本文介(jie)紹了一個單節鋰(li)離(li)子(Li+)電(dian)池(chi)的(de)(de)充(chong)電(dian)方案,該方案基于(yu)MAX5022芯片,適用于(yu)全球(qiu)的(de)(de)交流電(dian)源,而且無需軟件控制或微處理器干(gan)預就能實現所有(you)的(de)(de)充(chong)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(bi)須保證作用到(dao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池上的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)和(he)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)最(zui)(zui)(zui)大(da)值(zhi)在規定(ding)的(de)(de)(de)(de)范圍內,當(dang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)維持在最(zui)(zui)(zui)大(da)單節電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya),充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)下降(jiang)到(dao)一(yi)個較低(di)值(zhi)時,充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器終(zhong)止(zhi)充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)。Li+和(he)鉛酸(suan)(suan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器的(de)(de)(de)(de)區別在于所允許的(de)(de)(de)(de)充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)、最(zui)(zui)(zui)大(da)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya),另外鉛酸(suan)(suan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器一(yi)般具有浮(fu)充能力(浮(fu)充是為(wei)了補充鉛酸(suan)(suan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池的(de)(de)(de)(de)自放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian),而(er)對一(yi)個已經完(wan)全充滿的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池繼續輸(shu)入充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)的(de)(de)(de)(de)技術),Li+電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池的(de)(de)(de)(de)自放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)非常低(di),所以不需(xu)要浮(fu)充。   

    Li+電池   

    為(wei)(wei)便攜(xie)式(shi)產(chan)品選擇電池(chi)時,最重(zhong)要(yao)的(de)是(shi)高能量(liang)密度、尺寸、重(zhong)量(liang)和(he)價格。迄(qi)今為(wei)(wei)止,Li+電池(chi)在最為(wei)(wei)流行的(de)電池(chi)中(zhong)能夠(gou)提供最高的(de)體積和(he)能量(liang)密度,但其(qi)每瓦(wa)時的(de)成本也較(jiao)高。鉛酸電池(chi)的(de)每瓦(wa)時成本最低,但比較(jiao)笨(ben)重(zhong)。本文以Li+電池(chi)為(wei)(wei)例討論充(chong)電器的(de)設計,它在便攜(xie)式(shi)產(chan)品中(zhong)已(yi)經被普(pu)遍使用。我們(men)選擇780mA的(de)充(chong)電電流和(he)4.2V的(de)終(zhong)止電壓。

    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(chu)理器(qi)干預,輸(shu)入電(dian)(dian)(dian)源可以(yi)采用(yong)全(quan)球范(fan)圍(wei)的(de)(de)任(ren)何(he)交(jiao)流(liu)電(dian)(dian)(dian)源,省去了大體積60Hz變壓(ya)器(qi),而且可提供(gong)單(dan)節(jie)Li+電(dian)(dian)(dian)池(chi)所要求的(de)(de)精確的(de)(de)充電(dian)(dian)(dian)電(dian)(dian)(dian)壓(ya)和電(dian)(dian)(dian)流(liu)。充電(dian)(dian)(dian)器(qi)IC(U4)集成了全(quan)面的(de)(de)保(bao)護功能(neng)(電(dian)(dian)(dian)壓(ya)、電(dian)(dian)(dian)流(liu)和自保(bao)護),確保(bao)Li+電(dian)(dian)(dian)池(chi)的(de)(de)安全(quan)充電(dian)(dian)(dian)。   

    假設Li+電(dian)池(chi)帶有(you)(you)過(guo)流、過(guo)壓和過(guo)熱保護(hu),這(zhe)些保護(hu)特性在電(dian)池(chi)包里(li)是必需的(de),因為充電(dian)期(qi)間(jian)一旦電(dian)路出現故障,Li+電(dian)池(chi)將存在很大的(de)危險性,過(guo)流、過(guo)壓或溫度過(guo)高都有(you)(you)可能造成Li+電(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,內置調整管,結(jie)合(he)其熱控制環路提供快速充電。   

    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所示充電器的電源端(duan)MAX5022評(ping)估(gu)板不需要作任何修改,隔離端(duan)只需對評(ping)估(gu)板的電阻值稍作修改。   

