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(gong)能齊全(quan)、性能良好(hao)、電(dian)路簡單、占印制(zhi)版面積小，價格(ge)低廉(lian)，整個充電(dian)器可以在產(chan)品(pin)中。若采用(yong)USB端口充電(dian)，使(shi)用(yong)十分(fen)方便。

　　近年來(lai)，一些用電(dian)(dian)(dian)量(liang)(liang)稍大的便攜(xie)式電(dian)(dian)(dian)子產(chan)品(如便攜(xie)式DVD、礦(kuang)燈(deng)、攝像機、便攜(xie)式測量(liang)(liang)儀器、小型電(dian)(dian)(dian)動工具等)往(wang)往(wang)采(cai)用1500mAh到(dao)5400mAh容量(liang)(liang)的鋰(li)離(li)子電(dian)(dian)(dian)池(chi)。若(ruo)采(cai)用500～1000mA充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)，則充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)時間太(tai)長(chang)。若(ruo)按0.5C充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)率來(lai)充(chong)(chong)(chong)(chong)(chong)3000mAh及5400mA時的電(dian)(dian)(dian)池(chi)時，其充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)池(chi)的容量(liang)(liang)要求為1500mA及2700mA。

　　有人提出：能(neng)否在1A線性(xing)充電(dian)器(qi)(qi)電(dian)路(lu)(lu)中加一(yi)(yi)個擴流電(dian)路(lu)(lu)，使充電(dian)電(dian)流擴大到(dao)2～2.5A，解決3000～5400mAh容量鋰離子電(dian)池(chi)的(de)充電(dian)問(wen)題。如果擴流的(de)充電(dian)器(qi)(qi)性(xing)能(neng)不錯、電(dian)路(lu)(lu)簡(jian)單、成本不高，這是個好主意。筆者(zhe)就按(an)這一(yi)(yi)思路(lu)(lu)設計一(yi)(yi)個擴流電(dian)路(lu)(lu)。這電(dian)路(lu)(lu)采用型號為CN3056的(de)1A線性(xing)充電(dian)器(qi)(qi)為基(ji)礎，另外加上(shang)擴流電(dian)路(lu)(lu)及控制電(dian)路(lu)(lu)組(zu)成。

　　CN3056簡(jian)介

　　CN3056充(chong)電(dian)器(qi)已(yi)在本刊2006年12期(qi)及2007年電(dian)源增刊上介(jie)紹過(“線性鋰二次電(dian)池充(chong)電(dian)器(qi)芯片CN3056”)。這里僅作一簡介(jie)。

　　CN3056組成(cheng)的充電(dian)(dian)(dian)器按恒(heng)流(liu)(liu)、恒(heng)壓模式充電(dian)(dian)(dian)，若充電(dian)(dian)(dian)電(dian)(dian)(dian)池電(dian)(dian)(dian)壓<3V，則有小(xiao)電(dian)(dian)(dian)流(liu)(liu)預(yu)充電(dian)(dian)(dian)模式;充電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)(liu)可設定(ding)，最大(da)充電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)(liu)為(wei)1A;精電(dian)(dian)(dian)密度4.2V ±1%、有熱(re)調節、欠壓鎖(suo)存及電(dian)(dian)(dian)池溫(wen)度檢測(ce)、超(chao)溫(wen)保護及充電(dian)(dian)(dian)狀態和溫(wen)度超(chao)差指(zhi)示功能(neng);10引腳小(xiao)尺寸DFN封裝(3mm×3mm)。

