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(dian)并延(yan)長電(dian)池(chi)使用壽命的設(she)計考慮事項。

　　鋰(li)離子電(dian)池充電(dian) 

　　大部(bu)分(fen)專用鋰(li)離子(zi)充(chong)(chong)電(dian)(dian)(dian)(dian)集成(cheng)(cheng)電(dian)(dian)(dian)(dian)路 (IC) 都是通過(guo)圖1所示的(de)(de)方式充(chong)(chong)電(dian)(dian)(dian)(dian)。鋰(li)離子(zi)電(dian)(dian)(dian)(dian)池的(de)(de)充(chong)(chong)電(dian)(dian)(dian)(dian)過(guo)程由三個(ge)階段組成(cheng)(cheng)：預充(chong)(chong)電(dian)(dian)(dian)(dian)、恒(heng)流 (CC) 快速充(chong)(chong)電(dian)(dian)(dian)(dian)以(yi)(yi)及(ji)恒(heng)壓 (CV) 終端 (Termination)。在(zai)(zai)預充(chong)(chong)電(dian)(dian)(dian)(dian)階段，以(yi)(yi)低(di)(di)速率（一般是快速充(chong)(chong)電(dian)(dian)(dian)(dian)率的(de)(de) 1/10）對電(dian)(dian)(dian)(dian)池充(chong)(chong)電(dian)(dian)(dian)(dian)，這(zhe)時(shi)的(de)(de)電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓低(di)(di)于 3.0V。這(zhe)樣可以(yi)(yi)實現對鈍化層(ceng)的(de)(de)恢復(fu) - 鈍化層(ceng)在(zai)(zai)深度放(fang)電(dian)(dian)(dian)(dian)狀態下(xia)(xia)存(cun)儲時(shi)間過(guo)長會分(fen)解。另(ling)外，還可以(yi)(yi)在(zai)(zai)發生(sheng)陽(yang)極短路的(de)(de)過(guo)充(chong)(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)池出現部(bu)分(fen)銅分(fen)解的(de)(de)情況下(xia)(xia)防止 1C 充(chong)(chong)電(dian)(dian)(dian)(dian)過(guo)熱。在(zai)(zai)電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓達到 3.0V 時(shi)，電(dian)(dian)(dian)(dian)池充(chong)(chong)電(dian)(dian)(dian)(dian)器進入 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(shi)調(diao)節電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)電(dian)(dian)(dian)(dian)壓(ya)(ya)并且進(jin)入 CV 階段。輸(shu)出(chu)穩(wen)壓(ya)(ya)精(jing)(jing)度是提高電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)容(rong)量和(he)延長使用壽命的關鍵。較低(di)(di)(di)的穩(wen)壓(ya)(ya)精(jing)(jing)度會造(zao)成(cheng)(cheng)(cheng)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)不(bu)足，進(jin)而(er)(er)造(zao)成(cheng)(cheng)(cheng)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)容(rong)量大幅降(jiang)低(di)(di)(di)。充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)不(bu)足 1％ 電(dian)(dian)(dian)(dian)壓(ya)(ya)時，電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)即損失大約(yue) 8％ 的容(rong)量。較低(di)(di)(di)的電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)穩(wen)壓(ya)(ya)精(jing)(jing)度也(ye)會造(zao)成(cheng)(cheng)(cheng)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)過(guo)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)，從而(er)(er)縮短電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)使用壽命。為了安全(quan)地對(dui)鋰離子電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)，僅允許環境溫(wen)度在 0～45 C 之間。在更(geng)低(di)(di)(di)溫(wen)度時充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)會形成(cheng)(cheng)(cheng)金屬鋰，從而(er)(er)提高電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)阻抗(kang)并造(zao)成(cheng)(cheng)(cheng)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)降(jiang)質。在更(geng)高溫(wen)度時充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)會由于鋰電(dian)(dian)(dian)(dian)解(jie)反應(ying)而(er)(er)造(zao)成(cheng)(cheng)(cheng)加速降(jiang)質。

