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電(dian)并延長電(dian)池(chi)使用壽命的設計考慮事項。

　　鋰離子電池充電 

　　大(da)部(bu)分(fen)(fen)專用鋰離子充(chong)電(dian)(dian)(dian)(dian)集成(cheng)電(dian)(dian)(dian)(dian)路 (IC) 都是通過圖1所(suo)示的方式(shi)充(chong)電(dian)(dian)(dian)(dian)。鋰離子電(dian)(dian)(dian)(dian)池(chi)(chi)的充(chong)電(dian)(dian)(dian)(dian)過程由三個階(jie)(jie)段(duan)組成(cheng)：預(yu)充(chong)電(dian)(dian)(dian)(dian)、恒(heng)流 (CC) 快(kuai)速充(chong)電(dian)(dian)(dian)(dian)以(yi)及恒(heng)壓(ya) (CV) 終端 (Termination)。在(zai)預(yu)充(chong)電(dian)(dian)(dian)(dian)階(jie)(jie)段(duan)，以(yi)低速率（一般是快(kuai)速充(chong)電(dian)(dian)(dian)(dian)率的 1/10）對電(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)，這時的電(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)壓(ya)低于 3.0V。這樣可以(yi)實現對鈍化(hua)層(ceng)的恢復 - 鈍化(hua)層(ceng)在(zai)深度放(fang)電(dian)(dian)(dian)(dian)狀態下存儲時間過長(chang)會分(fen)(fen)解。另外(wai)，還可以(yi)在(zai)發生陽極短路的過充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)池(chi)(chi)出現部(bu)分(fen)(fen)銅分(fen)(fen)解的情況下防止(zhi) 1C 充(chong)電(dian)(dian)(dian)(dian)過熱。在(zai)電(dian)(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)(dian)壓(ya)達到 3.0V 時，電(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)器進(jin)入 CC 階(jie)(jie)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電(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)壓并且(qie)進入 CV 階段。輸出穩壓精(jing)度(du)(du)是提高電(dian)(dian)(dian)池(chi)(chi)容量和延長使(shi)用壽(shou)命(ming)的關鍵。較低(di)的穩壓精(jing)度(du)(du)會(hui)(hui)造(zao)成電(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)不足，進而造(zao)成電(dian)(dian)(dian)池(chi)(chi)容量大(da)幅降(jiang)低(di)。充(chong)電(dian)(dian)(dian)不足 1％ 電(dian)(dian)(dian)壓時(shi)，電(dian)(dian)(dian)池(chi)(chi)即損失大(da)約 8％ 的容量。較低(di)的電(dian)(dian)(dian)池(chi)(chi)穩壓精(jing)度(du)(du)也會(hui)(hui)造(zao)成電(dian)(dian)(dian)池(chi)(chi)過(guo)充(chong)電(dian)(dian)(dian)，從而縮短電(dian)(dian)(dian)池(chi)(chi)使(shi)用壽(shou)命(ming)。為了安全地(di)對鋰(li)(li)離子電(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)，僅(jin)允許(xu)環境溫(wen)度(du)(du)在(zai) 0～45 C 之間(jian)。在(zai)更低(di)溫(wen)度(du)(du)時(shi)充(chong)電(dian)(dian)(dian)會(hui)(hui)形成金屬(shu)鋰(li)(li)，從而提高電(dian)(dian)(dian)池(chi)(chi)阻抗并造(zao)成電(dian)(dian)(dian)池(chi)(chi)降(jiang)質。在(zai)更高溫(wen)度(du)(du)時(shi)充(chong)電(dian)(dian)(dian)會(hui)(hui)由于鋰(li)(li)電(dian)(dian)(dian)解反應而造(zao)成加速降(jiang)質。

　　低成本獨立線性電池(chi)充電器

　　許多IC 制造商通(tong)過開發用于(yu)低功耗便攜設備的低成本(ben)線性電(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)器來(lai)滿足市場對更精確和更安全(quan)充(chong)電(dian)(dian)(dian)的需求。圖(tu) 2 就是一種采(cai)用更少外部組件的低成本(ben)獨立線性電(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)器電(dian)(dian)(dian)路結構(gou)圖(tu)。

　　這種電(dian)(dian)池充電(dian)(dian)器簡(jian)便地把適配器的 DC 電(dian)(dian)壓(ya)降低到電(dian)(dian)池電(dian)(dian)壓(ya)。導通元件上的功(gong)率等于適配器電(dian)(dian)壓(ya)減去(qu)電(dian)(dian)池電(dian)(dian)壓(ya)再(zai)乘以充電(dian)(dian)電(dian)(dian)流(liu)，如下式所示(shi)：

