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(qi)全、性能良好、電(dian)路簡(jian)單、占印制版面積小(xiao)，價格低廉，整個(ge)充電(dian)器可以(yi)在(zai)產品中。若采用USB端(duan)口(kou)充電(dian)，使用十分方便。

　　近年來，一些用電(dian)(dian)(dian)量稍大的便攜式電(dian)(dian)(dian)子(zi)產品(如便攜式DVD、礦燈(deng)、攝像機、便攜式測(ce)量儀器(qi)(qi)、小(xiao)型電(dian)(dian)(dian)動工具(ju)等)往(wang)(wang)往(wang)(wang)采用1500mAh到(dao)5400mAh容量的鋰離子(zi)電(dian)(dian)(dian)池。若(ruo)采用500～1000mA充電(dian)(dian)(dian)電(dian)(dian)(dian)流充電(dian)(dian)(dian)器(qi)(qi)充電(dian)(dian)(dian)，則充電(dian)(dian)(dian)時(shi)間太長。若(ruo)按(an)0.5C充電(dian)(dian)(dian)率來充3000mAh及5400mA時(shi)的電(dian)(dian)(dian)池時(shi)，其充電(dian)(dian)(dian)電(dian)(dian)(dian)池的容量要求為1500mA及2700mA。

　　有人提出：能(neng)否在1A線性(xing)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)(qi)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)中加一(yi)個擴(kuo)流(liu)(liu)(liu)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)，使充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)擴(kuo)大到2～2.5A，解決3000～5400mAh容量鋰離(li)子(zi)電(dian)(dian)(dian)(dian)(dian)池的(de)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)問題。如果擴(kuo)流(liu)(liu)(liu)的(de)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)(qi)性(xing)能(neng)不(bu)(bu)錯、電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)簡單、成本不(bu)(bu)高，這(zhe)(zhe)是個好主意。筆者就按這(zhe)(zhe)一(yi)思路(lu)(lu)(lu)(lu)(lu)設計一(yi)個擴(kuo)流(liu)(liu)(liu)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)。這(zhe)(zhe)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)采用型(xing)號為(wei)CN3056的(de)1A線性(xing)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)(qi)為(wei)基礎，另外加上擴(kuo)流(liu)(liu)(liu)電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)及控(kong)制電(dian)(dian)(dian)(dian)(dian)路(lu)(lu)(lu)(lu)(lu)組成。

　　CN3056簡介

　　CN3056充(chong)電器已在本刊(kan)2006年12期及2007年電源增刊(kan)上(shang)介紹過(“線性鋰二次電池充(chong)電器芯片CN3056”)。這(zhe)里僅作一簡介。

　　CN3056組成的(de)充(chong)電(dian)器按恒流(liu)、恒壓模式(shi)充(chong)電(dian)，若(ruo)充(chong)電(dian)電(dian)池電(dian)壓<3V，則有(you)小電(dian)流(liu)預充(chong)電(dian)模式(shi);充(chong)電(dian)電(dian)流(liu)可設定，最大充(chong)電(dian)電(dian)流(liu)為1A;精電(dian)密度(du)4.2V ±1%、有(you)熱調(diao)節、欠壓鎖存(cun)及電(dian)池溫度(du)檢測、超(chao)溫保(bao)護及充(chong)電(dian)狀態和溫度(du)超(chao)差指示功能;10引腳(jiao)小尺(chi)寸DFN封裝(zhuang)(3mm×3mm)。

　　若(ruo)充(chong)(chong)電(dian)(dian)(dian)(dian)率在0.5～1C之間、電(dian)(dian)(dian)(dian)池(chi)的溫(wen)度在0～45℃之間(室溫(wen)充(chong)(chong)電(dian)(dian)(dian)(dian))，則CN3056充(chong)(chong)電(dian)(dian)(dian)(dian)器電(dian)(dian)(dian)(dian)路中可省(sheng)去電(dian)(dian)(dian)(dian)池(chi)溫(wen)度檢測電(dian)(dian)(dian)(dian)路及(ji)電(dian)(dian)(dian)(dian)池(chi)超溫(wen)指(zhi)示電(dian)(dian)(dian)(dian)路(引(yin)腳TEMP及(ji)FAULT端(duan)接地)。VIN是(shi)電(dian)(dian)(dian)(dian)源(yuan)輸(shu)(shu)入端(duan)、CE是(shi)使能端(duan)，(高(gao)電(dian)(dian)(dian)(dian)平有效);RISET為充(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流ICH設(she)定電(dian)(dian)(dian)(dian)阻，RISET(Ω)=1800(V)/ICH(A);CHRG為充(chong)(chong)電(dian)(dian)(dian)(dian)狀態(tai)信(xin)號輸(shu)(shu)出(chu)端(duan)：充(chong)(chong)電(dian)(dian)(dian)(dian)時此端(duan)為高(gao)電(dian)(dian)(dian)(dian)平，LED亮(liang);充(chong)(chong)電(dian)(dian)(dian)(dian)結(jie)束時此端(duan)為高(gao)阻抗(kang)，LED滅;電(dian)(dian)(dian)(dian)池(chi)未裝入或接觸不良，LED閃亮(liang)。VIN一般取4.5～5V，10μF及(ji)6.8μF為輸(shu)(shu)入、輸(shu)(shu)出(chu)電(dian)(dian)(dian)(dian)容(rong)，保證充(chong)(chong)電(dian)(dian)(dian)(dian)器穩定工作(zuo)。

