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(neng)(neng)齊全、性能(neng)(neng)良好、電路簡單(dan)、占印制版面積小(xiao)，價格低(di)廉，整個充電器可以在產(chan)品中。若采(cai)用(yong)USB端口充電，使用(yong)十分方便。

　　近年來，一些(xie)用(yong)(yong)電(dian)(dian)(dian)量(liang)稍(shao)大的便(bian)攜(xie)(xie)式電(dian)(dian)(dian)子產品(如便(bian)攜(xie)(xie)式DVD、礦燈、攝像機、便(bian)攜(xie)(xie)式測量(liang)儀器、小型(xing)電(dian)(dian)(dian)動工具等)往往采(cai)用(yong)(yong)1500mAh到5400mAh容(rong)量(liang)的鋰離(li)子電(dian)(dian)(dian)池。若采(cai)用(yong)(yong)500～1000mA充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流充(chong)(chong)電(dian)(dian)(dian)器充(chong)(chong)電(dian)(dian)(dian)，則充(chong)(chong)電(dian)(dian)(dian)時間太長。若按0.5C充(chong)(chong)電(dian)(dian)(dian)率來充(chong)(chong)3000mAh及5400mA時的電(dian)(dian)(dian)池時，其(qi)充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)池的容(rong)量(liang)要求為(wei)1500mA及2700mA。

　　有人提出：能(neng)(neng)否在(zai)1A線(xian)性充(chong)(chong)電(dian)(dian)器(qi)電(dian)(dian)路(lu)中加(jia)一(yi)個擴流(liu)電(dian)(dian)路(lu)，使(shi)充(chong)(chong)電(dian)(dian)電(dian)(dian)流(liu)擴大到2～2.5A，解決3000～5400mAh容量鋰離子電(dian)(dian)池的(de)充(chong)(chong)電(dian)(dian)問(wen)題(ti)。如果擴流(liu)的(de)充(chong)(chong)電(dian)(dian)器(qi)性能(neng)(neng)不錯、電(dian)(dian)路(lu)簡單、成本不高，這是個好主意(yi)。筆者(zhe)就按(an)這一(yi)思路(lu)設計一(yi)個擴流(liu)電(dian)(dian)路(lu)。這電(dian)(dian)路(lu)采用型(xing)號為CN3056的(de)1A線(xian)性充(chong)(chong)電(dian)(dian)器(qi)為基礎，另外加(jia)上擴流(liu)電(dian)(dian)路(lu)及(ji)控制電(dian)(dian)路(lu)組成。

　　CN3056簡介

　　CN3056充電(dian)(dian)器已在本刊2006年(nian)12期及2007年(nian)電(dian)(dian)源增(zeng)刊上介紹(shao)過(“線性(xing)鋰二次電(dian)(dian)池充電(dian)(dian)器芯片CN3056”)。這(zhe)里僅(jin)作一簡介。

　　CN3056組成的充電(dian)(dian)器按恒流、恒壓(ya)模(mo)式充電(dian)(dian)，若充電(dian)(dian)電(dian)(dian)池(chi)電(dian)(dian)壓(ya)<3V，則有(you)小電(dian)(dian)流預充電(dian)(dian)模(mo)式;充電(dian)(dian)電(dian)(dian)流可(ke)設定，最大(da)充電(dian)(dian)電(dian)(dian)流為(wei)1A;精電(dian)(dian)密度4.2V ±1%、有(you)熱(re)調(diao)節、欠壓(ya)鎖存及(ji)電(dian)(dian)池(chi)溫(wen)度檢測、超溫(wen)保護及(ji)充電(dian)(dian)狀態和(he)溫(wen)度超差指示(shi)功能(neng);10引腳小尺寸(cun)DFN封裝(3mm×3mm)。

