小型便攜式電子產品采用的鋰離子電池或鋰聚合物電池的容量較小，大部分在400～1000mAh范圍內，與之配套的充電器的最大充電電流為450～1000mAh。由于電流不大，一般采用線性充電器。

　　新型線性鋰離子電池充電器功能齊全(quan)、性能良(liang)好、電(dian)路簡單、占印制版面(mian)積小，價格低廉(lian)，整(zheng)個充電(dian)器(qi)可(ke)以(yi)在產(chan)品(pin)中。若采用(yong)USB端口(kou)充電(dian)，使用(yong)十分方便。

　　近年來，一些用電(dian)(dian)(dian)(dian)(dian)量稍大的(de)便(bian)攜(xie)式電(dian)(dian)(dian)(dian)(dian)子產品(如便(bian)攜(xie)式DVD、礦燈(deng)、攝像機、便(bian)攜(xie)式測量儀器、小(xiao)型電(dian)(dian)(dian)(dian)(dian)動工具等)往往采(cai)用1500mAh到5400mAh容量的(de)鋰離子電(dian)(dian)(dian)(dian)(dian)池。若采(cai)用500～1000mA充電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)充電(dian)(dian)(dian)(dian)(dian)器充電(dian)(dian)(dian)(dian)(dian)，則充電(dian)(dian)(dian)(dian)(dian)時間太長(chang)。若按0.5C充電(dian)(dian)(dian)(dian)(dian)率來充3000mAh及5400mA時的(de)電(dian)(dian)(dian)(dian)(dian)池時，其充電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)池的(de)容量要求為1500mA及2700mA。

　　有人提出：能否在1A線(xian)性(xing)充(chong)(chong)電(dian)(dian)器(qi)電(dian)(dian)路中加一(yi)(yi)個擴(kuo)流(liu)電(dian)(dian)路，使(shi)充(chong)(chong)電(dian)(dian)電(dian)(dian)流(liu)擴(kuo)大到2～2.5A，解決3000～5400mAh容量鋰離子(zi)電(dian)(dian)池的充(chong)(chong)電(dian)(dian)問題。如果擴(kuo)流(liu)的充(chong)(chong)電(dian)(dian)器(qi)性(xing)能不(bu)錯、電(dian)(dian)路簡(jian)單、成本不(bu)高，這(zhe)(zhe)是(shi)個好主意。筆者(zhe)就按這(zhe)(zhe)一(yi)(yi)思(si)路設(she)計一(yi)(yi)個擴(kuo)流(liu)電(dian)(dian)路。這(zhe)(zhe)電(dian)(dian)路采用型號為CN3056的1A線(xian)性(xing)充(chong)(chong)電(dian)(dian)器(qi)為基礎，另(ling)外加上擴(kuo)流(liu)電(dian)(dian)路及(ji)控制電(dian)(dian)路組成。

　　CN3056簡(jian)介

　　CN3056充(chong)電器已在本刊2006年(nian)12期及2007年(nian)電源(yuan)增刊上介(jie)紹過(“線性鋰二次電池充(chong)電器芯(xin)片CN3056”)。這里(li)僅作一簡(jian)介(jie)。

　　CN3056組成(cheng)的充電(dian)器按(an)恒(heng)流(liu)(liu)、恒(heng)壓模式充電(dian)，若充電(dian)電(dian)池電(dian)壓<3V，則(ze)有(you)小(xiao)電(dian)流(liu)(liu)預充電(dian)模式;充電(dian)電(dian)流(liu)(liu)可設(she)定，最大充電(dian)電(dian)流(liu)(liu)為1A;精電(dian)密度(du)4.2V ±1%、有(you)熱調節、欠壓鎖(suo)存及電(dian)池溫度(du)檢測、超(chao)溫保(bao)護(hu)及充電(dian)狀(zhuang)態和溫度(du)超(chao)差指示功能;10引腳小(xiao)尺寸DFN封裝(zhuang)(3mm×3mm)。

