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(ji)有效充電并延長電池使用(yong)壽命的設計考慮事項。

　　鋰離子電(dian)池充電(dian) 

　　大部分(fen)(fen)專(zhuan)用鋰(li)離(li)子充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)集(ji)成電(dian)(dian)(dian)(dian)(dian)路 (IC) 都是通過圖1所示的(de)方式充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)。鋰(li)離(li)子電(dian)(dian)(dian)(dian)(dian)池的(de)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)過程由三個階段(duan)組成：預充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)、恒(heng)流 (CC) 快(kuai)速(su)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)以及恒(heng)壓 (CV) 終(zhong)端(duan) (Termination)。在(zai)預充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)階段(duan)，以低速(su)率(lv)（一般是快(kuai)速(su)充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)率(lv)的(de) 1/10）對(dui)電(dian)(dian)(dian)(dian)(dian)池充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)，這(zhe)時的(de)電(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)壓低于 3.0V。這(zhe)樣可(ke)以實(shi)現對(dui)鈍(dun)化(hua)層的(de)恢復 - 鈍(dun)化(hua)層在(zai)深度放電(dian)(dian)(dian)(dian)(dian)狀態下存(cun)儲時間(jian)過長會分(fen)(fen)解。另外，還可(ke)以在(zai)發(fa)生陽極短路的(de)過充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)池出現部分(fen)(fen)銅分(fen)(fen)解的(de)情況(kuang)下防止 1C 充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)過熱(re)。在(zai)電(dian)(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)(dian)壓達到 3.0V 時，電(dian)(dian)(dian)(dian)(dian)池充(chong)(chong)(chong)(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)器進入(ru) CC 階段(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(kai)始(shi)調節電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)(dian)壓(ya)并且進入 CV 階段。輸出(chu)穩壓(ya)精度(du)(du)(du)是提(ti)高(gao)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)容量(liang)和延長(chang)使用壽(shou)命的(de)(de)關(guan)鍵。較(jiao)低(di)的(de)(de)穩壓(ya)精度(du)(du)(du)會造成(cheng)(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)不(bu)足，進而(er)造成(cheng)(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)容量(liang)大(da)幅降(jiang)低(di)。充(chong)電(dian)(dian)(dian)(dian)(dian)不(bu)足 1％ 電(dian)(dian)(dian)(dian)(dian)壓(ya)時，電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)即損失大(da)約(yue) 8％ 的(de)(de)容量(liang)。較(jiao)低(di)的(de)(de)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)穩壓(ya)精度(du)(du)(du)也會造成(cheng)(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)過充(chong)電(dian)(dian)(dian)(dian)(dian)，從(cong)而(er)縮短(duan)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)使用壽(shou)命。為了安(an)全地對鋰(li)(li)離子電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)，僅允許環境溫(wen)度(du)(du)(du)在 0～45 C 之間。在更(geng)低(di)溫(wen)度(du)(du)(du)時充(chong)電(dian)(dian)(dian)(dian)(dian)會形成(cheng)(cheng)(cheng)金屬鋰(li)(li)，從(cong)而(er)提(ti)高(gao)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)阻抗(kang)并造成(cheng)(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)降(jiang)質。在更(geng)高(gao)溫(wen)度(du)(du)(du)時充(chong)電(dian)(dian)(dian)(dian)(dian)會由于鋰(li)(li)電(dian)(dian)(dian)(dian)(dian)解反應(ying)而(er)造成(cheng)(cheng)(cheng)加速(su)降(jiang)質。

　　低(di)成本獨立線性(xing)電(dian)池充電(dian)器

　　許多IC 制造商通過開發用(yong)于低功耗便攜設備的(de)(de)低成(cheng)本線性電(dian)池(chi)(chi)充電(dian)器來滿(man)足市(shi)場(chang)對(dui)更(geng)精確和更(geng)安全充電(dian)的(de)(de)需求(qiu)。圖 2 就是一種(zhong)采(cai)用(yong)更(geng)少外部組件的(de)(de)低成(cheng)本獨立線性電(dian)池(chi)(chi)充電(dian)器電(dian)路結(jie)構圖。

