鋰離子電池充電器的(de)發熱(re)(re)問題(ti)一直是(shi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)子(zi)(zi)工(gong)(gong)(gong)程師在進行鋰(li)(li)離子(zi)(zi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)設計時(shi)(shi)的(de)難(nan)點之一，如果設計不(bu)周密，會帶來(lai)安(an)(an)全問題(ti)。從(cong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)量(liang)和容量(liang)兩(liang)方面(mian)來(lai)講，鋰(li)(li)離子(zi)(zi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的(de)能(neng)(neng)量(liang)密度(du)都很(hen)大，因此廣(guang)泛應(ying)用于便攜式(shi)設備，如PDA、MP3、手機(ji)、數碼相機(ji)等(deng)。由(you)于高集成度(du)線(xian)性(xing)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)簡單(dan)易(yi)用、成本(ben)低、體積小(xiao)，因此廣(guang)泛應(ying)用于為單(dan)體鋰(li)(li)離子(zi)(zi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)。但是(shi)，如果用不(bu)具備熱(re)(re)調節功能(neng)(neng)的(de)適配器(qi)給(gei)便攜式(shi)系(xi)統鋰(li)(li)離子(zi)(zi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)，線(xian)性(xing)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)的(de)散(san)熱(re)(re)難(nan)題(ti)就會凸顯出(chu)來(lai)，難(nan)以(yi)保(bao)證在安(an)(an)全散(san)熱(re)(re)范圍內(nei)工(gong)(gong)(gong)作。這里介紹一種基于TI公司的(de)鋰(li)(li)離子(zi)(zi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)專用芯(xin)片BQ24060、支持熱(re)(re)調節保(bao)護功能(neng)(neng)的(de)鋰(li)(li)離子(zi)(zi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器(qi)的(de)設計方法。它不(bu)僅能(neng)(neng)夠使工(gong)(gong)(gong)程師完善散(san)熱(re)(re)方面(mian)的(de)考慮，同(tong)時(shi)(shi)還能(neng)(neng)極大化充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)率，盡可能(neng)(neng)縮(suo)短充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)時(shi)(shi)間，同(tong)時(shi)(shi)具備輸入(ru)過壓保(bao)護(VOP)功能(neng)(neng)。有著較強的(de)實用性(xing)。

1 BQ24060芯片功能特點
    BQ24060是(shi)TI公(gong)司的(de)一個(ge)高(gao)集成(cheng)的(de)鋰離子電(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)管理IC，其(qi)功(gong)能(neng)引腳如圖(tu)1所示，表1介紹了(le)各引腳功(gong)能(neng)。BQ24060提(ti)供(gong)能(neng)在(zai)有限(xian)空間里(li)完成(cheng)多(duo)功(gong)能(neng)的(de)、安(an)全的(de)滿(man)充(chong)(chong)(chong)電(dian)(dian)(dian)的(de)鋰離子電(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)器(qi)(qi)設計，其(qi)內(nei)部集成(cheng)了(le)1 A功(gong)率FET以(yi)及電(dian)(dian)(dian)流傳感器(qi)(qi)，因而能(neng)夠承受(shou)高(gao)達26 V的(de)輸入電(dian)(dian)(dian)壓(ya)(ya)。該產(chan)品還提(ti)供(gong)具備獨(du)特(te)安(an)全與低壓(ya)(ya)降(jiang)特(te)性的(de)全面(mian)充(chong)(chong)(chong)電(dian)(dian)(dian)管理功(gong)能(neng)，以(yi)延長(chang)電(dian)(dian)(dian)池(chi)使(shi)用壽命(ming)。BQ24060可以(yi)使(shi)鋰離子電(dian)(dian)(dian)池(chi)分三(san)階(jie)段進行充(chong)(chong)(chong)電(dian)(dian)(dian)：預充(chong)(chong)(chong)電(dian)(dian)(dian)方(fang)式(shi)、恒流熱調節充(chong)(chong)(chong)電(dian)(dian)(dian)方(fang)式(shi)、精確(que)恒壓(ya)(ya)充(chong)(chong)(chong)電(dian)(dian)(dian)方(fang)式(shi)。充(chong)(chong)(chong)電(dian)(dian)(dian)終止是(shi)基于一個(ge)最小電(dian)(dian)(dian)流。內(nei)部可編程(cheng)充(chong)(chong)(chong)電(dian)(dian)(dian)定(ding)時(shi)器(qi)(qi)為充(chong)(chong)(chong)電(dian)(dian)(dian)終止和(he)在(zai)熱調節狀態動(dong)態調節提(ti)供(gong)安(an)全保障。

