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(chong)電(dian)(dian)并延長電(dian)(dian)池使(shi)用壽命的設計考(kao)慮事項。

　　鋰離子電池充電 

　　大部分專(zhuan)用(yong)鋰(li)離子(zi)充(chong)電(dian)(dian)(dian)(dian)集成電(dian)(dian)(dian)(dian)路 (IC) 都是通過圖1所(suo)示的方式充(chong)電(dian)(dian)(dian)(dian)。鋰(li)離子(zi)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的充(chong)電(dian)(dian)(dian)(dian)過程由三(san)個階(jie)段組成：預(yu)充(chong)電(dian)(dian)(dian)(dian)、恒流(liu) (CC) 快速(su)充(chong)電(dian)(dian)(dian)(dian)以及(ji)恒壓 (CV) 終端 (Termination)。在(zai)預(yu)充(chong)電(dian)(dian)(dian)(dian)階(jie)段，以低速(su)率（一般是快速(su)充(chong)電(dian)(dian)(dian)(dian)率的 1/10）對電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)，這(zhe)(zhe)時的電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)壓低于 3.0V。這(zhe)(zhe)樣可以實(shi)現對鈍化層的恢(hui)復 - 鈍化層在(zai)深度放電(dian)(dian)(dian)(dian)狀態下存儲(chu)時間過長會分解。另(ling)外(wai)，還(huan)可以在(zai)發生(sheng)陽極短(duan)路的過充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)出現部分銅分解的情(qing)況下防止 1C 充(chong)電(dian)(dian)(dian)(dian)過熱。在(zai)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)壓達(da)到 3.0V 時，電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)器進入 CC 階(jie)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電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)電(dian)(dian)(dian)(dian)壓(ya)(ya)并且進入 CV 階段。輸出穩壓(ya)(ya)精度(du)(du)是提高(gao)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)容量和延長使用(yong)壽命的(de)關鍵。較(jiao)低(di)的(de)穩壓(ya)(ya)精度(du)(du)會造(zao)(zao)成(cheng)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)不(bu)足，進而(er)(er)造(zao)(zao)成(cheng)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)容量大幅降低(di)。充(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)即損失(shi)大約 8％ 的(de)容量。較(jiao)低(di)的(de)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)穩壓(ya)(ya)精度(du)(du)也會造(zao)(zao)成(cheng)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)過充(chong)(chong)電(dian)(dian)(dian)(dian)，從(cong)而(er)(er)縮短電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)使用(yong)壽命。為(wei)了安全地(di)對(dui)鋰離子電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)(chong)電(dian)(dian)(dian)(dian)，僅(jin)允許環境(jing)溫度(du)(du)在(zai) 0～45 C 之間(jian)。在(zai)更(geng)低(di)溫度(du)(du)時充(chong)(chong)電(dian)(dian)(dian)(dian)會形成(cheng)金屬鋰，從(cong)而(er)(er)提高(gao)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)阻抗并造(zao)(zao)成(cheng)電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)降質(zhi)。在(zai)更(geng)高(gao)溫度(du)(du)時充(chong)(chong)電(dian)(dian)(dian)(dian)會由于(yu)鋰電(dian)(dian)(dian)(dian)解反應而(er)(er)造(zao)(zao)成(cheng)加速降質(zhi)。

　　低成本(ben)獨立線性電(dian)池充(chong)電(dian)器

　　許多IC 制造商通過開發用于低功耗便攜(xie)設(she)備的(de)低成(cheng)本(ben)(ben)線性電(dian)池(chi)充(chong)電(dian)器來滿足市場對(dui)更(geng)精(jing)確(que)和更(geng)安全充(chong)電(dian)的(de)需求。圖 2 就是一種采用更(geng)少外部組件的(de)低成(cheng)本(ben)(ben)獨立(li)線性電(dian)池(chi)充(chong)電(dian)器電(dian)路結(jie)構圖。

　　這種電(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)器簡便地把適配(pei)器的 DC 電(dian)(dian)(dian)壓降低(di)到電(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)壓。導通元件上的功率(lv)等于適配(pei)器電(dian)(dian)(dian)壓減去電(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)壓再乘(cheng)以充電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)，如(ru)下式所示：

