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、電路簡單、占印(yin)制版面積小(xiao)，價格低廉，整個(ge)充電器可以(yi)在產品中。若采用(yong)USB端口(kou)充電，使用(yong)十(shi)分方便。

　　近(jin)年來，一些(xie)用電量稍大的便(bian)攜式電子產品(pin)(如便(bian)攜式DVD、礦(kuang)燈、攝(she)像機、便(bian)攜式測量儀(yi)器、小型(xing)電動工具等)往(wang)(wang)往(wang)(wang)采用1500mAh到5400mAh容量的鋰(li)離子電池。若采用500～1000mA充(chong)電電流充(chong)電器充(chong)電，則充(chong)電時(shi)(shi)間太長。若按0.5C充(chong)電率來充(chong)3000mAh及5400mA時(shi)(shi)的電池時(shi)(shi)，其充(chong)電電池的容量要求為(wei)1500mA及2700mA。

　　有人提出：能否(fou)在1A線性充(chong)電(dian)(dian)(dian)器電(dian)(dian)(dian)路(lu)(lu)(lu)中加(jia)一個(ge)擴流(liu)電(dian)(dian)(dian)路(lu)(lu)(lu)，使充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)擴大到2～2.5A，解決3000～5400mAh容量鋰離子電(dian)(dian)(dian)池的充(chong)電(dian)(dian)(dian)問題。如果擴流(liu)的充(chong)電(dian)(dian)(dian)器性能不錯、電(dian)(dian)(dian)路(lu)(lu)(lu)簡(jian)單、成本不高，這(zhe)是個(ge)好主意。筆(bi)者就按這(zhe)一思(si)路(lu)(lu)(lu)設計(ji)一個(ge)擴流(liu)電(dian)(dian)(dian)路(lu)(lu)(lu)。這(zhe)電(dian)(dian)(dian)路(lu)(lu)(lu)采(cai)用型(xing)號為(wei)CN3056的1A線性充(chong)電(dian)(dian)(dian)器為(wei)基礎，另(ling)外加(jia)上擴流(liu)電(dian)(dian)(dian)路(lu)(lu)(lu)及控制電(dian)(dian)(dian)路(lu)(lu)(lu)組成。

　　CN3056簡介

　　CN3056充(chong)電器已(yi)在本刊2006年(nian)12期及2007年(nian)電源增(zeng)刊上(shang)介紹過(“線性鋰二次電池充(chong)電器芯片CN3056”)。這里僅作一簡介。

　　CN3056組成的充電(dian)(dian)(dian)(dian)器按恒流、恒壓模式充電(dian)(dian)(dian)(dian)，若充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓<3V，則有小電(dian)(dian)(dian)(dian)流預充電(dian)(dian)(dian)(dian)模式;充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流可設定(ding)，最大充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流為1A;精電(dian)(dian)(dian)(dian)密(mi)度4.2V ±1%、有熱調節、欠壓鎖存及(ji)電(dian)(dian)(dian)(dian)池溫度檢測、超溫保護及(ji)充電(dian)(dian)(dian)(dian)狀態和(he)溫度超差指(zhi)示功能(neng);10引腳小尺寸(cun)DFN封(feng)裝(3mm×3mm)。

