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BQ24060的鋰離子電池充電器

       鋰離子電池充電器的發(fa)熱問(wen)題(ti)一直是電(dian)(dian)(dian)子(zi)(zi)工(gong)程(cheng)師在進行鋰(li)離(li)子(zi)(zi)電(dian)(dian)(dian)池(chi)(chi)(chi)充電(dian)(dian)(dian)器設(she)計(ji)時(shi)(shi)(shi)的難(nan)(nan)(nan)點之一,如(ru)果設(she)計(ji)不(bu)周密(mi),會(hui)帶來(lai)安全問(wen)題(ti)。從(cong)電(dian)(dian)(dian)量(liang)和容量(liang)兩(liang)方面(mian)來(lai)講,鋰(li)離(li)子(zi)(zi)電(dian)(dian)(dian)池(chi)(chi)(chi)的能(neng)量(liang)密(mi)度都很(hen)大(da),因(yin)(yin)此廣泛應用(yong)(yong)于便(bian)攜式(shi)設(she)備(bei)(bei)(bei),如(ru)PDA、MP3、手機(ji)、數碼相(xiang)機(ji)等(deng)。由(you)于高集成度線(xian)(xian)性電(dian)(dian)(dian)池(chi)(chi)(chi)充電(dian)(dian)(dian)器簡(jian)單易(yi)用(yong)(yong)、成本低、體積小,因(yin)(yin)此廣泛應用(yong)(yong)于為單體鋰(li)離(li)子(zi)(zi)電(dian)(dian)(dian)池(chi)(chi)(chi)充電(dian)(dian)(dian)。但是,如(ru)果用(yong)(yong)不(bu)具(ju)(ju)備(bei)(bei)(bei)熱調(diao)節(jie)功(gong)能(neng)的適配器給便(bian)攜式(shi)系統鋰(li)離(li)子(zi)(zi)電(dian)(dian)(dian)池(chi)(chi)(chi)充電(dian)(dian)(dian),線(xian)(xian)性充電(dian)(dian)(dian)器的散熱難(nan)(nan)(nan)題(ti)就會(hui)凸(tu)顯出來(lai),難(nan)(nan)(nan)以(yi)保證在安全散熱范圍內工(gong)作。這里(li)介紹一種基于TI公(gong)司的鋰(li)離(li)子(zi)(zi)電(dian)(dian)(dian)池(chi)(chi)(chi)充電(dian)(dian)(dian)器專用(yong)(yong)芯片BQ24060、支持熱調(diao)節(jie)保護(hu)功(gong)能(neng)的鋰(li)離(li)子(zi)(zi)電(dian)(dian)(dian)池(chi)(chi)(chi)充電(dian)(dian)(dian)器的設(she)計(ji)方法。它不(bu)僅能(neng)夠使(shi)工(gong)程(cheng)師完善散熱方面(mian)的考慮,同(tong)時(shi)(shi)(shi)還(huan)能(neng)極大(da)化充電(dian)(dian)(dian)率,盡(jin)可能(neng)縮短充電(dian)(dian)(dian)時(shi)(shi)(shi)間,同(tong)時(shi)(shi)(shi)具(ju)(ju)備(bei)(bei)(bei)輸入過壓保護(hu)(VOP)功(gong)能(neng)。有著(zhu)較強的實用(yong)(yong)性。

1 BQ24060芯片功能特點
    BQ24060是TI公司(si)的(de)(de)一個(ge)高(gao)集(ji)成的(de)(de)鋰(li)離子(zi)電(dian)池充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)管理(li)IC,其功能(neng)(neng)引(yin)腳(jiao)如圖(tu)1所示,表(biao)1介(jie)紹了(le)各(ge)引(yin)腳(jiao)功能(neng)(neng)。BQ24060提(ti)供能(neng)(neng)在(zai)有限空間里完(wan)成多功能(neng)(neng)的(de)(de)、安全的(de)(de)滿充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)的(de)(de)鋰(li)離子(zi)電(dian)池充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)器(qi)設計,其內部(bu)(bu)集(ji)成了(le)1 A功率(lv)FET以及電(dian)流(liu)傳感(gan)器(qi),因而能(neng)(neng)夠承受高(gao)達(da)26 V的(de)(de)輸入(ru)電(dian)壓。該產品還提(ti)供具備獨特(te)安全與(yu)低壓降特(te)性的(de)(de)全面(mian)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)管理(li)功能(neng)(neng),以延長電(dian)池使用壽命。BQ24060可以使鋰(li)離子(zi)電(dian)池分三階段(duan)進行充(chong)(chong)(chong)(chong)(chong)(chong)電(dian):預充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)方式、恒流(liu)熱調(diao)節(jie)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)方式、精(jing)確恒壓充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)方式。充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)終止是基于(yu)一個(ge)最小電(dian)流(liu)。內部(bu)(bu)可編程充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)定時器(qi)為充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)終止和在(zai)熱調(diao)節(jie)狀態動態調(diao)節(jie)提(ti)供安全保障。


