隨著器件集成度的提高和尺寸的縮減，手機、PDA 及便攜 DVD 播放器等便攜設備的市場需求增長迅猛。電池功率密度的提高成為技術進步的瓶頸，而鋰離子電池在該方面的優勢使其得到廣泛應用。為了延長系統運行時間并降低器件尺寸，系統設計人員開始意識到利用高級電路拓撲提高系統功率轉換遠不能解決問題。電池充電已成為提高電池容量并延長使用壽命的重要方案。線性電池充電器成本合理、尺寸小，適用于低容量電池充電應用。但線性電池充電器由于功耗較高，已不能充分滿足充電需求。本文主要介紹兆赫同步開關電池充電器以及有效充(chong)電并延(yan)長電池使用壽(shou)命的設(she)計考慮事項。

　　鋰(li)離子電池充電 

　　大(da)部(bu)分專用鋰離子(zi)充(chong)(chong)電(dian)(dian)集成(cheng)電(dian)(dian)路(lu) (IC) 都是通過圖1所示(shi)的(de)(de)方式充(chong)(chong)電(dian)(dian)。鋰離子(zi)電(dian)(dian)池(chi)(chi)的(de)(de)充(chong)(chong)電(dian)(dian)過程由三個(ge)階段(duan)組(zu)成(cheng)：預充(chong)(chong)電(dian)(dian)、恒(heng)流 (CC) 快(kuai)速(su)(su)充(chong)(chong)電(dian)(dian)以及(ji)恒(heng)壓 (CV) 終(zhong)端 (Termination)。在(zai)預充(chong)(chong)電(dian)(dian)階段(duan)，以低速(su)(su)率（一般是快(kuai)速(su)(su)充(chong)(chong)電(dian)(dian)率的(de)(de) 1/10）對電(dian)(dian)池(chi)(chi)充(chong)(chong)電(dian)(dian)，這(zhe)時(shi)(shi)的(de)(de)電(dian)(dian)池(chi)(chi)電(dian)(dian)壓低于 3.0V。這(zhe)樣可以實現對鈍化層(ceng)的(de)(de)恢復 - 鈍化層(ceng)在(zai)深度放電(dian)(dian)狀態下存儲時(shi)(shi)間過長會(hui)分解。另外，還(huan)可以在(zai)發生陽極短路(lu)的(de)(de)過充(chong)(chong)電(dian)(dian)電(dian)(dian)池(chi)(chi)出現部(bu)分銅分解的(de)(de)情況下防止 1C 充(chong)(chong)電(dian)(dian)過熱。在(zai)電(dian)(dian)池(chi)(chi)電(dian)(dian)壓達到 3.0V 時(shi)(shi)，電(dian)(dian)池(chi)(chi)充(chong)(chong)電(dian)(dian)器進(jin)入 CC 階段(duan)。

　　快速充電電流應當限制在 1C 速率（0.7C 速率），以防止過熱以及因而造成的加速降質。不過，為高功率容量設計的電池可以容許更高的充電率。應當合理選擇充電率，使電池溫度在充電結束時不超過 50 C。電池以快速充電率充電，直到達到穩壓極限（一般是 4.2V/電池，不過碳素 (coke-based) 陽極鋰離子電池為 4.1V）。然后，在充電電流以指數方式降低到預定義終端電平時，電池充電器開始調節(jie)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)并且進入 CV 階段。輸出穩壓(ya)精度(du)是(shi)提高(gao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)容量(liang)和延長(chang)使(shi)用壽(shou)命的(de)關鍵(jian)。較低的(de)穩壓(ya)精度(du)會造成(cheng)(cheng)(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)不足(zu)，進而(er)(er)造成(cheng)(cheng)(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)容量(liang)大幅降低。充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)不足(zu) 1％ 電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)時(shi)，電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)即(ji)損失大約 8％ 的(de)容量(liang)。較低的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)穩壓(ya)精度(du)也(ye)會造成(cheng)(cheng)(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)過(guo)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)，從而(er)(er)縮(suo)短電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)使(shi)用壽(shou)命。為了安全地(di)對鋰離子電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)，僅允許環境溫度(du)在(zai) 0～45 C 之間。在(zai)更(geng)低溫度(du)時(shi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)會形成(cheng)(cheng)(cheng)(cheng)金屬鋰，從而(er)(er)提高(gao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)阻抗并造成(cheng)(cheng)(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)降質。在(zai)更(geng)高(gao)溫度(du)時(shi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)會由(you)于鋰電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)解反應而(er)(er)造成(cheng)(cheng)(cheng)(cheng)加(jia)速降質。