　　若充(chong)電(dian)(dian)(dian)(dian)率(lv)在0.5～1C之(zhi)間(jian)、電(dian)(dian)(dian)(dian)池的溫(wen)(wen)度(du)在0～45℃之(zhi)間(jian)(室(shi)溫(wen)(wen)充(chong)電(dian)(dian)(dian)(dian))，則CN3056充(chong)電(dian)(dian)(dian)(dian)器電(dian)(dian)(dian)(dian)路(lu)中可省去電(dian)(dian)(dian)(dian)池溫(wen)(wen)度(du)檢測電(dian)(dian)(dian)(dian)路(lu)及電(dian)(dian)(dian)(dian)池超溫(wen)(wen)指示電(dian)(dian)(dian)(dian)路(lu)(引腳TEMP及FAULT端接地(di))。VIN是(shi)電(dian)(dian)(dian)(dian)源(yuan)輸(shu)(shu)入(ru)端、CE是(shi)使(shi)能端，(高(gao)電(dian)(dian)(dian)(dian)平有效);RISET為充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流ICH設定電(dian)(dian)(dian)(dian)阻(zu)，RISET(Ω)=1800(V)/ICH(A);CHRG為充(chong)電(dian)(dian)(dian)(dian)狀態信號輸(shu)(shu)出(chu)端：充(chong)電(dian)(dian)(dian)(dian)時此(ci)端為高(gao)電(dian)(dian)(dian)(dian)平，LED亮(liang);充(chong)電(dian)(dian)(dian)(dian)結束時此(ci)端為高(gao)阻(zu)抗，LED滅;電(dian)(dian)(dian)(dian)池未裝入(ru)或接觸不(bu)良，LED閃亮(liang)。VIN一(yi)般取4.5～5V，10μF及6.8μF為輸(shu)(shu)入(ru)、輸(shu)(shu)出(chu)電(dian)(dian)(dian)(dian)容，保(bao)證充(chong)電(dian)(dian)(dian)(dian)器穩(wen)定工作(zuo)。

　　充電器擴流電路

　　充電(dian)(dian)(dian)器擴(kuo)流(liu)(liu)電(dian)(dian)(dian)路(lu)(lu)是在原充電(dian)(dian)(dian)器電(dian)(dian)(dian)路(lu)(lu)上加(jia)上擴(kuo)流(liu)(liu)電(dian)(dian)(dian)路(lu)(lu)組成的。擴(kuo)流(liu)(liu)電(dian)(dian)(dian)路(lu)(lu)由兩部(bu)(bu)分組成：擴(kuo)流(liu)(liu)部(bu)(bu)分及控(kong)制部(bu)(bu)分。采用(yong)CN3056充電(dian)(dian)(dian)器為基礎(chu)，加(jia)上擴(kuo)流(liu)(liu)部(bu)(bu)分及控(kong)制部(bu)(bu)分電(dian)(dian)(dian)路(lu)(lu)。現分別(bie)介紹(shao)其工作(zuo)原理。

　　1 擴(kuo)流部分(fen)電路

　　擴流部分電路。它由P溝道(dao)功率MOSFET(VT)、R及RP組(zu)成的(de)分壓器(qi)、肖特(te)(te)基二極管D4組(zu)成。利用分壓器(qi)調節P-MOSFET的(de)-VGS大小，使獲得所需擴流電流ID。P-MOSFET的(de)輸(shu)出特(te)(te)性(以Si9933DY為例)。在-VGS=2.1V、VDS>0.5V時(shi)，其輸(shu)出特(te)(te)性幾乎(hu)是一水平(ping)直(zhi)線;在不同的(de)VDS時(shi)，ID是恒流。從圖4也可以看出，在 -VGS增加時(shi)，ID也相應增加。

　　2 控制(zhi)部分(fen)電路

　　控制(zhi)部(bu)分電(dian)路的目的是要保持原(yuan)有的三階段(duan)充(chong)(chong)電(dian)模(mo)式(shi)，在預充(chong)(chong)電(dian)階段(duan)及恒壓充(chong)(chong)電(dian)階段(duan)不擴流(liu)(liu)，擴流(liu)(liu)僅在恒流(liu)(liu)階段(duan)。

　　原充電器以1A電(dian)(dian)流(liu)(liu)(liu)充(chong)電(dian)(dian)，若擴(kuo)流(liu)(liu)(liu)電(dian)(dian)流(liu)(liu)(liu)為(wei)1A，則在恒流(liu)(liu)(liu)充(chong)電(dian)(dian)階段時充(chong)電(dian)(dian)電(dian)(dian)流(liu)(liu)(liu)為(wei)2A。圖5中紅線(xian)(xian)為(wei)充(chong)電(dian)(dian)電(dian)(dian)池電(dian)(dian)壓特(te)性、黑線(xian)(xian)為(wei)充(chong)電(dian)(dian)電(dian)(dian)流(liu)(liu)(liu)特(te)性，實線(xian)(xian)為(wei)加(jia)擴(kuo)流(liu)(liu)(liu)特(te)性，虛線(xian)(xian)為(wei)未(wei)加(jia)擴(kuo)流(liu)(liu)(liu)特(te)性。　　