　　若充(chong)(chong)電(dian)(dian)率在0.5～1C之間、電(dian)(dian)池(chi)(chi)(chi)的溫(wen)(wen)(wen)度在0～45℃之間(室(shi)溫(wen)(wen)(wen)充(chong)(chong)電(dian)(dian))，則(ze)CN3056充(chong)(chong)電(dian)(dian)器電(dian)(dian)路(lu)中可省去電(dian)(dian)池(chi)(chi)(chi)溫(wen)(wen)(wen)度檢測電(dian)(dian)路(lu)及電(dian)(dian)池(chi)(chi)(chi)超溫(wen)(wen)(wen)指(zhi)示電(dian)(dian)路(lu)(引腳TEMP及FAULT端(duan)接地(di))。VIN是電(dian)(dian)源輸(shu)入端(duan)、CE是使能端(duan)，(高電(dian)(dian)平(ping)有(you)效(xiao));RISET為(wei)(wei)(wei)充(chong)(chong)電(dian)(dian)電(dian)(dian)流ICH設(she)定(ding)電(dian)(dian)阻，RISET(Ω)=1800(V)/ICH(A);CHRG為(wei)(wei)(wei)充(chong)(chong)電(dian)(dian)狀態(tai)信號(hao)輸(shu)出(chu)端(duan)：充(chong)(chong)電(dian)(dian)時此端(duan)為(wei)(wei)(wei)高電(dian)(dian)平(ping)，LED亮;充(chong)(chong)電(dian)(dian)結束時此端(duan)為(wei)(wei)(wei)高阻抗(kang)，LED滅;電(dian)(dian)池(chi)(chi)(chi)未裝入或接觸不良，LED閃亮。VIN一般取4.5～5V，10μF及6.8μF為(wei)(wei)(wei)輸(shu)入、輸(shu)出(chu)電(dian)(dian)容，保證充(chong)(chong)電(dian)(dian)器穩定(ding)工作。

　　充電器擴流電路

　　充電器擴(kuo)流電路(lu)是在原充電器電路(lu)上(shang)加(jia)上(shang)擴(kuo)流電路(lu)組成的(de)。擴(kuo)流電路(lu)由兩部(bu)(bu)分(fen)組成：擴(kuo)流部(bu)(bu)分(fen)及控制部(bu)(bu)分(fen)。采用CN3056充電器為基礎，加(jia)上(shang)擴(kuo)流部(bu)(bu)分(fen)及控制部(bu)(bu)分(fen)電路(lu)。現分(fen)別介(jie)紹其工(gong)作(zuo)原理。

　　1 擴流部分電路(lu)

　　擴流部分電(dian)路。它由P溝道功(gong)率MOSFET(VT)、R及RP組(zu)成的(de)分壓器(qi)、肖特基二極管D4組(zu)成。利用分壓器(qi)調(diao)節(jie)P-MOSFET的(de)-VGS大小，使獲得所需擴流電(dian)流ID。P-MOSFET的(de)輸出(chu)特性(xing)(以Si9933DY為例)。在-VGS=2.1V、VDS>0.5V時，其輸出(chu)特性(xing)幾乎是一水(shui)平(ping)直線(xian);在不同的(de)VDS時，ID是恒(heng)流。從圖4也可以看(kan)出(chu)，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(dian)路(lu)的目的是要保持(chi)原有的三階(jie)段(duan)(duan)充(chong)電(dian)模式，在預充(chong)電(dian)階(jie)段(duan)(duan)及恒壓充(chong)電(dian)階(jie)段(duan)(duan)不擴流，擴流僅在恒流階(jie)段(duan)(duan)。

　　原充電器以1A電(dian)(dian)流充(chong)(chong)電(dian)(dian)，若擴(kuo)流電(dian)(dian)流為(wei)1A，則在(zai)恒流充(chong)(chong)電(dian)(dian)階段時(shi)充(chong)(chong)電(dian)(dian)電(dian)(dian)流為(wei)2A。圖5中(zhong)紅線為(wei)充(chong)(chong)電(dian)(dian)電(dian)(dian)池電(dian)(dian)壓(ya)特(te)性(xing)、黑線為(wei)充(chong)(chong)電(dian)(dian)電(dian)(dian)流特(te)性(xing)，實(shi)線為(wei)加(jia)擴(kuo)流特(te)性(xing)，虛線為(wei)未加(jia)擴(kuo)流特(te)性(xing)。　　

　　為保證擴流在電(dian)(dian)池電(dian)(dian)壓3.0V開始，在電(dian)(dian)池電(dian)(dian)壓4.15V時(shi)結束(shu)，控制(zhi)電(dian)(dian)路設置了窗口比較器，在電(dian)(dian)池電(dian)(dian)壓(VBAT)為3.0～4.15V之間控制(zhi)P-MOSFET導通。在此(ci)窗口電(dian)(dian)壓外，P-MOSFET截止。