　　低成本獨立線性電池充電器

　　許(xu)多IC 制造商通過開發用(yong)于低功耗便攜(xie)設備的低成(cheng)本線(xian)(xian)性(xing)電(dian)(dian)池(chi)充電(dian)(dian)器來滿足市場(chang)對(dui)更精確(que)和更安全充電(dian)(dian)的需(xu)求。圖(tu) 2 就是(shi)一種采用(yong)更少外部組件的低成(cheng)本獨立線(xian)(xian)性(xing)電(dian)(dian)池(chi)充電(dian)(dian)器電(dian)(dian)路結構(gou)圖(tu)。

　　這種電(dian)(dian)池充電(dian)(dian)器(qi)簡便地把適配(pei)器(qi)的(de) DC 電(dian)(dian)壓(ya)降低到電(dian)(dian)池電(dian)(dian)壓(ya)。導(dao)通(tong)元件上的(de)功率等于適配(pei)器(qi)電(dian)(dian)壓(ya)減(jian)去(qu)電(dian)(dian)池電(dian)(dian)壓(ya)再乘以(yi)充電(dian)(dian)電(dian)(dian)流，如下式所(suo)示：

　　如果(guo)采用 5V 適配器(qi)對(dui) 1200mAh 或 2200mAh 單體鋰離子電池充電，則(ze)圖 3 說明 0.7C 充電率快速(su)充電電流情況下的功(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。

　　兆赫同步開(kai)關電池充電器(qi)

　　同步開(kai)關式充電(dian)(dian)(dian)(dian)解決方案一般用于具有(you)(you)高(gao)輸入(ru)-輸出壓差的應用或者高(gao)容量(liang)電(dian)(dian)(dian)(dian)池(chi)組。對(dui)于 2200mAh 鋰離(li)子電(dian)(dian)(dian)(dian)池(chi)組，很(hen)難采(cai)用線性電(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)器通過車載(zai)適配器 (12V) 在 0.5C～1C 的快速充電(dian)(dian)(dian)(dian)率(lv)情(qing)況下對(dui)單體電(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)。雖(sui)然可(ke)以(yi)采(cai)用具有(you)(you)散熱調節功能的線性電(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)器，但是低充電(dian)(dian)(dian)(dian)率(lv)情(qing)況下的充電(dian)(dian)(dian)(dian)時間過長。

　　圖 4 說明適用于 DVD 播(bo)放器(qi)和智能(neng)電(dian)話等(deng)設備的充電(dian)電(dian)流達(da)到 2A 的獨立(li)高效同步開關降壓(ya)電(dian)池(chi)充電(dian)器(qi)。

　　它采用(yong)1.1MHz 開關(guan)(guan)頻率(lv)電(dian)(dian)(dian)壓模(mo)式控制架構(gou)，利用(yong)內置III型環路補償器(qi)(qi)降低(di)外部組(zu)件數量。為了(le)進一步降低(di)電(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器(qi)(qi)尺寸，它在(zai)(zai)4 4 mm小型封(feng)裝的(de)(de) PWM 控制器(qi)(qi)中集成(cheng)了(le)兩(liang)個功(gong)率(lv) MOSFET。功(gong)率(lv) MOSFET Q1 和 Q2 交替關(guan)(guan)閉，具(ju)有最(zui)佳的(de)(de)停滯時間(jian)，以(yi)優化高開關(guan)(guan)頻率(lv)時的(de)(de)效率(lv)。Q1 用(yong)作 P 通道(dao) MOSFET，在(zai)(zai)用(yong)于(yu)高側(ce) N-MOSFET 柵極驅動(dong)器(qi)(qi)時可(ke)以(yi)消除(chu)外部自益(yi)放大電(dian)(dian)(dian)容器(qi)(qi) (boost strap capacitor) 和二極管。另外，通過完全打開 Q1，在(zai)(zai)輸入電(dian)(dian)(dian)壓非常接近電(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)壓時，易(yi)于(yu)實現 100％的(de)(de)占(zhan)空比。打開和關(guan)(guan)閉時間(jian)處于(yu)受控狀態，從而可(ke)以(yi)根據(ju)反饋控制環路調節電(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流（CC 階段(duan)）或電(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)壓（CV階段(duan)）。電(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)器(qi)(qi)具(ju)有高度(du)集成(cheng)的(de)(de)功(gong)能，能夠安全、高效地對鋰離子電(dian)(dian)(dian)池(chi)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)。它可(ke)以(yi)編程預充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流、快速(su)充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流、充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)壓、充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)定時器(qi)(qi)、電(dian)(dian)(dian)池(chi)溫度(du)監控、自動(dong)再充(chong)(chong)(chong)(chong)電(dian)(dian)(dian)、短路和過熱保護。電(dian)(dian)(dian)路參數設計用(yong)于(yu)下述設計示例中的(de)(de)以(yi)下規(gui)格。

