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(xiao)充電并延長電池使用壽命(ming)的設計考慮事項。

　　鋰離子電(dian)池充電(dian) 

　　大(da)部分專用(yong)鋰離(li)子充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)集成電(dian)(dian)(dian)(dian)(dian)路(lu) (IC) 都(dou)是通過圖1所示的方式充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)。鋰離(li)子電(dian)(dian)(dian)(dian)(dian)池(chi)的充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)過程由三個(ge)階(jie)段(duan)(duan)組(zu)成：預充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)、恒流(liu) (CC) 快(kuai)速充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)以(yi)及恒壓 (CV) 終(zhong)端 (Termination)。在預充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)階(jie)段(duan)(duan)，以(yi)低(di)速率(lv)（一(yi)般是快(kuai)速充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)率(lv)的 1/10）對電(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)，這時(shi)的電(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)壓低(di)于 3.0V。這樣可以(yi)實現(xian)對鈍(dun)化層(ceng)的恢(hui)復 - 鈍(dun)化層(ceng)在深度(du)放電(dian)(dian)(dian)(dian)(dian)狀態下(xia)存儲時(shi)間(jian)過長(chang)會分解(jie)。另外，還可以(yi)在發(fa)生陽極(ji)短路(lu)的過充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)池(chi)出現(xian)部分銅分解(jie)的情況下(xia)防止 1C 充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)過熱。在電(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)壓達(da)到 3.0V 時(shi)，電(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器進入(ru) CC 階(jie)段(duan)(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)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)并且進(jin)入 CV 階段。輸出(chu)穩(wen)壓(ya)(ya)精(jing)度是提高電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)容量和延長使用(yong)壽命的(de)(de)關(guan)鍵。較低的(de)(de)穩(wen)壓(ya)(ya)精(jing)度會造成電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)(dian)(dian)不足(zu)，進(jin)而造成電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)容量大(da)幅降(jiang)低。充電(dian)(dian)(dian)(dian)(dian)(dian)不足(zu) 1％ 電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)時(shi)，電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)即損失大(da)約(yue) 8％ 的(de)(de)容量。較低的(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)穩(wen)壓(ya)(ya)精(jing)度也會造成電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)過充電(dian)(dian)(dian)(dian)(dian)(dian)，從(cong)而縮短電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)使用(yong)壽命。為了安全地對鋰離(li)子電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)(dian)(dian)，僅允許環(huan)境溫(wen)(wen)度在(zai) 0～45 C 之間。在(zai)更低溫(wen)(wen)度時(shi)充電(dian)(dian)(dian)(dian)(dian)(dian)會形成金屬(shu)鋰，從(cong)而提高電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)阻抗(kang)并造成電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)降(jiang)質。在(zai)更高溫(wen)(wen)度時(shi)充電(dian)(dian)(dian)(dian)(dian)(dian)會由于(yu)鋰電(dian)(dian)(dian)(dian)(dian)(dian)解(jie)反應(ying)而造成加速降(jiang)質。

　　低成本(ben)獨(du)立線性電池充電器

　　許多IC 制造商通過開(kai)發用于(yu)低(di)功耗便攜設備的低(di)成(cheng)(cheng)本(ben)線性電(dian)池(chi)充電(dian)器(qi)來滿足市場對(dui)更精確(que)和更安(an)全充電(dian)的需求(qiu)。圖 2 就是一(yi)種采用更少外部組件的低(di)成(cheng)(cheng)本(ben)獨立線性電(dian)池(chi)充電(dian)器(qi)電(dian)路結(jie)構圖。

　　這(zhe)種電(dian)(dian)池(chi)(chi)充電(dian)(dian)器簡便地把適配器的(de) DC 電(dian)(dian)壓降(jiang)低到電(dian)(dian)池(chi)(chi)電(dian)(dian)壓。導通元件上的(de)功率等(deng)于適配器電(dian)(dian)壓減(jian)去電(dian)(dian)池(chi)(chi)電(dian)(dian)壓再乘(cheng)以充電(dian)(dian)電(dian)(dian)流，如下式所示(shi)：

　　如果采用 5V 適配器(qi)對 1200mAh 或 2200mAh 單(dan)體鋰離子電(dian)池充電(dian)，則圖 3 說(shuo)明(ming) 0.7C 充電(dian)率快(kuai)速充電(dian)電(dian)流情況下的功耗。