    BQ24060充電算法縮短了充電時間，實現了總充電量的最大化，并可保護電池免遭過熱損壞或電損壞。當電池電壓降到內部閾值以下時，BQ24060會重新開始充電，如果去除外部輸入電源，那么就會進入低功耗睡眠模式。BQ24060集成了反向阻斷保護機制，以避免在沒有DC供電的情況下發生電池漏電的情況。此外，BQ24060經過配置后，還能在LDO模式下工作，這樣，即使沒有電池，系(xi)統(tong)(tong)也(ye)能完全正常工作(zuo)。在終端設備(bei)制造過程中(zhong)，LDO模式的特性對全面系(xi)統(tong)(tong)測(ce)試前的電路板測(ce)試也(ye)相(xiang)當有用。

 BQ24060還提供過壓保(bao)護特性，在(zai)高壓情況下，該(gai)器(qi)件(jian)能夠迅(xun)速將充電(dian)器(qi)和系統與適配(pei)器(qi)相斷連，從而實現保(bao)護功能。此(ci)外，即使(shi)在(zai)環境(jing)溫(wen)度較高的惡劣環境(jing)下，增強型(xing)熱過載保(bao)護穩壓特性也(ye)能確保(bao)BQ24060長期正(zheng)常(chang)工(gong)作，比如夏季(ji)在(zai)汽(qi)車(che)中工(gong)作，或錯誤地連接至輸入電(dian)壓較高的適配(pei)器(qi)。

2 BQ24060的關鍵技術特性
    BQ24060的關鍵技術特性為：熱穩壓特性最大化充電率；計時器引腳浮動時在LDO模式下工作；通過過壓保護將最大輸入電壓設置為26 V；集成功率FET與電流傳感器，以滿足最大1 A充電應用的需求；反向漏電保護可避免電池漏電；穩壓精度在±0．5％以內；最小電流時充電終止；預充電調節具備安全計時器功能；針對LED或系統接口的狀態輸出可指示充電與故障情況，提供電源狀(zhuang)態良好指示(shi)；短(duan)路與熱保護。

3 BQ24060芯片熱調節保護功能
    一個典型的充電過程包括三個充電階段．如圖2所示：預充電階段(Pre-Conditioning Phase)、快充恒流(CC)階段(Current Regulation Phase)、恒壓(CV)終止階段(Voltage Regulation and Charge TerminationPhase)。在預充階段，在電池電壓低于某一定值．如3．O V時，電池以較低速率充電。通常情況下，當電池電壓達到3．O V時，充電器就會進入CC階段。快充CC階段通常限制在1 C電池額定值以下。如果充電率超過1 C，那么電池使用壽命就會縮短，因為節點上積存的金屬鋰會與電解質發生反應，造成永久損失。最后，充電器會進入CV階段，這時它將保持峰值電池電壓，并在充電電流下降到預定義大小時終止充電。線性充電器的功耗計算公式：

    如何改進設計才能確保充電器在安全散熱范圍內正常工作，TI公司的鋰離子電池充電專用芯片BQ24060，引入了熱調節環路，可避免充電器過熱，給工程師設計安全(quan)的(de)充電(dian)器提供了方便(bian)。BQ24060內部芯片溫(wen)度(du)達到(dao)(dao)預定(ding)義的(de)溫(wen)度(du)閾值后(如110℃)，IC溫(wen)度(du)只要進一步(bu)提升就會讓充電(dian)電(dian)流下降，這有助于(yu)限制功耗，并為(wei)充電(dian)器提供熱保護。使結溫(wen)升高到(dao)(dao)熱調節的(de)極大功耗取決(jue)于(yu)PCB板(ban)布局、散熱通孔的(de)數量以及環境溫(wen)度(du)。從圖3中看出，1．2 s之(zhi)后，熱環路會在2 s以內將有效充電(dian)電(dian)流從1．2 A降到(dao)(dao)600 mA。