    對于更大(da)(da)(da)功(gong)率的(de)應(ying)用(yong),只需選(xuan)取功(gong)率較(jiao)大(da)(da)(da)的(de)開(kai)關管和(he)能(neng)夠處理更大(da)(da)(da)功(gong)率的(de)變壓器(qi)即可。圖(tu)1所(suo)示電(dian)路要求(qiu)充(chong)電(dian)電(dian)流(liu)為(wei)780mA,最大(da)(da)(da)電(dian)池電(dian)壓為(wei)4.2V。對充(chong)電(dian)電(dian)源的(de)要求(qiu)與MAX5022評估(gu)板所(suo)提供(gong)的(de)輸出相吻合(he)：5V@1A,因此,不需要更換評估(gu)板的(de)晶體管和(he)變壓器(qi)。U4采用(yong)SOT23封裝,所(suo)以,充(chong)電(dian)器(qi)的(de)尺(chi)(chi)寸(cun)在很大(da)(da)(da)程度上(shang)取決于應(ying)用(yong)對充(chong)電(dian)電(dian)源的(de)需求(qiu)。變壓器(qi)是(shi)電(dian)路中尺(chi)(chi)寸(cun)最大(da)(da)(da)的(de)元(yuan)件(jian),由于開(kai)關控制器(qi)的(de)工(gong)作頻率大(da)(da)(da)于250kHz,允許系統選(xuan)用(yong)小尺(chi)(chi)寸(cun)的(de)變壓器(qi)。    

    上(shang)電(dian)(dian)(dian)時,在(zai)(zai)開關控制(zhi)器(qi)工作之前,U1調節器(qi)直接通過泄(xie)漏電(dian)(dian)(dian)阻R1、R2從整(zheng)流(liu)通路吸取少量(liang)的(de)(de)(de)(de)電(dian)(dian)(dian)流(liu),兩個(ge)電(dian)(dian)(dian)阻阻值(zhi)保證每個(ge)電(dian)(dian)(dian)阻上(shang)的(de)(de)(de)(de)壓(ya)降不(bu)會超出每個(ge)電(dian)(dian)(dian)阻的(de)(de)(de)(de)容(rong)限,Vin引(yin)腳電(dian)(dian)(dian)壓(ya)不(bu)會超出其28V額定(ding)值(zhi)(30V為絕對最大值(zhi)),U1在(zai)(zai)輸入電(dian)(dian)(dian)壓(ya)達到24V時開始開關操作,并吸取更大的(de)(de)(de)(de)電(dian)(dian)(dian)流(liu)(高于R1、R2所能(neng)提(ti)供的(de)(de)(de)(de)電(dian)(dian)(dian)流(liu)),在(zai)(zai)變壓(ya)器(qi)副邊(bian)線圈能(neng)夠(gou)提(ti)供足夠(gou)的(de)(de)(de)(de)能(neng)量(liang)之前,額外的(de)(de)(de)(de)電(dian)(dian)(dian)流(liu)由Vin引(yin)腳的(de)(de)(de)(de)旁路電(dian)(dian)(dian)容(rong)供給(gei)。這種處理(li)方式能(neng)夠(gou)適合(he)較(jiao)寬(kuan)的(de)(de)(de)(de)輸入電(dian)(dian)(dian)壓(ya)范(fan)圍,對效(xiao)率的(de)(de)(de)(de)影響也(ye)很小(xiao),這是因(yin)為在(zai)(zai)啟動過程中只需極小(xiao)的(de)(de)(de)(de)供電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)(典(dian)型值(zhi)為50uA)。  

    為(wei)了防止(zhi)U1的(de)Vin引(yin)腳出現過壓(ya),可(ke)(ke)以在Vin與地(di)之(zhi)間接一(yi)個(ge)26V至30V的(de)齊納管,電(dian)壓(ya)值(zhi)需(xu)要高于(yu)欠(qian)壓(ya)鎖存電(dian)壓(ya)的(de)最(zui)大值(zhi),低(di)于(yu)該引(yin)腳所(suo)允許的(de)最(zui)大絕對(dui)電(dian)壓(ya)。如(ru)果器件沒有啟動正(zheng)常的(de)開(kai)關操作,則(ze)可(ke)(ke)認為(wei)電(dian)路發(fa)生(sheng)了某些(xie)問題,所(suo)以,這一(yi)保護措(cuo)施對(dui)于(yu)U1來(lai)說(shuo)并(bing)不是必需(xu)的(de)。    

