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、電路簡單(dan)、占印制(zhi)版面積小，價格低廉，整個充電器可以(yi)在產品(pin)中(zhong)。若(ruo)采(cai)用USB端口充電，使用十分方(fang)便。

　　近年來(lai)，一些用(yong)電(dian)(dian)量(liang)(liang)稍大(da)的(de)(de)便攜式電(dian)(dian)子產品(pin)(如便攜式DVD、礦(kuang)燈、攝像機(ji)、便攜式測量(liang)(liang)儀器、小型電(dian)(dian)動工(gong)具等)往往采(cai)用(yong)1500mAh到5400mAh容量(liang)(liang)的(de)(de)鋰離子電(dian)(dian)池(chi)。若(ruo)采(cai)用(yong)500～1000mA充(chong)電(dian)(dian)電(dian)(dian)流充(chong)電(dian)(dian)器充(chong)電(dian)(dian)，則充(chong)電(dian)(dian)時(shi)間(jian)太長。若(ruo)按0.5C充(chong)電(dian)(dian)率來(lai)充(chong)3000mAh及5400mA時(shi)的(de)(de)電(dian)(dian)池(chi)時(shi)，其充(chong)電(dian)(dian)電(dian)(dian)池(chi)的(de)(de)容量(liang)(liang)要(yao)求為1500mA及2700mA。

　　有人(ren)提出：能否(fou)在1A線(xian)性(xing)充(chong)電(dian)(dian)器(qi)電(dian)(dian)路中加(jia)一個(ge)擴(kuo)(kuo)流(liu)(liu)電(dian)(dian)路，使充(chong)電(dian)(dian)電(dian)(dian)流(liu)(liu)擴(kuo)(kuo)大到2～2.5A，解(jie)決(jue)3000～5400mAh容量鋰離子電(dian)(dian)池的(de)充(chong)電(dian)(dian)問(wen)題。如果擴(kuo)(kuo)流(liu)(liu)的(de)充(chong)電(dian)(dian)器(qi)性(xing)能不錯、電(dian)(dian)路簡單、成(cheng)本不高(gao)，這(zhe)是個(ge)好主意。筆(bi)者就按這(zhe)一思路設計一個(ge)擴(kuo)(kuo)流(liu)(liu)電(dian)(dian)路。這(zhe)電(dian)(dian)路采(cai)用(yong)型號為CN3056的(de)1A線(xian)性(xing)充(chong)電(dian)(dian)器(qi)為基礎，另外加(jia)上擴(kuo)(kuo)流(liu)(liu)電(dian)(dian)路及控制(zhi)電(dian)(dian)路組成(cheng)。

　　CN3056簡(jian)介(jie)

　　CN3056充電(dian)(dian)器(qi)已在本刊(kan)(kan)2006年12期及2007年電(dian)(dian)源增(zeng)刊(kan)(kan)上(shang)介(jie)紹(shao)過(guo)(“線(xian)性(xing)鋰二次(ci)電(dian)(dian)池(chi)充電(dian)(dian)器(qi)芯片CN3056”)。這里僅作一簡(jian)介(jie)。

　　CN3056組(zu)成的充(chong)(chong)電(dian)器按恒(heng)流、恒(heng)壓模(mo)式充(chong)(chong)電(dian)，若充(chong)(chong)電(dian)電(dian)池(chi)電(dian)壓<3V，則有小電(dian)流預充(chong)(chong)電(dian)模(mo)式;充(chong)(chong)電(dian)電(dian)流可(ke)設(she)定，最大充(chong)(chong)電(dian)電(dian)流為(wei)1A;精電(dian)密度(du)4.2V ±1%、有熱調節、欠壓鎖存(cun)及(ji)電(dian)池(chi)溫(wen)度(du)檢測、超溫(wen)保護及(ji)充(chong)(chong)電(dian)狀態和溫(wen)度(du)超差指示(shi)功能(neng);10引腳小尺寸DFN封裝(3mm×3mm)。

