)(de)應(ying)用(yong)(yong),只(zhi)需選(xuan)取功(gong)率(lv)較大(da)(da)的(de)(de)(de)開關(guan)(guan)管和能夠處(chu)理更(geng)(geng)大(da)(da)功(gong)率(lv)的(de)(de)(de)變(bian)(bian)壓(ya)(ya)器即可(ke)。圖1所(suo)示電(dian)(dian)路要(yao)求(qiu)充電(dian)(dian)電(dian)(dian)流為780mA,最大(da)(da)電(dian)(dian)池電(dian)(dian)壓(ya)(ya)為4.2V。對充電(dian)(dian)電(dian)(dian)源的(de)(de)(de)要(yao)求(qiu)與(yu)MAX5022評估板(ban)所(suo)提供的(de)(de)(de)輸出(chu)相吻合：5V@1A,因此,不需要(yao)更(geng)(geng)換評估板(ban)的(de)(de)(de)晶體(ti)管和變(bian)(bian)壓(ya)(ya)器。U4采用(yong)(yong)SOT23封裝,所(suo)以,充電(dian)(dian)器的(de)(de)(de)尺(chi)寸在很大(da)(da)程度上取決于(yu)應(ying)用(yong)(yong)對充電(dian)(dian)電(dian)(dian)源的(de)(de)(de)需求(qiu)。變(bian)(bian)壓(ya)(ya)器是(shi)電(dian)(dian)路中尺(chi)寸最大(da)(da)的(de)(de)(de)元件(jian),由于(yu)開關(guan)(guan)控制器的(de)(de)(de)工作頻(pin)率(lv)大(da)(da)于(yu)250kHz,允許系統選(xuan)用(yong)(yong)小尺(chi)寸的(de)(de)(de)變(bian)(bian)壓(ya)(ya)器。    

    上電(dian)(dian)(dian)(dian)(dian)時,在(zai)(zai)開關控制器工作之前(qian)(qian),U1調節(jie)器直接(jie)通過泄漏電(dian)(dian)(dian)(dian)(dian)阻(zu)R1、R2從整流(liu)(liu)(liu)通路吸(xi)取少量的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu),兩個(ge)電(dian)(dian)(dian)(dian)(dian)阻(zu)阻(zu)值(zhi)(zhi)保(bao)證每(mei)個(ge)電(dian)(dian)(dian)(dian)(dian)阻(zu)上的(de)(de)(de)(de)壓(ya)(ya)降不會超出(chu)每(mei)個(ge)電(dian)(dian)(dian)(dian)(dian)阻(zu)的(de)(de)(de)(de)容限,Vin引腳電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)不會超出(chu)其28V額(e)定(ding)值(zhi)(zhi)(30V為(wei)絕對最(zui)大值(zhi)(zhi)),U1在(zai)(zai)輸(shu)入電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)達到24V時開始開關操作,并吸(xi)取更大的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(高于R1、R2所(suo)能(neng)提供的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)),在(zai)(zai)變壓(ya)(ya)器副邊線圈(quan)能(neng)夠(gou)提供足夠(gou)的(de)(de)(de)(de)能(neng)量之前(qian)(qian),額(e)外的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)由Vin引腳的(de)(de)(de)(de)旁路電(dian)(dian)(dian)(dian)(dian)容供給。這(zhe)種處理方式(shi)能(neng)夠(gou)適合(he)較寬(kuan)的(de)(de)(de)(de)輸(shu)入電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)范(fan)圍,對效(xiao)率的(de)(de)(de)(de)影響(xiang)也很小,這(zhe)是(shi)因為(wei)在(zai)(zai)啟動過程中只需極小的(de)(de)(de)(de)供電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(典型(xing)值(zhi)(zhi)為(wei)50uA)。  

    為(wei)了防止U1的(de)Vin引腳出現過(guo)壓,可(ke)以(yi)在(zai)Vin與地(di)之間接一個26V至30V的(de)齊納管,電(dian)壓值需要高于欠壓鎖存電(dian)壓的(de)最(zui)大值,低于該引腳所允(yun)許的(de)最(zui)大絕對(dui)電(dian)壓。如(ru)果器件沒有(you)啟動正常的(de)開關操(cao)作,則可(ke)認為(wei)電(dian)路發生了某些問題,所以(yi),這一保護(hu)措施對(dui)于U1來說并不是必需的(de)。    

    U1是一款(kuan)電(dian)流模式(shi)PWM控制(zhi)(zhi)器,具(ju)(ju)有(you)逐周期限(xian)流電(dian)路(lu),必要(yao)時其占(zhan)空比可以提高到75%,非(fei)常(chang)適合圖1所示的(de)反激拓撲(pu)。原邊開(kai)關電(dian)流檢測(ce)采(cai)用尺寸為1206的(de)1.78Ω電(dian)阻(R7),該電(dian)阻限(xian)制(zhi)(zhi)了變(bian)壓器原邊的(de)最(zui)大電(dian)流。固定開(kai)關頻率(lv)和最(zui)大占(zhan)空比限(xian)制(zhi)(zhi)使得最(zui)大限(xian)流具(ju)(ju)有(you)輸出短路(lu)保護(hu)功能。  

