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(liang)好、電路(lu)簡單、占印制(zhi)版面積小，價格低廉，整個充電器可以(yi)在(zai)產品中。若采(cai)用USB端口充電，使(shi)用十分方便。

　　近年來，一些用電(dian)(dian)(dian)(dian)量稍(shao)大的(de)便(bian)攜(xie)(xie)式(shi)(shi)電(dian)(dian)(dian)(dian)子產品(pin)(如(ru)便(bian)攜(xie)(xie)式(shi)(shi)DVD、礦燈、攝像機、便(bian)攜(xie)(xie)式(shi)(shi)測量儀器、小型(xing)電(dian)(dian)(dian)(dian)動工(gong)具等(deng))往(wang)往(wang)采用1500mAh到5400mAh容量的(de)鋰離子電(dian)(dian)(dian)(dian)池。若采用500～1000mA充(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流充(chong)(chong)電(dian)(dian)(dian)(dian)器充(chong)(chong)電(dian)(dian)(dian)(dian)，則充(chong)(chong)電(dian)(dian)(dian)(dian)時間(jian)太長(chang)。若按0.5C充(chong)(chong)電(dian)(dian)(dian)(dian)率來充(chong)(chong)3000mAh及5400mA時的(de)電(dian)(dian)(dian)(dian)池時，其充(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)池的(de)容量要求為(wei)1500mA及2700mA。

　　有人提出：能否在1A線性充(chong)電(dian)(dian)器(qi)電(dian)(dian)路(lu)(lu)中(zhong)加一(yi)個(ge)擴(kuo)流(liu)(liu)電(dian)(dian)路(lu)(lu)，使充(chong)電(dian)(dian)電(dian)(dian)流(liu)(liu)擴(kuo)大到2～2.5A，解決3000～5400mAh容量鋰離子電(dian)(dian)池(chi)的(de)充(chong)電(dian)(dian)問題(ti)。如果(guo)擴(kuo)流(liu)(liu)的(de)充(chong)電(dian)(dian)器(qi)性能不錯(cuo)、電(dian)(dian)路(lu)(lu)簡單、成本(ben)不高，這(zhe)是個(ge)好(hao)主(zhu)意。筆(bi)者就按這(zhe)一(yi)思(si)路(lu)(lu)設(she)計一(yi)個(ge)擴(kuo)流(liu)(liu)電(dian)(dian)路(lu)(lu)。這(zhe)電(dian)(dian)路(lu)(lu)采用型(xing)號(hao)為CN3056的(de)1A線性充(chong)電(dian)(dian)器(qi)為基礎，另(ling)外加上擴(kuo)流(liu)(liu)電(dian)(dian)路(lu)(lu)及控制(zhi)電(dian)(dian)路(lu)(lu)組成。

　　CN3056簡介(jie)

　　CN3056充電(dian)器(qi)已在本刊(kan)2006年12期(qi)及2007年電(dian)源增刊(kan)上介紹過(“線性(xing)鋰二(er)次(ci)電(dian)池充電(dian)器(qi)芯(xin)片CN3056”)。這里僅作一簡介。

　　CN3056組成的(de)充(chong)(chong)(chong)(chong)電(dian)器按(an)恒流(liu)、恒壓模式充(chong)(chong)(chong)(chong)電(dian)，若充(chong)(chong)(chong)(chong)電(dian)電(dian)池電(dian)壓<3V，則(ze)有小(xiao)電(dian)流(liu)預(yu)充(chong)(chong)(chong)(chong)電(dian)模式;充(chong)(chong)(chong)(chong)電(dian)電(dian)流(liu)可設(she)定，最大(da)充(chong)(chong)(chong)(chong)電(dian)電(dian)流(liu)為1A;精電(dian)密(mi)度4.2V ±1%、有熱調節、欠(qian)壓鎖存及電(dian)池溫度檢測、超溫保護及充(chong)(chong)(chong)(chong)電(dian)狀態和溫度超差指示功能(neng);10引腳小(xiao)尺(chi)寸DFN封裝(3mm×3mm)。

