鋰電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)必須(xu)考(kao)慮充電(dian)(dian)(dian)(dian)(dian)(dian)、放(fang)電(dian)(dian)(dian)(dian)(dian)(dian)時的安(an)(an)全性(xing),以防止特性(xing)劣(lie)化。但鋰離(li)子(zi)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)能(neng)(neng)量密度(du)高(gao),難(nan)以確(que)保電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的安(an)(an)全性(xing),在過度(du)充電(dian)(dian)(dian)(dian)(dian)(dian)狀態(tai)(tai)下,電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)溫度(du)上(shang)升后能(neng)(neng)量將過剩,于是電(dian)(dian)(dian)(dian)(dian)(dian)解液(ye)分(fen)解而(er)產生氣體(ti),容易使內(nei)壓(ya)上(shang)升而(er)產生自燃(ran)或破裂的危險;反(fan)之,在過度(du)放(fang)電(dian)(dian)(dian)(dian)(dian)(dian)狀態(tai)(tai)下,電(dian)(dian)(dian)(dian)(dian)(dian)解液(ye)因(yin)分(fen)解導(dao)致電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)特性(xing)及耐久性(xing)劣(lie)化,降低可(ke)充電(dian)(dian)(dian)(dian)(dian)(dian)次(ci)數(shu)。因(yin)此(ci)鋰電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的過充、過度(du)放(fang)電(dian)(dian)(dian)(dian)(dian)(dian)、過電(dian)(dian)(dian)(dian)(dian)(dian)流及短路(lu)保護(hu)很重要(yao),所以通(tong)常都(dou)會在電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)包內(nei)設計保護(hu)線路(lu),用(yong)以保護(hu)鋰電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)。

1 電路設計

1.1 電路概述

鋰離子電池保(bao)護(hu)電(dian)(dian)(dian)路包(bao)括過度充(chong)電(dian)(dian)(dian)保(bao)護(hu)、過電(dian)(dian)(dian)流/ 短路保(bao)護(hu)和過放(fang)電(dian)(dian)(dian)保(bao)護(hu)等,該電(dian)(dian)(dian)路就是要確保(bao)這樣的(de)(de)過度充(chong)電(dian)(dian)(dian)及放(fang)電(dian)(dian)(dian)狀態時(shi)的(de)(de)安全,并(bing)防止特(te)性(xing)劣化。它主要由(you)集成保(bao)護(hu)電(dian)(dian)(dian)路IC、貼片(pian)電(dian)(dian)(dian)阻、貼片(pian)電(dian)(dian)(dian)容(rong)、場效應(ying)管(MOSFET) 、有的(de)(de)還有熱(re)敏電(dian)(dian)(dian)阻(NTC) 、識別電(dian)(dian)(dian)阻( ID) 、保(bao)險絲( FUSE) 等構(gou)成。

其中(zhong)集(ji)成(cheng)保護電(dian)(dian)(dian)路IC 用來(lai)檢(jian)測(ce)保護電(dian)(dian)(dian)路當前(qian)的電(dian)(dian)(dian)壓、電(dian)(dian)(dian)流、時間(jian)(jian)等參數(shu)以(yi)此來(lai)控(kong)制(zhi)場(chang)效(xiao)應(ying)管的開(kai)關狀態(tai);場(chang)效(xiao)應(ying)管(MOSFET) 則根據保護IC 來(lai)控(kong)制(zhi)回路中(zhong)是(shi)否(fou)有需開(kai)或關; 貼(tie)片(pian)電(dian)(dian)(dian)阻用作限流; 貼(tie)片(pian)電(dian)(dian)(dian)容作用為濾波、調節延遲時間(jian)(jian);熱敏電(dian)(dian)(dian)阻用來(lai)檢(jian)測(ce)電(dian)(dian)(dian)池塊內的環境溫度(du); 保險絲(si)防(fang)止流過電(dian)(dian)(dian)池的電(dian)(dian)(dian)流過大,切斷電(dian)(dian)(dian)流回路。

1.2 電路(lu)原(yuan)理及(ji)參數確(que)定

1.2.1 過(guo)度充電(dian)保護(hu)