　　如果采(cai)用(yong) 5V 適配器對 1200mAh 或 2200mAh 單體鋰離子電(dian)(dian)池(chi)充(chong)電(dian)(dian)，則圖 3 說明 0.7C 充(chong)電(dian)(dian)率快速充(chong)電(dian)(dian)電(dian)(dian)流情況下的功耗(hao)。

　　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。

　　兆赫同(tong)步開關電池(chi)充電器

　　同步開(kai)關式(shi)充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)解決方(fang)案一般(ban)用(yong)于具有高(gao)輸入-輸出(chu)壓(ya)差的應(ying)用(yong)或者高(gao)容量電(dian)(dian)(dian)池(chi)(chi)(chi)組。對(dui)于 2200mAh 鋰(li)離子電(dian)(dian)(dian)池(chi)(chi)(chi)組，很難(nan)采用(yong)線性(xing)電(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器通過車載適配(pei)器 (12V) 在 0.5C～1C 的快速充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)率(lv)(lv)情況下(xia)(xia)對(dui)單體電(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)。雖然可(ke)以采用(yong)具有散熱(re)調節功能的線性(xing)電(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器，但(dan)是低充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)率(lv)(lv)情況下(xia)(xia)的充(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)時(shi)間過長。

　　圖(tu) 4 說明適用于(yu) DVD 播放器(qi)和智能電(dian)話等設(she)備的(de)充(chong)電(dian)電(dian)流達到 2A 的(de)獨立高效同步開關降(jiang)壓(ya)電(dian)池充(chong)電(dian)器(qi)。

　　它采用(yong)(yong)1.1MHz 開(kai)(kai)關頻(pin)率(lv)(lv)(lv)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)模式控制架構，利用(yong)(yong)內置III型環路(lu)補償器(qi)(qi)降(jiang)低外(wai)部組件數量。為了進(jin)一步降(jiang)低電(dian)(dian)(dian)(dian)(dian)池充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)(qi)尺寸，它在(zai)4 4 mm小型封裝的(de) PWM 控制器(qi)(qi)中(zhong)集成(cheng)了兩(liang)個功(gong)(gong)率(lv)(lv)(lv) MOSFET。功(gong)(gong)率(lv)(lv)(lv) MOSFET Q1 和(he)(he) Q2 交替關閉(bi)，具(ju)有最佳(jia)的(de)停(ting)滯時(shi)間(jian)，以優化高(gao)開(kai)(kai)關頻(pin)率(lv)(lv)(lv)時(shi)的(de)效率(lv)(lv)(lv)。Q1 用(yong)(yong)作 P 通道 MOSFET，在(zai)用(yong)(yong)于(yu)高(gao)側(ce) N-MOSFET 柵極(ji)驅動器(qi)(qi)時(shi)可(ke)(ke)以消除(chu)外(wai)部自益放大電(dian)(dian)(dian)(dian)(dian)容器(qi)(qi) (boost strap capacitor) 和(he)(he)二極(ji)管。另外(wai)，通過(guo)完全(quan)(quan)打(da)開(kai)(kai) Q1，在(zai)輸入電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)非常接近(jin)電(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)時(shi)，易于(yu)實現 100％的(de)占空比。打(da)開(kai)(kai)和(he)(he)關閉(bi)時(shi)間(jian)處于(yu)受控狀態，從而可(ke)(ke)以根據(ju)反饋控制環路(lu)調節電(dian)(dian)(dian)(dian)(dian)池充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流（CC 階段）或(huo)電(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)（CV階段）。電(dian)(dian)(dian)(dian)(dian)池充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)(qi)具(ju)有高(gao)度(du)集成(cheng)的(de)功(gong)(gong)能，能夠(gou)安全(quan)(quan)、高(gao)效地對鋰離子(zi)電(dian)(dian)(dian)(dian)(dian)池充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)。它可(ke)(ke)以編(bian)程預充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流、快速充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流、充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)、充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)定時(shi)器(qi)(qi)、電(dian)(dian)(dian)(dian)(dian)池溫度(du)監控、自動再充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)、短路(lu)和(he)(he)過(guo)熱(re)保護。電(dian)(dian)(dian)(dian)(dian)路(lu)參數設(she)計用(yong)(yong)于(yu)下(xia)述設(she)計示例中(zhong)的(de)以下(xia)規格。

