鋰離子電池充電器的發熱(re)問(wen)題一直(zhi)是(shi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)子(zi)工程師(shi)在進(jin)行鋰(li)(li)離子(zi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器設計時(shi)的難(nan)點之一，如果(guo)設計不(bu)(bu)周密，會(hui)(hui)帶來安全(quan)問(wen)題。從(cong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)量(liang)和容量(liang)兩方面來講(jiang)，鋰(li)(li)離子(zi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)的能(neng)(neng)量(liang)密度都(dou)很大(da)，因(yin)此廣(guang)泛應(ying)用(yong)于便攜式(shi)設備(bei)，如PDA、MP3、手(shou)機、數碼相(xiang)機等(deng)。由于高(gao)集(ji)成度線性(xing)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器簡單易用(yong)、成本低、體(ti)積小，因(yin)此廣(guang)泛應(ying)用(yong)于為單體(ti)鋰(li)(li)離子(zi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)。但是(shi)，如果(guo)用(yong)不(bu)(bu)具(ju)(ju)備(bei)熱(re)調(diao)節(jie)功(gong)(gong)能(neng)(neng)的適配器給便攜式(shi)系(xi)統鋰(li)(li)離子(zi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)，線性(xing)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器的散熱(re)難(nan)題就(jiu)會(hui)(hui)凸顯出來，難(nan)以保證(zheng)在安全(quan)散熱(re)范圍內工作。這里介紹一種基(ji)于TI公司的鋰(li)(li)離子(zi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器專用(yong)芯(xin)片BQ24060、支持熱(re)調(diao)節(jie)保護(hu)功(gong)(gong)能(neng)(neng)的鋰(li)(li)離子(zi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)器的設計方法。它不(bu)(bu)僅能(neng)(neng)夠(gou)使工程師(shi)完善散熱(re)方面的考慮，同時(shi)還(huan)能(neng)(neng)極大(da)化(hua)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)率(lv)，盡可能(neng)(neng)縮短充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)時(shi)間，同時(shi)具(ju)(ju)備(bei)輸入過(guo)壓保護(hu)(VOP)功(gong)(gong)能(neng)(neng)。有(you)著(zhu)較強的實用(yong)性(xing)。

1 BQ24060芯片功能特點
    BQ24060是(shi)TI公司的一(yi)個高集(ji)成(cheng)的鋰(li)離(li)子(zi)(zi)電(dian)池(chi)充(chong)(chong)(chong)(chong)(chong)電(dian)管理(li)IC，其(qi)功(gong)(gong)能(neng)(neng)引腳如圖1所示，表1介紹了各(ge)引腳功(gong)(gong)能(neng)(neng)。BQ24060提(ti)供能(neng)(neng)在(zai)有(you)限(xian)空間里(li)完成(cheng)多功(gong)(gong)能(neng)(neng)的、安(an)全(quan)的滿充(chong)(chong)(chong)(chong)(chong)電(dian)的鋰(li)離(li)子(zi)(zi)電(dian)池(chi)充(chong)(chong)(chong)(chong)(chong)電(dian)器設計，其(qi)內部集(ji)成(cheng)了1 A功(gong)(gong)率FET以及電(dian)流傳感(gan)器，因而能(neng)(neng)夠承受高達26 V的輸入(ru)電(dian)壓。該產(chan)品還提(ti)供具(ju)備獨特(te)安(an)全(quan)與(yu)低壓降特(te)性的全(quan)面充(chong)(chong)(chong)(chong)(chong)電(dian)管理(li)功(gong)(gong)能(neng)(neng)，以延長電(dian)池(chi)使用壽命(ming)。BQ24060可以使鋰(li)離(li)子(zi)(zi)電(dian)池(chi)分(fen)三(san)階段進行(xing)充(chong)(chong)(chong)(chong)(chong)電(dian)：預充(chong)(chong)(chong)(chong)(chong)電(dian)方(fang)式(shi)、恒流熱調節(jie)充(chong)(chong)(chong)(chong)(chong)電(dian)方(fang)式(shi)、精確恒壓充(chong)(chong)(chong)(chong)(chong)電(dian)方(fang)式(shi)。充(chong)(chong)(chong)(chong)(chong)電(dian)終(zhong)止(zhi)是(shi)基(ji)于一(yi)個最小電(dian)流。內部可編(bian)程(cheng)充(chong)(chong)(chong)(chong)(chong)電(dian)定時器為(wei)充(chong)(chong)(chong)(chong)(chong)電(dian)終(zhong)止(zhi)和在(zai)熱調節(jie)狀態動態調節(jie)提(ti)供安(an)全(quan)保(bao)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)(xi)統也能完全正常工作。在終端設備制造過程中，LDO模式的(de)特(te)性對全面系(xi)(xi)統測試前的(de)電(dian)路板測試也相當有用。