    U1是一款(kuan)電流(liu)模式PWM控(kong)制(zhi)器(qi)(qi),具(ju)(ju)有逐周(zhou)期限流(liu)電路,必要時其占空(kong)比(bi)可以提高(gao)到75%,非常適(shi)合圖1所示的(de)反激拓撲。原邊開關電流(liu)檢測采(cai)用(yong)尺寸為1206的(de)1.78Ω電阻(R7),該電阻限制(zhi)了變壓(ya)器(qi)(qi)原邊的(de)最(zui)(zui)大電流(liu)。固定開關頻率和最(zui)(zui)大占空(kong)比(bi)限制(zhi)使得最(zui)(zui)大限流(liu)具(ju)(ju)有輸出(chu)短路保(bao)護功能。  

    一旦短(duan)路(lu)輸出(chu)造成電路(lu)達到了上(shang)述限制條件,第三(san)組線圈將無法提供10V以上(shang)的電壓(UVLO的低電壓門限),U1將再次進入啟動(dong)狀態,停止開關操(cao)作(zuo)。短(duan)路(lu)故(gu)障解除后系統會自動(dong)恢復正(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有助于降低功率損耗(hao)。  

    當調(diao)(diao)節點電(dian)壓(ya)高于(yu)1.24V時,并(bing)聯(lian)型穩壓(ya)器將吸入(ru)(ru)電(dian)流,導通(tong)(tong)光(guang)(guang)耦LED,原邊的(de)光(guang)(guang)敏晶體(ti)(ti)管將處于(yu)發光(guang)(guang)狀態。U1的(de)OPTO輸(shu)入(ru)(ru)引(yin)腳用于(yu)直(zhi)接(jie)(jie)連(lian)接(jie)(jie)光(guang)(guang)敏晶體(ti)(ti)管,以(yi)極其簡(jian)單(dan)的(de)方式構成(cheng)閉(bi)合(he)環(huan)(huan)路。光(guang)(guang)敏晶體(ti)(ti)管導通(tong)(tong)時,OPTO引(yin)腳被拉(la)低,內部比(bi)(bi)較器在(zai)它的(de)一個(ge)輸(shu)入(ru)(ru)端檢測(ce)到較低的(de)電(dian)壓(ya),比(bi)(bi)較器的(de)另一輸(shu)入(ru)(ru)電(dian)流檢測(ce)引(yin)腳(CS)的(de)峰值電(dian)壓(ya),OPTO引(yin)腳的(de)低電(dian)壓(ya)將在(zai)每個(ge)周(zhou)期直(zhi)接(jie)(jie)限制功率MOSFETN1的(de)峰值電(dian)流。由于(yu)U1是一個(ge)PWM控(kong)制器,電(dian)流限制使得電(dian)壓(ya)調(diao)(diao)節受作用在(zai)N1上的(de)占空(kong)比(bi)(bi)(導通(tong)(tong)時間與斷開時間的(de)比(bi)(bi)值)的(de)影響,環(huan)(huan)路穩定(ding)性(xing)通(tong)(tong)過調(diao)(diao)節R11和C12來保(bao)證。R10和C5(連(lian)接(jie)(jie)在(zai)U3的(de)第3和第4引(yin)腳)可消除穩定(ding)工作情況(kuang)下的(de)噪聲干擾。   

    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)(dian)(dian)器(qi),采用(yong)線性充(chong)(chong)電(dian)(dian)(dian)(dian)模式,內置(zhi)調整管和電(dian)(dian)(dian)(dian)流(liu)檢測電(dian)(dian)(dian)(dian)路。另外,它還集(ji)成了一個(ge)反向抑制(zhi)二極管,在(zai)(zai)去掉輸(shu)入電(dian)(dian)(dian)(dian)源時可(ke)防止電(dian)(dian)(dian)(dian)池(chi)方向放電(dian)(dian)(dian)(dian)。接(jie)入一個(ge)深(shen)度(du)放電(dian)(dian)(dian)(dian)的(de)電(dian)(dian)(dian)(dian)池(chi)時,U4能夠檢測到(dao)低電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓(ya)(低于2.5V),這樣,U4將首先啟(qi)動涓(juan)流(liu)充(chong)(chong)電(dian)(dian)(dian)(dian)過程,在(zai)(zai)快充(chong)(chong)周(zhou)期的(de)1/10時間(jian)段(duan)對電(dian)(dian)(dian)(dian)池(chi)進行涓(juan)流(liu)充(chong)(chong)電(dian)(dian)(dian)(dian),直到(dao)電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓(ya)上升(sheng)(sheng)到(dao)2.5V以上。在(zai)(zai)涓(juan)流(liu)充(chong)(chong)電(dian)(dian)(dian)(dian)到(dao)快速(su)充(chong)(chong)電(dian)(dian)(dian)(dian)轉換(huan)時,軟啟(qi)動電(dian)(dian)(dian)(dian)路迫使(shi)充(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)在(zai)(zai)3ms時間(jian)內緩(huan)慢上升(sheng)(sheng),避免較大的(de)浪涌電(dian)(dian)(dian)(dian)流(liu)拉低輸(shu)入電(dian)(dian)(dian)(dian)源。   