　　這種電(dian)(dian)(dian)(dian)池充電(dian)(dian)(dian)(dian)器簡(jian)便地把適(shi)配器的 DC 電(dian)(dian)(dian)(dian)壓(ya)降低到電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓(ya)。導通元件上的功率(lv)等(deng)于適(shi)配器電(dian)(dian)(dian)(dian)壓(ya)減去電(dian)(dian)(dian)(dian)池電(dian)(dian)(dian)(dian)壓(ya)再(zai)乘以充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)，如下式所(suo)示：

　　如(ru)果采(cai)用 5V 適(shi)配器對 1200mAh 或 2200mAh 單(dan)體鋰(li)離子電(dian)(dian)池充電(dian)(dian)，則圖 3 說明 0.7C 充電(dian)(dian)率快速充電(dian)(dian)電(dian)(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(tong)步(bu)開關電(dian)池充電(dian)器

　　同步開關式充(chong)電(dian)(dian)(dian)(dian)解決方(fang)案一般用(yong)(yong)于(yu)具有(you)高輸入(ru)-輸出壓差的(de)應用(yong)(yong)或者(zhe)高容量電(dian)(dian)(dian)(dian)池(chi)組。對于(yu) 2200mAh 鋰離子(zi)電(dian)(dian)(dian)(dian)池(chi)組，很難(nan)采用(yong)(yong)線性(xing)電(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)器(qi)通過車載適配器(qi) (12V) 在 0.5C～1C 的(de)快速(su)充(chong)電(dian)(dian)(dian)(dian)率情(qing)況下(xia)對單體電(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)。雖然可(ke)以(yi)采用(yong)(yong)具有(you)散熱調節功(gong)能的(de)線性(xing)電(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)器(qi)，但是低(di)充(chong)電(dian)(dian)(dian)(dian)率情(qing)況下(xia)的(de)充(chong)電(dian)(dian)(dian)(dian)時間過長(chang)。

　　圖 4 說明適用于 DVD 播放器(qi)(qi)和智能(neng)電(dian)(dian)(dian)話等設備的(de)充電(dian)(dian)(dian)電(dian)(dian)(dian)流達到 2A 的(de)獨立高效同(tong)步開關降壓電(dian)(dian)(dian)池充電(dian)(dian)(dian)器(qi)(qi)。

　　它(ta)(ta)采用(yong)(yong)1.1MHz 開(kai)(kai)關頻(pin)(pin)率(lv)(lv)電(dian)(dian)(dian)壓(ya)模式控制架構(gou)，利用(yong)(yong)內(nei)置III型(xing)環路(lu)補償(chang)器(qi)(qi)降低(di)外(wai)部(bu)組件(jian)數(shu)量。為了進一(yi)步降低(di)電(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)器(qi)(qi)尺寸，它(ta)(ta)在4 4 mm小型(xing)封裝(zhuang)的 PWM 控制器(qi)(qi)中集(ji)成了兩個功率(lv)(lv) MOSFET。功率(lv)(lv) MOSFET Q1 和 Q2 交替關閉，具有(you)最佳的停滯時間(jian)，以(yi)優化高開(kai)(kai)關頻(pin)(pin)率(lv)(lv)時的效率(lv)(lv)。Q1 用(yong)(yong)作 P 通(tong)道(dao) MOSFET，在用(yong)(yong)于高側 N-MOSFET 柵極(ji)驅動器(qi)(qi)時可(ke)以(yi)消除(chu)外(wai)部(bu)自益(yi)放(fang)大電(dian)(dian)(dian)容器(qi)(qi) (boost strap capacitor) 和二極(ji)管(guan)。另外(wai)，通(tong)過(guo)完全(quan)打開(kai)(kai) Q1，在輸入電(dian)(dian)(dian)壓(ya)非常接(jie)近電(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)壓(ya)時，易于實現(xian) 100％的占空(kong)比。打開(kai)(kai)和關閉時間(jian)處于受控狀(zhuang)態，從而可(ke)以(yi)根據(ju)反饋控制環路(lu)調節電(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流（CC 階(jie)段）或電(dian)(dian)(dian)池(chi)(chi)電(dian)(dian)(dian)壓(ya)（CV階(jie)段）。電(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)器(qi)(qi)具有(you)高度集(ji)成的功能，能夠安(an)全(quan)、高效地對(dui)鋰離子(zi)電(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)。它(ta)(ta)可(ke)以(yi)編程預充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流、快速充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)流、充(chong)電(dian)(dian)(dian)電(dian)(dian)(dian)壓(ya)、充(chong)電(dian)(dian)(dian)定(ding)時器(qi)(qi)、電(dian)(dian)(dian)池(chi)(chi)溫度監控、自動再充(chong)電(dian)(dian)(dian)、短路(lu)和過(guo)熱保護。電(dian)(dian)(dian)路(lu)參數(shu)設計(ji)(ji)用(yong)(yong)于下(xia)述設計(ji)(ji)示例(li)中的以(yi)下(xia)規格。