　　若充電(dian)(dian)(dian)(dian)率在0.5～1C之(zhi)間、電(dian)(dian)(dian)(dian)池的溫(wen)(wen)度(du)在0～45℃之(zhi)間(室溫(wen)(wen)充電(dian)(dian)(dian)(dian))，則CN3056充電(dian)(dian)(dian)(dian)器電(dian)(dian)(dian)(dian)路中可省去電(dian)(dian)(dian)(dian)池溫(wen)(wen)度(du)檢測電(dian)(dian)(dian)(dian)路及電(dian)(dian)(dian)(dian)池超(chao)溫(wen)(wen)指(zhi)示(shi)電(dian)(dian)(dian)(dian)路(引腳TEMP及FAULT端(duan)(duan)接地)。VIN是電(dian)(dian)(dian)(dian)源輸(shu)(shu)入(ru)端(duan)(duan)、CE是使能端(duan)(duan)，(高(gao)電(dian)(dian)(dian)(dian)平(ping)有效);RISET為(wei)(wei)充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流ICH設定電(dian)(dian)(dian)(dian)阻，RISET(Ω)=1800(V)/ICH(A);CHRG為(wei)(wei)充電(dian)(dian)(dian)(dian)狀態信號輸(shu)(shu)出端(duan)(duan)：充電(dian)(dian)(dian)(dian)時(shi)此端(duan)(duan)為(wei)(wei)高(gao)電(dian)(dian)(dian)(dian)平(ping)，LED亮(liang);充電(dian)(dian)(dian)(dian)結束時(shi)此端(duan)(duan)為(wei)(wei)高(gao)阻抗，LED滅;電(dian)(dian)(dian)(dian)池未裝入(ru)或接觸不良，LED閃亮(liang)。VIN一般取4.5～5V，10μF及6.8μF為(wei)(wei)輸(shu)(shu)入(ru)、輸(shu)(shu)出電(dian)(dian)(dian)(dian)容，保(bao)證充電(dian)(dian)(dian)(dian)器穩定工作(zuo)。

　　充電器擴流電路

　　充電(dian)(dian)器(qi)擴流(liu)電(dian)(dian)路(lu)(lu)(lu)是在(zai)原充電(dian)(dian)器(qi)電(dian)(dian)路(lu)(lu)(lu)上加上擴流(liu)電(dian)(dian)路(lu)(lu)(lu)組成的。擴流(liu)電(dian)(dian)路(lu)(lu)(lu)由(you)兩部(bu)(bu)分(fen)(fen)組成：擴流(liu)部(bu)(bu)分(fen)(fen)及(ji)控制(zhi)部(bu)(bu)分(fen)(fen)。采(cai)用CN3056充電(dian)(dian)器(qi)為(wei)基礎，加上擴流(liu)部(bu)(bu)分(fen)(fen)及(ji)控制(zhi)部(bu)(bu)分(fen)(fen)電(dian)(dian)路(lu)(lu)(lu)。現分(fen)(fen)別介紹其(qi)工作原理。

　　1 擴流部(bu)分電路(lu)

　　擴流(liu)部(bu)分(fen)電(dian)路。它由P溝道功率MOSFET(VT)、R及(ji)RP組成(cheng)的(de)分(fen)壓(ya)器、肖特(te)基二(er)極管(guan)D4組成(cheng)。利用分(fen)壓(ya)器調節P-MOSFET的(de)-VGS大(da)小，使(shi)獲(huo)得所需(xu)擴流(liu)電(dian)流(liu)ID。P-MOSFET的(de)輸出特(te)性(xing)(以Si9933DY為例)。在-VGS=2.1V、VDS>0.5V時(shi)，其輸出特(te)性(xing)幾乎是一水平(ping)直線;在不同的(de)VDS時(shi)，ID是恒流(liu)。從(cong)圖4也可以看出，在 -VGS增加(jia)時(shi)，ID也相(xiang)應增加(jia)。

　　2 控制部分電路

　　控(kong)制部分(fen)電路(lu)的目的是(shi)要保持原有的三階(jie)(jie)段充電模式(shi)，在預充電階(jie)(jie)段及恒壓充電階(jie)(jie)段不擴流，擴流僅在恒流階(jie)(jie)段。

　　原充電器以(yi)1A電(dian)流充(chong)電(dian)，若擴流電(dian)流為(wei)(wei)1A，則在恒(heng)流充(chong)電(dian)階(jie)段時充(chong)電(dian)電(dian)流為(wei)(wei)2A。圖5中紅線(xian)(xian)為(wei)(wei)充(chong)電(dian)電(dian)池電(dian)壓特(te)性(xing)、黑(hei)線(xian)(xian)為(wei)(wei)充(chong)電(dian)電(dian)流特(te)性(xing)，實線(xian)(xian)為(wei)(wei)加(jia)擴流特(te)性(xing)，虛(xu)線(xian)(xian)為(wei)(wei)未加(jia)擴流特(te)性(xing)。　　