　　若充(chong)電(dian)(dian)(dian)(dian)率在0.5～1C之間、電(dian)(dian)(dian)(dian)池的溫(wen)(wen)(wen)度(du)在0～45℃之間(室(shi)溫(wen)(wen)(wen)充(chong)電(dian)(dian)(dian)(dian))，則CN3056充(chong)電(dian)(dian)(dian)(dian)器(qi)電(dian)(dian)(dian)(dian)路(lu)(lu)中可(ke)省去電(dian)(dian)(dian)(dian)池溫(wen)(wen)(wen)度(du)檢測電(dian)(dian)(dian)(dian)路(lu)(lu)及(ji)電(dian)(dian)(dian)(dian)池超溫(wen)(wen)(wen)指示(shi)電(dian)(dian)(dian)(dian)路(lu)(lu)(引腳TEMP及(ji)FAULT端(duan)(duan)(duan)(duan)接地)。VIN是電(dian)(dian)(dian)(dian)源輸(shu)入(ru)端(duan)(duan)(duan)(duan)、CE是使能端(duan)(duan)(duan)(duan)，(高(gao)電(dian)(dian)(dian)(dian)平有(you)效);RISET為充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流ICH設定(ding)電(dian)(dian)(dian)(dian)阻，RISET(Ω)=1800(V)/ICH(A);CHRG為充(chong)電(dian)(dian)(dian)(dian)狀(zhuang)態信號輸(shu)出端(duan)(duan)(duan)(duan)：充(chong)電(dian)(dian)(dian)(dian)時此端(duan)(duan)(duan)(duan)為高(gao)電(dian)(dian)(dian)(dian)平，LED亮(liang);充(chong)電(dian)(dian)(dian)(dian)結(jie)束時此端(duan)(duan)(duan)(duan)為高(gao)阻抗，LED滅;電(dian)(dian)(dian)(dian)池未裝入(ru)或接觸不良，LED閃亮(liang)。VIN一(yi)般取4.5～5V，10μF及(ji)6.8μF為輸(shu)入(ru)、輸(shu)出電(dian)(dian)(dian)(dian)容，保(bao)證充(chong)電(dian)(dian)(dian)(dian)器(qi)穩定(ding)工作。

　　充電器擴流電路

　　充電器擴流(liu)電路(lu)(lu)是在原充電器電路(lu)(lu)上(shang)加(jia)上(shang)擴流(liu)電路(lu)(lu)組(zu)(zu)成(cheng)的。擴流(liu)電路(lu)(lu)由(you)兩部(bu)分(fen)(fen)組(zu)(zu)成(cheng)：擴流(liu)部(bu)分(fen)(fen)及控制部(bu)分(fen)(fen)。采(cai)用(yong)CN3056充電器為基礎，加(jia)上(shang)擴流(liu)部(bu)分(fen)(fen)及控制部(bu)分(fen)(fen)電路(lu)(lu)。現(xian)分(fen)(fen)別介紹其工作原理。

　　1 擴流(liu)部分電路

　　擴(kuo)流(liu)(liu)部分電路(lu)。它由P溝道功率(lv)MOSFET(VT)、R及RP組成(cheng)的分壓器(qi)、肖特(te)基二極(ji)管D4組成(cheng)。利用(yong)分壓器(qi)調節(jie)P-MOSFET的-VGS大小，使獲得所需擴(kuo)流(liu)(liu)電流(liu)(liu)ID。P-MOSFET的輸(shu)出(chu)特(te)性(以Si9933DY為例)。在-VGS=2.1V、VDS>0.5V時(shi)，其輸(shu)出(chu)特(te)性幾乎是(shi)一水平直線;在不(bu)同的VDS時(shi)，ID是(shi)恒流(liu)(liu)。從圖4也可(ke)以看(kan)出(chu)，在 -VGS增加(jia)(jia)時(shi)，ID也相應增加(jia)(jia)。

　　2 控制部分電路

　　控制部分電路的(de)(de)目的(de)(de)是要保(bao)持原有的(de)(de)三階(jie)段充電模式(shi)，在(zai)預(yu)充電階(jie)段及恒壓充電階(jie)段不擴(kuo)流，擴(kuo)流僅在(zai)恒流階(jie)段。

