鋰(li)電(dian)池(chi)必(bi)須考慮充(chong)(chong)電(dian)、放電(dian)時的(de)安全性(xing),以(yi)防(fang)止(zhi)特性(xing)劣(lie)化(hua)。但(dan)鋰(li)離子電(dian)池(chi)能(neng)量(liang)密(mi)度(du)高(gao),難(nan)以(yi)確保(bao)電(dian)池(chi)的(de)安全性(xing),在過度(du)充(chong)(chong)電(dian)狀(zhuang)(zhuang)態(tai)下(xia),電(dian)池(chi)溫度(du)上升(sheng)后能(neng)量(liang)將過剩(sheng),于是電(dian)解(jie)液(ye)分(fen)解(jie)而產(chan)生氣(qi)體,容易使內壓上升(sheng)而產(chan)生自(zi)燃或破裂的(de)危險;反之(zhi),在過度(du)放電(dian)狀(zhuang)(zhuang)態(tai)下(xia),電(dian)解(jie)液(ye)因(yin)分(fen)解(jie)導致電(dian)池(chi)特性(xing)及(ji)耐(nai)久性(xing)劣(lie)化(hua),降(jiang)低可充(chong)(chong)電(dian)次數。因(yin)此鋰(li)電(dian)池(chi)的(de)過充(chong)(chong)、過度(du)放電(dian)、過電(dian)流及(ji)短路保(bao)護(hu)很重(zhong)要,所(suo)以(yi)通常都(dou)會在電(dian)池(chi)包內設計保(bao)護(hu)線路,用以(yi)保(bao)護(hu)鋰(li)電(dian)池(chi)。

1 電路設計

1.1 電路概述

鋰離子電池保(bao)護(hu)(hu)電路(lu)包括(kuo)過度充(chong)電保(bao)護(hu)(hu)、過電流/ 短路(lu)保(bao)護(hu)(hu)和過放電保(bao)護(hu)(hu)等,該(gai)電路(lu)就是要(yao)確(que)保(bao)這樣(yang)的過度充(chong)電及放電狀態時(shi)的安全,并防止(zhi)特性劣化。它主要(yao)由(you)集成(cheng)(cheng)保(bao)護(hu)(hu)電路(lu)IC、貼(tie)片電阻、貼(tie)片電容、場效(xiao)應(ying)管(guan)(MOSFET) 、有(you)的還有(you)熱敏電阻(NTC) 、識別(bie)電阻( ID) 、保(bao)險絲( FUSE) 等構成(cheng)(cheng)。

其中集成(cheng)保護(hu)電(dian)(dian)路IC 用(yong)來(lai)檢測保護(hu)電(dian)(dian)路當前的(de)電(dian)(dian)壓、電(dian)(dian)流、時(shi)間等參數以此來(lai)控制場(chang)效應(ying)(ying)管(guan)的(de)開關狀態;場(chang)效應(ying)(ying)管(guan)(MOSFET) 則根據(ju)保護(hu)IC 來(lai)控制回路中是否有需開或關; 貼片(pian)電(dian)(dian)阻(zu)用(yong)作(zuo)限流; 貼片(pian)電(dian)(dian)容作(zuo)用(yong)為(wei)濾波、調(diao)節延遲時(shi)間;熱敏電(dian)(dian)阻(zu)用(yong)來(lai)檢測電(dian)(dian)池(chi)塊(kuai)內的(de)環境溫度; 保險絲防止流過電(dian)(dian)池(chi)的(de)電(dian)(dian)流過大,切(qie)斷(duan)電(dian)(dian)流回路。

1.2 電路原(yuan)理及參數確(que)定(ding)

1.2.1 過(guo)度充(chong)電保(bao)護

當充電器對鋰電池過度充電時,鋰電池會因溫度(du)(du)上(shang)升而(er)導(dao)致(zhi)內壓(ya)上(shang)升,需(xu)終止當前(qian)充(chong)(chong)電(dian)(dian)(dian)的狀態。此時(shi),集成保護電(dian)(dian)(dian)路IC 需(xu)檢測電(dian)(dian)(dian)池電(dian)(dian)(dian)壓(ya),當到達4.25V 時(shi)(假(jia)設電(dian)(dian)(dian)池過(guo)充(chong)(chong)電(dian)(dian)(dian)壓(ya)臨界(jie)點為(wei)4.25 V) 即激活過(guo)度(du)(du)充(chong)(chong)電(dian)(dian)(dian)保護,將(jiang)功率MOS 由開轉為(wei)切斷,進而(er)截止充(chong)(chong)電(dian)(dian)(dian)。另外,為(wei)防(fang)止由于(yu)噪音(yin)所產生的過(guo)度(du)(du)充(chong)(chong)電(dian)(dian)(dian)而(er)誤(wu)判(pan)為(wei)過(guo)充(chong)(chong)保護,因此需(xu)要設定延(yan)遲時(shi)間,并(bing)且延(yan)遲時(shi)間不能短(duan)于(yu)噪音(yin)的持續(xu)時(shi)間以免誤(wu)判(pan)。過(guo)充(chong)(chong)電(dian)(dian)(dian)保護延(yan)時(shi)時(shi)間tvdet1計算公式(shi)為(wei):

t vdet1 = { C3 ×( Vdd - 0. 7) }/ (0. 48 ×10 - 6 ) (1)