   上(shang)述功能的(de)實(shi)施不需要(yao)微控制器的(de)監管,而且,U4還具有可(ke)編程(cheng)的(de)溫(wen)度限制功能,我們會(hui)在下面就這一點進行詳細(xi)討(tao)論。用一個電(dian)(dian)阻(zu)設置最大充電(dian)(dian)電(dian)(dian)流(liu)(liu)(liu),電(dian)(dian)阻(zu)兩端的(de)電(dian)(dian)壓(ya)與充電(dian)(dian)電(dian)(dian)流(liu)(liu)(liu)成(cheng)正(zheng)比,通(tong)過監視(shi)該(gai)電(dian)(dian)壓(ya)即可(ke)連續跟蹤(zong)充電(dian)(dian)電(dian)(dian)流(liu)(liu)(liu),更為復雜的(de)系(xi)統(tong)也可(ke)以采用ADC跟蹤(zong)監視(shi)充電(dian)(dian)電(dian)(dian)流(liu)(liu)(liu)。   

    Maxim還(huan)提供有(you)另一款(kuan)與MAX1507非(fei)常(chang)相似的充(chong)電器―MAX1555,這(zhe)款(kuan)IC能夠(gou)在沒有(you)交流電源(yuan)(yuan)的情況下從USB主機(ji)獲得(de)充(chong)電電源(yuan)(yuan)。  

    電流調節

    如果把Li+電(dian)(dian)(dian)池(chi)直(zhi)接(jie)與一個低阻電(dian)(dian)(dian)壓源連(lian)接(jie),充電(dian)(dian)(dian)電(dian)(dian)(dian)流將(jiang)不受任何限制,這種狀(zhuang)況會對(dui)電(dian)(dian)(dian)池(chi)及(ji)其周圍環境造(zao)成(cheng)災難(nan)性的損害,從(cong)Li+電(dian)(dian)(dian)池(chi)的安全性考慮,必須將(jiang)其充電(dian)(dian)(dian)電(dian)(dian)(dian)流限制在額定范(fan)圍以內。U4根據連(lian)接(jie)在ISET(第5引腳)的電(dian)(dian)(dian)阻限制充電(dian)(dian)(dian)電(dian)(dian)(dian)流,圖1電(dian)(dian)(dian)路中選(xuan)用了一個1.87kΩ的電(dian)(dian)(dian)阻,將(jiang)最大(da)電(dian)(dian)(dian)流設置在780mA:

    Ifast=1461V/Rset=1461/1870≈780mA

    電(dian)(dian)池電(dian)(dian)壓最(zui)小時,由于(yu)輸(shu)(shu)入穩定在4.5V,U4輸(shu)(shu)入與輸(shu)(shu)出的壓差最(zui)大(da),如果此時電(dian)(dian)池是以最(zui)大(da)電(dian)(dian)流(liu)充電(dian)(dian),U4功(gong)耗將達到最(zui)大(da)值。例如,假設電(dian)(dian)池電(dian)(dian)壓為2.5V,則對應(ying)的功(gong)耗為：

