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(you)效充電并(bing)延長(chang)電池使用壽(shou)命的(de)設計考慮(lv)事項(xiang)。

　　鋰離子(zi)電池充電 

　　大(da)部(bu)分專用(yong)鋰離(li)子(zi)充(chong)電(dian)(dian)集成電(dian)(dian)路(lu) (IC) 都是(shi)通過圖1所示(shi)的(de)方式充(chong)電(dian)(dian)。鋰離(li)子(zi)電(dian)(dian)池(chi)的(de)充(chong)電(dian)(dian)過程(cheng)由三(san)個階段組(zu)成：預(yu)充(chong)電(dian)(dian)、恒流 (CC) 快速(su)充(chong)電(dian)(dian)以(yi)(yi)及恒壓(ya) (CV) 終端 (Termination)。在(zai)預(yu)充(chong)電(dian)(dian)階段，以(yi)(yi)低(di)速(su)率(lv)（一(yi)般是(shi)快速(su)充(chong)電(dian)(dian)率(lv)的(de) 1/10）對電(dian)(dian)池(chi)充(chong)電(dian)(dian)，這時的(de)電(dian)(dian)池(chi)電(dian)(dian)壓(ya)低(di)于 3.0V。這樣可(ke)以(yi)(yi)實現對鈍化(hua)(hua)層的(de)恢復(fu) - 鈍化(hua)(hua)層在(zai)深度放(fang)電(dian)(dian)狀(zhuang)態下(xia)存儲時間過長會分解。另(ling)外，還(huan)可(ke)以(yi)(yi)在(zai)發生陽(yang)極短路(lu)的(de)過充(chong)電(dian)(dian)電(dian)(dian)池(chi)出(chu)現部(bu)分銅分解的(de)情況下(xia)防止 1C 充(chong)電(dian)(dian)過熱(re)。在(zai)電(dian)(dian)池(chi)電(dian)(dian)壓(ya)達到 3.0V 時，電(dian)(dian)池(chi)充(chong)電(dian)(dian)器(qi)進入(ru) CC 階段。

　　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)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)(dian)壓(ya)并(bing)且進(jin)入 CV 階段。輸出穩(wen)壓(ya)精度(du)(du)(du)是提高電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)容(rong)量和(he)延長使用壽(shou)命的關鍵(jian)。較(jiao)(jiao)低(di)的穩(wen)壓(ya)精度(du)(du)(du)會(hui)造成(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充電(dian)(dian)(dian)(dian)(dian)不足(zu)，進(jin)而(er)(er)造成(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)容(rong)量大(da)幅降低(di)。充電(dian)(dian)(dian)(dian)(dian)不足(zu) 1％ 電(dian)(dian)(dian)(dian)(dian)壓(ya)時(shi)，電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)即(ji)損失大(da)約 8％ 的容(rong)量。較(jiao)(jiao)低(di)的電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)穩(wen)壓(ya)精度(du)(du)(du)也(ye)會(hui)造成(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)過充電(dian)(dian)(dian)(dian)(dian)，從而(er)(er)縮短(duan)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)使用壽(shou)命。為了安全地對鋰(li)離子電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充電(dian)(dian)(dian)(dian)(dian)，僅允許(xu)環境溫(wen)度(du)(du)(du)在(zai)(zai) 0～45 C 之間。在(zai)(zai)更(geng)低(di)溫(wen)度(du)(du)(du)時(shi)充電(dian)(dian)(dian)(dian)(dian)會(hui)形成(cheng)(cheng)金屬鋰(li)，從而(er)(er)提高電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)阻抗并(bing)造成(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)降質。在(zai)(zai)更(geng)高溫(wen)度(du)(du)(du)時(shi)充電(dian)(dian)(dian)(dian)(dian)會(hui)由于(yu)鋰(li)電(dian)(dian)(dian)(dian)(dian)解反應而(er)(er)造成(cheng)(cheng)加速降質。

　　低成本(ben)獨立(li)線性(xing)電池充電器

　　許多IC 制造(zao)商(shang)通過開發(fa)用(yong)于低功耗便(bian)攜設備的(de)(de)低成(cheng)本線性(xing)電池(chi)充電器來滿足(zu)市(shi)場(chang)對(dui)更(geng)(geng)精確和更(geng)(geng)安(an)全充電的(de)(de)需求(qiu)。圖(tu) 2 就(jiu)是一種采用(yong)更(geng)(geng)少外部(bu)組件的(de)(de)低成(cheng)本獨立線性(xing)電池(chi)充電器電路結構圖(tu)。