    一旦短(duan)路(lu)輸(shu)出造成電路(lu)達(da)到(dao)了上(shang)述限(xian)制條(tiao)件,第三組線圈將(jiang)無法提供(gong)10V以上(shang)的電壓(UVLO的低電壓門(men)限(xian)),U1將(jiang)再(zai)次進(jin)入啟(qi)動狀態,停止開關操作(zuo)。短(duan)路(lu)故(gu)障解除后(hou)系統會自動恢復(fu)正(zheng)常的工作(zuo)狀態。  

    電壓調節
 
    變壓器副邊的電壓經過整流、濾波,然后由R4、R5分壓在并聯型穩壓器U3的調節點產生1.24V(對應于4.5V輸出)的反饋電壓。為保證電池充電器正常工作,4.5V是最低電壓(包括容差)。因為充電器采用的是線性結構,充電效率與輸入、輸出電壓的差值成反比,用最低電壓為充電器供電(dian)有(you)助于降(jiang)低功率損耗。  

    當(dang)調節(jie)點(dian)電(dian)(dian)壓高于(yu)1.24V時,并聯型穩(wen)(wen)壓器(qi)將吸入(ru)電(dian)(dian)流(liu)(liu),導(dao)通光(guang)(guang)(guang)耦LED,原(yuan)邊的(de)(de)光(guang)(guang)(guang)敏(min)(min)晶(jing)(jing)體(ti)管將處于(yu)發光(guang)(guang)(guang)狀(zhuang)態。U1的(de)(de)OPTO輸入(ru)引腳用(yong)于(yu)直(zhi)接連(lian)接光(guang)(guang)(guang)敏(min)(min)晶(jing)(jing)體(ti)管,以極其簡(jian)單的(de)(de)方式構成閉(bi)合(he)環路。光(guang)(guang)(guang)敏(min)(min)晶(jing)(jing)體(ti)管導(dao)通時,OPTO引腳被拉低(di),內部比較(jiao)器(qi)在它的(de)(de)一(yi)個輸入(ru)端檢(jian)測(ce)到較(jiao)低(di)的(de)(de)電(dian)(dian)壓,比較(jiao)器(qi)的(de)(de)另一(yi)輸入(ru)電(dian)(dian)流(liu)(liu)檢(jian)測(ce)引腳(CS)的(de)(de)峰值電(dian)(dian)壓,OPTO引腳的(de)(de)低(di)電(dian)(dian)壓將在每個周(zhou)期直(zhi)接限(xian)制(zhi)(zhi)功率MOSFETN1的(de)(de)峰值電(dian)(dian)流(liu)(liu)。由于(yu)U1是一(yi)個PWM控(kong)制(zhi)(zhi)器(qi),電(dian)(dian)流(liu)(liu)限(xian)制(zhi)(zhi)使得電(dian)(dian)壓調節(jie)受作(zuo)(zuo)用(yong)在N1上的(de)(de)占空比(導(dao)通時間與(yu)斷開時間的(de)(de)比值)的(de)(de)影響,環路穩(wen)(wen)定性通過調節(jie)R11和(he)C12來保證(zheng)。R10和(he)C5(連(lian)接在U3的(de)(de)第3和(he)第4引腳)可消除(chu)穩(wen)(wen)定工作(zuo)(zuo)情(qing)況下的(de)(de)噪(zao)聲干(gan)擾。   