式中: Vdd為保(bao)護N1 的(de)過充電檢測(ce)電壓(ya)值。

簡便計算延時時間: t = C3/ 0. 01 ×77 (ms) (2)

如若C3 容值為0.22 F ,則延時值為:0. 22 /0. 01 ×77 = 1694 (ms)

1.2.2 過度放電保護

在過度放電的情況下,電解液因分解而導致電池特性劣化,并造成充電次數的降低。過度放電保護IC 原理:為了防止鋰電池的過度放電狀態,假設鋰電池接上負載,當鋰電池電壓低于其過度放電電壓檢測點(假定為2.3 V) 時將激活過度放電保護,使功率MOS FET 由開轉變為切斷而截止放電,以避免電池過度放電現象產生,并將電池保持在低靜態電流的待機模式,此時的電流僅0.1μA 。當鋰電池接上充電器,且(qie)此時(shi)鋰電(dian)(dian)池電(dian)(dian)壓高于過度放(fang)電(dian)(dian)電(dian)(dian)壓時(shi),過度放(fang)電(dian)(dian)保(bao)護功能方可解除。另外,考(kao)慮到(dao)脈沖放(fang)電(dian)(dian)的情(qing)況,過放(fang)電(dian)(dian)檢測電(dian)(dian)路(lu)設有延遲時(shi)間以避免產生誤動作(zuo)。

1.2.3 過電流及短路電流保護(hu)

因(yin)為不明原因(yin)(放電(dian)(dian)時或正負極遭(zao)金屬物誤觸) 造成過(guo)(guo)電(dian)(dian)流(liu)(liu)或短路(lu)(lu),為確保安全,必須使(shi)其立(li)即停止放電(dian)(dian)。過(guo)(guo)電(dian)(dian)流(liu)(liu)保護IC 原理為,當(dang)放電(dian)(dian)電(dian)(dian)流(liu)(liu)過(guo)(guo)大或短路(lu)(lu)情(qing)況產生(sheng)時,保護IC 將激(ji)活過(guo)(guo)(短路(lu)(lu)) 電(dian)(dian)流(liu)(liu)保護,此時過(guo)(guo)電(dian)(dian)流(liu)(liu)的(de)(de)檢測是將功率MOSFET 的(de)(de)Rds (on) 當(dang)成感(gan)應阻抗用以監(jian)測其電(dian)(dian)壓的(de)(de)下降(jiang)情(qing)形,如果比(bi)所定的(de)(de)過(guo)(guo)電(dian)(dian)流(liu)(liu)檢測電(dian)(dian)壓還高(gao)則停止放電(dian)(dian),運算公式為:

V_ = I ×Rds ( on) ×2 ( V_為(wei)過電(dian)流(liu)檢測電(dian)壓(ya), I 為(wei)放電(dian)電(dian)流(liu)) (3)假(jia)設V_ = 0. 2V , Rds (on) = 25 mΩ,則(ze)保(bao)護電(dian)流(liu)的(de)大小(xiao)為(wei)I = 4 A 。

同樣,過(guo)(guo)電(dian)(dian)流(liu)檢測(ce)也必須(xu)設有延遲時間以防有突發電(dian)(dian)流(liu)流(liu)入時產生誤(wu)動作。通(tong)常(chang)在過(guo)(guo)電(dian)(dian)流(liu)產生后,若能去除過(guo)(guo)電(dian)(dian)流(liu)因素(su)(例如(ru)馬上(shang)與(yu)負載脫離) ,將會恢復(fu)其正(zheng)常(chang)狀態,可以再(zai)進行(xing)正(zheng)常(chang)的(de)充放(fang)電(dian)(dian)動作。

2 結束語

在(zai)(zai)進行保護(hu)電(dian)路(lu)設計時(shi)(shi)(shi)使電(dian)池充電(dian)到(dao)飽滿的(de)狀態是使用(yong)者很關心的(de)問(wen)題(ti),同時(shi)(shi)(shi)兼顧到(dao)安(an)全性問(wen)題(ti),因(yin)此需(xu)(xu)要在(zai)(zai)達到(dao)容許(xu)電(dian)壓(ya)時(shi)(shi)(shi)截(jie)止充電(dian)狀態。要同時(shi)(shi)(shi)符合(he)這兩個條件,必須有高精密度(du)的(de)檢(jian)測(ce)器,目(mu)前(qian)檢(jian)測(ce)器的(de)精密度(du)為25 mV 。另外還必須考(kao)慮到(dao)集成保護(hu)電(dian)路(lu)IC 功耗、耐高電(dian)壓(ya)問(wen)題(ti)。此外為了(le)使功率MOSFET的(de)Rds ( on) 在(zai)(zai)充電(dian)電(dian)流(liu)與放電(dian)電(dian)流(liu)時(shi)(shi)(shi)有效應用(yong), 需(xu)(xu)使該阻(zu)抗(kang)(kang)值盡量(liang)低, 目(mu)前(qian)該阻(zu)抗(kang)(kang)約為20～30 mΩ,這樣過(guo)電(dian)流(liu)檢(jian)測(ce)電(dian)壓(ya)就可(ke)較低。