　　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。

　　兆赫同步開關電池充電器

　　同(tong)步開關式充(chong)(chong)(chong)(chong)(chong)電解決方案一般用于(yu)具有高輸(shu)入-輸(shu)出(chu)壓差的(de)應用或者高容量電池(chi)組。對于(yu) 2200mAh 鋰離子電池(chi)組，很難采用線性電池(chi)充(chong)(chong)(chong)(chong)(chong)電器(qi)通過車(che)載適配器(qi) (12V) 在 0.5C～1C 的(de)快速充(chong)(chong)(chong)(chong)(chong)電率情(qing)況下對單(dan)體電池(chi)充(chong)(chong)(chong)(chong)(chong)電。雖(sui)然(ran)可以采用具有散熱調節功能的(de)線性電池(chi)充(chong)(chong)(chong)(chong)(chong)電器(qi)，但是低充(chong)(chong)(chong)(chong)(chong)電率情(qing)況下的(de)充(chong)(chong)(chong)(chong)(chong)電時間(jian)過長。

　　圖 4 說明(ming)適(shi)用于(yu) DVD 播放(fang)器和智(zhi)能電(dian)話(hua)等設備(bei)的充電(dian)電(dian)流達(da)到(dao) 2A 的獨(du)立高效同步開(kai)關降(jiang)壓電(dian)池(chi)充電(dian)器。

　　它采用(yong)(yong)1.1MHz 開關頻(pin)率(lv)電(dian)(dian)(dian)壓(ya)(ya)模式控制架構，利用(yong)(yong)內置III型(xing)(xing)環路(lu)補(bu)償器(qi)降低(di)外部(bu)組(zu)件數量。為了進(jin)一步降低(di)電(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)器(qi)尺寸，它在(zai)4 4 mm小(xiao)型(xing)(xing)封裝的(de) PWM 控制器(qi)中集(ji)成(cheng)了兩個(ge)功(gong)率(lv) MOSFET。功(gong)率(lv) MOSFET Q1 和 Q2 交(jiao)替關閉(bi)(bi)，具(ju)有(you)最佳的(de)停滯(zhi)時(shi)間(jian)(jian)，以優(you)化(hua)高(gao)開關頻(pin)率(lv)時(shi)的(de)效率(lv)。Q1 用(yong)(yong)作(zuo) P 通(tong)(tong)道(dao) MOSFET，在(zai)用(yong)(yong)于(yu)高(gao)側(ce) N-MOSFET 柵極驅動(dong)器(qi)時(shi)可(ke)以消除外部(bu)自益放大電(dian)(dian)(dian)容器(qi) (boost strap capacitor) 和二極管(guan)。另外，通(tong)(tong)過(guo)完全打開 Q1，在(zai)輸入電(dian)(dian)(dian)壓(ya)(ya)非常(chang)接近電(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)壓(ya)(ya)時(shi)，易于(yu)實現 100％的(de)占空比。打開和關閉(bi)(bi)時(shi)間(jian)(jian)處于(yu)受控狀(zhuang)態，從而可(ke)以根(gen)據反(fan)饋控制環路(lu)調節(jie)電(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)（CC 階段）或電(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)壓(ya)(ya)（CV階段）。電(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)器(qi)具(ju)有(you)高(gao)度(du)(du)集(ji)成(cheng)的(de)功(gong)能(neng)，能(neng)夠安全、高(gao)效地對鋰離(li)子電(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)。它可(ke)以編程預充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)、快速充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)、充(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)壓(ya)(ya)、充(chong)(chong)電(dian)(dian)(dian)定時(shi)器(qi)、電(dian)(dian)(dian)池(chi)(chi)(chi)溫度(du)(du)監控、自動(dong)再充(chong)(chong)電(dian)(dian)(dian)、短(duan)路(lu)和過(guo)熱(re)保(bao)護。電(dian)(dian)(dian)路(lu)參(can)數設計用(yong)(yong)于(yu)下(xia)述設計示(shi)例(li)中的(de)以下(xia)規(gui)格。

適配(pei)器(qi) DC 電壓：12 V

雙體(ti)鋰離子(zi)電池組(zu)：4.2 V/電池，1900mAh/電池

預(yu)充電(dian)電(dian)流：IPRE-CHG=133 mA

快速充電電流：ICHG=1.33 A

充電(dian)時(shi)間限制：tCHG = 5-hour

開始充電的(de)溫度(du)范(fan)圍：T= 0℃～45 C。

　　由于(yu)電池充(chong)電器的(de)(de)尺寸對(dui)便攜設備極其重要，因(yin)此需(xu)要采用盡(jin)可能小的(de)(de)輸(shu)出(chu)(chu)電感(gan)器。對(dui)于(yu)給定的(de)(de)電感(gan)器紋(wen)波電流，所需(xu)的(de)(de)電感(gan)由下(xia)式得出(chu)(chu)：