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

　　由(you)R5、R6及(ji)R7、R8組(zu)成(cheng)兩個(ge)電(dian)壓(ya)分(fen)壓(ya)器(qi)(檢測電(dian)池的電(dian)壓(ya)VBAT)，并分(fen)別(bie)將其檢測的電(dian)壓(ya)輸(shu)入比較器(qi)P1及(ji)比較器(qi)P2組(zu)成(cheng)的窗口比較器(qi)。R3、R4分(fen)別(bie)為(wei)(wei)P1及(ji)P2的上拉電(dian)阻，D2、D3為(wei)(wei)隔離二極(ji)管。充電(dian)電(dian)池電(dian)壓(ya)VBAT與P1、P2的輸(shu)出及(ji)P-MOSFET的工作(zuo)狀態。

　　P-MOSFET的(de)(de)(de)-VGS電(dian)(dian)(dian)(dian)壓是(shi)(shi)由R2、RP往D1提供的(de)(de)(de)，則(ze)(ze)P-MOSFET在上(shang)電(dian)(dian)(dian)(dian)后應是(shi)(shi)一直導通(tong)(tong)(tong)的(de)(de)(de)。現(xian)要(yao)求(qiu)(qiu)在電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓(VBAT)小(xiao)于(yu)(yu)3.0V及(ji)大(da)于(yu)(yu)4.15V時(shi)P-MOSFET要(yao)關斷(duan)，則(ze)(ze)控(kong)(kong)制(zhi)電(dian)(dian)(dian)(dian)路要(yao)在VBAT<3.0V及(ji)VBAT> 4.15V時(shi)，在P-MOSFET的(de)(de)(de)柵(zha)極(ji)G上(shang)加上(shang)高電(dian)(dian)(dian)(dian)平，使其(qi)-VGS=0.7V，小(xiao)于(yu)(yu)導通(tong)(tong)(tong)閾值電(dian)(dian)(dian)(dian)壓-VGS(th)，則(ze)(ze)P-MOSFET截(jie)止(關斷(duan))。現(xian)由P1、P2比較器及(ji)其(qi)他元器件組成窗口比較器實現(xian)了這(zhe)一控(kong)(kong)制(zhi)要(yao)求(qiu)(qiu)：無論(lun)是(shi)(shi)P1或(huo)P2輸出高電(dian)(dian)(dian)(dian)平時(shi)，VIN通(tong)(tong)(tong)過R4或(huo)R3及(ji)D3或(huo)D2加在P-MOSFET的(de)(de)(de)柵(zha)極(ji)上(shang)，迫(po)使柵(zha)極(ji)電(dian)(dian)(dian)(dian)壓為VIN=0.7V，則(ze)(ze)-VDS=0.7V而截(jie)止，滿足(zu)了控(kong)(kong)制(zhi)的(de)(de)(de)要(yao)求(qiu)(qiu)。

　　P-MOSFET的(de)功耗及散熱

　　1 擴(kuo)流管P-MOSFET的(de)功耗計算

　　P-MOSFET在(zai)擴流(liu)時的功耗PD與輸(shu)出電壓(ya)VIN電池(chi)電壓(ya)VBAT、肖特基二極管的正(zheng)向壓(ya)降VF及擴流(liu)電流(liu)ID有關，其(qi)計(ji)算公式(shi)如下：

　　PD=VIN-(VBAT+VF)×ID (1)

　　其最大(da)的(de)功耗是(shi)在(zai)VIN(max)及(ji)VBAT(min)時，即在(zai)擴流開始時(VBAT=3V)，則(ze)上式可寫成：

　　PDmax=VIN(max)-(3V+VF)×ID (2)

　　若VIN(max)=5.2V、在ID=1A時(shi)，VF=0.4V，則PDmax=1.8W。選(xuan)擇的(de)P-MOSFET的(de)最大(da)允許功(gong)耗應大(da)于(yu)計算出的(de)最大(da)功(gong)耗。

　　2 P-MOSFET的散熱

　　貼片式功率MOSFET采用(yong)印(yin)制(zhi)板的(de)敷銅(tong)層來散熱(re)，即在設(she)計印(yin)制(zhi)板時要留(liu)出(chu)(chu)一定(ding)的(de)散熱(re)面(mian)積。例如，采用(yong)DPAK封裝的(de)MTD2955E在計算出(chu)(chu)PDmax=1.75W時，需(xu)11mm2散熱(re)面(mian)積;若PDmax=3W時，需(xu)26mm2