　　這(zhe)種電(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)器簡便地把適配器的(de) DC 電(dian)(dian)(dian)壓(ya)降低到電(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)壓(ya)。導(dao)通元件上的(de)功率等(deng)于適配器電(dian)(dian)(dian)壓(ya)減去電(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)壓(ya)再(zai)乘以(yi)充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流，如下(xia)式(shi)所示：

　　如果(guo)采用 5V 適配器對 1200mAh 或(huo) 2200mAh 單體鋰離(li)子電(dian)池充(chong)(chong)電(dian)，則圖 3 說明 0.7C 充(chong)(chong)電(dian)率快速充(chong)(chong)電(dian)電(dian)流(liu)情況下(xia)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(guan)電池(chi)充電器

　　同步(bu)開關(guan)式充(chong)電(dian)(dian)(dian)解決(jue)方案一般用于具(ju)有高(gao)輸(shu)入(ru)-輸(shu)出壓差的(de)(de)(de)應用或者高(gao)容量電(dian)(dian)(dian)池(chi)組。對于 2200mAh 鋰離(li)子電(dian)(dian)(dian)池(chi)組，很難采(cai)用線(xian)性電(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)器(qi)通(tong)過車載適配(pei)器(qi) (12V) 在 0.5C～1C 的(de)(de)(de)快(kuai)速充(chong)電(dian)(dian)(dian)率(lv)情(qing)況下對單體電(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)。雖然(ran)可以采(cai)用具(ju)有散熱調節功能的(de)(de)(de)線(xian)性電(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)器(qi)，但是低充(chong)電(dian)(dian)(dian)率(lv)情(qing)況下的(de)(de)(de)充(chong)電(dian)(dian)(dian)時間過長(chang)。

　　圖 4 說明適(shi)用于 DVD 播放器和智(zhi)能電(dian)(dian)話(hua)等設備的(de)充電(dian)(dian)電(dian)(dian)流(liu)達(da)到(dao) 2A 的(de)獨立高效同步開關降(jiang)壓(ya)電(dian)(dian)池充電(dian)(dian)器。

　　它(ta)采(cai)用(yong)1.1MHz 開關(guan)頻(pin)率電(dian)(dian)(dian)壓(ya)(ya)模式控制(zhi)架構，利用(yong)內置III型環(huan)路(lu)補償器(qi)降(jiang)低(di)外(wai)部組件數(shu)量(liang)。為(wei)了(le)(le)進一(yi)步降(jiang)低(di)電(dian)(dian)(dian)池(chi)(chi)(chi)(chi)充電(dian)(dian)(dian)器(qi)尺(chi)寸，它(ta)在4 4 mm小型封裝的(de) PWM 控制(zhi)器(qi)中集成了(le)(le)兩(liang)個功率 MOSFET。功率 MOSFET Q1 和(he) Q2 交替關(guan)閉(bi)(bi)，具有(you)最(zui)佳的(de)停滯時(shi)(shi)間，以(yi)優化高開關(guan)頻(pin)率時(shi)(shi)的(de)效率。Q1 用(yong)作 P 通道 MOSFET，在用(yong)于高側(ce) N-MOSFET 柵極(ji)(ji)驅動器(qi)時(shi)(shi)可以(yi)消除外(wai)部自益放大(da)電(dian)(dian)(dian)容(rong)器(qi) (boost strap capacitor) 和(he)二極(ji)(ji)管。另外(wai)，通過完全打開 Q1，在輸入電(dian)(dian)(dian)壓(ya)(ya)非常接(jie)近電(dian)(dian)(dian)池(chi)(chi)(chi)(chi)電(dian)(dian)(dian)壓(ya)(ya)時(shi)(shi)，易于實現 100％的(de)占(zhan)空(kong)比(bi)。打開和(he)關(guan)閉(bi)(bi)時(shi)(shi)間處于受(shou)控狀態，從而可以(yi)根據(ju)反(fan)饋控制(zhi)環(huan)路(lu)調節電(dian)(dian)(dian)池(chi)(chi)(chi)(chi)充電(dian)(dian)(dian)電(dian)(dian)(dian)流（CC 階段）或電(dian)(dian)(dian)池(chi)(chi)(chi)(chi)電(dian)(dian)(dian)壓(ya)(ya)（CV階段）。電(dian)(dian)(dian)池(chi)(chi)(chi)(chi)充電(dian)(dian)(dian)器(qi)具有(you)高度(du)集成的(de)功能，能夠(gou)安全、高效地對(dui)鋰離子電(dian)(dian)(dian)池(chi)(chi)(chi)(chi)充電(dian)(dian)(dian)。它(ta)可以(yi)編程(cheng)預充電(dian)(dian)(dian)電(dian)(dian)(dian)流、快速充電(dian)(dian)(dian)電(dian)(dian)(dian)流、充電(dian)(dian)(dian)電(dian)(dian)(dian)壓(ya)(ya)、充電(dian)(dian)(dian)定時(shi)(shi)器(qi)、電(dian)(dian)(dian)池(chi)(chi)(chi)(chi)溫度(du)監(jian)控、自動再充電(dian)(dian)(dian)、短路(lu)和(he)過熱保護。電(dian)(dian)(dian)路(lu)參(can)數(shu)設(she)計(ji)用(yong)于下述設(she)計(ji)示(shi)例(li)中的(de)以(yi)下規格。