    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,系統也能完全正常工作(zuo)。在終端設備(bei)制造過程中,LDO模式的特性(xing)對全面系統測試(shi)前的電路板(ban)測試(shi)也相當有用。

 BQ24060還提(ti)供過壓保(bao)護(hu)特性(xing),在高(gao)(gao)壓情況下,該(gai)器(qi)件能(neng)夠迅速將(jiang)充電(dian)器(qi)和系統與適(shi)配器(qi)相斷連(lian),從(cong)而實現保(bao)護(hu)功能(neng)。此外,即(ji)使在環(huan)境溫度較(jiao)高(gao)(gao)的惡(e)劣環(huan)境下,增強(qiang)型熱(re)過載(zai)保(bao)護(hu)穩壓特性(xing)也能(neng)確保(bao)BQ24060長期正常工作,比如夏季在汽車中工作,或錯誤地(di)連(lian)接至輸入(ru)電(dian)壓較(jiao)高(gao)(gao)的適(shi)配器(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(yu)故障(zhang)情況,提供電源狀態良好(hao)指(zhi)示;短路與(yu)熱保(bao)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(re)(re),給工程(cheng)師設計(ji)安全的(de)充電器提供了方便(bian)。BQ24060內部芯片溫度達(da)到預定義的(de)溫度閾值后(hou)(如(ru)110℃),IC溫度只要進一步提升就會讓(rang)充電電流(liu)(liu)下降(jiang),這有助于限(xian)制功(gong)耗,并(bing)為充電器提供熱(re)(re)保(bao)護(hu)。使結(jie)溫升高到熱(re)(re)調(diao)節的(de)極大功(gong)耗取決于PCB板布局、散熱(re)(re)通(tong)孔的(de)數量(liang)以(yi)及(ji)環境溫度。從圖3中看出,1.2 s之后(hou),熱(re)(re)環路會在2 s以(yi)內將有效充電電流(liu)(liu)從1.2 A降(jiang)到600 mA。


    采用BQ24060專用芯片(pian)(pian)設計(ji)的充(chong)(chong)電(dian)器,其充(chong)(chong)電(dian)過程如圖4所示(shi),充(chong)(chong)電(dian)器從預充(chong)(chong)電(dian)階段轉向快充(chong)(chong)模(mo)式(shi)時(shi),充(chong)(chong)電(dian)電(dian)流(liu)IBAT受芯片(pian)(pian)結(jie)溫TJ的限制而緩慢上升,其間經歷(li)了一個熱調節過程后,充(chong)(chong)電(dian)電(dian)流(liu)IBAT達到快充(chong)(chong)電(dian)流(liu),進入恒流(liu)充(chong)(chong)電(dian)狀態(tai),此后結(jie)溫開始下(xia)降,保證了充(chong)(chong)電(dian)器工作于安全狀態(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(chong)電(dian)安(an)(an)全(quan)計時(shi)器有(you)固(gu)定設(she)置(zhi)的(de)話,就會過早終止充(chong)電(dian)。BQ24060采用動態安(an)(an)全(quan)計時(shi)器控制電(dian)路(lu),能(neng)(neng)在熱(re)調(diao)節階段有(you)效(xiao)延長安(an)(an)全(quan)時(shi)間,并盡可(ke)能(neng)(neng)降低安(an)(an)全(quan)計時(shi)器的(de)故障幾率。從圖5中可(ke)以看出(chu),熱(re)調(diao)節模式下的(de)安(an)(an)全(quan)計時(shi)器的(de)響應與有(you)效(xiao)充(chong)電(dian)電(dian)流成反比。

 

 外部熱敏(min)(min)電(dian)阻RT1和RT2可(ke)以用來設(she)置選(xuan)擇溫度(du)控(kong)制充電(dian)窗口,如圖6所(suo)示。假設(she)RTH和RTC是熱敏(min)(min)電(dian)阻的高低溫極限值(zhi),RT1和RT2是負溫度(du)系數(shu)熱敏(min)(min)電(dian)阻,則RT1和RT2可(ke)以用如下(xia)公式計算(suan),先計算(suan)RT2,然后將RT2代入計算(suan)RT1:

    對于(yu)某熱(re)敏電阻,如果0℃和45℃對應的阻值分別為27.28 kΩ和4.912 kΩ,代入上面兩式,得:

