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(yi)及有效充(chong)電(dian)并延(yan)長電(dian)池使用壽命的設計考(kao)慮(lv)事項。

　　鋰離子電池(chi)充電 

　　大部(bu)分專用(yong)鋰離子充(chong)(chong)(chong)電(dian)(dian)集成(cheng)電(dian)(dian)路 (IC) 都是通過(guo)圖1所示(shi)的(de)方式充(chong)(chong)(chong)電(dian)(dian)。鋰離子電(dian)(dian)池(chi)(chi)的(de)充(chong)(chong)(chong)電(dian)(dian)過(guo)程由(you)三(san)個階(jie)段(duan)(duan)組成(cheng)：預充(chong)(chong)(chong)電(dian)(dian)、恒流 (CC) 快(kuai)速(su)充(chong)(chong)(chong)電(dian)(dian)以(yi)(yi)及恒壓(ya) (CV) 終端 (Termination)。在預充(chong)(chong)(chong)電(dian)(dian)階(jie)段(duan)(duan)，以(yi)(yi)低速(su)率(lv)（一般是快(kuai)速(su)充(chong)(chong)(chong)電(dian)(dian)率(lv)的(de) 1/10）對電(dian)(dian)池(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)，這時(shi)的(de)電(dian)(dian)池(chi)(chi)電(dian)(dian)壓(ya)低于 3.0V。這樣可(ke)(ke)以(yi)(yi)實現(xian)對鈍化層的(de)恢(hui)復 - 鈍化層在深度放(fang)電(dian)(dian)狀態下存儲時(shi)間過(guo)長會分解。另外，還可(ke)(ke)以(yi)(yi)在發生陽極短路的(de)過(guo)充(chong)(chong)(chong)電(dian)(dian)電(dian)(dian)池(chi)(chi)出現(xian)部(bu)分銅分解的(de)情況下防止(zhi) 1C 充(chong)(chong)(chong)電(dian)(dian)過(guo)熱(re)。在電(dian)(dian)池(chi)(chi)電(dian)(dian)壓(ya)達(da)到 3.0V 時(shi)，電(dian)(dian)池(chi)(chi)充(chong)(chong)(chong)電(dian)(dian)器進入 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(kai)始調節(jie)電(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)壓(ya)(ya)(ya)并且進入 CV 階段。輸出穩壓(ya)(ya)(ya)精(jing)度(du)是提高(gao)電(dian)(dian)(dian)池(chi)(chi)(chi)容(rong)(rong)量和延長(chang)使用壽(shou)命的(de)(de)關鍵。較低的(de)(de)穩壓(ya)(ya)(ya)精(jing)度(du)會造(zao)(zao)成電(dian)(dian)(dian)池(chi)(chi)(chi)充電(dian)(dian)(dian)不足，進而造(zao)(zao)成電(dian)(dian)(dian)池(chi)(chi)(chi)容(rong)(rong)量大幅降低。充電(dian)(dian)(dian)不足 1％ 電(dian)(dian)(dian)壓(ya)(ya)(ya)時，電(dian)(dian)(dian)池(chi)(chi)(chi)即(ji)損失(shi)大約 8％ 的(de)(de)容(rong)(rong)量。較低的(de)(de)電(dian)(dian)(dian)池(chi)(chi)(chi)穩壓(ya)(ya)(ya)精(jing)度(du)也會造(zao)(zao)成電(dian)(dian)(dian)池(chi)(chi)(chi)過充電(dian)(dian)(dian)，從而縮短(duan)電(dian)(dian)(dian)池(chi)(chi)(chi)使用壽(shou)命。為了安全地對(dui)鋰(li)離子電(dian)(dian)(dian)池(chi)(chi)(chi)充電(dian)(dian)(dian)，僅允許環境(jing)溫(wen)度(du)在 0～45 C 之間。在更低溫(wen)度(du)時充電(dian)(dian)(dian)會形成金屬(shu)鋰(li)，從而提高(gao)電(dian)(dian)(dian)池(chi)(chi)(chi)阻抗并造(zao)(zao)成電(dian)(dian)(dian)池(chi)(chi)(chi)降質(zhi)。在更高(gao)溫(wen)度(du)時充電(dian)(dian)(dian)會由于鋰(li)電(dian)(dian)(dian)解反(fan)應(ying)而造(zao)(zao)成加(jia)速降質(zhi)。

　　低(di)成本獨立線性電池充(chong)電器(qi)

