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、性能良好、電(dian)(dian)路簡單、占印制(zhi)版面積(ji)小，價格低廉，整個充(chong)電(dian)(dian)器可以(yi)在產品中。若采用USB端(duan)口充(chong)電(dian)(dian)，使用十分方便。

　　近(jin)年來(lai)，一些用(yong)電(dian)量稍(shao)大的(de)便(bian)攜式電(dian)子產品(如便(bian)攜式DVD、礦燈、攝像機、便(bian)攜式測(ce)量儀器、小型電(dian)動工具等)往往采用(yong)1500mAh到5400mAh容量的(de)鋰離子電(dian)池(chi)(chi)。若(ruo)采用(yong)500～1000mA充電(dian)電(dian)流充電(dian)器充電(dian)，則充電(dian)時間(jian)太(tai)長。若(ruo)按(an)0.5C充電(dian)率來(lai)充3000mAh及5400mA時的(de)電(dian)池(chi)(chi)時，其充電(dian)電(dian)池(chi)(chi)的(de)容量要求為1500mA及2700mA。

　　有(you)人提出：能(neng)(neng)否在1A線性(xing)充電(dian)(dian)(dian)器電(dian)(dian)(dian)路(lu)(lu)(lu)中加(jia)一個擴(kuo)流電(dian)(dian)(dian)路(lu)(lu)(lu)，使充電(dian)(dian)(dian)電(dian)(dian)(dian)流擴(kuo)大到2～2.5A，解決(jue)3000～5400mAh容(rong)量鋰離子電(dian)(dian)(dian)池的(de)充電(dian)(dian)(dian)問題。如果擴(kuo)流的(de)充電(dian)(dian)(dian)器性(xing)能(neng)(neng)不錯、電(dian)(dian)(dian)路(lu)(lu)(lu)簡單、成本不高，這(zhe)是(shi)個好主意(yi)。筆(bi)者就按這(zhe)一思路(lu)(lu)(lu)設計一個擴(kuo)流電(dian)(dian)(dian)路(lu)(lu)(lu)。這(zhe)電(dian)(dian)(dian)路(lu)(lu)(lu)采(cai)用型號為(wei)CN3056的(de)1A線性(xing)充電(dian)(dian)(dian)器為(wei)基礎(chu)，另外加(jia)上擴(kuo)流電(dian)(dian)(dian)路(lu)(lu)(lu)及控制(zhi)電(dian)(dian)(dian)路(lu)(lu)(lu)組成。

　　CN3056簡(jian)介

　　CN3056充電器已在本刊2006年12期及(ji)2007年電源(yuan)增(zeng)刊上介紹過(“線(xian)性(xing)鋰二次電池充電器芯(xin)片CN3056”)。這里(li)僅作一簡介。

　　CN3056組(zu)成的充(chong)電(dian)(dian)(dian)器按恒流(liu)、恒壓(ya)(ya)模式充(chong)電(dian)(dian)(dian)，若(ruo)充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)池電(dian)(dian)(dian)壓(ya)(ya)<3V，則有(you)(you)小電(dian)(dian)(dian)流(liu)預充(chong)電(dian)(dian)(dian)模式;充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)可設(she)定(ding)，最(zui)大充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)為1A;精電(dian)(dian)(dian)密度(du)(du)4.2V ±1%、有(you)(you)熱調節、欠(qian)壓(ya)(ya)鎖存及電(dian)(dian)(dian)池溫(wen)度(du)(du)檢測(ce)、超溫(wen)保護及充(chong)電(dian)(dian)(dian)狀態和溫(wen)度(du)(du)超差指示功能;10引腳小尺寸(cun)DFN封裝(3mm×3mm)。