4.2 快充電流750 mA的設計
    快充電流的(de)(de)設置是通過調節BQ24060的(de)(de)第(di)6腳ISET端(duan)與地Vss間的(de)(de)阻抗(kang)RISET的(de)(de)值來決定的(de)(de),其計算公式如(ru)下:

    通過查BQ24060的電特性表可(ke)(ke)得VSET=2.5 V,KSET=335,當(dang),IOUT=O.75 A時(shi),可(ke)(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(an)全計時(shi)器(qi)工作時(shi)間是通過設置其(qi)第2腳TMR與地間的阻抗RTMR來決定的。其(qi)計算公式如下:

    通過查BQ24060的電(dian)(dian)特性表可得KCHG=0.1 h/kΩ,則(ze)當TCHR=5h時,有(you)RTMR=5/0.1=50 kΩ。因此,在BQ24060的第2腳(jiao)TMR與地間連接了一(yi)個49.9 kΩ的電(dian)(dian)阻,如(ru)圖6所示。

5 PCB設計
    BQ24060采用有(you)利(li)于(yu)(yu)(yu)散(san)熱(re)的(de)(de)(de)MLP封(feng)裝形(xing)(xing)式(shi),這種封(feng)裝形(xing)(xing)式(shi)可提(ti)供一(yi)個熱(re)端面(mian),通過(guo)(guo)直(zhi)接(jie)焊(han)接(jie)的(de)(de)(de)方(fang)式(shi)使IC與(yu)(yu)(yu)印刷線(xian)(xian)路(lu)板(ban)間(jian)有(you)效熱(re)接(jie)觸(chu),可靠焊(han)接(jie)后,通過(guo)(guo)熱(re)傳(chuan)遞PCB就相當于(yu)(yu)(yu)一(yi)個蓄熱(re)裝置(zhi)(zhi)(zhi)。這樣在(zai)設(she)計(ji)(ji)PCB時就可以合理優化設(she)計(ji)(ji),從而最(zui)大(da)限(xian)度(du)地(di)(di)(di)(di)保證(zheng)良好的(de)(de)(de)熱(re)傳(chuan)導路(lu)徑和蓄熱(re)特性,從而使系統獲(huo)得極佳的(de)(de)(de)熱(re)穩定性能。本著就近原則(ze),在(zai)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)輸入(ru)端與(yu)(yu)(yu)地(di)(di)(di)(di)之(zhi)間(jian)一(yi)般放(fang)置(zhi)(zhi)(zhi)一(yi)個1 μF以上的(de)(de)(de)陶瓷電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong),以加(jia)強高頻濾(lv)波,并有(you)利(li)于(yu)(yu)(yu)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)器短暫熱(re)插拔。該(gai)設(she)計(ji)(ji)放(fang)置(zhi)(zhi)(zhi)了一(yi)個4.7μF的(de)(de)(de)陶瓷電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong),最(zui)好通過(guo)(guo)過(guo)(guo)孔直(zhi)接(jie)與(yu)(yu)(yu)地(di)(di)(di)(di)層相連。該(gai)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)值(zhi)應大(da)于(yu)(yu)(yu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)輸出端放(fang)置(zhi)(zhi)(zhi)的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)值(zhi)。在(zai)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)輸出端放(fang)置(zhi)(zhi)(zhi)一(yi)個2.2μF的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong),同樣在(zai)布板(ban)時該(gai)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)要(yao)(yao)求就近與(yu)(yu)(yu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)輸出端相連,并要(yao)(yao)求短距入(ru)地(di)(di)(di)(di),最(zui)好通過(guo)(guo)過(guo)(guo)孔直(zhi)接(jie)與(yu)(yu)(yu)地(di)(di)(di)(di)層相連。BAT和ISET端間(jian),放(fang)置(zhi)(zhi)(zhi)一(yi)個0.47 μF的(de)(de)(de)陶瓷電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong),有(you)助于(yu)(yu)(yu)回路(lu)的(de)(de)(de)穩定性。在(zai)布板(ban)時,所(suo)有(you)小(xiao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)的(de)(de)(de)地(di)(di)(di)(di)應與(yu)(yu)(yu)大(da)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)地(di)(di)(di)(di)相互隔離,可采用單點地(di)(di)(di)(di)技術(shu)使小(xiao)信號地(di)(di)(di)(di)與(yu)(yu)(yu)大(da)信號地(di)(di)(di)(di)連接(jie),在(zai)走線(xian)(xian)時要(yao)(yao)注意小(xiao)信號地(di)(di)(di)(di)與(yu)(yu)(yu)大(da)信號地(di)(di)(di)(di)的(de)(de)(de)前后關系,避免共模(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(shi)具備輸(shu)入過(guo)壓保護(VOP)功能,有著較好的實用性。

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