　　式(shi)中(zhong)，f_{s}和(he) DIripple,L 分(fen)別是開(kai)關(guan)(guan)(guan)(guan)頻(pin)率(lv)和(he)電(dian)(dian)感(gan)器(qi)(qi)紋波電(dian)(dian)流。在(zai)(zai)上式(shi)中(zhong)代(dai)入VIN=12V、VBAT=6.0V（3.0V/電(dian)(dian)池）、Iripple,L=30％ICHG、ICHG=1.33A以及fs=1MHz ，可以得出L=7.5 H。可以選(xuan)擇L=10 H的(de)(de)屏蔽電(dian)(dian)感(gan)器(qi)(qi)。請注意：屏蔽電(dian)(dian)感(gan)器(qi)(qi)在(zai)(zai)把磁(ci)通量限制在(zai)(zai)電(dian)(dian)感(gan)器(qi)(qi)內部和(he)降低輻(fu)射電(dian)(dian)磁(ci)干擾 (EMI) 方(fang)面具有更高(gao)能力。所需的(de)(de)電(dian)(dian)感(gan)與(yu)開(kai)關(guan)(guan)(guan)(guan)頻(pin)率(lv)成反(fan)比。另(ling)一(yi)方(fang)面，電(dian)(dian)感(gan)可以降低 10 倍，在(zai)(zai) 1MHz 時(shi)的(de)(de)尺(chi)寸(cun)低于 100kHz 時(shi)的(de)(de)尺(chi)寸(cun)，開(kai)關(guan)(guan)(guan)(guan)頻(pin)率(lv)越高(gao)，Q1 和(he) Q2 上的(de)(de)開(kai)關(guan)(guan)(guan)(guan)損耗越高(gao)，同時(shi)電(dian)(dian)感(gan)器(qi)(qi)內核損耗也(ye)越高(gao)。因此，1MHz 開(kai)關(guan)(guan)(guan)(guan)頻(pin)率(lv)是實際(ji)設(she)計中(zhong)電(dian)(dian)感(gan)器(qi)(qi)尺(chi)寸(cun)和(he)功率(lv)轉換效率(lv)之(zhi)間的(de)(de)理想取舍。

　　電(dian)感器(qi)額(e)定電(dian)流的選(xuan)擇對實現預期效率也很重要。峰值電(dian)感器(qi)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。
關鍵是(shi)選擇較小的(de)、具有(you)良好(hao)溫度特征的(de)陶瓷(ci)輸出電容器，如：X7R 和 X5R 陶瓷(ci)電容器。進入(ru)電池的(de)紋波(bo)電流由下(xia)式得出：

　　式中，ESR、RSNS和RBAT分(fen)別是輸出(chu)電(dian)(dian)(dian)(dian)(dian)容(rong)器(qi)(qi)等效(xiao)串連(lian)電(dian)(dian)(dian)(dian)(dian)阻、電(dian)(dian)(dian)(dian)(dian)流(liu)感測電(dian)(dian)(dian)(dian)(dian)阻器(qi)(qi)和電(dian)(dian)(dian)(dian)(dian)池(chi)內部(bu)阻抗，包(bao)括電(dian)(dian)(dian)(dian)(dian)池(chi)組中保護 MOSFET 的(de)(de) Rdson。輸出(chu)電(dian)(dian)(dian)(dian)(dian)容(rong)器(qi)(qi)的(de)(de) ESR 越(yue)低(di)，進入(ru)電(dian)(dian)(dian)(dian)(dian)池(chi)的(de)(de)紋(wen)波(bo)(bo)電(dian)(dian)(dian)(dian)(dian)流(liu)也越(yue)低(di)。進入(ru)電(dian)(dian)(dian)(dian)(dian)池(chi)的(de)(de)紋(wen)波(bo)(bo)電(dian)(dian)(dian)(dian)(dian)流(liu)應當低(di)于電(dian)(dian)(dian)(dian)(dian)感器(qi)(qi)紋(wen)波(bo)(bo)電(dian)(dian)(dian)(dian)(dian)流(liu)的(de)(de)十(shi)分(fen)之一(yi)，一(yi)般情況下 10 F/10m ESR 陶(tao)瓷電(dian)(dian)(dian)(dian)(dian)容(rong)器(qi)(qi)即可滿足上述(shu)需求(qiu)。