　　由(you)R5、R6及R7、R8組成(cheng)兩個電(dian)壓分(fen)壓器(qi)(qi)(檢測(ce)電(dian)池的(de)(de)電(dian)壓VBAT)，并分(fen)別將其(qi)檢測(ce)的(de)(de)電(dian)壓輸入比(bi)較器(qi)(qi)P1及比(bi)較器(qi)(qi)P2組成(cheng)的(de)(de)窗口比(bi)較器(qi)(qi)。R3、R4分(fen)別為(wei)P1及P2的(de)(de)上拉電(dian)阻，D2、D3為(wei)隔離二極管。充電(dian)電(dian)池電(dian)壓VBAT與P1、P2的(de)(de)輸出及P-MOSFET的(de)(de)工作狀態(tai)。

　　P-MOSFET的-VGS電(dian)壓(ya)是(shi)由(you)(you)R2、RP往D1提供(gong)的，則P-MOSFET在(zai)(zai)上(shang)電(dian)后應是(shi)一(yi)直導通的。現要(yao)(yao)(yao)(yao)求(qiu)(qiu)在(zai)(zai)電(dian)池電(dian)壓(ya)(VBAT)小于(yu)3.0V及(ji)(ji)大于(yu)4.15V時P-MOSFET要(yao)(yao)(yao)(yao)關斷，則控(kong)制電(dian)路要(yao)(yao)(yao)(yao)在(zai)(zai)VBAT<3.0V及(ji)(ji)VBAT> 4.15V時，在(zai)(zai)P-MOSFET的柵(zha)極G上(shang)加上(shang)高電(dian)平，使(shi)其-VGS=0.7V，小于(yu)導通閾值電(dian)壓(ya)-VGS(th)，則P-MOSFET截(jie)止(關斷)。現由(you)(you)P1、P2比較器及(ji)(ji)其他(ta)元(yuan)器件組(zu)成窗口比較器實現了這一(yi)控(kong)制要(yao)(yao)(yao)(yao)求(qiu)(qiu)：無論是(shi)P1或P2輸出高電(dian)平時，VIN通過(guo)R4或R3及(ji)(ji)D3或D2加在(zai)(zai)P-MOSFET的柵(zha)極上(shang)，迫使(shi)柵(zha)極電(dian)壓(ya)為VIN=0.7V，則-VDS=0.7V而截(jie)止，滿足了控(kong)制的要(yao)(yao)(yao)(yao)求(qiu)(qiu)。

　　P-MOSFET的功耗(hao)及散(san)熱

　　1 擴流管P-MOSFET的功耗計算

　　P-MOSFET在擴(kuo)流時的(de)功耗PD與輸出(chu)電壓(ya)VIN電池電壓(ya)VBAT、肖特基二(er)極管的(de)正(zheng)向壓(ya)降(jiang)VF及擴(kuo)流電流ID有(you)關，其計(ji)算公式如下：

　　PD=VIN-(VBAT+VF)×ID (1)

　　其最大的功耗(hao)是(shi)在(zai)VIN(max)及VBAT(min)時(shi)，即(ji)在(zai)擴(kuo)流開始時(shi)(VBAT=3V)，則(ze)上式可(ke)寫成(cheng)：

　　PDmax=VIN(max)-(3V+VF)×ID (2)

　　若VIN(max)=5.2V、在(zai)ID=1A時，VF=0.4V，則(ze)PDmax=1.8W。選擇的P-MOSFET的最(zui)大(da)允許功耗應大(da)于計(ji)算出的最(zui)大(da)功耗。

　　2 P-MOSFET的散熱

　　貼(tie)片式(shi)功率MOSFET采(cai)(cai)用印制板(ban)的敷銅(tong)層來(lai)散熱，即(ji)在設計印制板(ban)時(shi)要留出一定的散熱面積。例如(ru)，采(cai)(cai)用DPAK封(feng)裝的MTD2955E在計算出PDmax=1.75W時(shi)，需11mm2散熱面積;若PDmax=3W時(shi)，需26mm2