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BQ24060的鋰離子電池充電器

       鋰離子電池充電器的(de)(de)(de)發熱(re)(re)問題(ti)一(yi)直是(shi)電(dian)(dian)(dian)(dian)(dian)子(zi)工(gong)(gong)程師在(zai)進行鋰(li)離子(zi)電(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)設(she)計(ji)時(shi)的(de)(de)(de)難點之一(yi),如(ru)果設(she)計(ji)不周密,會(hui)帶來(lai)(lai)安全問題(ti)。從電(dian)(dian)(dian)(dian)(dian)量和容量兩方面來(lai)(lai)講,鋰(li)離子(zi)電(dian)(dian)(dian)(dian)(dian)池(chi)的(de)(de)(de)能(neng)(neng)(neng)量密度都很大,因此廣泛(fan)應用(yong)(yong)于便攜(xie)式(shi)設(she)備(bei),如(ru)PDA、MP3、手機、數碼相機等(deng)。由于高集成度線性電(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)簡單(dan)易用(yong)(yong)、成本低、體積小,因此廣泛(fan)應用(yong)(yong)于為單(dan)體鋰(li)離子(zi)電(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)。但是(shi),如(ru)果用(yong)(yong)不具(ju)備(bei)熱(re)(re)調節功能(neng)(neng)(neng)的(de)(de)(de)適配器(qi)給(gei)便攜(xie)式(shi)系統鋰(li)離子(zi)電(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian),線性充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)的(de)(de)(de)散熱(re)(re)難題(ti)就會(hui)凸顯出來(lai)(lai),難以保(bao)證在(zai)安全散熱(re)(re)范圍內工(gong)(gong)作。這里介紹一(yi)種(zhong)基于TI公司的(de)(de)(de)鋰(li)離子(zi)電(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)專用(yong)(yong)芯(xin)片BQ24060、支持(chi)熱(re)(re)調節保(bao)護功能(neng)(neng)(neng)的(de)(de)(de)鋰(li)離子(zi)電(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)的(de)(de)(de)設(she)計(ji)方法。它不僅能(neng)(neng)(neng)夠使工(gong)(gong)程師完善散熱(re)(re)方面的(de)(de)(de)考慮,同(tong)時(shi)還(huan)能(neng)(neng)(neng)極(ji)大化充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)率,盡(jin)可能(neng)(neng)(neng)縮短充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)時(shi)間,同(tong)時(shi)具(ju)備(bei)輸入過壓保(bao)護(VOP)功能(neng)(neng)(neng)。有著較強的(de)(de)(de)實(shi)用(yong)(yong)性。

1 BQ24060芯片功能特點
    BQ24060是(shi)TI公司的(de)一(yi)個高(gao)集(ji)(ji)成(cheng)(cheng)的(de)鋰(li)離子電(dian)(dian)池(chi)充(chong)(chong)(chong)電(dian)(dian)管理(li)IC,其功能(neng)引腳如圖1所示(shi),表1介紹了各(ge)引腳功能(neng)。BQ24060提供能(neng)在有限空間里完成(cheng)(cheng)多(duo)功能(neng)的(de)、安全(quan)的(de)滿充(chong)(chong)(chong)電(dian)(dian)的(de)鋰(li)離子電(dian)(dian)池(chi)充(chong)(chong)(chong)電(dian)(dian)器設計(ji),其內部集(ji)(ji)成(cheng)(cheng)了1 A功率FET以及電(dian)(dian)流(liu)傳感器,因而能(neng)夠承受(shou)高(gao)達26 V的(de)輸入電(dian)(dian)壓。該(gai)產品(pin)還(huan)提供具備獨特安全(quan)與低壓降(jiang)特性的(de)全(quan)面充(chong)(chong)(chong)電(dian)(dian)管理(li)功能(neng),以延長電(dian)(dian)池(chi)使用壽命。BQ24060可以使鋰(li)離子電(dian)(dian)池(chi)分三階段(duan)進(jin)行充(chong)(chong)(chong)電(dian)(dian):預充(chong)(chong)(chong)電(dian)(dian)方式、恒流(liu)熱(re)調(diao)節充(chong)(chong)(chong)電(dian)(dian)方式、精確恒壓充(chong)(chong)(chong)電(dian)(dian)方式。充(chong)(chong)(chong)電(dian)(dian)終止是(shi)基于一(yi)個最小電(dian)(dian)流(liu)。內部可編程充(chong)(chong)(chong)電(dian)(dian)定時器為充(chong)(chong)(chong)電(dian)(dian)終止和在熱(re)調(diao)節狀(zhuang)態動態調(diao)節提供安全(quan)保障(zhang)。


    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)統也(ye)能完全正(zheng)常工作。在終端(duan)設備(bei)制造過程(cheng)中,LDO模(mo)式的特性對全面(mian)系(xi)統測(ce)試前的電路(lu)板測(ce)試也(ye)相當有用。