　　許多IC 制造商(shang)通過開發用于(yu)低(di)功耗便攜設備(bei)的低(di)成(cheng)本(ben)線性電(dian)(dian)池充電(dian)(dian)器來(lai)滿(man)足(zu)市場對更精確和更安(an)全(quan)充電(dian)(dian)的需(xu)求(qiu)。圖 2 就是一(yi)種采用更少(shao)外(wai)部組件(jian)的低(di)成(cheng)本(ben)獨(du)立(li)線性電(dian)(dian)池充電(dian)(dian)器電(dian)(dian)路結構(gou)圖。

　　這種(zhong)電池充電器簡便地把(ba)適配(pei)器的 DC 電壓降低(di)到(dao)電池電壓。導通元件上的功率等于(yu)適配(pei)器電壓減去電池電壓再乘(cheng)以(yi)充電電流，如下式所示：

　　如(ru)果(guo)采用 5V 適配(pei)器對 1200mAh 或 2200mAh 單體(ti)鋰離子(zi)電(dian)(dian)池充(chong)電(dian)(dian)，則圖(tu) 3 說明 0.7C 充(chong)電(dian)(dian)率快速充(chong)電(dian)(dian)電(dian)(dian)流(liu)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。

　　兆赫同步(bu)開關電池(chi)充電器

　　同步開關式充(chong)(chong)電(dian)(dian)(dian)(dian)解決(jue)方(fang)案(an)一(yi)般(ban)用(yong)于(yu)具有高(gao)輸入-輸出壓(ya)差的(de)(de)應(ying)用(yong)或者高(gao)容(rong)量電(dian)(dian)(dian)(dian)池(chi)組。對于(yu) 2200mAh 鋰(li)離(li)子(zi)電(dian)(dian)(dian)(dian)池(chi)組，很(hen)難采用(yong)線性(xing)電(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)器通過車(che)載適配器 (12V) 在(zai) 0.5C～1C 的(de)(de)快速充(chong)(chong)電(dian)(dian)(dian)(dian)率(lv)情況下對單(dan)體電(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)。雖(sui)然可以(yi)采用(yong)具有散熱調節功能的(de)(de)線性(xing)電(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)器，但(dan)是低(di)充(chong)(chong)電(dian)(dian)(dian)(dian)率(lv)情況下的(de)(de)充(chong)(chong)電(dian)(dian)(dian)(dian)時間過長。

　　圖 4 說明適用于 DVD 播放器和智能電(dian)(dian)話等設備(bei)的充電(dian)(dian)電(dian)(dian)流達到 2A 的獨立高效同步開關降(jiang)壓電(dian)(dian)池充電(dian)(dian)器。

　　它(ta)采(cai)用(yong)1.1MHz 開(kai)(kai)關(guan)頻率電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)模式控制(zhi)架(jia)構，利(li)用(yong)內置(zhi)III型(xing)環路(lu)補(bu)償(chang)器(qi)(qi)降低外(wai)部組件(jian)數量。為了進一步降低電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)尺寸，它(ta)在4 4 mm小型(xing)封(feng)裝的(de)(de) PWM 控制(zhi)器(qi)(qi)中集成(cheng)了兩個功率 MOSFET。功率 MOSFET Q1 和 Q2 交替關(guan)閉，具有(you)最佳(jia)的(de)(de)停滯時(shi)間(jian)(jian)，以(yi)優化高開(kai)(kai)關(guan)頻率時(shi)的(de)(de)效率。Q1 用(yong)作 P 通道 MOSFET，在用(yong)于(yu)(yu)高側 N-MOSFET 柵極(ji)驅動器(qi)(qi)時(shi)可(ke)以(yi)消除外(wai)部自益放大電(dian)(dian)(dian)(dian)(dian)(dian)容器(qi)(qi) (boost strap capacitor) 和二(er)極(ji)管。另外(wai)，通過完全打開(kai)(kai) Q1，在輸(shu)入電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)非常接近(jin)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)時(shi)，易于(yu)(yu)實現(xian) 100％的(de)(de)占(zhan)空比。打開(kai)(kai)和關(guan)閉時(shi)間(jian)(jian)處于(yu)(yu)受控狀態，從而(er)可(ke)以(yi)根據反(fan)饋控制(zhi)環路(lu)調節(jie)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流（CC 階(jie)段）或電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)（CV階(jie)段）。電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)器(qi)(qi)具有(you)高度(du)集成(cheng)的(de)(de)功能(neng)，能(neng)夠安全、高效地對鋰離子(zi)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)。它(ta)可(ke)以(yi)編程(cheng)預充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流、快速(su)充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流、充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)、充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)定時(shi)器(qi)(qi)、電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)溫(wen)度(du)監控、自動再充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)、短路(lu)和過熱保護。電(dian)(dian)(dian)(dian)(dian)(dian)路(lu)參數設計用(yong)于(yu)(yu)下(xia)述(shu)設計示(shi)例中的(de)(de)以(yi)下(xia)規(gui)格。