當充電器對鋰電池過度充電時,鋰電池會(hui)因溫(wen)度(du)上升而(er)導致內壓上升,需終止(zhi)當前(qian)充(chong)(chong)電(dian)(dian)(dian)的(de)狀態。此(ci)時(shi),集成保(bao)護(hu)(hu)電(dian)(dian)(dian)路(lu)IC 需檢測電(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)壓,當到達4.25V 時(shi)(假設(she)電(dian)(dian)(dian)池(chi)過(guo)充(chong)(chong)電(dian)(dian)(dian)壓臨界點(dian)為(wei)(wei)4.25 V) 即激活過(guo)度(du)充(chong)(chong)電(dian)(dian)(dian)保(bao)護(hu)(hu),將功率MOS 由開轉(zhuan)為(wei)(wei)切斷,進而(er)截止(zhi)充(chong)(chong)電(dian)(dian)(dian)。另(ling)外,為(wei)(wei)防止(zhi)由于噪音所產生(sheng)的(de)過(guo)度(du)充(chong)(chong)電(dian)(dian)(dian)而(er)誤判為(wei)(wei)過(guo)充(chong)(chong)保(bao)護(hu)(hu),因此(ci)需要設(she)定延遲(chi)(chi)時(shi)間(jian),并(bing)且延遲(chi)(chi)時(shi)間(jian)不能(neng)短(duan)于噪音的(de)持續時(shi)間(jian)以免誤判。過(guo)充(chong)(chong)電(dian)(dian)(dian)保(bao)護(hu)(hu)延時(shi)時(shi)間(jian)tvdet1計算(suan)公式為(wei)(wei):

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

式中: Vdd為(wei)保護N1 的過(guo)充電檢(jian)測電壓值。

簡便計算延時時間(jian): t = C3/ 0. 01 ×77 (ms) (2)

如若C3 容(rong)值(zhi)為0.22 F ,則延時值(zhi)為:0. 22 /0. 01 ×77 = 1694 (ms)

1.2.2 過(guo)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)此時鋰電(dian)(dian)(dian)池電(dian)(dian)(dian)壓高于過度放電(dian)(dian)(dian)電(dian)(dian)(dian)壓時,過度放電(dian)(dian)(dian)保護功能方可解除(chu)。另外,考慮到脈(mo)沖放電(dian)(dian)(dian)的情(qing)況(kuang),過放電(dian)(dian)(dian)檢測(ce)電(dian)(dian)(dian)路(lu)設有延(yan)遲(chi)時間以避免產(chan)生誤動作。

1.2.3 過(guo)電流(liu)及(ji)短路電流(liu)保護

因為不明原因(放(fang)(fang)電(dian)(dian)(dian)(dian)時或(huo)(huo)正負極遭金屬(shu)物誤觸(chu)) 造成過(guo)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)或(huo)(huo)短(duan)路(lu),為確保(bao)安全,必(bi)須使(shi)其立即(ji)停止放(fang)(fang)電(dian)(dian)(dian)(dian)。過(guo)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)保(bao)護IC 原理為,當放(fang)(fang)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)過(guo)大或(huo)(huo)短(duan)路(lu)情況產生時,保(bao)護IC 將(jiang)激活過(guo)(短(duan)路(lu)) 電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)保(bao)護,此時過(guo)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)的(de)(de)(de)檢測(ce)是將(jiang)功率MOSFET 的(de)(de)(de)Rds (on) 當成感(gan)應阻抗用以(yi)監測(ce)其電(dian)(dian)(dian)(dian)壓的(de)(de)(de)下降情形(xing),如果比所定的(de)(de)(de)過(guo)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)檢測(ce)電(dian)(dian)(dian)(dian)壓還高則停止放(fang)(fang)電(dian)(dian)(dian)(dian),運算公式為:

V_ = I ×Rds ( on) ×2 ( V_為過電(dian)流(liu)檢測電(dian)壓, I 為放電(dian)電(dian)流(liu)) (3)假設V_ = 0. 2V , Rds (on) = 25 mΩ,則保護電(dian)流(liu)的大小為I = 4 A 。

同樣(yang),過(guo)電(dian)流檢(jian)測(ce)也必須設有(you)延遲時間以防(fang)有(you)突發(fa)電(dian)流流入時產生(sheng)誤動作。通常(chang)在過(guo)電(dian)流產生(sheng)后,若能(neng)去除(chu)過(guo)電(dian)流因素(例(li)如馬上與負載脫離) ,將會恢復其(qi)正常(chang)狀(zhuang)態,可以再進行正常(chang)的充放電(dian)動作。

2 結束語

在進行保護(hu)電路設計(ji)時(shi)使電池充電到(dao)飽(bao)滿的(de)狀態是使用者很關心的(de)問(wen)題,同(tong)時(shi)兼顧到(dao)安全性問(wen)題,因(yin)此需要(yao)在達到(dao)容許電壓時(shi)截止(zhi)充電狀態。要(yao)同(tong)時(shi)符合這(zhe)兩個條件,必須(xu)有高(gao)精密(mi)度(du)的(de)檢(jian)測器,目前檢(jian)測器的(de)精密(mi)度(du)為25 mV 。另外(wai)還(huan)必須(xu)考(kao)慮到(dao)集成保護(hu)電路IC 功耗、耐高(gao)電壓問(wen)題。此外(wai)為了使功率MOSFET的(de)Rds ( on) 在充電電流與(yu)放電電流時(shi)有效應用, 需使該阻抗(kang)值盡量低, 目前該阻抗(kang)約為20～30 mΩ,這(zhe)樣過電流檢(jian)測電壓就可較(jiao)低。