　　低成本獨立線性(xing)電池充電器

　　許多IC 制造(zao)商通過開發(fa)用(yong)于低(di)功(gong)耗(hao)便攜設備(bei)的低(di)成(cheng)本線(xian)性(xing)電(dian)池充(chong)(chong)電(dian)器來滿足(zu)市場對更(geng)精(jing)確和更(geng)安全充(chong)(chong)電(dian)的需求。圖 2 就是一種采用(yong)更(geng)少外部組件(jian)的低(di)成(cheng)本獨(du)立線(xian)性(xing)電(dian)池充(chong)(chong)電(dian)器電(dian)路結(jie)構圖。

　　這(zhe)種電(dian)(dian)池充(chong)電(dian)(dian)器簡(jian)便地把適(shi)(shi)配(pei)器的 DC 電(dian)(dian)壓降低到電(dian)(dian)池電(dian)(dian)壓。導(dao)通元件上的功(gong)率等于適(shi)(shi)配(pei)器電(dian)(dian)壓減去電(dian)(dian)池電(dian)(dian)壓再(zai)乘以充(chong)電(dian)(dian)電(dian)(dian)流，如下式所示：

　　如果采(cai)用 5V 適配(pei)器(qi)對 1200mAh 或(huo) 2200mAh 單體鋰離子電池充(chong)電，則圖 3 說明 0.7C 充(chong)電率快速充(chong)電電流情(qing)況下的功(gong)耗。

　　在電池從預充電向快速充電階段過渡時，最大功耗分別為 1.68W 和 3.0W。對于具有 47℃/W 熱阻的 3 3mmQFN 封裝而言，3.0W 的功耗會造成 141℃ 的溫度升高。這肯定會超過 25℃環境溫度時的最高 125℃硅芯片接點工作溫度。快速充電電流調節和 AC 適配器電壓容差在線性電池充電器中同樣至關重要。如果穩壓容差較寬松，則導通晶體管和封裝需要更大的尺寸，從而增加尺寸和成本。線性電池充電器的主要問題是其高功耗。必須對充電系統的充電電流、尺寸、成本和散熱需求做出取舍。因此，由于其突出的尺寸、成本和散熱問題，線性電池充電器一般適用于低容量（低于 1300mAh）鋰離子電池應用。那么，如何解決高容量電池組或高輸入-輸出壓差應用的散熱問題？答案是高效率同步開關電池充電器。

　　兆(zhao)赫(he)同步(bu)開關電池充電器

　　同步開關(guan)式充(chong)(chong)電(dian)(dian)(dian)解決方案(an)一(yi)般用(yong)于具(ju)有高輸(shu)入-輸(shu)出壓差的(de)應用(yong)或者高容量電(dian)(dian)(dian)池(chi)組(zu)(zu)。對(dui)于 2200mAh 鋰離子(zi)電(dian)(dian)(dian)池(chi)組(zu)(zu)，很(hen)難采用(yong)線性(xing)電(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)器(qi)通過(guo)車載適(shi)配(pei)器(qi) (12V) 在 0.5C～1C 的(de)快速充(chong)(chong)電(dian)(dian)(dian)率情況下對(dui)單(dan)體(ti)電(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)。雖然(ran)可以采用(yong)具(ju)有散熱調節功能的(de)線性(xing)電(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)器(qi)，但是低(di)充(chong)(chong)電(dian)(dian)(dian)率情況下的(de)充(chong)(chong)電(dian)(dian)(dian)時間(jian)過(guo)長。

　　圖 4 說明適用(yong)于(yu) DVD 播放器(qi)和智能電話(hua)等設備的充電電流達到(dao) 2A 的獨立高(gao)效同步開關降壓電池(chi)充電器(qi)。

