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)齊全、性能(neng)良好(hao)、電路簡單(dan)、占印制版面積小，價格(ge)低廉，整個(ge)充(chong)(chong)電器可以(yi)在(zai)產品中。若采用(yong)USB端口(kou)充(chong)(chong)電，使用(yong)十分(fen)方便。

　　近年(nian)來(lai)，一些(xie)用電(dian)量稍大的便(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(qi)、小型電(dian)動工(gong)具等)往往采(cai)用1500mAh到5400mAh容(rong)量的鋰離子電(dian)池。若采(cai)用500～1000mA充(chong)(chong)電(dian)電(dian)流充(chong)(chong)電(dian)器(qi)充(chong)(chong)電(dian)，則充(chong)(chong)電(dian)時(shi)(shi)間(jian)太長。若按(an)0.5C充(chong)(chong)電(dian)率來(lai)充(chong)(chong)3000mAh及5400mA時(shi)(shi)的電(dian)池時(shi)(shi)，其充(chong)(chong)電(dian)電(dian)池的容(rong)量要求(qiu)為1500mA及2700mA。

　　有人提出：能否(fou)在(zai)1A線(xian)性(xing)充(chong)(chong)電器電路(lu)中加一(yi)個(ge)擴(kuo)流(liu)(liu)電路(lu)，使充(chong)(chong)電電流(liu)(liu)擴(kuo)大到2～2.5A，解決3000～5400mAh容(rong)量鋰(li)離(li)子電池的(de)充(chong)(chong)電問題。如果(guo)擴(kuo)流(liu)(liu)的(de)充(chong)(chong)電器性(xing)能不(bu)錯、電路(lu)簡單、成本(ben)不(bu)高(gao)，這是個(ge)好主意。筆(bi)者就(jiu)按這一(yi)思路(lu)設計一(yi)個(ge)擴(kuo)流(liu)(liu)電路(lu)。這電路(lu)采用型號為CN3056的(de)1A線(xian)性(xing)充(chong)(chong)電器為基礎(chu)，另(ling)外加上擴(kuo)流(liu)(liu)電路(lu)及控制電路(lu)組(zu)成。

　　CN3056簡介

　　CN3056充(chong)電(dian)(dian)器已在本刊2006年12期及2007年電(dian)(dian)源增刊上介紹(shao)過(“線性鋰二次(ci)電(dian)(dian)池充(chong)電(dian)(dian)器芯片(pian)CN3056”)。這里僅作一(yi)簡介。

　　CN3056組成(cheng)的(de)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器按恒流、恒壓模式充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)，若(ruo)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)壓<3V，則有小電(dian)(dian)(dian)(dian)(dian)流預充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)模式;充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流可設定(ding)，最大充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流為1A;精電(dian)(dian)(dian)(dian)(dian)密度4.2V ±1%、有熱(re)調節、欠壓鎖存及(ji)電(dian)(dian)(dian)(dian)(dian)池溫(wen)(wen)度檢測、超溫(wen)(wen)保護及(ji)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)狀(zhuang)態和溫(wen)(wen)度超差指示功能(neng);10引腳(jiao)小尺寸DFN封裝(3mm×3mm)。

　　若充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)率在0.5～1C之間(jian)、電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的(de)溫(wen)(wen)度在0～45℃之間(jian)(室溫(wen)(wen)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian))，則CN3056充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器電(dian)(dian)(dian)(dian)(dian)路中可省(sheng)去電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)溫(wen)(wen)度檢測電(dian)(dian)(dian)(dian)(dian)路及(ji)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)超溫(wen)(wen)指示電(dian)(dian)(dian)(dian)(dian)路(引(yin)腳TEMP及(ji)FAULT端接地)。VIN是(shi)電(dian)(dian)(dian)(dian)(dian)源輸(shu)入(ru)端、CE是(shi)使能端，(高電(dian)(dian)(dian)(dian)(dian)平(ping)有效(xiao));RISET為充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流ICH設定(ding)電(dian)(dian)(dian)(dian)(dian)阻，RISET(Ω)=1800(V)/ICH(A);CHRG為充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)狀態信(xin)號輸(shu)出端：充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)時此(ci)端為高電(dian)(dian)(dian)(dian)(dian)平(ping)，LED亮;充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)結束時此(ci)端為高阻抗，LED滅(mie);電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)未裝入(ru)或接觸不良，LED閃亮。VIN一般取4.5～5V，10μF及(ji)6.8μF為輸(shu)入(ru)、輸(shu)出電(dian)(dian)(dian)(dian)(dian)容，保(bao)證充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器穩定(ding)工(gong)作。

　　充電器擴流電路

　　充電(dian)(dian)(dian)器擴流電(dian)(dian)(dian)路(lu)是在原充電(dian)(dian)(dian)器電(dian)(dian)(dian)路(lu)上加上擴流電(dian)(dian)(dian)路(lu)組成的(de)。擴流電(dian)(dian)(dian)路(lu)由兩部(bu)分組成：擴流部(bu)分及(ji)控(kong)制(zhi)部(bu)分。采用CN3056充電(dian)(dian)(dian)器為(wei)基礎，加上擴流部(bu)分及(ji)控(kong)制(zhi)部(bu)分電(dian)(dian)(dian)路(lu)。現(xian)分別介紹其工作原理。

　　1 擴流(liu)部分電路(lu)