　　若充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)率(lv)在0.5～1C之(zhi)間(jian)、電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池的溫度(du)在0～45℃之(zhi)間(jian)(室溫充電(dian)(dian)(dian)(dian)(dian)(dian)(dian))，則(ze)CN3056充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路中(zhong)可省(sheng)去電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池溫度(du)檢測電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路及電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池超溫指示(shi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(引(yin)腳TEMP及FAULT端(duan)(duan)接地(di))。VIN是電(dian)(dian)(dian)(dian)(dian)(dian)(dian)源輸入端(duan)(duan)、CE是使能端(duan)(duan)，(高電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平有(you)效);RISET為充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流ICH設定電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻，RISET(Ω)=1800(V)/ICH(A);CHRG為充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)狀(zhuang)態信(xin)號輸出(chu)(chu)端(duan)(duan)：充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)時此端(duan)(duan)為高電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平，LED亮;充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)結(jie)束時此端(duan)(duan)為高阻抗，LED滅(mie);電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池未裝入或接觸不良(liang)，LED閃亮。VIN一般取4.5～5V，10μF及6.8μF為輸入、輸出(chu)(chu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容，保證充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器穩定工作(zuo)。

　　充電器擴流電路

　　充(chong)電(dian)器擴流(liu)電(dian)路(lu)是在原(yuan)充(chong)電(dian)器電(dian)路(lu)上加(jia)上擴流(liu)電(dian)路(lu)組成的(de)。擴流(liu)電(dian)路(lu)由兩部分(fen)(fen)(fen)組成：擴流(liu)部分(fen)(fen)(fen)及控制(zhi)部分(fen)(fen)(fen)。采用(yong)CN3056充(chong)電(dian)器為基礎，加(jia)上擴流(liu)部分(fen)(fen)(fen)及控制(zhi)部分(fen)(fen)(fen)電(dian)路(lu)。現分(fen)(fen)(fen)別介紹其(qi)工作(zuo)原(yuan)理。

　　1 擴(kuo)流部分電(dian)路

　　擴(kuo)流(liu)部分電(dian)路。它由P溝道(dao)功率MOSFET(VT)、R及RP組成的(de)(de)分壓器、肖特(te)基(ji)二極管(guan)D4組成。利用分壓器調節(jie)P-MOSFET的(de)(de)-VGS大小，使獲(huo)得(de)所需擴(kuo)流(liu)電(dian)流(liu)ID。P-MOSFET的(de)(de)輸(shu)出特(te)性(xing)(以Si9933DY為(wei)例)。在-VGS=2.1V、VDS>0.5V時(shi)，其輸(shu)出特(te)性(xing)幾乎(hu)是(shi)(shi)一(yi)水平直線;在不同的(de)(de)VDS時(shi)，ID是(shi)(shi)恒(heng)流(liu)。從圖(tu)4也(ye)可以看出，在 -VGS增加時(shi)，ID也(ye)相應(ying)增加。

　　2 控制部(bu)分電(dian)路

　　控制部分電路的(de)目(mu)的(de)是(shi)要保持原有的(de)三階(jie)段充電模式，在預充電階(jie)段及恒(heng)(heng)壓充電階(jie)段不擴流，擴流僅在恒(heng)(heng)流階(jie)段。

　　原充電器以1A電(dian)(dian)(dian)流(liu)(liu)充(chong)(chong)電(dian)(dian)(dian)，若擴(kuo)(kuo)流(liu)(liu)電(dian)(dian)(dian)流(liu)(liu)為(wei)(wei)1A，則在恒流(liu)(liu)充(chong)(chong)電(dian)(dian)(dian)階段(duan)時充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)(liu)為(wei)(wei)2A。圖(tu)5中紅(hong)線(xian)為(wei)(wei)充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)池電(dian)(dian)(dian)壓(ya)特性(xing)(xing)、黑線(xian)為(wei)(wei)充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)(liu)特性(xing)(xing)，實線(xian)為(wei)(wei)加(jia)擴(kuo)(kuo)流(liu)(liu)特性(xing)(xing)，虛線(xian)為(wei)(wei)未加(jia)擴(kuo)(kuo)流(liu)(liu)特性(xing)(xing)。　　

　　為(wei)保證擴流(liu)在電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)3.0V開始，在電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)4.15V時(shi)結束，控制(zhi)電(dian)(dian)(dian)(dian)路設置了窗口(kou)比較器，在電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)(VBAT)為(wei)3.0～4.15V之間控制(zhi)P-MOSFET導通(tong)。在此窗口(kou)電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)外(wai)，P-MOSFET截止。