    采用(yong)BQ24060專用(yong)芯(xin)片(pian)設計(ji)的充電(dian)(dian)器，其(qi)充電(dian)(dian)過(guo)程(cheng)如圖4所示，充電(dian)(dian)器從預充電(dian)(dian)階段(duan)轉(zhuan)向快(kuai)充模式時，充電(dian)(dian)電(dian)(dian)流(liu)(liu)IBAT受芯(xin)片(pian)結溫TJ的限制而緩慢上升，其(qi)間經歷了一個熱調節過(guo)程(cheng)后，充電(dian)(dian)電(dian)(dian)流(liu)(liu)IBAT達到快(kuai)充電(dian)(dian)流(liu)(liu)，進入恒流(liu)(liu)充電(dian)(dian)狀(zhuang)態(tai)，此后結溫開始下(xia)降(jiang)，保證了充電(dian)(dian)器工作于安(an)全狀(zhuang)態(tai)。

    如果非理想的工作條件引起IC結溫達到極限值Tj，充電循環激活內部控制回路系統將被調節，并控制其它充電控制回路，進而減小充電電流，直到IC結溫小于Tj，使IC工作于安全溫度下。
    熱調節通常在快充早期階段進行，不過如果在CV模式下它仍然工作的話，充電電流就會過早達到充電終止閾值。為了避免錯誤充電終止，只要散熱調節回路在工作，電池充電終止功能就會被禁用。此外，有效充電電流降低會延長電池充電時間。如果充電(dian)(dian)(dian)(dian)安(an)(an)全計時器有(you)固定設置的話，就會(hui)過早(zao)終止充電(dian)(dian)(dian)(dian)。BQ24060采(cai)用動態安(an)(an)全計時器控(kong)制電(dian)(dian)(dian)(dian)路，能在熱(re)調(diao)節階段有(you)效延長安(an)(an)全時間，并盡可能降低安(an)(an)全計時器的故障幾率。從圖5中(zhong)可以看出，熱(re)調(diao)節模(mo)式下的安(an)(an)全計時器的響應與有(you)效充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流成反比。

 外(wai)部熱敏(min)電(dian)(dian)阻(zu)RT1和RT2可(ke)以(yi)用來設置選擇溫度(du)(du)控制(zhi)充電(dian)(dian)窗(chuang)口(kou)，如圖6所(suo)示(shi)。假設RTH和RTC是熱敏(min)電(dian)(dian)阻(zu)的高低溫極限值，RT1和RT2是負(fu)溫度(du)(du)系數熱敏(min)電(dian)(dian)阻(zu)，則RT1和RT2可(ke)以(yi)用如下公(gong)式計(ji)算，先計(ji)算RT2，然后(hou)將(jiang)RT2代入計(ji)算RT1：

    對(dui)于某熱敏電(dian)阻，如(ru)果0℃和(he)45℃對(dui)應(ying)的阻值分別為27．28 kΩ和(he)4．912 kΩ，代(dai)入上(shang)面(mian)兩(liang)式，得：

4．2 快充電流750 mA的設計
    快充電流的(de)設(she)置(zhi)是(shi)通過(guo)調節BQ24060的(de)第(di)6腳ISET端與地Vss間(jian)的(de)阻抗RISET的(de)值來決定的(de)，其計算公(gong)式(shi)如下：

    通(tong)過查(cha)BQ24060的電特(te)性表可(ke)得VSET=2．5 V，KSET=335，當，IOUT=O．75 A時，可(ke)得：

    因此，在BQ24060的第6腳ISET端與地間連接了一個1．13 kΩ的電阻，如圖6所示。
4．3 5h安全計時器工作時間的設計
    BQ24060的安全計時(shi)器工作時(shi)間是通過設(she)置(zhi)其(qi)第2腳TMR與地間的阻抗RTMR來決定的。其(qi)計算公式如下(xia)：

    通過查BQ24060的(de)電特性表可得KCHG=0．1 h／kΩ，則當(dang)TCHR=5h時，有RTMR=5／0．1=50 kΩ。因此，在(zai)BQ24060的(de)第2腳TMR與(yu)地間連接了(le)一個49．9 kΩ的(de)電阻，如圖6所示。