　　擴流部分電(dian)(dian)路(lu)。它(ta)由P溝道功(gong)率(lv)MOSFET(VT)、R及RP組成(cheng)的分壓器、肖特基二極管D4組成(cheng)。利用(yong)分壓器調(diao)節(jie)P-MOSFET的-VGS大(da)小，使獲(huo)得所需擴流電(dian)(dian)流ID。P-MOSFET的輸(shu)出(chu)(chu)特性(以(yi)Si9933DY為(wei)例)。在(zai)-VGS=2.1V、VDS>0.5V時(shi)，其輸(shu)出(chu)(chu)特性幾乎是(shi)一水平直線;在(zai)不同的VDS時(shi)，ID是(shi)恒(heng)流。從圖4也可以(yi)看出(chu)(chu)，在(zai) -VGS增加(jia)時(shi)，ID也相應(ying)增加(jia)。

　　2 控制部(bu)分電路

　　控制部分電路的目(mu)的是要(yao)保持原有的三(san)階(jie)段(duan)(duan)(duan)充電模式，在預充電階(jie)段(duan)(duan)(duan)及(ji)恒(heng)壓充電階(jie)段(duan)(duan)(duan)不(bu)擴流(liu)，擴流(liu)僅在恒(heng)流(liu)階(jie)段(duan)(duan)(duan)。

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

　　為(wei)保證(zheng)擴(kuo)流(liu)在電(dian)(dian)(dian)池電(dian)(dian)(dian)壓(ya)3.0V開始，在電(dian)(dian)(dian)池電(dian)(dian)(dian)壓(ya)4.15V時結束，控制電(dian)(dian)(dian)路(lu)設(she)置了(le)窗口(kou)比(bi)較器，在電(dian)(dian)(dian)池電(dian)(dian)(dian)壓(ya)(VBAT)為(wei)3.0～4.15V之間控制P-MOSFET導通。在此(ci)窗口(kou)電(dian)(dian)(dian)壓(ya)外，P-MOSFET截止。

　　由R5、R6及R7、R8組(zu)成兩(liang)個(ge)電(dian)(dian)壓分(fen)(fen)壓器(qi)(檢(jian)測電(dian)(dian)池的(de)(de)電(dian)(dian)壓VBAT)，并分(fen)(fen)別將(jiang)其檢(jian)測的(de)(de)電(dian)(dian)壓輸入比較(jiao)器(qi)P1及比較(jiao)器(qi)P2組(zu)成的(de)(de)窗口比較(jiao)器(qi)。R3、R4分(fen)(fen)別為(wei)P1及P2的(de)(de)上拉電(dian)(dian)阻，D2、D3為(wei)隔離二極管。充(chong)電(dian)(dian)電(dian)(dian)池電(dian)(dian)壓VBAT與(yu)P1、P2的(de)(de)輸出及P-MOSFET的(de)(de)工作(zuo)狀態。

　　P-MOSFET的(de)-VGS電(dian)壓(ya)是(shi)由(you)R2、RP往D1提供的(de)，則(ze)P-MOSFET在(zai)上(shang)(shang)電(dian)后應是(shi)一(yi)直導(dao)通(tong)的(de)。現要(yao)求在(zai)電(dian)池電(dian)壓(ya)(VBAT)小于(yu)3.0V及(ji)大于(yu)4.15V時P-MOSFET要(yao)關(guan)斷(duan)，則(ze)控制電(dian)路要(yao)在(zai)VBAT<3.0V及(ji)VBAT> 4.15V時，在(zai)P-MOSFET的(de)柵(zha)(zha)極G上(shang)(shang)加上(shang)(shang)高電(dian)平，使其-VGS=0.7V，小于(yu)導(dao)通(tong)閾(yu)值(zhi)電(dian)壓(ya)-VGS(th)，則(ze)P-MOSFET截(jie)止(關(guan)斷(duan))。現由(you)P1、P2比較器(qi)(qi)及(ji)其他元器(qi)(qi)件組成窗口比較器(qi)(qi)實現了(le)這一(yi)控制要(yao)求：無論是(shi)P1或(huo)P2輸出(chu)高電(dian)平時，VIN通(tong)過R4或(huo)R3及(ji)D3或(huo)D2加在(zai)P-MOSFET的(de)柵(zha)(zha)極上(shang)(shang)，迫使柵(zha)(zha)極電(dian)壓(ya)為(wei)VIN=0.7V，則(ze)-VDS=0.7V而截(jie)止，滿足了(le)控制的(de)要(yao)求。

　　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(liu)時的(de)功耗PD與輸出電(dian)壓(ya)VIN電(dian)池電(dian)壓(ya)VBAT、肖特(te)基二極管的(de)正(zheng)向壓(ya)降VF及擴(kuo)流(liu)電(dian)流(liu)ID有(you)關，其計算公(gong)式(shi)如下：

　　PD=VIN-(VBAT+VF)×ID (1)

　　其(qi)最(zui)大的功(gong)耗是(shi)在VIN(max)及VBAT(min)時(shi)(shi)，即在擴(kuo)流開始時(shi)(shi)(VBAT=3V)，則(ze)上(shang)式可(ke)寫(xie)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(da)允許功耗應(ying)大(da)于計算出(chu)的最(zui)大(da)功耗。

　　2 P-MOSFET的散熱

　　貼(tie)片(pian)式功率(lv)MOSFET采用(yong)印(yin)制(zhi)板的(de)敷銅層來散熱，即在設計(ji)印(yin)制(zhi)板時要留出一定的(de)散熱面積。例如，采用(yong)DPAK封裝(zhuang)的(de)MTD2955E在計(ji)算出PDmax=1.75W時，需11mm2散熱面積;若PDmax=3W時，需26mm2