適(shi)配器 DC 電壓：12 V

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

預(yu)充電電流：IPRE-CHG=133 mA

快速充電(dian)電(dian)流：ICHG=1.33 A

充(chong)電時間限制：tCHG = 5-hour

開始充電的溫度(du)范圍：T= 0℃～45 C。

　　由于(yu)電池充(chong)電器的尺寸對便攜設(she)備極(ji)其重要，因此需要采用(yong)盡可(ke)能小的輸出電感器。對于(yu)給定的電感器紋(wen)波(bo)電流，所需的電感由下(xia)式得出：

　　式中(zhong)，f_{s}和(he)(he) DIripple,L 分(fen)別(bie)是(shi)(shi)開(kai)(kai)關(guan)(guan)頻(pin)率(lv)(lv)和(he)(he)電(dian)(dian)感(gan)(gan)(gan)器紋波電(dian)(dian)流。在上式中(zhong)代入(ru)VIN=12V、VBAT=6.0V（3.0V/電(dian)(dian)池）、Iripple,L=30％ICHG、ICHG=1.33A以及fs=1MHz ，可(ke)以得出L=7.5 H。可(ke)以選擇(ze)L=10 H的屏蔽電(dian)(dian)感(gan)(gan)(gan)器。請(qing)注意：屏蔽電(dian)(dian)感(gan)(gan)(gan)器在把磁(ci)通(tong)量限制在電(dian)(dian)感(gan)(gan)(gan)器內部和(he)(he)降低輻射電(dian)(dian)磁(ci)干(gan)擾 (EMI) 方(fang)面(mian)具有更高能力(li)。所(suo)需的電(dian)(dian)感(gan)(gan)(gan)與開(kai)(kai)關(guan)(guan)頻(pin)率(lv)(lv)成反比。另一方(fang)面(mian)，電(dian)(dian)感(gan)(gan)(gan)可(ke)以降低 10 倍，在 1MHz 時(shi)的尺(chi)(chi)寸(cun)低于 100kHz 時(shi)的尺(chi)(chi)寸(cun)，開(kai)(kai)關(guan)(guan)頻(pin)率(lv)(lv)越(yue)高，Q1 和(he)(he) Q2 上的開(kai)(kai)關(guan)(guan)損耗越(yue)高，同(tong)時(shi)電(dian)(dian)感(gan)(gan)(gan)器內核損耗也越(yue)高。因此，1MHz 開(kai)(kai)關(guan)(guan)頻(pin)率(lv)(lv)是(shi)(shi)實際設計(ji)中(zhong)電(dian)(dian)感(gan)(gan)(gan)器尺(chi)(chi)寸(cun)和(he)(he)功(gong)率(lv)(lv)轉換效率(lv)(lv)之間的理(li)想取舍。

　　電感器(qi)額定電流(liu)的選(xuan)擇對實現預(yu)期效(xiao)率也很重要。峰值電感器(qi)電流(liu) 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。
關鍵是選擇較小(xiao)的(de)、具(ju)有良好溫度特征的(de)陶(tao)瓷(ci)輸出(chu)電(dian)(dian)容(rong)(rong)器，如：X7R 和(he) X5R 陶(tao)瓷(ci)電(dian)(dian)容(rong)(rong)器。進入電(dian)(dian)池的(de)紋波電(dian)(dian)流由下式得出(chu)：