適配器 DC 電壓：12 V

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

預充(chong)電(dian)電(dian)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(chong)電的(de)溫度(du)范圍(wei)：T= 0℃～45 C。

　　由于電(dian)池充(chong)電(dian)器(qi)的尺寸(cun)對(dui)便攜設備極其(qi)重要，因(yin)此需要采用盡(jin)可(ke)能(neng)小(xiao)的輸出電(dian)感器(qi)。對(dui)于給定的電(dian)感器(qi)紋波(bo)電(dian)流，所需的電(dian)感由下式(shi)得出：

　　式中(zhong)，f_{s}和(he)(he) DIripple,L 分別是開(kai)關(guan)(guan)頻率(lv)(lv)和(he)(he)電(dian)(dian)(dian)感(gan)(gan)(gan)器(qi)(qi)紋波電(dian)(dian)(dian)流。在上式中(zhong)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(ke)以得出(chu)L=7.5 H。可(ke)以選擇(ze)L=10 H的屏蔽(bi)(bi)電(dian)(dian)(dian)感(gan)(gan)(gan)器(qi)(qi)。請注(zhu)意：屏蔽(bi)(bi)電(dian)(dian)(dian)感(gan)(gan)(gan)器(qi)(qi)在把磁通量(liang)限制在電(dian)(dian)(dian)感(gan)(gan)(gan)器(qi)(qi)內部和(he)(he)降(jiang)低輻射(she)電(dian)(dian)(dian)磁干擾(rao) (EMI) 方面(mian)具有更高(gao)能力。所需的電(dian)(dian)(dian)感(gan)(gan)(gan)與(yu)開(kai)關(guan)(guan)頻率(lv)(lv)成反比。另一(yi)方面(mian)，電(dian)(dian)(dian)感(gan)(gan)(gan)可(ke)以降(jiang)低 10 倍，在 1MHz 時(shi)的尺(chi)寸低于 100kHz 時(shi)的尺(chi)寸，開(kai)關(guan)(guan)頻率(lv)(lv)越高(gao)，Q1 和(he)(he) Q2 上的開(kai)關(guan)(guan)損耗(hao)(hao)越高(gao)，同時(shi)電(dian)(dian)(dian)感(gan)(gan)(gan)器(qi)(qi)內核(he)損耗(hao)(hao)也越高(gao)。因此(ci)，1MHz 開(kai)關(guan)(guan)頻率(lv)(lv)是實(shi)際設(she)計中(zhong)電(dian)(dian)(dian)感(gan)(gan)(gan)器(qi)(qi)尺(chi)寸和(he)(he)功(gong)率(lv)(lv)轉換效率(lv)(lv)之(zhi)間(jian)的理(li)想取舍。

　　電感(gan)器額定電流的選擇對實(shi)現預期效率(lv)也(ye)很重要。峰值電感(gan)器電流 IPeak 通過下式計算：

　　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(de)、具有良好溫度(du)特征(zheng)的(de)陶瓷(ci)(ci)輸出電容器(qi)，如：X7R 和(he) X5R 陶瓷(ci)(ci)電容器(qi)。進(jin)入電池的(de)紋波電流(liu)由下式得出：