適配器(qi) DC 電壓：12 V

雙體鋰(li)離子電(dian)(dian)池組：4.2 V/電(dian)(dian)池，1900mAh/電(dian)(dian)池

預充電電流：IPRE-CHG=133 mA

快(kuai)速(su)充電電流：ICHG=1.33 A

充電時間限制：tCHG = 5-hour

開(kai)始充電的溫度范(fan)圍(wei)：T= 0℃～45 C。

　　由于(yu)電(dian)(dian)池(chi)充電(dian)(dian)器的(de)尺寸對(dui)便攜設備(bei)極其重要，因此(ci)需(xu)要采用盡可能小的(de)輸出電(dian)(dian)感(gan)器。對(dui)于(yu)給定的(de)電(dian)(dian)感(gan)器紋波電(dian)(dian)流，所(suo)需(xu)的(de)電(dian)(dian)感(gan)由下式得出：

　　式(shi)(shi)中(zhong)(zhong)，f_{s}和(he)(he)(he) DIripple,L 分別是開(kai)關頻率(lv)(lv)和(he)(he)(he)電(dian)感(gan)(gan)器紋波電(dian)流。在(zai)上(shang)式(shi)(shi)中(zhong)(zhong)代入(ru)VIN=12V、VBAT=6.0V（3.0V/電(dian)池）、Iripple,L=30％ICHG、ICHG=1.33A以(yi)(yi)(yi)及fs=1MHz ，可以(yi)(yi)(yi)得出L=7.5 H。可以(yi)(yi)(yi)選擇L=10 H的屏蔽電(dian)感(gan)(gan)器。請(qing)注(zhu)意：屏蔽電(dian)感(gan)(gan)器在(zai)把磁通量限制在(zai)電(dian)感(gan)(gan)器內部和(he)(he)(he)降(jiang)低(di)輻射(she)電(dian)磁干擾 (EMI) 方(fang)面具有(you)更(geng)高(gao)能力。所需的電(dian)感(gan)(gan)與(yu)開(kai)關頻率(lv)(lv)成反比。另一(yi)方(fang)面，電(dian)感(gan)(gan)可以(yi)(yi)(yi)降(jiang)低(di) 10 倍，在(zai) 1MHz 時的尺寸低(di)于 100kHz 時的尺寸，開(kai)關頻率(lv)(lv)越高(gao)，Q1 和(he)(he)(he) Q2 上(shang)的開(kai)關損(sun)耗越高(gao)，同時電(dian)感(gan)(gan)器內核損(sun)耗也越高(gao)。因(yin)此，1MHz 開(kai)關頻率(lv)(lv)是實際設計中(zhong)(zhong)電(dian)感(gan)(gan)器尺寸和(he)(he)(he)功率(lv)(lv)轉(zhuan)換效率(lv)(lv)之間的理想取(qu)舍。

　　電感(gan)器額定電流的(de)選擇(ze)對實(shi)現(xian)預(yu)期效率也很(hen)重要。峰值電感(gan)器電流 IPeak 通過下式(shi)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(xuan)擇較小的(de)、具(ju)有良好溫度(du)特(te)征(zheng)的(de)陶(tao)瓷輸出(chu)電(dian)容器(qi)(qi)，如：X7R 和(he) X5R 陶(tao)瓷電(dian)容器(qi)(qi)。進(jin)入電(dian)池的(de)紋波電(dian)流由下式得出(chu)：