    P=IV=(0.78A)×(4.5V-2.5V)=1.56W

    U4封裝(8引腳(jiao)薄型DFN-EP,EP表示裸露焊盤(pan))在(zai)溫度高達(da)70℃時可(ke)以(yi)耗散的(de)(de)最(zui)(zui)大功率是1.951W,這樣,在(zai)整個電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)范(fan)圍內以(yi)780mA的(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)應(ying)該沒(mei)有任何問(wen)題。但是,如(ru)果IC管芯由(you)于較高的(de)(de)環境溫度或不良的(de)(de)散熱條件(jian)(jian)達(da)到(dao)了70℃,U4內部的(de)(de)熱調節環路將自動降(jiang)低充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu),這一過程有別于其它(ta)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)IC中(zhong)的(de)(de)“熱關斷”,“熱關斷”方式(shi)只(zhi)是簡單地(di)中(zhong)止(zhi)芯片(pian)中(zhong)的(de)(de)所有電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路工作,直到(dao)晶(jing)片(pian)溫度降(jiang)至規定(ding)的(de)(de)數值。MAX1507所采用的(de)(de)方案能夠保(bao)證在(zai)芯片(pian)溫度不超出最(zui)(zui)大限制范(fan)圍的(de)(de)條件(jian)(jian)下提供最(zui)(zui)快的(de)(de)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)速(su)率,隨著電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)的(de)(de)升高,充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)在(zai)保(bao)持最(zui)(zui)高溫度的(de)(de)前提下逐漸升高,直到(dao)達(da)到(dao)最(zui)(zui)大充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)。在(zai)此之(zhi)后(hou),充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器在(zai)滿足芯片(pian)溫度限制的(de)(de)條件(jian)(jian)下將以(yi)最(zui)(zui)大充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)對(dui)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian),直到(dao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)達(da)到(dao)內部設(she)置的(de)(de)4.2V±1%。電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)達(da)到(dao)門(men)限值后(hou),U4將輸出電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)保(bao)持在(zai)4.2V,從(cong)而使(shi)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)開始下降(jiang)。充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)降(jiang)低到(dao)快充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)的(de)(de)10%時,充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)指示燈關閉,表示充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)周期結束(shu)。

    如果充(chong)電過程中(zhong)輸出(chu)負(fu)載增大(da),例如,用戶在(zai)電池充(chong)電時啟(qi)動了(le)其它電路(lu),U4允(yun)許返(fan)回到限流模式。

    測試結果

    圖1電(dian)路(lu)輸入為(wei)110V交流(liu)電(dian)源,輸出為(wei)4.14V、740mA負(fu)載,效率可達到46%。當(dang)負(fu)載電(dian)流(liu)增大使電(dian)池電(dian)壓(ya)降至2.7V時(shi),電(dian)路(lu)將輸出電(dian)流(liu)限(xian)制在(zai)802mA,比設置的780mA限(xian)流(liu)門限(xian)提高不到3%,在(zai)規定的參(can)數(shu)指(zhi)標(biao)以內。MAX1507允許±8%的變(bian)化(hua)。當(dang)電(dian)池電(dian)壓(ya)拉(la)至2.5V時(shi),輸出電(dian)流(liu)降至76.1mA,在(zai)78mA(780mA的1/10)的3%以內。U4數(shu)據手冊中規定允許±5%的范圍。輸出短路(lu)時(shi)電(dian)路(lu)還允許返回到該模式。

    空(kong)載時(shi),輸出穩定在4.2V左右,形成(cheng)一個三角波,在U4輸出接一個負載電阻可以消(xiao)除這個偏(pian)差,保持其連續(xu)性,但(dan)會降低(di)系統效率,但(dan)對于交流(liu)電供電系統來說(shuo),效率并不重要。

    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(fang)案不需(xu)要任何額外的(de)控制電路(如微控制器(qi)的(de)干預(yu)),也不需(xu)要軟件開(kai)發(fa)。硬件開(kai)發(fa)只需(xu)對Maxim提供的(de)標準評估板稍(shao)作(zuo)修改即(ji)可(ke),具有成本低、開(kai)發(fa)周期短(duan)等優(you)勢(shi)。

    由于系(xi)統(tong)沒有60Hz的隔離變壓器(qi)(qi),具有尺寸小、輕(qing)便等(deng)特點,可用于手機或其它手持(chi)設備(bei)的充(chong)電(dian)。所(suo)有熱量均集中在交流適配器(qi)(qi)內,與內置(zhi)充(chong)電(dian)器(qi)(qi)的手持(chi)設備(bei)相比(bi),不需要過多考慮散熱問題。