　　若(ruo)充(chong)(chong)電(dian)(dian)(dian)率在0.5～1C之間、電(dian)(dian)(dian)池(chi)(chi)的(de)溫(wen)(wen)度在0～45℃之間(室(shi)溫(wen)(wen)充(chong)(chong)電(dian)(dian)(dian))，則CN3056充(chong)(chong)電(dian)(dian)(dian)器電(dian)(dian)(dian)路中(zhong)可省(sheng)去(qu)電(dian)(dian)(dian)池(chi)(chi)溫(wen)(wen)度檢(jian)測電(dian)(dian)(dian)路及電(dian)(dian)(dian)池(chi)(chi)超溫(wen)(wen)指示電(dian)(dian)(dian)路(引腳TEMP及FAULT端(duan)(duan)(duan)接地)。VIN是電(dian)(dian)(dian)源輸(shu)入端(duan)(duan)(duan)、CE是使能端(duan)(duan)(duan)，(高(gao)電(dian)(dian)(dian)平(ping)有效);RISET為(wei)(wei)充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流ICH設定電(dian)(dian)(dian)阻，RISET(Ω)=1800(V)/ICH(A);CHRG為(wei)(wei)充(chong)(chong)電(dian)(dian)(dian)狀態信(xin)號輸(shu)出端(duan)(duan)(duan)：充(chong)(chong)電(dian)(dian)(dian)時此端(duan)(duan)(duan)為(wei)(wei)高(gao)電(dian)(dian)(dian)平(ping)，LED亮(liang);充(chong)(chong)電(dian)(dian)(dian)結束時此端(duan)(duan)(duan)為(wei)(wei)高(gao)阻抗，LED滅;電(dian)(dian)(dian)池(chi)(chi)未裝入或接觸(chu)不良(liang)，LED閃亮(liang)。VIN一般取4.5～5V，10μF及6.8μF為(wei)(wei)輸(shu)入、輸(shu)出電(dian)(dian)(dian)容，保證充(chong)(chong)電(dian)(dian)(dian)器穩定工作。

　　充電器擴流電路

　　充(chong)電(dian)(dian)器擴流電(dian)(dian)路(lu)是在原(yuan)充(chong)電(dian)(dian)器電(dian)(dian)路(lu)上(shang)加上(shang)擴流電(dian)(dian)路(lu)組成(cheng)的。擴流電(dian)(dian)路(lu)由兩部(bu)(bu)(bu)分(fen)(fen)組成(cheng)：擴流部(bu)(bu)(bu)分(fen)(fen)及控制部(bu)(bu)(bu)分(fen)(fen)。采(cai)用(yong)CN3056充(chong)電(dian)(dian)器為基礎(chu)，加上(shang)擴流部(bu)(bu)(bu)分(fen)(fen)及控制部(bu)(bu)(bu)分(fen)(fen)電(dian)(dian)路(lu)。現(xian)分(fen)(fen)別介紹其工作原(yuan)理(li)。

　　1 擴流(liu)部分電路

　　擴流部分電(dian)路。它由P溝道功率MOSFET(VT)、R及RP組(zu)成的(de)分壓器(qi)、肖特(te)(te)基二極(ji)管(guan)D4組(zu)成。利(li)用分壓器(qi)調節P-MOSFET的(de)-VGS大小，使(shi)獲(huo)得所需擴流電(dian)流ID。P-MOSFET的(de)輸出特(te)(te)性(以Si9933DY為例)。在-VGS=2.1V、VDS>0.5V時，其輸出特(te)(te)性幾乎是一水平直(zhi)線;在不同(tong)的(de)VDS時，ID是恒流。從圖(tu)4也可以看出，在 -VGS增(zeng)加時，ID也相(xiang)應增(zeng)加。

　　2 控制部分電路

　　控制部分電路(lu)的目(mu)的是要(yao)保持(chi)原有的三階(jie)段(duan)(duan)(duan)充電模式，在預充電階(jie)段(duan)(duan)(duan)及恒壓充電階(jie)段(duan)(duan)(duan)不擴流(liu)，擴流(liu)僅在恒流(liu)階(jie)段(duan)(duan)(duan)。

　　原充電器以1A電(dian)(dian)流(liu)充電(dian)(dian)，若擴(kuo)流(liu)電(dian)(dian)流(liu)為1A，則在(zai)恒流(liu)充電(dian)(dian)階段時充電(dian)(dian)電(dian)(dian)流(liu)為2A。圖5中紅線(xian)為充電(dian)(dian)電(dian)(dian)池電(dian)(dian)壓特(te)性、黑(hei)線(xian)為充電(dian)(dian)電(dian)(dian)流(liu)特(te)性，實線(xian)為加擴(kuo)流(liu)特(te)性，虛線(xian)為未(wei)加擴(kuo)流(liu)特(te)性。　　