　　它(ta)采用(yong)1.1MHz 開(kai)關(guan)(guan)頻率(lv)電(dian)(dian)(dian)(dian)(dian)壓(ya)模式控(kong)制架構，利(li)用(yong)內置III型環(huan)路(lu)補償器(qi)降(jiang)(jiang)低外(wai)(wai)部(bu)組件數量。為了進一(yi)步降(jiang)(jiang)低電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)尺寸(cun)，它(ta)在(zai)4 4 mm小型封裝的(de)(de) PWM 控(kong)制器(qi)中(zhong)集成了兩個功率(lv) MOSFET。功率(lv) MOSFET Q1 和 Q2 交替(ti)關(guan)(guan)閉(bi)，具有(you)最佳的(de)(de)停滯時間(jian)(jian)，以(yi)(yi)優化高(gao)(gao)開(kai)關(guan)(guan)頻率(lv)時的(de)(de)效(xiao)(xiao)率(lv)。Q1 用(yong)作 P 通(tong)道 MOSFET，在(zai)用(yong)于(yu)高(gao)(gao)側(ce) N-MOSFET 柵極驅動器(qi)時可(ke)以(yi)(yi)消除外(wai)(wai)部(bu)自益放大電(dian)(dian)(dian)(dian)(dian)容(rong)器(qi) (boost strap capacitor) 和二極管。另外(wai)(wai)，通(tong)過完全(quan)打(da)開(kai) Q1，在(zai)輸入電(dian)(dian)(dian)(dian)(dian)壓(ya)非常接近電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)電(dian)(dian)(dian)(dian)(dian)壓(ya)時，易于(yu)實現(xian) 100％的(de)(de)占空比。打(da)開(kai)和關(guan)(guan)閉(bi)時間(jian)(jian)處于(yu)受(shou)控(kong)狀態(tai)，從而(er)可(ke)以(yi)(yi)根據反饋控(kong)制環(huan)路(lu)調節電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)（CC 階(jie)段）或(huo)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)電(dian)(dian)(dian)(dian)(dian)壓(ya)（CV階(jie)段）。電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)具有(you)高(gao)(gao)度(du)集成的(de)(de)功能，能夠(gou)安(an)全(quan)、高(gao)(gao)效(xiao)(xiao)地對鋰離子電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)。它(ta)可(ke)以(yi)(yi)編程預(yu)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)、快速充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)、充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)壓(ya)、充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)定時器(qi)、電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)溫度(du)監控(kong)、自動再充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)、短路(lu)和過熱保(bao)護。電(dian)(dian)(dian)(dian)(dian)路(lu)參數設計用(yong)于(yu)下述設計示(shi)例中(zhong)的(de)(de)以(yi)(yi)下規格。

適配(pei)器 DC 電壓(ya)：12 V

雙(shuang)體鋰離(li)子電池(chi)組：4.2 V/電池(chi)，1900mAh/電池(chi)

預充電電流(liu)：IPRE-CHG=133 mA

快(kuai)速(su)充(chong)電(dian)電(dian)流：ICHG=1.33 A

充電時間限制：tCHG = 5-hour

開始充電的溫(wen)度范圍：T= 0℃～45 C。

　　由于(yu)電池充電器(qi)(qi)的(de)(de)(de)尺寸(cun)對便(bian)攜設(she)備極其(qi)重要，因此需(xu)要采用(yong)盡可能小的(de)(de)(de)輸出電感器(qi)(qi)。對于(yu)給定的(de)(de)(de)電感器(qi)(qi)紋波(bo)電流，所需(xu)的(de)(de)(de)電感由下式得(de)出：

　　式中，f_{s}和 DIripple,L 分(fen)別是開(kai)關頻率和電感(gan)器紋波電流。在(zai)上式中代入VIN=12V、VBAT=6.0V（3.0V/電池）、Iripple,L=30％ICHG、ICHG=1.33A以及fs=1MHz ，可(ke)(ke)以得出(chu)L=7.5 H。可(ke)(ke)以選擇(ze)L=10 H的屏(ping)蔽(bi)電感(gan)器。請注意(yi)：屏(ping)蔽(bi)電感(gan)器在(zai)把磁通量限制在(zai)電感(gan)器內(nei)(nei)部和降低輻射電磁干(gan)擾 (EMI) 方面具(ju)有更高(gao)(gao)能力。所需的電感(gan)與開(kai)關頻率成反比。另一方面，電感(gan)可(ke)(ke)以降低 10 倍，在(zai) 1MHz 時的尺寸(cun)低于(yu) 100kHz 時的尺寸(cun)，開(kai)關頻率越高(gao)(gao)，Q1 和 Q2 上的開(kai)關損(sun)耗越高(gao)(gao)，同時電感(gan)器內(nei)(nei)核(he)損(sun)耗也越高(gao)(gao)。因此，1MHz 開(kai)關頻率是實際(ji)設計中電感(gan)器尺寸(cun)和功率轉(zhuan)換效率之間的理(li)想取舍。

　　電(dian)感器額定電(dian)流的選擇(ze)對實現預期效(xiao)率也很重(zhong)要(yao)。峰值電(dian)感器電(dian)流 IPeak 通過(guo)下式計算：

　　電池電壓為輸入電壓一半時電感器具有最高的紋波電流。因此，在所有工作情況下電感器飽和額定電流都應當始終大于最高峰值電感器電流。
關鍵(jian)是選擇較小(xiao)的、具有良好溫度特征的陶瓷(ci)輸出電(dian)容(rong)器，如：X7R 和 X5R 陶瓷(ci)電(dian)容(rong)器。進入電(dian)池的紋波電(dian)流由下式得出：