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

　　為保(bao)證(zheng)擴(kuo)流在電池電壓3.0V開始，在電池電壓4.15V時(shi)結束，控制電路(lu)設(she)置了(le)窗口比較器，在電池電壓(VBAT)為3.0～4.15V之間控制P-MOSFET導通。在此窗口電壓外，P-MOSFET截止(zhi)。

　　由R5、R6及(ji)R7、R8組成兩個電壓(ya)分壓(ya)器(qi)(檢(jian)測(ce)電池的電壓(ya)VBAT)，并分別將其檢(jian)測(ce)的電壓(ya)輸入比較(jiao)器(qi)P1及(ji)比較(jiao)器(qi)P2組成的窗口比較(jiao)器(qi)。R3、R4分別為P1及(ji)P2的上拉電阻，D2、D3為隔(ge)離(li)二(er)極(ji)管。充電電池電壓(ya)VBAT與P1、P2的輸出及(ji)P-MOSFET的工作狀態。

　　P-MOSFET的(de)-VGS電壓是(shi)由R2、RP往(wang)D1提供的(de)，則(ze)P-MOSFET在(zai)上(shang)(shang)電后應是(shi)一(yi)直導(dao)通的(de)。現要(yao)求(qiu)在(zai)電池電壓(VBAT)小于(yu)3.0V及(ji)(ji)大于(yu)4.15V時P-MOSFET要(yao)關斷，則(ze)控(kong)制電路(lu)要(yao)在(zai)VBAT<3.0V及(ji)(ji)VBAT> 4.15V時，在(zai)P-MOSFET的(de)柵極(ji)G上(shang)(shang)加上(shang)(shang)高電平，使(shi)(shi)其-VGS=0.7V，小于(yu)導(dao)通閾值電壓-VGS(th)，則(ze)P-MOSFET截止(關斷)。現由P1、P2比(bi)較器(qi)及(ji)(ji)其他(ta)元器(qi)件組成窗口比(bi)較器(qi)實現了這一(yi)控(kong)制要(yao)求(qiu)：無論是(shi)P1或P2輸出高電平時，VIN通過(guo)R4或R3及(ji)(ji)D3或D2加在(zai)P-MOSFET的(de)柵極(ji)上(shang)(shang)，迫使(shi)(shi)柵極(ji)電壓為(wei)VIN=0.7V，則(ze)-VDS=0.7V而截止，滿足了控(kong)制的(de)要(yao)求(qiu)。

　　P-MOSFET的功耗及散熱

　　1 擴流管P-MOSFET的功耗計算

　　P-MOSFET在擴(kuo)流時(shi)的功耗PD與(yu)輸出電(dian)(dian)(dian)壓(ya)VIN電(dian)(dian)(dian)池電(dian)(dian)(dian)壓(ya)VBAT、肖特(te)基二極管(guan)的正向壓(ya)降VF及擴(kuo)流電(dian)(dian)(dian)流ID有關，其計算(suan)公式如下(xia)：

　　PD=VIN-(VBAT+VF)×ID (1)

　　其最大的功耗是(shi)在VIN(max)及VBAT(min)時(shi)，即在擴流開始(shi)時(shi)(VBAT=3V)，則上式可寫成：

　　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P-MOSFET的最大允許(xu)功(gong)耗應(ying)大于計(ji)算(suan)出(chu)的最大功(gong)耗。

　　2 P-MOSFET的散(san)熱

　　貼(tie)片式功率MOSFET采(cai)用印制(zhi)板的敷銅(tong)層來散熱(re)(re)，即在(zai)(zai)設計(ji)印制(zhi)板時(shi)要留出一定(ding)的散熱(re)(re)面積(ji)。例(li)如，采(cai)用DPAK封裝的MTD2955E在(zai)(zai)計(ji)算出PDmax=1.75W時(shi)，需11mm2散熱(re)(re)面積(ji);若PDmax=3W時(shi)，需26mm2