適配器 DC 電(dian)壓：12 V

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

預(yu)充(chong)電電流：IPRE-CHG=133 mA

快速充電電流：ICHG=1.33 A

充電時間限制：tCHG = 5-hour

開始充電(dian)的溫(wen)度(du)范圍：T= 0℃～45 C。

　　由(you)于電(dian)池充(chong)電(dian)器的(de)尺(chi)寸對(dui)(dui)便(bian)攜設備極其重要，因(yin)此(ci)需(xu)要采(cai)用盡(jin)可能小的(de)輸出(chu)電(dian)感器。對(dui)(dui)于給定的(de)電(dian)感器紋波電(dian)流，所需(xu)的(de)電(dian)感由(you)下式(shi)得(de)出(chu)：

　　式(shi)中，f_{s}和(he) DIripple,L 分別是開(kai)關(guan)頻(pin)率(lv)和(he)電(dian)(dian)(dian)感(gan)器(qi)紋波電(dian)(dian)(dian)流(liu)。在上(shang)式(shi)中代入VIN=12V、VBAT=6.0V（3.0V/電(dian)(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)屏(ping)蔽電(dian)(dian)(dian)感(gan)器(qi)。請注意：屏(ping)蔽電(dian)(dian)(dian)感(gan)器(qi)在把磁(ci)通量(liang)限制在電(dian)(dian)(dian)感(gan)器(qi)內部和(he)降低(di)(di)輻射電(dian)(dian)(dian)磁(ci)干擾 (EMI) 方面(mian)具有更(geng)高能力(li)。所需的(de)電(dian)(dian)(dian)感(gan)與(yu)開(kai)關(guan)頻(pin)率(lv)成反比。另一方面(mian)，電(dian)(dian)(dian)感(gan)可以降低(di)(di) 10 倍，在 1MHz 時的(de)尺(chi)寸低(di)(di)于 100kHz 時的(de)尺(chi)寸，開(kai)關(guan)頻(pin)率(lv)越(yue)高，Q1 和(he) Q2 上(shang)的(de)開(kai)關(guan)損耗越(yue)高，同時電(dian)(dian)(dian)感(gan)器(qi)內核損耗也(ye)越(yue)高。因此，1MHz 開(kai)關(guan)頻(pin)率(lv)是實際設(she)計中電(dian)(dian)(dian)感(gan)器(qi)尺(chi)寸和(he)功率(lv)轉(zhuan)換(huan)效率(lv)之間的(de)理想取舍。

　　電(dian)感器額(e)定電(dian)流(liu)的選擇對實現預期效(xiao)率也(ye)很重要。峰值(zhi)電(dian)感器電(dian)流(liu) IPeak 通過下(xia)式計(ji)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(de)、具有良好(hao)溫(wen)度特征的(de)陶瓷輸出(chu)電(dian)容(rong)器，如(ru)：X7R 和 X5R 陶瓷電(dian)容(rong)器。進(jin)入電(dian)池的(de)紋波電(dian)流(liu)由下式(shi)得出(chu)：