　　如(ru)果采用 5V 適配(pei)器對 1200mAh 或 2200mAh 單(dan)體鋰(li)離子電(dian)(dian)池充電(dian)(dian)，則(ze)圖 3 說明(ming) 0.7C 充電(dian)(dian)率快速充電(dian)(dian)電(dian)(dian)流情況(kuang)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(dian)池充電(dian)器

　　同步開關(guan)式(shi)充(chong)電(dian)(dian)(dian)(dian)解(jie)決方案一般用(yong)于(yu)具有(you)高輸(shu)(shu)入-輸(shu)(shu)出(chu)壓差的應用(yong)或者高容量電(dian)(dian)(dian)(dian)池(chi)組(zu)。對于(yu) 2200mAh 鋰(li)離子電(dian)(dian)(dian)(dian)池(chi)組(zu)，很難采用(yong)線性電(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)器(qi)通過車(che)載適配器(qi) (12V) 在(zai) 0.5C～1C 的快速(su)充(chong)電(dian)(dian)(dian)(dian)率情況下(xia)對單(dan)體(ti)電(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)。雖然可(ke)以采用(yong)具有(you)散(san)熱調(diao)節功能的線性電(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)器(qi)，但是(shi)低(di)充(chong)電(dian)(dian)(dian)(dian)率情況下(xia)的充(chong)電(dian)(dian)(dian)(dian)時間過長。

　　圖 4 說明適(shi)用于 DVD 播放器(qi)和智(zhi)能電(dian)話(hua)等設備的充電(dian)電(dian)流(liu)達到 2A 的獨(du)立高效同(tong)步開關降壓電(dian)池充電(dian)器(qi)。

　　它(ta)采用(yong)1.1MHz 開關頻率(lv)電(dian)壓模式控(kong)制(zhi)架構(gou)，利用(yong)內置III型環路(lu)補償器(qi)降低外(wai)部組件數量(liang)。為了進一步(bu)降低電(dian)池(chi)(chi)(chi)充(chong)電(dian)器(qi)尺寸，它(ta)在4 4 mm小型封裝的 PWM 控(kong)制(zhi)器(qi)中集成了兩個功率(lv) MOSFET。功率(lv) MOSFET Q1 和(he)(he) Q2 交替關閉(bi)，具有(you)最佳(jia)的停(ting)滯時間，以(yi)(yi)優(you)化高開關頻率(lv)時的效率(lv)。Q1 用(yong)作 P 通道 MOSFET，在用(yong)于(yu)高側 N-MOSFET 柵極驅動器(qi)時可(ke)以(yi)(yi)消除(chu)外(wai)部自(zi)益放大(da)電(dian)容器(qi) (boost strap capacitor) 和(he)(he)二(er)極管。另外(wai)，通過完全打開 Q1，在輸入電(dian)壓非常接(jie)近電(dian)池(chi)(chi)(chi)電(dian)壓時，易于(yu)實(shi)現 100％的占空比。打開和(he)(he)關閉(bi)時間處于(yu)受控(kong)狀態，從(cong)而(er)可(ke)以(yi)(yi)根據(ju)反饋控(kong)制(zhi)環路(lu)調(diao)節電(dian)池(chi)(chi)(chi)充(chong)電(dian)電(dian)流(liu)（CC 階段）或電(dian)池(chi)(chi)(chi)電(dian)壓（CV階段）。電(dian)池(chi)(chi)(chi)充(chong)電(dian)器(qi)具有(you)高度(du)集成的功能，能夠安(an)全、高效地(di)對鋰離子電(dian)池(chi)(chi)(chi)充(chong)電(dian)。它(ta)可(ke)以(yi)(yi)編程預充(chong)電(dian)電(dian)流(liu)、快速充(chong)電(dian)電(dian)流(liu)、充(chong)電(dian)電(dian)壓、充(chong)電(dian)定時器(qi)、電(dian)池(chi)(chi)(chi)溫度(du)監(jian)控(kong)、自(zi)動再充(chong)電(dian)、短路(lu)和(he)(he)過熱保護。電(dian)路(lu)參數設(she)計(ji)(ji)用(yong)于(yu)下述設(she)計(ji)(ji)示例中的以(yi)(yi)下規格。