　　由R5、R6及(ji)(ji)(ji)R7、R8組成(cheng)兩(liang)個電(dian)壓(ya)(ya)(ya)分(fen)壓(ya)(ya)(ya)器(qi)(qi)(檢(jian)測(ce)電(dian)池的電(dian)壓(ya)(ya)(ya)VBAT)，并(bing)分(fen)別將其檢(jian)測(ce)的電(dian)壓(ya)(ya)(ya)輸入比(bi)較(jiao)(jiao)器(qi)(qi)P1及(ji)(ji)(ji)比(bi)較(jiao)(jiao)器(qi)(qi)P2組成(cheng)的窗口比(bi)較(jiao)(jiao)器(qi)(qi)。R3、R4分(fen)別為(wei)P1及(ji)(ji)(ji)P2的上(shang)拉電(dian)阻(zu)，D2、D3為(wei)隔離(li)二極(ji)管。充(chong)電(dian)電(dian)池電(dian)壓(ya)(ya)(ya)VBAT與(yu)P1、P2的輸出及(ji)(ji)(ji)P-MOSFET的工作狀態。

　　P-MOSFET的(de)-VGS電(dian)壓(ya)(ya)是(shi)由R2、RP往(wang)D1提供的(de)，則(ze)P-MOSFET在(zai)(zai)上(shang)電(dian)后應是(shi)一直導(dao)通的(de)。現要求(qiu)在(zai)(zai)電(dian)池電(dian)壓(ya)(ya)(VBAT)小于3.0V及(ji)大于4.15V時(shi)(shi)P-MOSFET要關斷(duan)，則(ze)控(kong)制電(dian)路要在(zai)(zai)VBAT<3.0V及(ji)VBAT> 4.15V時(shi)(shi)，在(zai)(zai)P-MOSFET的(de)柵極(ji)G上(shang)加(jia)(jia)上(shang)高(gao)電(dian)平，使(shi)(shi)其-VGS=0.7V，小于導(dao)通閾值電(dian)壓(ya)(ya)-VGS(th)，則(ze)P-MOSFET截止(關斷(duan))。現由P1、P2比(bi)較(jiao)器(qi)及(ji)其他元(yuan)器(qi)件組成窗口(kou)比(bi)較(jiao)器(qi)實現了(le)這一控(kong)制要求(qiu)：無論是(shi)P1或(huo)P2輸出高(gao)電(dian)平時(shi)(shi)，VIN通過R4或(huo)R3及(ji)D3或(huo)D2加(jia)(jia)在(zai)(zai)P-MOSFET的(de)柵極(ji)上(shang)，迫(po)使(shi)(shi)柵極(ji)電(dian)壓(ya)(ya)為VIN=0.7V，則(ze)-VDS=0.7V而(er)截止，滿足了(le)控(kong)制的(de)要求(qiu)。

　　P-MOSFET的功耗及(ji)散熱(re)

　　1 擴(kuo)流管P-MOSFET的功耗計算

　　P-MOSFET在擴流(liu)時的功耗(hao)PD與輸出電(dian)壓(ya)(ya)VIN電(dian)池電(dian)壓(ya)(ya)VBAT、肖特基二極管(guan)的正(zheng)向壓(ya)(ya)降VF及擴流(liu)電(dian)流(liu)ID有關，其計算公式如(ru)下：

　　PD=VIN-(VBAT+VF)×ID (1)

　　其最大(da)的功耗(hao)是在VIN(max)及VBAT(min)時，即在擴流(liu)開始(shi)時(VBAT=3V)，則上式可寫成：

　　PDmax=VIN(max)-(3V+VF)×ID (2)

　　若VIN(max)=5.2V、在ID=1A時，VF=0.4V，則PDmax=1.8W。選(xuan)擇的P-MOSFET的最(zui)大(da)允許功(gong)耗應大(da)于計算(suan)出的最(zui)大(da)功(gong)耗。

　　2 P-MOSFET的散熱

　　貼片(pian)式功率MOSFET采(cai)(cai)用印制板的(de)敷(fu)銅層來散(san)熱(re)，即在(zai)(zai)設計(ji)(ji)印制板時要留出(chu)一(yi)定的(de)散(san)熱(re)面積(ji)。例如，采(cai)(cai)用DPAK封裝(zhuang)的(de)MTD2955E在(zai)(zai)計(ji)(ji)算出(chu)PDmax=1.75W時，需11mm2散(san)熱(re)面積(ji);若PDmax=3W時，需26mm2