　　?選擇電流感(gan)測電阻器RSNS

　　根據感(gan)測電(dian)阻(zu)器的調節閾(yu)值 VIREG 選擇 RSNS。為了(le)取得標準的感(gan)測電(dian)阻(zu)器值，使(shi) VIREG=133mV，則求(qiu)得 RSNS：

　　感測電阻器的功(gong)耗(hao)為I2CHGRSNS=I2CHGRsns=0.18W。選擇(ze) 0.5W 時(shi)的 1206 額定尺寸(cun)。

　　?選擇快(kuai)速充電電流(liu) 設(she)定電阻(zu)器RSET1.

　　RSET1 用于設定(ding)快速充電(dian)電(dian)流，RSET1 由下式(shi)求得：

　　?選擇預充電(dian)(dian)電(dian)(dian)流設定電(dian)(dian)阻(zu)器 RSET2.

　　RSET2用于設定預充(chong)電電流，由下(xia)式(shi)求得(de)：

　　?選(xuan)擇最長充電時(shi)間設定電容(rong)器(qi) CTTC

　　如果電池未(wei)充(chong)滿，充(chong)電定時器(qi)可以檢測“壞”電池組，此時充(chong)電定時器(qi)失(shi)效(xiao)。CTTC 用于(yu)對充(chong)電定時器(qi)進行(xing)編程，規定每 nF 為 2.6 分鐘(zhong)。

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

　　可(ke)以(yi)選用(yong) 0.1 F 陶瓷(ci)電(dian)容(rong)器(qi)。

　　?選擇最低與(yu)最高充電溫度(du)設定電阻器 RT1 與(yu) RT2

　　RT1 與 RT2 用(yong)于在 0 C～45 C 間充電(dian)溫度范(fan)圍(wei)內進行(xing)編(bian)程，以啟動電(dian)池充電(dian)器。對于電(dian)池組中常用(yong)的 103AT-2 熱敏電(dian)阻，RT(0℃)=RTL=27.28k ，RT(45℃)=RTH=4.911 k ，RT1與RT2由下式確定：

　　在上式中代入 RTL 與 RTH 可以求得 RT1=9.31kW，RT2=442 kW。

　　在16V輸入電(dian)(dian)壓下(xia)(xia)仍然具有超過 90％ 的(de)(de)效率。與(yu)(yu)線性充(chong)(chong)電(dian)(dian)器相(xiang)比，功(gong)耗低得多，而且可以(yi)(yi)在電(dian)(dian)池組(zu)側設計(ji)同(tong)步開關(guan)充(chong)(chong)電(dian)(dian)器，以(yi)(yi)降低對主板空(kong)間的(de)(de)占用(yong)，由于(yu)(yu)以(yi)(yi) MHz 頻率進行(xing)工(gong)作，電(dian)(dian)感器的(de)(de)尺寸較(jiao)小。需要(yao)牢記的(de)(de)是(shi)，電(dian)(dian)池的(de)(de)使(shi)(shi)用(yong)壽(shou)命主要(yao)取決于(yu)(yu)其溫(wen)度。利(li)用(yong)同(tong)步開關(guan)電(dian)(dian)池充(chong)(chong)電(dian)(dian)器對鋰離子(zi)電(dian)(dian)池充(chong)(chong)電(dian)(dian)一般(ban)情況下(xia)(xia)產生的(de)(de)熱量更(geng)低。因(yin)此(ci)，與(yu)(yu)線性電(dian)(dian)池充(chong)(chong)電(dian)(dian)器相(xiang)比，它具有更(geng)長的(de)(de)使(shi)(shi)用(yong)壽(shou)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，線性電池充電器不再(zai)能(neng)夠高效(xiao)的(de)對鋰離子電(dian)(dian)池充電(dian)(dian)。集(ji)成 MOSFET 的(de)高效(xiao)率同(tong)步開(kai)關電(dian)(dian)池充電(dian)(dian)器(qi)為這些高級便(bian)攜(xie)設(she)備提供高效(xiao)的(de)充電(dian)(dian)解(jie)決方案，從而實現更(geng)低的(de)熱(re)量與更(geng)長的(de)電(dian)(dian)池使用壽命。