　　式中，ESR、RSNS和RBAT分別是輸出電(dian)容器(qi)(qi)等效串連電(dian)阻(zu)、電(dian)流(liu)感測電(dian)阻(zu)器(qi)(qi)和電(dian)池(chi)內部阻(zu)抗，包括(kuo)電(dian)池(chi)組中保護 MOSFET 的(de) Rdson。輸出電(dian)容器(qi)(qi)的(de) ESR 越低，進(jin)入電(dian)池(chi)的(de)紋波電(dian)流(liu)也越低。進(jin)入電(dian)池(chi)的(de)紋波電(dian)流(liu)應當低于電(dian)感器(qi)(qi)紋波電(dian)流(liu)的(de)十分之一，一般(ban)情況下 10 F/10m ESR 陶瓷電(dian)容器(qi)(qi)即可滿足上述需求(qiu)。

　　?選擇(ze)電流(liu)感測電阻器RSNS

　　根據感測電阻(zu)器的調節閾值 VIREG 選擇(ze) RSNS。為了取(qu)得標準(zhun)的感測電阻(zu)器值，使 VIREG=133mV，則求得 RSNS：

　　感測電阻器的(de)功耗為I2CHGRSNS=I2CHGRsns=0.18W。選擇(ze) 0.5W 時的(de) 1206 額(e)定尺寸(cun)。

　　?選(xuan)擇(ze)快速充電電流 設定電阻器RSET1.

　　RSET1 用(yong)于設定快速充電電流，RSET1 由下(xia)式(shi)求得(de)：

　　?選擇(ze)預充電電流設定電阻器 RSET2.

　　RSET2用于設定預充(chong)電電流，由(you)下式求(qiu)得：

　　?選擇最長充電(dian)時間設定電(dian)容器 CTTC

　　如果電池未充(chong)(chong)滿，充(chong)(chong)電定(ding)時(shi)器可以檢測“壞”電池組，此時(shi)充(chong)(chong)電定(ding)時(shi)器失效(xiao)。CTTC 用(yong)于對充(chong)(chong)電定(ding)時(shi)器進行編程，規定(ding)每 nF 為 2.6 分鐘。

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

　　可以選(xuan)用 0.1 F 陶瓷電(dian)容(rong)器。

　　?選擇(ze)最低(di)與最高充電溫度(du)設定(ding)電阻器 RT1 與 RT2

　　RT1 與 RT2 用于在(zai) 0 C～45 C 間充(chong)電(dian)溫度范圍(wei)內進行編程，以啟(qi)動電(dian)池(chi)充(chong)電(dian)器。對于電(dian)池(chi)組中常用的 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(zhong)代入 RTL 與 RTH 可(ke)以求得 RT1=9.31kW，RT2=442 kW。

　　在16V輸入電(dian)壓下仍(reng)然具(ju)有(you)超過 90％ 的(de)(de)效率。與(yu)(yu)線性充電(dian)器(qi)(qi)相比(bi)，功(gong)耗(hao)低得多，而且可以(yi)在電(dian)池組側設計同步開關(guan)(guan)充電(dian)器(qi)(qi)，以(yi)降低對(dui)主板空(kong)間(jian)的(de)(de)占用，由于以(yi) MHz 頻率進(jin)行工(gong)作，電(dian)感(gan)器(qi)(qi)的(de)(de)尺寸(cun)較小(xiao)。需要牢記的(de)(de)是，電(dian)池的(de)(de)使用壽命主要取決于其(qi)溫度(du)。利(li)用同步開關(guan)(guan)電(dian)池充電(dian)器(qi)(qi)對(dui)鋰離子電(dian)池充電(dian)一般(ban)情況下產生的(de)(de)熱量(liang)更低。因此，與(yu)(yu)線性電(dian)池充電(dian)器(qi)(qi)相比(bi)，它具(ju)有(you)更長的(de)(de)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，線性電池充電器不再(zai)能夠(gou)高(gao)效的(de)對鋰離子電(dian)(dian)池充電(dian)(dian)。集(ji)成 MOSFET 的(de)高(gao)效率同步開關電(dian)(dian)池充電(dian)(dian)器為這些高(gao)級便(bian)攜(xie)設備(bei)提供高(gao)效的(de)充電(dian)(dian)解決(jue)方(fang)案(an)，從而實現更低的(de)熱量與更長的(de)電(dian)(dian)池使(shi)用(yong)壽(shou)命。