 BQ24060還提供過(guo)壓(ya)保護特性,在(zai)高壓(ya)情況下,該器(qi)(qi)件能(neng)(neng)夠迅速將充電器(qi)(qi)和系(xi)統與(yu)適配器(qi)(qi)相(xiang)斷(duan)連,從而(er)實現保護功能(neng)(neng)。此外,即(ji)使在(zai)環境(jing)溫度較(jiao)高的(de)惡劣(lie)環境(jing)下,增強型熱過(guo)載(zai)保護穩壓(ya)特性也(ye)能(neng)(neng)確保BQ24060長期正(zheng)常工作,比(bi)如夏季(ji)在(zai)汽車中工作,或錯誤地連接至輸入電壓(ya)較(jiao)高的(de)適配器(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(yu)故(gu)障情況,提供電源狀態良(liang)好指示;短路與(yu)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(guo)熱,給工程師設計安全的(de)充(chong)(chong)電(dian)(dian)器提供了方便(bian)。BQ24060內部芯(xin)片溫度(du)達到預定義的(de)溫度(du)閾值后(hou)(如110℃),IC溫度(du)只要進一步提升就(jiu)會(hui)讓(rang)充(chong)(chong)電(dian)(dian)電(dian)(dian)流(liu)下(xia)降(jiang)(jiang),這(zhe)有助于限制功(gong)耗(hao),并為(wei)充(chong)(chong)電(dian)(dian)器提供熱保護。使結溫升高到熱調節(jie)的(de)極大功(gong)耗(hao)取決于PCB板布局、散熱通孔(kong)的(de)數量以及環境溫度(du)。從(cong)圖(tu)3中看出,1.2 s之后(hou),熱環路(lu)會(hui)在2 s以內將有效充(chong)(chong)電(dian)(dian)電(dian)(dian)流(liu)從(cong)1.2 A降(jiang)(jiang)到600 mA。


    采用(yong)BQ24060專用(yong)芯(xin)片設計的充(chong)電(dian)(dian)(dian)(dian)器,其充(chong)電(dian)(dian)(dian)(dian)過程(cheng)如(ru)圖4所(suo)示,充(chong)電(dian)(dian)(dian)(dian)器從(cong)預充(chong)電(dian)(dian)(dian)(dian)階(jie)段轉(zhuan)向快(kuai)(kuai)充(chong)模式時,充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流IBAT受芯(xin)片結(jie)溫(wen)TJ的限制而(er)緩慢(man)上升,其間(jian)經歷了(le)一個熱(re)調節(jie)過程(cheng)后,充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流IBAT達到快(kuai)(kuai)充(chong)電(dian)(dian)(dian)(dian)流,進(jin)入恒流充(chong)電(dian)(dian)(dian)(dian)狀態,此后結(jie)溫(wen)開始下降,保證了(le)充(chong)電(dian)(dian)(dian)(dian)器工作于(yu)安全狀態。


    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。如果充電安全(quan)計時器(qi)有(you)(you)固(gu)定設置的話(hua),就會(hui)過(guo)早終止充電。BQ24060采(cai)用動態(tai)安全(quan)計時器(qi)控制電路(lu),能在熱調(diao)節階段有(you)(you)效延(yan)長安全(quan)時間,并盡可(ke)能降低(di)安全(quan)計時器(qi)的故(gu)障幾率。從圖5中可(ke)以看出,熱調(diao)節模式下(xia)的安全(quan)計時器(qi)的響應與有(you)(you)效充電電流成(cheng)反比。

 

 外部熱(re)敏(min)(min)電(dian)阻(zu)RT1和(he)RT2可以用來設(she)(she)置選擇溫(wen)度(du)控制充電(dian)窗口(kou),如(ru)圖6所(suo)示。假設(she)(she)RTH和(he)RTC是熱(re)敏(min)(min)電(dian)阻(zu)的高低溫(wen)極限值,RT1和(he)RT2是負溫(wen)度(du)系(xi)數(shu)熱(re)敏(min)(min)電(dian)阻(zu),則RT1和(he)RT2可以用如(ru)下公式計(ji)算(suan),先計(ji)算(suan)RT2,然后將RT2代(dai)入計(ji)算(suan)RT1:

    對(dui)于(yu)某熱敏(min)電阻,如果0℃和45℃對(dui)應的阻值分別為27.28 kΩ和4.912 kΩ,代入上面兩(liang)式,得:

4.2 快充電流750 mA的設計
    快充(chong)電流(liu)的(de)設置是通過調節BQ24060的(de)第6腳ISET端(duan)與地Vss間(jian)的(de)阻(zu)抗RISET的(de)值(zhi)來決定的(de),其計算公式(shi)如下(xia):

    通過查BQ24060的電特性(xing)表可(ke)得VSET=2.5 V,KSET=335,當,IOUT=O.75 A時(shi),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(ji)(ji)時器工作時間是通(tong)過設置其(qi)(qi)第2腳(jiao)TMR與地間的阻抗RTMR來(lai)決定的。其(qi)(qi)計(ji)(ji)算公式如下:

    通過查BQ24060的(de)電特性(xing)表可(ke)得KCHG=0.1 h/kΩ,則當TCHR=5h時,有RTMR=5/0.1=50 kΩ。因此(ci),在BQ24060的(de)第2腳TMR與(yu)地間連(lian)接了一(yi)個49.9 kΩ的(de)電阻,如圖6所示(shi)。

5 PCB設計
    BQ24060采用(yong)有(you)利(li)于(yu)(yu)(yu)散熱(re)(re)(re)的(de)(de)(de)MLP封(feng)裝(zhuang)(zhuang)形式(shi),這種封(feng)裝(zhuang)(zhuang)形式(shi)可提供一(yi)(yi)(yi)個熱(re)(re)(re)端(duan)(duan)(duan)面(mian),通(tong)過(guo)直(zhi)接(jie)(jie)焊接(jie)(jie)的(de)(de)(de)方式(shi)使(shi)IC與(yu)(yu)(yu)(yu)印刷線(xian)路(lu)(lu)板間有(you)效熱(re)(re)(re)接(jie)(jie)觸,可靠(kao)焊接(jie)(jie)后,通(tong)過(guo)熱(re)(re)(re)傳遞PCB就(jiu)(jiu)(jiu)相當于(yu)(yu)(yu)一(yi)(yi)(yi)個蓄(xu)熱(re)(re)(re)裝(zhuang)(zhuang)置(zhi)。這樣在設計PCB時(shi)就(jiu)(jiu)(jiu)可以(yi)合理優(you)化設計,從(cong)而最(zui)大(da)限度地(di)(di)保(bao)證良好的(de)(de)(de)熱(re)(re)(re)傳導(dao)路(lu)(lu)徑和蓄(xu)熱(re)(re)(re)特性(xing),從(cong)而使(shi)系統獲得極佳(jia)的(de)(de)(de)熱(re)(re)(re)穩定(ding)性(xing)能。本(ben)著就(jiu)(jiu)(jiu)近(jin)原則(ze),在電(dian)(dian)(dian)源(yuan)輸(shu)(shu)入端(duan)(duan)(duan)與(yu)(yu)(yu)(yu)地(di)(di)之(zhi)間一(yi)(yi)(yi)般放(fang)(fang)置(zhi)一(yi)(yi)(yi)個1 μF以(yi)上的(de)(de)(de)陶(tao)瓷電(dian)(dian)(dian)容(rong),以(yi)加(jia)強高(gao)頻濾波(bo),并有(you)利(li)于(yu)(yu)(yu)充電(dian)(dian)(dian)器短暫熱(re)(re)(re)插(cha)拔。該(gai)設計放(fang)(fang)置(zhi)了一(yi)(yi)(yi)個4.7μF的(de)(de)(de)陶(tao)瓷電(dian)(dian)(dian)容(rong),最(zui)好通(tong)過(guo)過(guo)孔直(zhi)接(jie)(jie)與(yu)(yu)(yu)(yu)地(di)(di)層相連。該(gai)電(dian)(dian)(dian)容(rong)值應(ying)大(da)于(yu)(yu)(yu)電(dian)(dian)(dian)源(yuan)輸(shu)(shu)出(chu)端(duan)(duan)(duan)放(fang)(fang)置(zhi)的(de)(de)(de)電(dian)(dian)(dian)容(rong)值。在電(dian)(dian)(dian)源(yuan)輸(shu)(shu)出(chu)端(duan)(duan)(duan)放(fang)(fang)置(zhi)一(yi)(yi)(yi)個2.2μF的(de)(de)(de)電(dian)(dian)(dian)容(rong),同樣在布板時(shi)該(gai)電(dian)(dian)(dian)容(rong)要求就(jiu)(jiu)(jiu)近(jin)與(yu)(yu)(yu)(yu)電(dian)(dian)(dian)源(yuan)輸(shu)(shu)出(chu)端(duan)(duan)(duan)相連,并要求短距入地(di)(di),最(zui)好通(tong)過(guo)過(guo)孔直(zhi)接(jie)(jie)與(yu)(yu)(yu)(yu)地(di)(di)層相連。BAT和ISET端(duan)(duan)(duan)間,放(fang)(fang)置(zhi)一(yi)(yi)(yi)個0.47 μF的(de)(de)(de)陶(tao)瓷電(dian)(dian)(dian)容(rong),有(you)助于(yu)(yu)(yu)回(hui)路(lu)(lu)的(de)(de)(de)穩定(ding)性(xing)。在布板時(shi),所有(you)小電(dian)(dian)(dian)流的(de)(de)(de)地(di)(di)應(ying)與(yu)(yu)(yu)(yu)大(da)電(dian)(dian)(dian)流地(di)(di)相互隔離,可采用(yong)單點地(di)(di)技術使(shi)小信號地(di)(di)與(yu)(yu)(yu)(yu)大(da)信號地(di)(di)連接(jie)(jie),在走線(xian)時(shi)要注(zhu)意(yi)小信號地(di)(di)與(yu)(yu)(yu)(yu)大(da)信號地(di)(di)的(de)(de)(de)前后關系,避免共(gong)模(mo)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)輸入過壓保護(VOP)功能,有(you)著較好(hao)的實用性。

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