    U4的工作特點   

    U4是一款用于單節Li+電池充電、功能完備的(de)充(chong)(chong)電(dian)(dian)器,采用(yong)線性(xing)充(chong)(chong)電(dian)(dian)模式,內置(zhi)調整管(guan)(guan)和(he)電(dian)(dian)流(liu)(liu)(liu)檢(jian)測電(dian)(dian)路(lu)。另外,它還(huan)集成了一個反向抑(yi)制二極管(guan)(guan),在(zai)去掉輸(shu)入電(dian)(dian)源(yuan)(yuan)時(shi)可防止電(dian)(dian)池(chi)(chi)(chi)方向放(fang)電(dian)(dian)。接(jie)入一個深度放(fang)電(dian)(dian)的(de)電(dian)(dian)池(chi)(chi)(chi)時(shi),U4能夠檢(jian)測到(dao)(dao)低電(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)壓(低于(yu)2.5V),這樣,U4將首先啟動(dong)涓(juan)流(liu)(liu)(liu)充(chong)(chong)電(dian)(dian)過程,在(zai)快充(chong)(chong)周期的(de)1/10時(shi)間(jian)(jian)段對電(dian)(dian)池(chi)(chi)(chi)進行涓(juan)流(liu)(liu)(liu)充(chong)(chong)電(dian)(dian),直到(dao)(dao)電(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)壓上升到(dao)(dao)2.5V以(yi)上。在(zai)涓(juan)流(liu)(liu)(liu)充(chong)(chong)電(dian)(dian)到(dao)(dao)快速充(chong)(chong)電(dian)(dian)轉換(huan)時(shi),軟(ruan)啟動(dong)電(dian)(dian)路(lu)迫使充(chong)(chong)電(dian)(dian)電(dian)(dian)流(liu)(liu)(liu)在(zai)3ms時(shi)間(jian)(jian)內緩慢上升,避免(mian)較大的(de)浪涌電(dian)(dian)流(liu)(liu)(liu)拉低輸(shu)入電(dian)(dian)源(yuan)(yuan)。   

   上(shang)述功能的實施不(bu)需要微(wei)控(kong)制(zhi)器(qi)的監(jian)管,而且,U4還具有可編程的溫(wen)度(du)限制(zhi)功能,我(wo)們會(hui)在下面(mian)就這一點進行詳細討(tao)論。用一個電(dian)(dian)阻設(she)置最大(da)充(chong)(chong)電(dian)(dian)電(dian)(dian)流(liu)(liu),電(dian)(dian)阻兩端的電(dian)(dian)壓與(yu)充(chong)(chong)電(dian)(dian)電(dian)(dian)流(liu)(liu)成正(zheng)比,通(tong)過監(jian)視(shi)該電(dian)(dian)壓即可連續跟蹤(zong)充(chong)(chong)電(dian)(dian)電(dian)(dian)流(liu)(liu),更為復雜的系統也可以采用ADC跟蹤(zong)監(jian)視(shi)充(chong)(chong)電(dian)(dian)電(dian)(dian)流(liu)(liu)。   

    Maxim還提供(gong)有另一(yi)款與MAX1507非常相似的充電器―MAX1555,這款IC能夠(gou)在(zai)沒有交流電源的情況下從(cong)USB主機獲(huo)得充電電源。  

    電流調節

    如果把Li+電(dian)(dian)(dian)池直接與一(yi)個(ge)低(di)阻電(dian)(dian)(dian)壓源(yuan)連接,充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)將(jiang)不受任何限(xian)(xian)制,這種狀況會對電(dian)(dian)(dian)池及其周圍環境造成(cheng)災難(nan)性的(de)損害,從Li+電(dian)(dian)(dian)池的(de)安全性考慮,必須將(jiang)其充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)限(xian)(xian)制在(zai)額定范圍以(yi)內。U4根據(ju)連接在(zai)ISET(第5引腳)的(de)電(dian)(dian)(dian)阻限(xian)(xian)制充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu),圖1電(dian)(dian)(dian)路(lu)中選用了一(yi)個(ge)1.87kΩ的(de)電(dian)(dian)(dian)阻,將(jiang)最大電(dian)(dian)(dian)流(liu)設置在(zai)780mA:

    Ifast=1461V/Rset=1461/1870≈780mA

    電(dian)(dian)池(chi)電(dian)(dian)壓最小(xiao)時,由于輸入(ru)穩定在4.5V,U4輸入(ru)與(yu)輸出的壓差(cha)最大(da),如果此時電(dian)(dian)池(chi)是(shi)以(yi)最大(da)電(dian)(dian)流充電(dian)(dian),U4功耗將(jiang)達到最大(da)值。例如,假設電(dian)(dian)池(chi)電(dian)(dian)壓為(wei)2.5V,則(ze)對應的功耗為(wei)：