適配器(qi) DC 電(dian)壓：12 V

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

預充電電流(liu)：IPRE-CHG=133 mA

快速充電電流(liu)：ICHG=1.33 A

充電時間(jian)限制：tCHG = 5-hour

開始(shi)充電的(de)溫度范(fan)圍：T= 0℃～45 C。

　　由于電池充電器的(de)尺(chi)寸對便攜設備極其重要，因此(ci)需(xu)要采用(yong)盡可能小的(de)輸出(chu)電感器。對于給定的(de)電感器紋波(bo)電流，所需(xu)的(de)電感由下(xia)式(shi)得出(chu)：

　　式中(zhong)(zhong)，f_{s}和 DIripple,L 分別是(shi)開(kai)(kai)(kai)關(guan)(guan)(guan)頻(pin)率和電(dian)(dian)(dian)感(gan)(gan)器紋(wen)波電(dian)(dian)(dian)流。在上(shang)式中(zhong)(zhong)代入VIN=12V、VBAT=6.0V（3.0V/電(dian)(dian)(dian)池）、Iripple,L=30％ICHG、ICHG=1.33A以(yi)及fs=1MHz ，可以(yi)得出L=7.5 H。可以(yi)選擇L=10 H的(de)(de)屏蔽(bi)電(dian)(dian)(dian)感(gan)(gan)器。請注(zhu)意：屏蔽(bi)電(dian)(dian)(dian)感(gan)(gan)器在把磁通量限制(zhi)在電(dian)(dian)(dian)感(gan)(gan)器內部(bu)和降(jiang)低(di)(di)輻射電(dian)(dian)(dian)磁干(gan)擾(rao) (EMI) 方面(mian)具(ju)有更高能力。所需的(de)(de)電(dian)(dian)(dian)感(gan)(gan)與開(kai)(kai)(kai)關(guan)(guan)(guan)頻(pin)率成反(fan)比。另一方面(mian)，電(dian)(dian)(dian)感(gan)(gan)可以(yi)降(jiang)低(di)(di) 10 倍(bei)，在 1MHz 時(shi)的(de)(de)尺寸(cun)低(di)(di)于 100kHz 時(shi)的(de)(de)尺寸(cun)，開(kai)(kai)(kai)關(guan)(guan)(guan)頻(pin)率越(yue)高，Q1 和 Q2 上(shang)的(de)(de)開(kai)(kai)(kai)關(guan)(guan)(guan)損耗越(yue)高，同時(shi)電(dian)(dian)(dian)感(gan)(gan)器內核損耗也越(yue)高。因此，1MHz 開(kai)(kai)(kai)關(guan)(guan)(guan)頻(pin)率是(shi)實際(ji)設計中(zhong)(zhong)電(dian)(dian)(dian)感(gan)(gan)器尺寸(cun)和功率轉(zhuan)換效(xiao)率之間的(de)(de)理想(xiang)取舍。

　　電感器(qi)額定電流的(de)選擇(ze)對實(shi)現預期效率也很重要。峰(feng)值電感器(qi)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(jiao)小(xiao)的、具(ju)有良(liang)好溫度(du)特征的陶瓷輸(shu)出(chu)電容器，如：X7R 和 X5R 陶瓷電容器。進入電池的紋波電流(liu)由下(xia)式得出(chu)：

　　式中，ESR、RSNS和RBAT分別是輸出電(dian)(dian)(dian)(dian)容(rong)(rong)器(qi)等效串(chuan)連(lian)電(dian)(dian)(dian)(dian)阻、電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)感測電(dian)(dian)(dian)(dian)阻器(qi)和電(dian)(dian)(dian)(dian)池內(nei)部阻抗(kang)，包括電(dian)(dian)(dian)(dian)池組(zu)中保護 MOSFET 的 Rdson。輸出電(dian)(dian)(dian)(dian)容(rong)(rong)器(qi)的 ESR 越低，進(jin)入電(dian)(dian)(dian)(dian)池的紋波(bo)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)也越低。進(jin)入電(dian)(dian)(dian)(dian)池的紋波(bo)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)應當低于電(dian)(dian)(dian)(dian)感器(qi)紋波(bo)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)的十分之一(yi)，一(yi)般情況(kuang)下 10 F/10m ESR 陶瓷電(dian)(dian)(dian)(dian)容(rong)(rong)器(qi)即(ji)可滿足上述需求。