　　式中，ESR、RSNS和(he)RBAT分別(bie)是輸出(chu)(chu)電(dian)容(rong)器(qi)等(deng)效串連電(dian)阻、電(dian)流(liu)感(gan)測電(dian)阻器(qi)和(he)電(dian)池(chi)內部(bu)阻抗(kang)，包括電(dian)池(chi)組中保護(hu) MOSFET 的(de) Rdson。輸出(chu)(chu)電(dian)容(rong)器(qi)的(de) ESR 越低(di)(di)，進(jin)入(ru)電(dian)池(chi)的(de)紋波(bo)電(dian)流(liu)也越低(di)(di)。進(jin)入(ru)電(dian)池(chi)的(de)紋波(bo)電(dian)流(liu)應當低(di)(di)于電(dian)感(gan)器(qi)紋波(bo)電(dian)流(liu)的(de)十分之(zhi)一，一般情(qing)況下(xia) 10 F/10m ESR 陶瓷(ci)電(dian)容(rong)器(qi)即可滿足上(shang)述(shu)需求(qiu)。

　　?選擇電流感測電阻器(qi)RSNS

　　根據感測電(dian)阻(zu)器的(de)調節(jie)閾值 VIREG 選擇 RSNS。為了取得標準的(de)感測電(dian)阻(zu)器值，使(shi) VIREG=133mV，則(ze)求得 RSNS：

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

　　?選擇快速充(chong)電(dian)電(dian)流 設定電(dian)阻器RSET1.

　　RSET1 用于設定快速充電電流，RSET1 由下式求得：

　　?選擇預充電(dian)電(dian)流設定電(dian)阻器 RSET2.

　　RSET2用于設(she)定預充(chong)電電流，由下式(shi)求得(de)：

　　?選擇(ze)最(zui)長(chang)充(chong)電時間設定(ding)電容器 CTTC

　　如果(guo)電(dian)池(chi)未充(chong)滿，充(chong)電(dian)定時器可以檢測(ce)“壞”電(dian)池(chi)組，此時充(chong)電(dian)定時器失(shi)效。CTTC 用于(yu)對(dui)充(chong)電(dian)定時器進行編(bian)程，規定每 nF 為 2.6 分鐘。

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

　　可以選用 0.1 F 陶瓷(ci)電(dian)容器。

　　?選擇最(zui)低與最(zui)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(dian)(dian)溫(wen)度范圍內進(jin)行(xing)編程，以啟動電(dian)(dian)池充電(dian)(dian)器。對(dui)于電(dian)(dian)池組中常用(yong)的 103AT-2 熱敏(min)電(dian)(dian)阻，RT(0℃)=RTL=27.28k ，RT(45℃)=RTH=4.911 k ，RT1與(yu)RT2由下式(shi)確定：

　　在上式中(zhong)代(dai)入 RTL 與 RTH 可以求得 RT1=9.31kW，RT2=442 kW。

　　在16V輸入電(dian)(dian)(dian)(dian)(dian)壓(ya)下仍然具(ju)有(you)超過 90％ 的(de)(de)效率(lv)。與線性充(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)相比，功耗低(di)(di)得多，而且可以(yi)在電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)組側設計同步(bu)開關充(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)，以(yi)降低(di)(di)對主板空間的(de)(de)占(zhan)用，由(you)于以(yi) MHz 頻率(lv)進行工作，電(dian)(dian)(dian)(dian)(dian)感器(qi)的(de)(de)尺寸(cun)較小。需要牢(lao)記的(de)(de)是，電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)的(de)(de)使用壽(shou)命(ming)主要取(qu)決于其溫度。利用同步(bu)開關電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)對鋰離子電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)一般情況下產生的(de)(de)熱量更低(di)(di)。因此，與線性電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)相比，它具(ju)有(you)更長的(de)(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效的(de)(de)對鋰離子電池充電。集(ji)成 MOSFET 的(de)(de)高效率同步開關電池充電器為這些高級便攜設備(bei)提(ti)供高效的(de)(de)充電解(jie)決方(fang)案，從而實現更低的(de)(de)熱量與更長的(de)(de)電池使(shi)用壽命(ming)。