　　式(shi)中，ESR、RSNS和RBAT分別是輸(shu)出電(dian)(dian)(dian)(dian)容(rong)器等效(xiao)串連電(dian)(dian)(dian)(dian)阻(zu)、電(dian)(dian)(dian)(dian)流感(gan)測電(dian)(dian)(dian)(dian)阻(zu)器和電(dian)(dian)(dian)(dian)池(chi)內(nei)部(bu)阻(zu)抗，包括電(dian)(dian)(dian)(dian)池(chi)組中保護 MOSFET 的 Rdson。輸(shu)出電(dian)(dian)(dian)(dian)容(rong)器的 ESR 越低(di)，進(jin)入電(dian)(dian)(dian)(dian)池(chi)的紋波(bo)電(dian)(dian)(dian)(dian)流也越低(di)。進(jin)入電(dian)(dian)(dian)(dian)池(chi)的紋波(bo)電(dian)(dian)(dian)(dian)流應(ying)當低(di)于電(dian)(dian)(dian)(dian)感(gan)器紋波(bo)電(dian)(dian)(dian)(dian)流的十分之一(yi)，一(yi)般情況下 10 F/10m ESR 陶瓷電(dian)(dian)(dian)(dian)容(rong)器即可滿足上述需(xu)求。

　　?選擇電(dian)流(liu)感測電(dian)阻器(qi)RSNS

　　根據感測(ce)電(dian)阻器的調節閾值 VIREG 選擇(ze) RSNS。為了取(qu)得標準的感測(ce)電(dian)阻器值，使 VIREG=133mV，則求(qiu)得 RSNS：

　　感測電阻(zu)器的(de)功(gong)耗為I2CHGRSNS=I2CHGRsns=0.18W。選擇 0.5W 時的(de) 1206 額定(ding)尺(chi)寸。

　　?選(xuan)擇(ze)快速充電電流 設定電阻器RSET1.

　　RSET1 用(yong)于設定(ding)快速充(chong)電電流，RSET1 由(you)下式求得(de)：

　　?選擇預充電電流(liu)設定電阻器 RSET2.

　　RSET2用(yong)于設(she)定預充(chong)電電流(liu)，由下式求得：

　　?選擇最長(chang)充電(dian)時(shi)間設定電(dian)容器 CTTC

　　如(ru)果電(dian)池(chi)未充(chong)(chong)滿，充(chong)(chong)電(dian)定時(shi)器可以檢測“壞”電(dian)池(chi)組，此時(shi)充(chong)(chong)電(dian)定時(shi)器失效。CTTC 用于對充(chong)(chong)電(dian)定時(shi)器進行編程，規定每(mei) nF 為(wei) 2.6 分鐘。

　　C_{TTC}=\frac{t_{CHG}}{K_{TTC}}=\frac{5 60}{2.6}=115nF

　　可(ke)以選用(yong) 0.1 F 陶瓷(ci)電容器。

　　?選擇最(zui)低(di)與最(zui)高充電(dian)溫度(du)設(she)定(ding)電(dian)阻器(qi) RT1 與 RT2

　　RT1 與(yu) RT2 用于(yu)在 0 C～45 C 間充電溫度范圍內進行編程，以啟動(dong)電池充電器。對(dui)于(yu)電池組中常(chang)用的(de) 103AT-2 熱敏電阻，RT(0℃)=RTL=27.28k ，RT(45℃)=RTH=4.911 k ，RT1與(yu)RT2由(you)下式確(que)定：

　　在上式中代(dai)入 RTL 與 RTH 可以求得 RT1=9.31kW，RT2=442 kW。

　　在16V輸入(ru)電壓下仍然具有超過 90％ 的效率(lv)。與線性(xing)充(chong)電器(qi)相(xiang)比(bi)，功耗低(di)得多，而且可以在電池組側設(she)計同步開關充(chong)電器(qi)，以降(jiang)低(di)對(dui)(dui)主(zhu)(zhu)板空間(jian)的占(zhan)用，由于以 MHz 頻率(lv)進行(xing)工作(zuo)，電感器(qi)的尺寸(cun)較(jiao)小。需要牢記(ji)的是，電池的使用壽命主(zhu)(zhu)要取決于其溫度。利(li)用同步開關電池充(chong)電器(qi)對(dui)(dui)鋰離子(zi)電池充(chong)電一般(ban)情況下產生的熱量更(geng)低(di)。因此(ci)，與線性(xing)電池充(chong)電器(qi)相(xiang)比(bi)，它具有更(geng)長的使用壽命。

　　線性電池充電器適用于具有低成本和小尺寸優勢的低容量電池充電應用。隨著便攜式 DVD 播放器和智能電話等便攜設備對功率需求的不斷提高，由于其內在的高功耗限制，線性電池充電器不(bu)再能夠高(gao)效(xiao)(xiao)的(de)對(dui)鋰離子電池(chi)充電。集成(cheng) MOSFET 的(de)高(gao)效(xiao)(xiao)率同步開關電池(chi)充電器為這些(xie)高(gao)級(ji)便攜設(she)備提供高(gao)效(xiao)(xiao)的(de)充電解決方案，從而實現更(geng)低的(de)熱量與更(geng)長的(de)電池(chi)使用(yong)壽命。