適配器(qi) DC 電壓：12 V

雙(shuang)體(ti)鋰離子電(dian)池組：4.2 V/電(dian)池，1900mAh/電(dian)池

預充電(dian)電(dian)流(liu)：IPRE-CHG=133 mA

快速充電(dian)(dian)電(dian)(dian)流：ICHG=1.33 A

充電時間限制：tCHG = 5-hour

開始充電(dian)的溫度范圍：T= 0℃～45 C。

　　由(you)(you)于電(dian)池充電(dian)器(qi)(qi)的(de)尺寸對(dui)(dui)便攜設(she)備極其重(zhong)要，因此需要采用盡可能(neng)小(xiao)的(de)輸出電(dian)感器(qi)(qi)。對(dui)(dui)于給定的(de)電(dian)感器(qi)(qi)紋波電(dian)流，所需的(de)電(dian)感由(you)(you)下式得出：

　　式(shi)中，f_{s}和(he)(he) DIripple,L 分別是開(kai)(kai)關(guan)頻(pin)率(lv)和(he)(he)電(dian)(dian)感(gan)(gan)器(qi)(qi)紋波電(dian)(dian)流。在(zai)(zai)上式(shi)中代入VIN=12V、VBAT=6.0V（3.0V/電(dian)(dian)池）、Iripple,L=30％ICHG、ICHG=1.33A以及fs=1MHz ，可以得(de)出L=7.5 H。可以選擇(ze)L=10 H的(de)(de)(de)屏蔽電(dian)(dian)感(gan)(gan)器(qi)(qi)。請(qing)注意：屏蔽電(dian)(dian)感(gan)(gan)器(qi)(qi)在(zai)(zai)把(ba)磁(ci)通(tong)量(liang)限制在(zai)(zai)電(dian)(dian)感(gan)(gan)器(qi)(qi)內部和(he)(he)降低輻射(she)電(dian)(dian)磁(ci)干擾 (EMI) 方(fang)面具有更高能力。所需的(de)(de)(de)電(dian)(dian)感(gan)(gan)與開(kai)(kai)關(guan)頻(pin)率(lv)成反(fan)比。另一方(fang)面，電(dian)(dian)感(gan)(gan)可以降低 10 倍，在(zai)(zai) 1MHz 時的(de)(de)(de)尺(chi)寸低于 100kHz 時的(de)(de)(de)尺(chi)寸，開(kai)(kai)關(guan)頻(pin)率(lv)越(yue)(yue)高，Q1 和(he)(he) Q2 上的(de)(de)(de)開(kai)(kai)關(guan)損(sun)耗越(yue)(yue)高，同時電(dian)(dian)感(gan)(gan)器(qi)(qi)內核損(sun)耗也越(yue)(yue)高。因此，1MHz 開(kai)(kai)關(guan)頻(pin)率(lv)是實際設(she)計中電(dian)(dian)感(gan)(gan)器(qi)(qi)尺(chi)寸和(he)(he)功率(lv)轉換效率(lv)之間的(de)(de)(de)理想取舍。

　　電(dian)感(gan)器(qi)額定電(dian)流的選(xuan)擇對實現預期效率也(ye)很重要。峰值電(dian)感(gan)器(qi)電(dian)流 IPeak 通(tong)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。
關鍵是選擇較小的、具有良好溫度特征的陶(tao)瓷(ci)(ci)輸出電(dian)容器，如：X7R 和 X5R 陶(tao)瓷(ci)(ci)電(dian)容器。進入(ru)電(dian)池(chi)的紋(wen)波(bo)電(dian)流(liu)由下式得出：

　　式中，ESR、RSNS和RBAT分別是輸出電(dian)(dian)(dian)(dian)容器(qi)(qi)等效串連電(dian)(dian)(dian)(dian)阻、電(dian)(dian)(dian)(dian)流感測電(dian)(dian)(dian)(dian)阻器(qi)(qi)和電(dian)(dian)(dian)(dian)池(chi)(chi)內部阻抗，包括電(dian)(dian)(dian)(dian)池(chi)(chi)組中保護 MOSFET 的 Rdson。輸出電(dian)(dian)(dian)(dian)容器(qi)(qi)的 ESR 越(yue)低，進入電(dian)(dian)(dian)(dian)池(chi)(chi)的紋波(bo)電(dian)(dian)(dian)(dian)流也越(yue)低。進入電(dian)(dian)(dian)(dian)池(chi)(chi)的紋波(bo)電(dian)(dian)(dian)(dian)流應當低于電(dian)(dian)(dian)(dian)感器(qi)(qi)紋波(bo)電(dian)(dian)(dian)(dian)流的十分之一，一般情(qing)況下 10 F/10m ESR 陶瓷電(dian)(dian)(dian)(dian)容器(qi)(qi)即可(ke)滿(man)足上述需求。