　　式中，ESR、RSNS和RBAT分別是輸(shu)出(chu)電(dian)(dian)(dian)(dian)容器等效串(chuan)連(lian)電(dian)(dian)(dian)(dian)阻、電(dian)(dian)(dian)(dian)流(liu)感(gan)測電(dian)(dian)(dian)(dian)阻器和電(dian)(dian)(dian)(dian)池(chi)內部阻抗，包括電(dian)(dian)(dian)(dian)池(chi)組中保護 MOSFET 的 Rdson。輸(shu)出(chu)電(dian)(dian)(dian)(dian)容器的 ESR 越低(di)(di)，進(jin)入(ru)電(dian)(dian)(dian)(dian)池(chi)的紋(wen)波電(dian)(dian)(dian)(dian)流(liu)也越低(di)(di)。進(jin)入(ru)電(dian)(dian)(dian)(dian)池(chi)的紋(wen)波電(dian)(dian)(dian)(dian)流(liu)應當(dang)低(di)(di)于電(dian)(dian)(dian)(dian)感(gan)器紋(wen)波電(dian)(dian)(dian)(dian)流(liu)的十分之(zhi)一，一般情況下 10 F/10m ESR 陶瓷電(dian)(dian)(dian)(dian)容器即可(ke)滿足(zu)上述需(xu)求。

　　?選擇(ze)電(dian)流感測電(dian)阻器RSNS

　　根據(ju)感(gan)測電阻(zu)器(qi)的調節閾值 VIREG 選擇(ze) RSNS。為了取得(de)標準的感(gan)測電阻(zu)器(qi)值，使 VIREG=133mV，則求得(de) RSNS：

　　感(gan)測電(dian)阻器的(de)功(gong)耗為(wei)I2CHGRSNS=I2CHGRsns=0.18W。選(xuan)擇 0.5W 時的(de) 1206 額定尺寸(cun)。

　　?選擇快速充(chong)電電流(liu) 設定電阻器(qi)RSET1.

　　RSET1 用于設定(ding)快速充電電流(liu)，RSET1 由下式求得：

　　?選擇預充電電流設定電阻器 RSET2.

　　RSET2用于設(she)定預充(chong)電(dian)電(dian)流，由下式求(qiu)得(de)：

　　?選擇(ze)最長(chang)充電時間設定電容器 CTTC

　　如(ru)果電池未充(chong)(chong)滿(man)，充(chong)(chong)電定(ding)(ding)時器(qi)可以檢(jian)測“壞”電池組，此時充(chong)(chong)電定(ding)(ding)時器(qi)失效(xiao)。CTTC 用于對充(chong)(chong)電定(ding)(ding)時器(qi)進行編程(cheng)，規(gui)定(ding)(ding)每(mei) nF 為 2.6 分(fen)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(zui)低(di)與(yu)最(zui)高充電溫度設定電阻(zu)器 RT1 與(yu) RT2

　　RT1 與(yu) RT2 用(yong)于(yu)在 0 C～45 C 間充(chong)(chong)電(dian)溫度(du)范圍(wei)內進行(xing)編(bian)程，以(yi)啟(qi)動電(dian)池充(chong)(chong)電(dian)器。對于(yu)電(dian)池組中常用(yong)的 103AT-2 熱敏電(dian)阻(zu)，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 可以求得 RT1=9.31kW，RT2=442 kW。

　　在16V輸入電(dian)(dian)(dian)(dian)壓下仍然(ran)具有(you)超過 90％ 的(de)(de)(de)(de)效(xiao)率(lv)。與線(xian)性充電(dian)(dian)(dian)(dian)器相比(bi)，功耗低得多，而且可以(yi)(yi)在電(dian)(dian)(dian)(dian)池(chi)組側設計同(tong)步(bu)開關(guan)充電(dian)(dian)(dian)(dian)器，以(yi)(yi)降低對主板空(kong)間的(de)(de)(de)(de)占用(yong)，由于以(yi)(yi) MHz 頻率(lv)進行(xing)工(gong)作，電(dian)(dian)(dian)(dian)感器的(de)(de)(de)(de)尺寸(cun)較(jiao)小(xiao)。需(xu)要牢記(ji)的(de)(de)(de)(de)是(shi)，電(dian)(dian)(dian)(dian)池(chi)的(de)(de)(de)(de)使用(yong)壽(shou)命主要取決于其溫度。利用(yong)同(tong)步(bu)開關(guan)電(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)器對鋰(li)離子電(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)一般(ban)情況下產生的(de)(de)(de)(de)熱量更低。因此，與線(xian)性電(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)器相比(bi)，它具有(you)更長的(de)(de)(de)(de)使用(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(bu)再能夠高(gao)效的(de)對鋰離子(zi)電池充(chong)電。集成 MOSFET 的(de)高(gao)效率同步(bu)開關(guan)電池充(chong)電器為這些高(gao)級便攜設備提供高(gao)效的(de)充(chong)電解決方(fang)案，從而實現更低的(de)熱量與更長的(de)電池使用壽命。