5 PCB設計
    BQ24060采用有利于(yu)散(san)熱(re)(re)(re)的(de)(de)MLP封裝形式(shi)，這種(zhong)封裝形式(shi)可(ke)提(ti)供一(yi)(yi)(yi)個熱(re)(re)(re)端(duan)面(mian)，通(tong)過(guo)(guo)直接(jie)焊接(jie)的(de)(de)方式(shi)使IC與印刷(shua)線路(lu)板(ban)間有效熱(re)(re)(re)接(jie)觸，可(ke)靠(kao)焊接(jie)后(hou)，通(tong)過(guo)(guo)熱(re)(re)(re)傳遞PCB就(jiu)(jiu)相(xiang)(xiang)當于(yu)一(yi)(yi)(yi)個蓄熱(re)(re)(re)裝置(zhi)。這樣(yang)(yang)在(zai)(zai)(zai)設計(ji)PCB時(shi)(shi)就(jiu)(jiu)可(ke)以(yi)合(he)理優化設計(ji)，從而(er)最大(da)(da)限度地(di)(di)(di)保證良好(hao)的(de)(de)熱(re)(re)(re)傳導(dao)路(lu)徑和蓄熱(re)(re)(re)特性，從而(er)使系(xi)統獲(huo)得極佳的(de)(de)熱(re)(re)(re)穩定(ding)性能(neng)。本著就(jiu)(jiu)近原則，在(zai)(zai)(zai)電(dian)(dian)源(yuan)輸(shu)(shu)入(ru)端(duan)與地(di)(di)(di)之(zhi)間一(yi)(yi)(yi)般放(fang)置(zhi)一(yi)(yi)(yi)個1 μF以(yi)上的(de)(de)陶瓷電(dian)(dian)容(rong)(rong)，以(yi)加強高(gao)頻濾波，并有利于(yu)充電(dian)(dian)器短暫熱(re)(re)(re)插(cha)拔。該設計(ji)放(fang)置(zhi)了一(yi)(yi)(yi)個4．7μF的(de)(de)陶瓷電(dian)(dian)容(rong)(rong)，最好(hao)通(tong)過(guo)(guo)過(guo)(guo)孔直接(jie)與地(di)(di)(di)層相(xiang)(xiang)連(lian)。該電(dian)(dian)容(rong)(rong)值(zhi)應(ying)大(da)(da)于(yu)電(dian)(dian)源(yuan)輸(shu)(shu)出端(duan)放(fang)置(zhi)的(de)(de)電(dian)(dian)容(rong)(rong)值(zhi)。在(zai)(zai)(zai)電(dian)(dian)源(yuan)輸(shu)(shu)出端(duan)放(fang)置(zhi)一(yi)(yi)(yi)個2．2μF的(de)(de)電(dian)(dian)容(rong)(rong)，同樣(yang)(yang)在(zai)(zai)(zai)布板(ban)時(shi)(shi)該電(dian)(dian)容(rong)(rong)要求就(jiu)(jiu)近與電(dian)(dian)源(yuan)輸(shu)(shu)出端(duan)相(xiang)(xiang)連(lian)，并要求短距入(ru)地(di)(di)(di)，最好(hao)通(tong)過(guo)(guo)過(guo)(guo)孔直接(jie)與地(di)(di)(di)層相(xiang)(xiang)連(lian)。BAT和ISET端(duan)間，放(fang)置(zhi)一(yi)(yi)(yi)個0．47 μF的(de)(de)陶瓷電(dian)(dian)容(rong)(rong)，有助于(yu)回路(lu)的(de)(de)穩定(ding)性。在(zai)(zai)(zai)布板(ban)時(shi)(shi)，所有小(xiao)電(dian)(dian)流的(de)(de)地(di)(di)(di)應(ying)與大(da)(da)電(dian)(dian)流地(di)(di)(di)相(xiang)(xiang)互隔離，可(ke)采用單點(dian)地(di)(di)(di)技術使小(xiao)信號地(di)(di)(di)與大(da)(da)信號地(di)(di)(di)連(lian)接(jie)，在(zai)(zai)(zai)走線時(shi)(shi)要注意小(xiao)信號地(di)(di)(di)與大(da)(da)信號地(di)(di)(di)的(de)(de)前后(hou)關(guan)系(xi)，避(bi)免共模干擾。

     實際上，鋰離子電池充電器的要求比較高，既要實現快充，又要保證安全，其技術難點就在于電池的發熱問題。這里介紹的支持熱調節保護功能的鋰離子電池充電器的設計方法，不僅能夠使工程師完善散熱方面的考慮，同時還能極大化充電率，盡可能縮短充電時間，同時具備(bei)輸(shu)入過壓(ya)保護(VOP)功能，有著較好的實用(yong)性。