　　若充(chong)電(dian)(dian)(dian)率在(zai)0.5～1C之(zhi)間(jian)、電(dian)(dian)(dian)池(chi)的溫(wen)(wen)度在(zai)0～45℃之(zhi)間(jian)(室(shi)溫(wen)(wen)充(chong)電(dian)(dian)(dian))，則(ze)CN3056充(chong)電(dian)(dian)(dian)器(qi)電(dian)(dian)(dian)路(lu)中可省去(qu)電(dian)(dian)(dian)池(chi)溫(wen)(wen)度檢(jian)測電(dian)(dian)(dian)路(lu)及電(dian)(dian)(dian)池(chi)超溫(wen)(wen)指示電(dian)(dian)(dian)路(lu)(引(yin)腳TEMP及FAULT端接(jie)地)。VIN是電(dian)(dian)(dian)源輸入(ru)端、CE是使能(neng)端，(高電(dian)(dian)(dian)平有(you)效);RISET為充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)ICH設定電(dian)(dian)(dian)阻(zu)，RISET(Ω)=1800(V)/ICH(A);CHRG為充(chong)電(dian)(dian)(dian)狀態(tai)信號輸出端：充(chong)電(dian)(dian)(dian)時此(ci)端為高電(dian)(dian)(dian)平，LED亮;充(chong)電(dian)(dian)(dian)結束(shu)時此(ci)端為高阻(zu)抗，LED滅(mie);電(dian)(dian)(dian)池(chi)未裝入(ru)或接(jie)觸不良，LED閃亮。VIN一般取4.5～5V，10μF及6.8μF為輸入(ru)、輸出電(dian)(dian)(dian)容，保(bao)證充(chong)電(dian)(dian)(dian)器(qi)穩定工作。

　　充電器擴流電路

　　充電(dian)器(qi)擴流電(dian)路(lu)(lu)(lu)是在原充電(dian)器(qi)電(dian)路(lu)(lu)(lu)上加上擴流電(dian)路(lu)(lu)(lu)組(zu)成(cheng)的。擴流電(dian)路(lu)(lu)(lu)由兩(liang)部分(fen)組(zu)成(cheng)：擴流部分(fen)及控制(zhi)部分(fen)。采用(yong)CN3056充電(dian)器(qi)為基礎(chu)，加上擴流部分(fen)及控制(zhi)部分(fen)電(dian)路(lu)(lu)(lu)。現分(fen)別(bie)介紹其工作(zuo)原理。

　　1 擴流部(bu)分電(dian)路(lu)

　　擴(kuo)流部分電路。它由P溝道功率(lv)MOSFET(VT)、R及RP組(zu)成(cheng)的分壓器、肖特基二極管D4組(zu)成(cheng)。利(li)用分壓器調節P-MOSFET的-VGS大小，使(shi)獲(huo)得所需擴(kuo)流電流ID。P-MOSFET的輸出(chu)特性(以Si9933DY為例)。在-VGS=2.1V、VDS>0.5V時(shi)，其輸出(chu)特性幾乎是一(yi)水平直線;在不同的VDS時(shi)，ID是恒流。從圖4也(ye)可以看出(chu)，在 -VGS增加(jia)時(shi)，ID也(ye)相應增加(jia)。

　　2 控(kong)制部分(fen)電路

　　控(kong)制(zhi)部分電(dian)路(lu)的(de)目的(de)是要保持原有(you)的(de)三階(jie)段(duan)充(chong)電(dian)模式，在(zai)預充(chong)電(dian)階(jie)段(duan)及恒壓充(chong)電(dian)階(jie)段(duan)不(bu)擴(kuo)流，擴(kuo)流僅在(zai)恒流階(jie)段(duan)。

　　原充電器以(yi)1A電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)充(chong)電(dian)(dian)(dian)(dian)，若擴(kuo)流(liu)(liu)(liu)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)為(wei)(wei)1A，則在恒流(liu)(liu)(liu)充(chong)電(dian)(dian)(dian)(dian)階段時充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)為(wei)(wei)2A。圖5中紅線(xian)為(wei)(wei)充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓特性(xing)、黑線(xian)為(wei)(wei)充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)特性(xing)，實線(xian)為(wei)(wei)加擴(kuo)流(liu)(liu)(liu)特性(xing)，虛線(xian)為(wei)(wei)未加擴(kuo)流(liu)(liu)(liu)特性(xing)。　　

　　為保證(zheng)擴流(liu)在電(dian)(dian)池(chi)電(dian)(dian)壓3.0V開始，在電(dian)(dian)池(chi)電(dian)(dian)壓4.15V時結束，控制(zhi)電(dian)(dian)路設置了窗口(kou)比(bi)較器(qi)，在電(dian)(dian)池(chi)電(dian)(dian)壓(VBAT)為3.0～4.15V之間控制(zhi)P-MOSFET導通。在此窗口(kou)電(dian)(dian)壓外，P-MOSFET截止。