    P=IV=(0.78A)×(4.5V-2.5V)=1.56W

    U4封(feng)裝(8引(yin)腳薄型DFN-EP,EP表(biao)(biao)示裸(luo)露焊盤)在(zai)溫(wen)(wen)度(du)高(gao)(gao)達(da)70℃時可以(yi)耗散的(de)(de)最(zui)(zui)(zui)(zui)大(da)(da)功率(lv)是(shi)1.951W,這樣,在(zai)整個(ge)電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓范(fan)圍(wei)內以(yi)780mA的(de)(de)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)應該(gai)沒有(you)任何問題。但是(shi),如果IC管芯由于(yu)較高(gao)(gao)的(de)(de)環境溫(wen)(wen)度(du)或(huo)不良的(de)(de)散熱(re)(re)條件達(da)到(dao)了70℃,U4內部的(de)(de)熱(re)(re)調節環路(lu)將(jiang)自(zi)動降低充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu),這一過程有(you)別于(yu)其它充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)IC中的(de)(de)“熱(re)(re)關(guan)斷”,“熱(re)(re)關(guan)斷”方式只是(shi)簡單地中止芯片中的(de)(de)所有(you)電(dian)(dian)(dian)(dian)路(lu)工作,直到(dao)晶片溫(wen)(wen)度(du)降至規定的(de)(de)數(shu)值。MAX1507所采(cai)用的(de)(de)方案能夠保證在(zai)芯片溫(wen)(wen)度(du)不超(chao)出最(zui)(zui)(zui)(zui)大(da)(da)限制范(fan)圍(wei)的(de)(de)條件下(xia)提(ti)供最(zui)(zui)(zui)(zui)快的(de)(de)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)速率(lv),隨著(zhu)電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓的(de)(de)升(sheng)高(gao)(gao),充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)在(zai)保持(chi)最(zui)(zui)(zui)(zui)高(gao)(gao)溫(wen)(wen)度(du)的(de)(de)前提(ti)下(xia)逐漸升(sheng)高(gao)(gao),直到(dao)達(da)到(dao)最(zui)(zui)(zui)(zui)大(da)(da)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)。在(zai)此(ci)之后,充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)器(qi)在(zai)滿足(zu)芯片溫(wen)(wen)度(du)限制的(de)(de)條件下(xia)將(jiang)以(yi)最(zui)(zui)(zui)(zui)大(da)(da)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)對電(dian)(dian)(dian)(dian)池充(chong)(chong)(chong)電(dian)(dian)(dian)(dian),直到(dao)電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓達(da)到(dao)內部設置的(de)(de)4.2V±1%。電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓達(da)到(dao)門限值后,U4將(jiang)輸出電(dian)(dian)(dian)(dian)壓保持(chi)在(zai)4.2V,從而使充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)開始下(xia)降。充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)降低到(dao)快充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)的(de)(de)10%時,充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)指示燈關(guan)閉(bi),表(biao)(biao)示充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)周期結束(shu)。

    如果充電過程中輸(shu)出負載(zai)增大,例如,用戶在電池充電時啟動了(le)其它電路(lu),U4允許返回到(dao)限流模式(shi)。

    測試結果

    圖(tu)1電(dian)路(lu)輸入為110V交流(liu)(liu)電(dian)源,輸出(chu)為4.14V、740mA負載,效率可達(da)到(dao)46%。當負載電(dian)流(liu)(liu)增大使電(dian)池電(dian)壓降至2.7V時,電(dian)路(lu)將輸出(chu)電(dian)流(liu)(liu)限制在802mA,比設(she)置的(de)780mA限流(liu)(liu)門限提高不到(dao)3%,在規(gui)(gui)定(ding)的(de)參數(shu)指標以內(nei)。MAX1507允(yun)許(xu)±8%的(de)變化。當電(dian)池電(dian)壓拉至2.5V時,輸出(chu)電(dian)流(liu)(liu)降至76.1mA,在78mA(780mA的(de)1/10)的(de)3%以內(nei)。U4數(shu)據手冊中規(gui)(gui)定(ding)允(yun)許(xu)±5%的(de)范(fan)圍。輸出(chu)短路(lu)時電(dian)路(lu)還允(yun)許(xu)返回(hui)到(dao)該(gai)模式。

    空載時,輸出穩定在4.2V左右(you),形成一個(ge)三角(jiao)波,在U4輸出接一個(ge)負載電(dian)阻可(ke)以(yi)消除(chu)這個(ge)偏差,保持其連續性,但(dan)會降低(di)系統效率(lv),但(dan)對于交流電(dian)供電(dian)系統來說,效率(lv)并不(bu)重要。

    本文小結

    利用電流模式PWM控制器(U1)構成隔離電源,結合Li+電池充電IC(U4)和其它元件可以構成一個由通用的交流電源供電的Li+電池充電器。該方案(an)不需(xu)要任何(he)額(e)外的控(kong)制電路(如微(wei)控(kong)制器的干預),也不需(xu)要軟件開(kai)(kai)發。硬件開(kai)(kai)發只(zhi)需(xu)對Maxim提供的標準(zhun)評估板(ban)稍作修(xiu)改即(ji)可(ke),具有成本低、開(kai)(kai)發周期(qi)短(duan)等優勢。

    由(you)于系統(tong)沒有60Hz的(de)隔離變(bian)壓器(qi)(qi),具有尺寸小、輕便等(deng)特點,可用(yong)于手機(ji)或其它手持設備(bei)的(de)充電(dian)。所(suo)有熱量均集(ji)中在(zai)交流適(shi)配器(qi)(qi)內,與內置充電(dian)器(qi)(qi)的(de)手持設備(bei)相比,不(bu)需要(yao)過多(duo)考(kao)慮散熱問題(ti)。