　　充電器擴流電路

　　充(chong)(chong)電(dian)(dian)(dian)器(qi)擴(kuo)流(liu)(liu)(liu)電(dian)(dian)(dian)路(lu)(lu)是在原(yuan)(yuan)充(chong)(chong)電(dian)(dian)(dian)器(qi)電(dian)(dian)(dian)路(lu)(lu)上加上擴(kuo)流(liu)(liu)(liu)電(dian)(dian)(dian)路(lu)(lu)組(zu)成的。擴(kuo)流(liu)(liu)(liu)電(dian)(dian)(dian)路(lu)(lu)由兩部分(fen)(fen)(fen)組(zu)成：擴(kuo)流(liu)(liu)(liu)部分(fen)(fen)(fen)及控制部分(fen)(fen)(fen)。采用CN3056充(chong)(chong)電(dian)(dian)(dian)器(qi)為基礎，加上擴(kuo)流(liu)(liu)(liu)部分(fen)(fen)(fen)及控制部分(fen)(fen)(fen)電(dian)(dian)(dian)路(lu)(lu)。現分(fen)(fen)(fen)別介紹其工(gong)作原(yuan)(yuan)理(li)。

　　1 擴(kuo)流部分電路(lu)

　　擴流(liu)(liu)部分(fen)電路。它由P溝道功率MOSFET(VT)、R及RP組成的(de)(de)分(fen)壓器(qi)、肖特基二(er)極(ji)管D4組成。利用分(fen)壓器(qi)調節P-MOSFET的(de)(de)-VGS大小，使(shi)獲(huo)得(de)所需擴流(liu)(liu)電流(liu)(liu)ID。P-MOSFET的(de)(de)輸出特性(以(yi)Si9933DY為例)。在(zai)-VGS=2.1V、VDS>0.5V時(shi)(shi)，其(qi)輸出特性幾乎是(shi)(shi)一水平直線;在(zai)不同的(de)(de)VDS時(shi)(shi)，ID是(shi)(shi)恒流(liu)(liu)。從圖4也(ye)可(ke)以(yi)看(kan)出，在(zai) -VGS增加(jia)(jia)時(shi)(shi)，ID也(ye)相應增加(jia)(jia)。

　　2 控制部分電路

　　控制部分電(dian)路的(de)目的(de)是要保持原(yuan)有的(de)三階(jie)(jie)段充(chong)電(dian)模(mo)式，在預充(chong)電(dian)階(jie)(jie)段及恒壓充(chong)電(dian)階(jie)(jie)段不擴流(liu)，擴流(liu)僅在恒流(liu)階(jie)(jie)段。

　　原充電器以1A電(dian)(dian)(dian)流(liu)充電(dian)(dian)(dian)，若(ruo)擴(kuo)流(liu)電(dian)(dian)(dian)流(liu)為(wei)1A，則(ze)在恒流(liu)充電(dian)(dian)(dian)階段時充電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)為(wei)2A。圖(tu)5中(zhong)紅線(xian)為(wei)充電(dian)(dian)(dian)電(dian)(dian)(dian)池電(dian)(dian)(dian)壓特(te)(te)性(xing)、黑(hei)線(xian)為(wei)充電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)特(te)(te)性(xing)，實線(xian)為(wei)加(jia)擴(kuo)流(liu)特(te)(te)性(xing)，虛線(xian)為(wei)未加(jia)擴(kuo)流(liu)特(te)(te)性(xing)。　　