　　式中，ESR、RSNS和(he)RBAT分別是輸出電(dian)(dian)(dian)容(rong)器(qi)(qi)等(deng)效串連電(dian)(dian)(dian)阻、電(dian)(dian)(dian)流感測電(dian)(dian)(dian)阻器(qi)(qi)和(he)電(dian)(dian)(dian)池(chi)(chi)內部阻抗，包括電(dian)(dian)(dian)池(chi)(chi)組中保護(hu) MOSFET 的(de) Rdson。輸出電(dian)(dian)(dian)容(rong)器(qi)(qi)的(de) ESR 越(yue)低，進(jin)入電(dian)(dian)(dian)池(chi)(chi)的(de)紋波(bo)電(dian)(dian)(dian)流也越(yue)低。進(jin)入電(dian)(dian)(dian)池(chi)(chi)的(de)紋波(bo)電(dian)(dian)(dian)流應當低于電(dian)(dian)(dian)感器(qi)(qi)紋波(bo)電(dian)(dian)(dian)流的(de)十分之一，一般情況下 10 F/10m ESR 陶瓷電(dian)(dian)(dian)容(rong)器(qi)(qi)即可滿足上述需(xu)求。

　　?選擇電(dian)(dian)流(liu)感測電(dian)(dian)阻(zu)器RSNS

　　根據感測(ce)電(dian)阻器的調(diao)節(jie)閾值 VIREG 選擇(ze) RSNS。為(wei)了取得標準的感測(ce)電(dian)阻器值，使 VIREG=133mV，則求(qiu)得 RSNS：

　　感測電阻(zu)器(qi)的功(gong)耗為I2CHGRSNS=I2CHGRsns=0.18W。選擇 0.5W 時的 1206 額定尺(chi)寸。

　　?選(xuan)擇快(kuai)速(su)充電電流 設定電阻器RSET1.

　　RSET1 用于(yu)設(she)定(ding)快速(su)充電電流(liu)，RSET1 由下式求得：

　　?選擇預(yu)充電電流設定(ding)電阻器 RSET2.

　　RSET2用于設定(ding)預充電(dian)電(dian)流，由下式(shi)求得：

　　?選擇(ze)最長充電時(shi)間(jian)設(she)定電容(rong)器 CTTC

　　如果電(dian)池未(wei)充滿，充電(dian)定時(shi)器(qi)(qi)可以檢(jian)測“壞”電(dian)池組，此時(shi)充電(dian)定時(shi)器(qi)(qi)失效。CTTC 用于(yu)對充電(dian)定時(shi)器(qi)(qi)進行編程，規(gui)定每 nF 為 2.6 分鐘(zhong)。

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

　　可以(yi)選用 0.1 F 陶瓷電(dian)容器。

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

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

　　在上式中代(dai)入 RTL 與 RTH 可(ke)以求(qiu)得 RT1=9.31kW，RT2=442 kW。

　　在(zai)16V輸入電(dian)(dian)壓下仍(reng)然具有(you)超過 90％ 的效率(lv)。與(yu)線性充電(dian)(dian)器相(xiang)比，功耗低得(de)多，而(er)且(qie)可(ke)以在(zai)電(dian)(dian)池(chi)(chi)組側設計(ji)同步(bu)開(kai)關(guan)充電(dian)(dian)器，以降(jiang)低對主板空間的占用(yong)(yong)，由(you)于以 MHz 頻率(lv)進行工作，電(dian)(dian)感(gan)器的尺(chi)寸較小。需要牢記的是，電(dian)(dian)池(chi)(chi)的使用(yong)(yong)壽命主要取決于其溫度(du)。利用(yong)(yong)同步(bu)開(kai)關(guan)電(dian)(dian)池(chi)(chi)充電(dian)(dian)器對鋰離子電(dian)(dian)池(chi)(chi)充電(dian)(dian)一般情況下產生(sheng)的熱量更(geng)低。因此，與(yu)線性電(dian)(dian)池(chi)(chi)充電(dian)(dian)器相(xiang)比，它具有(you)更(geng)長的使用(yong)(yong)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)(gao)效(xiao)的(de)(de)對鋰(li)離子電(dian)(dian)池充電(dian)(dian)。集成 MOSFET 的(de)(de)高(gao)(gao)效(xiao)率(lv)同步開(kai)關電(dian)(dian)池充電(dian)(dian)器為這些高(gao)(gao)級便攜設備提供高(gao)(gao)效(xiao)的(de)(de)充電(dian)(dian)解決方案，從而實(shi)現更低的(de)(de)熱量與更長的(de)(de)電(dian)(dian)池使用(yong)壽(shou)命。