 BQ24060還提供過(guo)壓保(bao)護特性，在(zai)(zai)高(gao)壓情況下(xia)，該器(qi)(qi)件能夠迅速將充電器(qi)(qi)和系統與適(shi)配器(qi)(qi)相斷連，從而實現保(bao)護功能。此外，即使在(zai)(zai)環境(jing)溫度較(jiao)(jiao)高(gao)的惡劣環境(jing)下(xia)，增(zeng)強型(xing)熱過(guo)載保(bao)護穩壓特性也能確保(bao)BQ24060長期(qi)正常(chang)工作，比如(ru)夏季在(zai)(zai)汽(qi)車中(zhong)工作，或錯誤(wu)地連接至輸(shu)入電壓較(jiao)(jiao)高(gao)的適(shi)配器(qi)(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(qing)況(kuang)，提供電源(yuan)狀(zhuang)態良(liang)好指示；短路與熱(re)保護(hu)。

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(gong)程師設計(ji)安全的(de)充電(dian)(dian)器提(ti)(ti)供了方(fang)便(bian)。BQ24060內部芯片溫(wen)度(du)(du)達到(dao)預定義的(de)溫(wen)度(du)(du)閾值(zhi)后(如(ru)110℃)，IC溫(wen)度(du)(du)只要進一步提(ti)(ti)升(sheng)就會讓(rang)充電(dian)(dian)電(dian)(dian)流(liu)下(xia)降(jiang)，這(zhe)有助于(yu)限制功(gong)耗，并(bing)為充電(dian)(dian)器提(ti)(ti)供熱保護。使結溫(wen)升(sheng)高到(dao)熱調節的(de)極大功(gong)耗取決(jue)于(yu)PCB板布局、散熱通孔(kong)的(de)數量以(yi)及環(huan)境溫(wen)度(du)(du)。從圖3中看出，1．2 s之后，熱環(huan)路會在(zai)2 s以(yi)內將有效充電(dian)(dian)電(dian)(dian)流(liu)從1．2 A降(jiang)到(dao)600 mA。

    采用BQ24060專用芯片設(she)計的(de)充(chong)(chong)電(dian)(dian)(dian)器(qi)，其充(chong)(chong)電(dian)(dian)(dian)過程(cheng)如圖(tu)4所示，充(chong)(chong)電(dian)(dian)(dian)器(qi)從(cong)預(yu)充(chong)(chong)電(dian)(dian)(dian)階段轉(zhuan)向快充(chong)(chong)模式時，充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)IBAT受芯片結(jie)溫(wen)TJ的(de)限制而緩慢上(shang)升(sheng)，其間經歷了(le)(le)一個熱(re)調(diao)節(jie)過程(cheng)后，充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)IBAT達到(dao)快充(chong)(chong)電(dian)(dian)(dian)流(liu)，進入(ru)恒(heng)流(liu)充(chong)(chong)電(dian)(dian)(dian)狀(zhuang)態，此后結(jie)溫(wen)開始下降，保證(zheng)了(le)(le)充(chong)(chong)電(dian)(dian)(dian)器(qi)工作(zuo)于安(an)全(quan)狀(zhuang)態。

    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)安(an)(an)全計時(shi)(shi)器有(you)固定(ding)設置的話，就會過早終(zhong)止充電(dian)。BQ24060采用動態安(an)(an)全計時(shi)(shi)器控制電(dian)路(lu)，能在熱調節階(jie)段有(you)效(xiao)延長(chang)安(an)(an)全時(shi)(shi)間，并盡可(ke)能降低安(an)(an)全計時(shi)(shi)器的故障幾率。從圖(tu)5中可(ke)以(yi)看出，熱調節模式下的安(an)(an)全計時(shi)(shi)器的響應與有(you)效(xiao)充電(dian)電(dian)流成(cheng)反比。

 外部熱敏(min)電(dian)阻RT1和RT2可以(yi)用來設置選(xuan)擇溫(wen)度(du)控制(zhi)充(chong)電(dian)窗口(kou)，如圖(tu)6所(suo)示。假(jia)設RTH和RTC是熱敏(min)電(dian)阻的高低溫(wen)極(ji)限(xian)值，RT1和RT2是負(fu)溫(wen)度(du)系數(shu)熱敏(min)電(dian)阻，則RT1和RT2可以(yi)用如下公(gong)式計(ji)(ji)算(suan)(suan)，先計(ji)(ji)算(suan)(suan)RT2，然(ran)后(hou)將(jiang)RT2代入(ru)計(ji)(ji)算(suan)(suan)RT1：

    對(dui)于某熱敏電(dian)阻(zu)，如(ru)果(guo)0℃和45℃對(dui)應(ying)的阻(zu)值分(fen)別為27．28 kΩ和4．912 kΩ，代入(ru)上面兩(liang)式，得(de)：

4．2 快充電流750 mA的設計
    快充電流(liu)的設(she)置是通過(guo)調(diao)節(jie)BQ24060的第6腳ISET端與地Vss間的阻抗RISET的值(zhi)來決定(ding)的，其(qi)計算公式如(ru)下：

    通過查BQ24060的電特性(xing)表(biao)可得VSET=2．5 V，KSET=335，當(dang)，IOUT=O．75 A時，可得：

    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(de)(de)(de)安全計(ji)時器工作(zuo)時間(jian)(jian)是通(tong)過(guo)設置其第2腳(jiao)TMR與地間(jian)(jian)的(de)(de)(de)阻抗RTMR來決(jue)定(ding)的(de)(de)(de)。其計(ji)算公式如下(xia)：