　　由R5、R6及(ji)(ji)R7、R8組(zu)成(cheng)兩個電(dian)壓(ya)分(fen)壓(ya)器(檢(jian)測電(dian)池(chi)的(de)(de)(de)(de)電(dian)壓(ya)VBAT)，并分(fen)別(bie)將其檢(jian)測的(de)(de)(de)(de)電(dian)壓(ya)輸入比(bi)較(jiao)器P1及(ji)(ji)比(bi)較(jiao)器P2組(zu)成(cheng)的(de)(de)(de)(de)窗口比(bi)較(jiao)器。R3、R4分(fen)別(bie)為P1及(ji)(ji)P2的(de)(de)(de)(de)上(shang)拉電(dian)阻，D2、D3為隔離二極(ji)管。充電(dian)電(dian)池(chi)電(dian)壓(ya)VBAT與P1、P2的(de)(de)(de)(de)輸出及(ji)(ji)P-MOSFET的(de)(de)(de)(de)工作狀態。

　　P-MOSFET的-VGS電(dian)壓(ya)是由(you)(you)R2、RP往(wang)D1提供的，則(ze)(ze)(ze)P-MOSFET在(zai)(zai)上(shang)電(dian)后應是一直導(dao)通的。現要(yao)求在(zai)(zai)電(dian)池電(dian)壓(ya)(VBAT)小(xiao)(xiao)于(yu)3.0V及(ji)大于(yu)4.15V時P-MOSFET要(yao)關斷(duan)，則(ze)(ze)(ze)控(kong)制(zhi)(zhi)電(dian)路要(yao)在(zai)(zai)VBAT<3.0V及(ji)VBAT> 4.15V時，在(zai)(zai)P-MOSFET的柵極(ji)G上(shang)加上(shang)高(gao)電(dian)平，使其-VGS=0.7V，小(xiao)(xiao)于(yu)導(dao)通閾值電(dian)壓(ya)-VGS(th)，則(ze)(ze)(ze)P-MOSFET截止(zhi)(關斷(duan))。現由(you)(you)P1、P2比較(jiao)器(qi)及(ji)其他元器(qi)件組成(cheng)窗口比較(jiao)器(qi)實現了這一控(kong)制(zhi)(zhi)要(yao)求：無論(lun)是P1或P2輸出高(gao)電(dian)平時，VIN通過R4或R3及(ji)D3或D2加在(zai)(zai)P-MOSFET的柵極(ji)上(shang)，迫使柵極(ji)電(dian)壓(ya)為VIN=0.7V，則(ze)(ze)(ze)-VDS=0.7V而截止(zhi)，滿足(zu)了控(kong)制(zhi)(zhi)的要(yao)求。

　　P-MOSFET的功(gong)耗及散熱

　　1 擴流管P-MOSFET的功(gong)耗計(ji)算(suan)

　　P-MOSFET在擴流(liu)(liu)時的(de)功耗PD與輸出電(dian)壓VIN電(dian)池電(dian)壓VBAT、肖特基二(er)極管的(de)正向壓降VF及擴流(liu)(liu)電(dian)流(liu)(liu)ID有關，其計(ji)算公式(shi)如下：

　　PD=VIN-(VBAT+VF)×ID (1)

　　其最大(da)的功(gong)耗是(shi)在VIN(max)及VBAT(min)時(shi)，即在擴流開(kai)始(shi)時(shi)(VBAT=3V)，則上式(shi)可寫成：

　　PDmax=VIN(max)-(3V+VF)×ID (2)

　　若(ruo)VIN(max)=5.2V、在ID=1A時，VF=0.4V，則PDmax=1.8W。選擇的P-MOSFET的最(zui)大(da)允許功耗應(ying)大(da)于計算(suan)出(chu)的最(zui)大(da)功耗。

　　2 P-MOSFET的散(san)熱(re)

　　貼片式(shi)功率MOSFET采用印制板(ban)的(de)敷(fu)銅層來散(san)熱，即(ji)在(zai)設計印制板(ban)時要(yao)留出(chu)一定的(de)散(san)熱面積。例如，采用DPAK封裝的(de)MTD2955E在(zai)計算出(chu)PDmax=1.75W時，需11mm2散(san)熱面積;若PDmax=3W時，需26mm2