　　為(wei)保證擴(kuo)流在電(dian)(dian)池(chi)電(dian)(dian)壓(ya)(ya)3.0V開始，在電(dian)(dian)池(chi)電(dian)(dian)壓(ya)(ya)4.15V時結束，控制電(dian)(dian)路設(she)置了(le)窗口比較器，在電(dian)(dian)池(chi)電(dian)(dian)壓(ya)(ya)(VBAT)為(wei)3.0～4.15V之間(jian)控制P-MOSFET導通。在此窗口電(dian)(dian)壓(ya)(ya)外，P-MOSFET截止。

　　由R5、R6及(ji)(ji)(ji)R7、R8組成(cheng)(cheng)兩(liang)個(ge)電(dian)(dian)(dian)壓(ya)分(fen)壓(ya)器(檢測電(dian)(dian)(dian)池的(de)電(dian)(dian)(dian)壓(ya)VBAT)，并分(fen)別(bie)(bie)將其(qi)檢測的(de)電(dian)(dian)(dian)壓(ya)輸入比(bi)較器P1及(ji)(ji)(ji)比(bi)較器P2組成(cheng)(cheng)的(de)窗口比(bi)較器。R3、R4分(fen)別(bie)(bie)為P1及(ji)(ji)(ji)P2的(de)上拉電(dian)(dian)(dian)阻，D2、D3為隔離二極管。充電(dian)(dian)(dian)電(dian)(dian)(dian)池電(dian)(dian)(dian)壓(ya)VBAT與P1、P2的(de)輸出及(ji)(ji)(ji)P-MOSFET的(de)工(gong)作狀態。

　　P-MOSFET的(de)(de)-VGS電(dian)壓(ya)是(shi)由R2、RP往D1提(ti)供的(de)(de)，則(ze)P-MOSFET在(zai)上電(dian)后應(ying)是(shi)一直導(dao)通(tong)的(de)(de)。現要(yao)求在(zai)電(dian)池(chi)電(dian)壓(ya)(VBAT)小于(yu)(yu)3.0V及大于(yu)(yu)4.15V時(shi)P-MOSFET要(yao)關(guan)斷，則(ze)控(kong)制電(dian)路要(yao)在(zai)VBAT<3.0V及VBAT> 4.15V時(shi)，在(zai)P-MOSFET的(de)(de)柵(zha)(zha)極(ji)(ji)G上加上高電(dian)平(ping)，使其(qi)-VGS=0.7V，小于(yu)(yu)導(dao)通(tong)閾(yu)值電(dian)壓(ya)-VGS(th)，則(ze)P-MOSFET截止(zhi)(關(guan)斷)。現由P1、P2比較器及其(qi)他(ta)元器件組成(cheng)窗口(kou)比較器實現了這(zhe)一控(kong)制要(yao)求：無論(lun)是(shi)P1或P2輸出高電(dian)平(ping)時(shi)，VIN通(tong)過(guo)R4或R3及D3或D2加在(zai)P-MOSFET的(de)(de)柵(zha)(zha)極(ji)(ji)上，迫使柵(zha)(zha)極(ji)(ji)電(dian)壓(ya)為VIN=0.7V，則(ze)-VDS=0.7V而截止(zhi)，滿(man)足了控(kong)制的(de)(de)要(yao)求。

　　P-MOSFET的功耗及散熱

　　1 擴流管P-MOSFET的功耗計(ji)算

　　P-MOSFET在(zai)擴流時的功(gong)耗PD與輸出電(dian)壓VIN電(dian)池電(dian)壓VBAT、肖特基二極管(guan)的正(zheng)向壓降VF及擴流電(dian)流ID有關，其計算公式(shi)如下(xia)：

　　PD=VIN-(VBAT+VF)×ID (1)

　　其最大的(de)功耗是在VIN(max)及VBAT(min)時，即在擴流開(kai)始時(VBAT=3V)，則(ze)上式可(ke)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(de)P-MOSFET的(de)最大(da)允(yun)許功耗(hao)(hao)應大(da)于(yu)計(ji)算出(chu)的(de)最大(da)功耗(hao)(hao)。

　　2 P-MOSFET的散熱(re)

　　貼(tie)片式功(gong)率MOSFET采(cai)用(yong)印制(zhi)(zhi)板(ban)的(de)敷銅層(ceng)來散(san)(san)熱(re)(re)，即在設計印制(zhi)(zhi)板(ban)時要留出(chu)一定(ding)的(de)散(san)(san)熱(re)(re)面(mian)(mian)積(ji)。例如(ru)，采(cai)用(yong)DPAK封裝(zhuang)的(de)MTD2955E在計算出(chu)PDmax=1.75W時，需11mm2散(san)(san)熱(re)(re)面(mian)(mian)積(ji);若PDmax=3W時，需26mm2