　　?選擇電(dian)流感測電(dian)阻器RSNS

　　根(gen)據(ju)感(gan)測電(dian)阻(zu)器的調節閾值 VIREG 選(xuan)擇 RSNS。為(wei)了(le)取得標準的感(gan)測電(dian)阻(zu)器值，使 VIREG=133mV，則求得 RSNS：

　　感測電阻器的功耗為I2CHGRSNS=I2CHGRsns=0.18W。選擇 0.5W 時的 1206 額定(ding)尺寸。

　　?選(xuan)擇快速充電(dian)電(dian)流 設定電(dian)阻器RSET1.

　　RSET1 用(yong)于設定快速充電(dian)電(dian)流，RSET1 由下式求得(de)：

　　?選擇(ze)預充(chong)電(dian)電(dian)流設定電(dian)阻(zu)器 RSET2.

　　RSET2用于設定預充電電流，由下式求得：

　　?選擇最長充電時間設定電容器 CTTC

　　如果電(dian)池未充(chong)(chong)滿，充(chong)(chong)電(dian)定時(shi)(shi)器(qi)可以檢(jian)測“壞”電(dian)池組，此(ci)時(shi)(shi)充(chong)(chong)電(dian)定時(shi)(shi)器(qi)失效。CTTC 用于對(dui)充(chong)(chong)電(dian)定時(shi)(shi)器(qi)進(jin)行編程(cheng)，規定每 nF 為 2.6 分鐘(zhong)。

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

　　可以選用(yong) 0.1 F 陶瓷電(dian)容器。

　　?選擇(ze)最低與最高充電溫(wen)度設定電阻器 RT1 與 RT2

　　RT1 與(yu) RT2 用(yong)于在(zai) 0 C～45 C 間充電溫度范(fan)圍內進行編(bian)程，以啟動電池充電器。對(dui)于電池組中常(chang)用(yong)的(de) 103AT-2 熱敏電阻，RT(0℃)=RTL=27.28k ，RT(45℃)=RTH=4.911 k ，RT1與(yu)RT2由下(xia)式(shi)確定：

　　在上式中代入 RTL 與 RTH 可以求(qiu)得 RT1=9.31kW，RT2=442 kW。

　　在16V輸入電(dian)(dian)(dian)壓下仍然(ran)具(ju)(ju)有超過 90％ 的(de)效率。與線性充(chong)(chong)電(dian)(dian)(dian)器(qi)相(xiang)比(bi)，功耗低(di)得多(duo)，而且(qie)可以在電(dian)(dian)(dian)池(chi)(chi)組側(ce)設計(ji)同步開關(guan)充(chong)(chong)電(dian)(dian)(dian)器(qi)，以降低(di)對主板空間的(de)占用，由于以 MHz 頻率進行工作，電(dian)(dian)(dian)感(gan)器(qi)的(de)尺寸較小。需要(yao)(yao)牢記的(de)是，電(dian)(dian)(dian)池(chi)(chi)的(de)使用壽(shou)命(ming)主要(yao)(yao)取決(jue)于其(qi)溫度。利用同步開關(guan)電(dian)(dian)(dian)池(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)器(qi)對鋰離子電(dian)(dian)(dian)池(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)一般情(qing)況(kuang)下產生的(de)熱(re)量更低(di)。因此，與線性電(dian)(dian)(dian)池(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)器(qi)相(xiang)比(bi)，它具(ju)(ju)有更長的(de)使用壽(shou)命(ming)。

　　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，線性電池充電器不再能夠高(gao)效(xiao)的(de)(de)對鋰(li)離子電(dian)池(chi)充電(dian)。集(ji)成 MOSFET 的(de)(de)高(gao)效(xiao)率同步開關電(dian)池(chi)充電(dian)器為這些高(gao)級(ji)便攜(xie)設(she)備(bei)提供高(gao)效(xiao)的(de)(de)充電(dian)解(jie)決(jue)方案，從(cong)而實現(xian)更(geng)低(di)的(de)(de)熱量與更(geng)長的(de)(de)電(dian)池(chi)使(shi)用壽命。