　　為(wei)保證擴流(liu)在電(dian)(dian)池電(dian)(dian)壓3.0V開始(shi)，在電(dian)(dian)池電(dian)(dian)壓4.15V時結束(shu)，控(kong)制電(dian)(dian)路(lu)設置了窗(chuang)口比(bi)較(jiao)器，在電(dian)(dian)池電(dian)(dian)壓(VBAT)為(wei)3.0～4.15V之間(jian)控(kong)制P-MOSFET導(dao)通(tong)。在此窗(chuang)口電(dian)(dian)壓外，P-MOSFET截止。

　　由(you)R5、R6及(ji)(ji)R7、R8組(zu)成(cheng)兩個電(dian)(dian)(dian)壓(ya)分壓(ya)器(檢(jian)測(ce)電(dian)(dian)(dian)池(chi)的(de)電(dian)(dian)(dian)壓(ya)VBAT)，并分別將其檢(jian)測(ce)的(de)電(dian)(dian)(dian)壓(ya)輸入比較(jiao)器P1及(ji)(ji)比較(jiao)器P2組(zu)成(cheng)的(de)窗口比較(jiao)器。R3、R4分別為P1及(ji)(ji)P2的(de)上拉電(dian)(dian)(dian)阻，D2、D3為隔離(li)二極管(guan)。充電(dian)(dian)(dian)電(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)壓(ya)VBAT與P1、P2的(de)輸出及(ji)(ji)P-MOSFET的(de)工作狀態。

　　P-MOSFET的(de)(de)(de)-VGS電(dian)壓是由(you)R2、RP往D1提供的(de)(de)(de)，則P-MOSFET在(zai)上電(dian)后應是一(yi)直導(dao)通的(de)(de)(de)。現要(yao)求(qiu)(qiu)(qiu)在(zai)電(dian)池電(dian)壓(VBAT)小(xiao)于(yu)3.0V及大于(yu)4.15V時(shi)P-MOSFET要(yao)關斷，則控(kong)(kong)制(zhi)電(dian)路要(yao)在(zai)VBAT<3.0V及VBAT> 4.15V時(shi)，在(zai)P-MOSFET的(de)(de)(de)柵極G上加上高電(dian)平，使其-VGS=0.7V，小(xiao)于(yu)導(dao)通閾值(zhi)電(dian)壓-VGS(th)，則P-MOSFET截(jie)止(關斷)。現由(you)P1、P2比較(jiao)(jiao)器及其他(ta)元器件(jian)組成窗口比較(jiao)(jiao)器實現了這(zhe)一(yi)控(kong)(kong)制(zhi)要(yao)求(qiu)(qiu)(qiu)：無論(lun)是P1或(huo)P2輸出高電(dian)平時(shi)，VIN通過R4或(huo)R3及D3或(huo)D2加在(zai)P-MOSFET的(de)(de)(de)柵極上，迫使柵極電(dian)壓為VIN=0.7V，則-VDS=0.7V而(er)截(jie)止，滿足了控(kong)(kong)制(zhi)的(de)(de)(de)要(yao)求(qiu)(qiu)(qiu)。

　　P-MOSFET的功耗及散熱

　　1 擴(kuo)流(liu)管P-MOSFET的(de)功耗(hao)計算(suan)

　　P-MOSFET在擴流時(shi)的功耗PD與輸出(chu)電壓VIN電池電壓VBAT、肖特基二極(ji)管的正向壓降(jiang)VF及擴流電流ID有關，其計(ji)算公(gong)式如下：

　　PD=VIN-(VBAT+VF)×ID (1)

　　其最大的功耗(hao)是在VIN(max)及VBAT(min)時，即在擴(kuo)流開始時(VBAT=3V)，則上式可(ke)寫成：

　　PDmax=VIN(max)-(3V+VF)×ID (2)

　　若VIN(max)=5.2V、在ID=1A時，VF=0.4V，則PDmax=1.8W。選擇的P-MOSFET的最(zui)大允許功(gong)(gong)耗應(ying)大于計算出的最(zui)大功(gong)(gong)耗。

　　2 P-MOSFET的(de)散熱(re)

　　貼片(pian)式功率(lv)MOSFET采(cai)用印(yin)制板(ban)的敷銅(tong)層來散(san)熱(re)，即在設計(ji)印(yin)制板(ban)時(shi)要(yao)留出(chu)一定的散(san)熱(re)面積(ji)。例如(ru)，采(cai)用DPAK封裝的MTD2955E在計(ji)算出(chu)PDmax=1.75W時(shi)，需11mm2散(san)熱(re)面積(ji);若PDmax=3W時(shi)，需26mm2