    通過查BQ24060的(de)(de)電(dian)特性(xing)表可得KCHG=0．1 h／kΩ，則當TCHR=5h時，有RTMR=5／0．1=50 kΩ。因此，在(zai)BQ24060的(de)(de)第2腳TMR與地間連(lian)接了一個49．9 kΩ的(de)(de)電(dian)阻，如圖(tu)6所示。

5 PCB設計
    BQ24060采用有(you)利(li)于(yu)(yu)散熱(re)(re)(re)的(de)(de)MLP封(feng)裝(zhuang)形式，這種封(feng)裝(zhuang)形式可提(ti)供一(yi)(yi)個(ge)(ge)熱(re)(re)(re)端(duan)面，通過(guo)直接(jie)(jie)(jie)焊接(jie)(jie)(jie)的(de)(de)方(fang)式使(shi)IC與(yu)(yu)(yu)(yu)(yu)印刷線(xian)路板(ban)間有(you)效熱(re)(re)(re)接(jie)(jie)(jie)觸，可靠焊接(jie)(jie)(jie)后，通過(guo)熱(re)(re)(re)傳(chuan)遞PCB就相當(dang)于(yu)(yu)一(yi)(yi)個(ge)(ge)蓄熱(re)(re)(re)裝(zhuang)置(zhi)。這樣在(zai)(zai)設(she)計PCB時就可以(yi)合理優化設(she)計，從(cong)而(er)最(zui)大(da)限度(du)地(di)保證良好(hao)的(de)(de)熱(re)(re)(re)傳(chuan)導路徑和蓄熱(re)(re)(re)特性，從(cong)而(er)使(shi)系統(tong)獲得極佳(jia)的(de)(de)熱(re)(re)(re)穩定性能。本著就近原則(ze)，在(zai)(zai)電(dian)(dian)(dian)源(yuan)輸入(ru)端(duan)與(yu)(yu)(yu)(yu)(yu)地(di)之間一(yi)(yi)般(ban)放(fang)置(zhi)一(yi)(yi)個(ge)(ge)1 μF以(yi)上的(de)(de)陶(tao)瓷(ci)電(dian)(dian)(dian)容，以(yi)加強高(gao)頻濾波，并有(you)利(li)于(yu)(yu)充(chong)電(dian)(dian)(dian)器短暫熱(re)(re)(re)插拔。該(gai)(gai)設(she)計放(fang)置(zhi)了一(yi)(yi)個(ge)(ge)4．7μF的(de)(de)陶(tao)瓷(ci)電(dian)(dian)(dian)容，最(zui)好(hao)通過(guo)過(guo)孔直接(jie)(jie)(jie)與(yu)(yu)(yu)(yu)(yu)地(di)層(ceng)相連(lian)。該(gai)(gai)電(dian)(dian)(dian)容值(zhi)(zhi)應(ying)大(da)于(yu)(yu)電(dian)(dian)(dian)源(yuan)輸出端(duan)放(fang)置(zhi)的(de)(de)電(dian)(dian)(dian)容值(zhi)(zhi)。在(zai)(zai)電(dian)(dian)(dian)源(yuan)輸出端(duan)放(fang)置(zhi)一(yi)(yi)個(ge)(ge)2．2μF的(de)(de)電(dian)(dian)(dian)容，同(tong)樣在(zai)(zai)布(bu)板(ban)時該(gai)(gai)電(dian)(dian)(dian)容要(yao)求就近與(yu)(yu)(yu)(yu)(yu)電(dian)(dian)(dian)源(yuan)輸出端(duan)相連(lian)，并要(yao)求短距入(ru)地(di)，最(zui)好(hao)通過(guo)過(guo)孔直接(jie)(jie)(jie)與(yu)(yu)(yu)(yu)(yu)地(di)層(ceng)相連(lian)。BAT和ISET端(duan)間，放(fang)置(zhi)一(yi)(yi)個(ge)(ge)0．47 μF的(de)(de)陶(tao)瓷(ci)電(dian)(dian)(dian)容，有(you)助(zhu)于(yu)(yu)回(hui)路的(de)(de)穩定性。在(zai)(zai)布(bu)板(ban)時，所(suo)有(you)小電(dian)(dian)(dian)流的(de)(de)地(di)應(ying)與(yu)(yu)(yu)(yu)(yu)大(da)電(dian)(dian)(dian)流地(di)相互(hu)隔離(li)，可采用單點地(di)技術使(shi)小信號(hao)地(di)與(yu)(yu)(yu)(yu)(yu)大(da)信號(hao)地(di)連(lian)接(jie)(jie)(jie)，在(zai)(zai)走線(xian)時要(yao)注意小信號(hao)地(di)與(yu)(yu)(yu)(yu)(yu)大(da)信號(hao)地(di)的(de)(de)前后關(guan)系，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，同時具備輸(shu)入過壓保護(hu)(VOP)功能，有著較好的實用性。