　　?選擇(ze)電流感測電阻器RSNS

　　根據感測電(dian)阻器的調節閾值 VIREG 選擇 RSNS。為了(le)取得(de)標準的感測電(dian)阻器值，使 VIREG=133mV，則求得(de) RSNS：

　　感測電阻器的功(gong)耗為I2CHGRSNS=I2CHGRsns=0.18W。選(xuan)擇(ze) 0.5W 時的 1206 額定尺寸。

　　?選擇快(kuai)速(su)充電(dian)電(dian)流 設定電(dian)阻器RSET1.

　　RSET1 用于(yu)設(she)定快速充電電流，RSET1 由下式(shi)求(qiu)得：

　　?選擇(ze)預充電電流(liu)設定電阻器 RSET2.

　　RSET2用于設定預充電電流，由下式(shi)求得(de)：

　　?選擇最長充(chong)電(dian)時間設(she)定電(dian)容器(qi) CTTC

　　如(ru)果電池未充(chong)滿，充(chong)電定時器可(ke)以(yi)檢測“壞”電池組，此時充(chong)電定時器失效(xiao)。CTTC 用于對充(chong)電定時器進行(xing)編程，規(gui)定每 nF 為 2.6 分(fen)鐘。

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

　　可(ke)以(yi)選用 0.1 F 陶瓷電容器。

　　?選(xuan)擇最(zui)低與最(zui)高充(chong)電溫(wen)度(du)設定(ding)電阻(zu)器 RT1 與 RT2

　　RT1 與(yu) RT2 用于(yu)在 0 C～45 C 間充(chong)電(dian)溫度范圍內進行編程，以啟動電(dian)池(chi)充(chong)電(dian)器。對于(yu)電(dian)池(chi)組中常用的(de) 103AT-2 熱敏電(dian)阻，RT(0℃)=RTL=27.28k ，RT(45℃)=RTH=4.911 k ，RT1與(yu)RT2由下式確定：

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

　　在16V輸入(ru)電壓下仍然具(ju)有超過 90％ 的(de)(de)(de)效(xiao)率(lv)。與(yu)線(xian)性充(chong)(chong)電器(qi)(qi)相比，功耗低得(de)多(duo)，而且(qie)可以在電池(chi)(chi)組(zu)側設計(ji)同(tong)步開關(guan)充(chong)(chong)電器(qi)(qi)，以降低對(dui)主(zhu)板空(kong)間(jian)的(de)(de)(de)占(zhan)用，由(you)于以 MHz 頻率(lv)進(jin)行工(gong)作，電感器(qi)(qi)的(de)(de)(de)尺寸(cun)較(jiao)小。需要牢記的(de)(de)(de)是，電池(chi)(chi)的(de)(de)(de)使(shi)用壽(shou)(shou)命主(zhu)要取決于其(qi)溫度。利用同(tong)步開關(guan)電池(chi)(chi)充(chong)(chong)電器(qi)(qi)對(dui)鋰離子(zi)電池(chi)(chi)充(chong)(chong)電一般情況下產生的(de)(de)(de)熱量(liang)更(geng)低。因(yin)此，與(yu)線(xian)性電池(chi)(chi)充(chong)(chong)電器(qi)(qi)相比，它具(ju)有更(geng)長的(de)(de)(de)使(shi)用壽(shou)(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能(neng)夠(gou)高效(xiao)的(de)對鋰離子電池(chi)充(chong)(chong)電。集成 MOSFET 的(de)高效(xiao)率同(tong)步(bu)開關電池(chi)充(chong)(chong)電器為(wei)這些高級便攜設備提(ti)供高效(xiao)的(de)充(chong)(chong)電解決方案(an)，從(cong)而實現更(geng)低的(de)熱量與(yu)更(geng)長的(de)電池(chi)使(shi)用壽(shou)命。