　　為保證(zheng)擴(kuo)流在(zai)(zai)電池電壓3.0V開始，在(zai)(zai)電池電壓4.15V時結束，控制電路設置(zhi)了窗(chuang)口比較器(qi)，在(zai)(zai)電池電壓(VBAT)為3.0～4.15V之間(jian)控制P-MOSFET導通。在(zai)(zai)此窗(chuang)口電壓外(wai)，P-MOSFET截止(zhi)。

　　由(you)R5、R6及R7、R8組成兩個電(dian)(dian)(dian)(dian)(dian)壓分壓器(qi)(檢(jian)(jian)測電(dian)(dian)(dian)(dian)(dian)池(chi)的(de)電(dian)(dian)(dian)(dian)(dian)壓VBAT)，并分別將(jiang)其檢(jian)(jian)測的(de)電(dian)(dian)(dian)(dian)(dian)壓輸入比(bi)較(jiao)器(qi)P1及比(bi)較(jiao)器(qi)P2組成的(de)窗口比(bi)較(jiao)器(qi)。R3、R4分別為(wei)P1及P2的(de)上拉(la)電(dian)(dian)(dian)(dian)(dian)阻，D2、D3為(wei)隔(ge)離二極管。充電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)壓VBAT與P1、P2的(de)輸出(chu)及P-MOSFET的(de)工(gong)作狀態。

　　P-MOSFET的(de)(de)-VGS電壓(ya)(ya)是(shi)由R2、RP往D1提供的(de)(de)，則P-MOSFET在上(shang)電后(hou)應是(shi)一直導(dao)通的(de)(de)。現(xian)(xian)要(yao)求(qiu)在電池電壓(ya)(ya)(VBAT)小于(yu)(yu)3.0V及大于(yu)(yu)4.15V時P-MOSFET要(yao)關(guan)斷(duan)，則控(kong)(kong)制(zhi)電路(lu)要(yao)在VBAT<3.0V及VBAT> 4.15V時，在P-MOSFET的(de)(de)柵(zha)極G上(shang)加上(shang)高電平(ping)，使其-VGS=0.7V，小于(yu)(yu)導(dao)通閾值電壓(ya)(ya)-VGS(th)，則P-MOSFET截止(關(guan)斷(duan))。現(xian)(xian)由P1、P2比較器及其他元器件組成窗口比較器實(shi)現(xian)(xian)了這一控(kong)(kong)制(zhi)要(yao)求(qiu)：無論是(shi)P1或P2輸出高電平(ping)時，VIN通過R4或R3及D3或D2加在P-MOSFET的(de)(de)柵(zha)極上(shang)，迫使柵(zha)極電壓(ya)(ya)為VIN=0.7V，則-VDS=0.7V而截止，滿足了控(kong)(kong)制(zhi)的(de)(de)要(yao)求(qiu)。

　　P-MOSFET的功耗及(ji)散熱

　　1 擴流管P-MOSFET的功(gong)耗計算

　　P-MOSFET在(zai)擴(kuo)流(liu)時的(de)功耗PD與輸(shu)出電(dian)壓(ya)VIN電(dian)池電(dian)壓(ya)VBAT、肖特基二(er)極(ji)管的(de)正向壓(ya)降(jiang)VF及擴(kuo)流(liu)電(dian)流(liu)ID有關，其計算公式如下：

　　PD=VIN-(VBAT+VF)×ID (1)

　　其(qi)最大的功耗是在VIN(max)及(ji)VBAT(min)時(shi)，即在擴流開始時(shi)(VBAT=3V)，則上(shang)式可(ke)寫成：

　　PDmax=VIN(max)-(3V+VF)×ID (2)

　　若VIN(max)=5.2V、在ID=1A時(shi)，VF=0.4V，則PDmax=1.8W。選(xuan)擇的(de)P-MOSFET的(de)最大(da)允(yun)許(xu)功耗(hao)應大(da)于計算出的(de)最大(da)功耗(hao)。

　　2 P-MOSFET的散熱

　　貼片式(shi)功率MOSFET采用印制板的(de)(de)敷銅層來散熱，即在(zai)(zai)設計印制板時(shi)(shi)要留出一定的(de)(de)散熱面積。例如，采用DPAK封裝的(de)(de)MTD2955E在(zai)(zai)計算出PDmax=1.75W時(shi)(shi)，需(xu)11mm2散熱面積;若(ruo)PDmax=3W時(shi)(shi)，需(xu)26mm2