鋰(li)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)必須考慮充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)、放電(dian)(dian)(dian)(dian)(dian)(dian)時的安全(quan)(quan)性,以防止特(te)性劣(lie)化。但鋰(li)離(li)子電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)能(neng)(neng)量密(mi)度(du)(du)高(gao),難以確保電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的安全(quan)(quan)性,在過度(du)(du)充(chong)(chong)(chong)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)(du)上(shang)(shang)升(sheng)后能(neng)(neng)量將過剩,于是(shi)電(dian)(dian)(dian)(dian)(dian)(dian)解液分解而產(chan)生氣體,容易使內(nei)壓上(shang)(shang)升(sheng)而產(chan)生自(zi)燃或破裂的危險;反之,在過度(du)(du)放電(dian)(dian)(dian)(dian)(dian)(dian)狀態(tai)(tai)下,電(dian)(dian)(dian)(dian)(dian)(dian)解液因分解導致(zhi)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)特(te)性及(ji)耐久性劣(lie)化,降低(di)可充(chong)(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)次數。因此鋰(li)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)的過充(chong)(chong)(chong)、過度(du)(du)放電(dian)(dian)(dian)(dian)(dian)(dian)、過電(dian)(dian)(dian)(dian)(dian)(dian)流及(ji)短(duan)路(lu)(lu)保護很重要,所以通常(chang)都(dou)會在電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)(chi)包內(nei)設計(ji)保護線(xian)路(lu)(lu),用以保護鋰(li)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)護電(dian)(dian)(dian)路包括過度(du)充(chong)電(dian)(dian)(dian)保(bao)護、過電(dian)(dian)(dian)流/ 短路保(bao)護和(he)過放(fang)電(dian)(dian)(dian)保(bao)護等,該(gai)電(dian)(dian)(dian)路就是要(yao)確(que)保(bao)這樣的(de)過度(du)充(chong)電(dian)(dian)(dian)及(ji)放(fang)電(dian)(dian)(dian)狀態(tai)時的(de)安(an)全(quan),并防(fang)止特性(xing)劣(lie)化。它主要(yao)由集成(cheng)(cheng)保(bao)護電(dian)(dian)(dian)路IC、貼(tie)片電(dian)(dian)(dian)阻、貼(tie)片電(dian)(dian)(dian)容、場效(xiao)應管(guan)(MOSFET) 、有(you)的(de)還有(you)熱敏(min)電(dian)(dian)(dian)阻(NTC) 、識別(bie)電(dian)(dian)(dian)阻( ID) 、保(bao)險絲( FUSE) 等構(gou)成(cheng)(cheng)。

其中(zhong)集(ji)成保護(hu)電(dian)(dian)(dian)(dian)(dian)(dian)路IC 用(yong)(yong)來(lai)檢(jian)測(ce)保護(hu)電(dian)(dian)(dian)(dian)(dian)(dian)路當(dang)前的(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)壓、電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)、時間等參數以此來(lai)控制場(chang)效應管的(de)(de)開關(guan)狀態;場(chang)效應管(MOSFET) 則根據保護(hu)IC 來(lai)控制回路中(zhong)是否有需開或關(guan); 貼(tie)片(pian)電(dian)(dian)(dian)(dian)(dian)(dian)阻用(yong)(yong)作限流(liu)(liu); 貼(tie)片(pian)電(dian)(dian)(dian)(dian)(dian)(dian)容作用(yong)(yong)為濾波、調節延(yan)遲時間;熱敏電(dian)(dian)(dian)(dian)(dian)(dian)阻用(yong)(yong)來(lai)檢(jian)測(ce)電(dian)(dian)(dian)(dian)(dian)(dian)池塊內的(de)(de)環境溫度(du); 保險絲防止流(liu)(liu)過電(dian)(dian)(dian)(dian)(dian)(dian)池的(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)過大(da),切斷電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)回路。

1.2 電路(lu)原(yuan)理及(ji)參(can)數(shu)確定

1.2.1 過度(du)充電保護

當充電器對鋰電池過度充電時,鋰電池會因溫度(du)上升而導致內壓上升,需終(zhong)止(zhi)當前充(chong)(chong)電(dian)(dian)的狀態。此時(shi)(shi),集(ji)成保(bao)護(hu)電(dian)(dian)路IC 需檢測電(dian)(dian)池電(dian)(dian)壓,當到達(da)4.25V 時(shi)(shi)(假(jia)設(she)電(dian)(dian)池過(guo)充(chong)(chong)電(dian)(dian)壓臨界點為4.25 V) 即激活過(guo)度(du)充(chong)(chong)電(dian)(dian)保(bao)護(hu),將(jiang)功率MOS 由(you)開轉為切(qie)斷,進而截止(zhi)充(chong)(chong)電(dian)(dian)。另(ling)外,為防止(zhi)由(you)于(yu)噪(zao)音所(suo)產生(sheng)的過(guo)度(du)充(chong)(chong)電(dian)(dian)而誤判(pan)為過(guo)充(chong)(chong)保(bao)護(hu),因此需要設(she)定延遲(chi)(chi)時(shi)(shi)間,并且延遲(chi)(chi)時(shi)(shi)間不能短于(yu)噪(zao)音的持續時(shi)(shi)間以免誤判(pan)。過(guo)充(chong)(chong)電(dian)(dian)保(bao)護(hu)延時(shi)(shi)時(shi)(shi)間tvdet1計算(suan)公式為:

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

式中: Vdd為保(bao)護N1 的過充電(dian)檢測電(dian)壓(ya)值(zhi)。

簡便計算延時(shi)時(shi)間: t = C3/ 0. 01 ×77 (ms) (2)

如(ru)若C3 容值為0.22 F ,則(ze)延時值為:0. 22 /0. 01 ×77 = 1694 (ms)

1.2.2 過(guo)度放電保護(hu)

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,且此時(shi)(shi)鋰電(dian)池(chi)電(dian)壓(ya)高于(yu)過(guo)(guo)度放(fang)電(dian)電(dian)壓(ya)時(shi)(shi),過(guo)(guo)度放(fang)電(dian)保護(hu)功能方可解除(chu)。另外,考慮到脈(mo)沖(chong)放(fang)電(dian)的情況,過(guo)(guo)放(fang)電(dian)檢測電(dian)路設有延遲時(shi)(shi)間以避(bi)免產生誤(wu)動作。

1.2.3 過(guo)電(dian)流及短路(lu)電(dian)流保護

因(yin)為不明原因(yin)(放(fang)電(dian)(dian)(dian)時(shi)(shi)或正負極遭金屬物誤(wu)觸) 造成過(guo)(guo)(guo)電(dian)(dian)(dian)流(liu)(liu)或短路,為確保安(an)全,必(bi)須使其立(li)即停止(zhi)放(fang)電(dian)(dian)(dian)。過(guo)(guo)(guo)電(dian)(dian)(dian)流(liu)(liu)保護IC 原理(li)為,當(dang)放(fang)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)(liu)過(guo)(guo)(guo)大或短路情況產生時(shi)(shi),保護IC 將(jiang)激活過(guo)(guo)(guo)(短路) 電(dian)(dian)(dian)流(liu)(liu)保護,此時(shi)(shi)過(guo)(guo)(guo)電(dian)(dian)(dian)流(liu)(liu)的(de)檢測(ce)(ce)是將(jiang)功(gong)率(lv)MOSFET 的(de)Rds (on) 當(dang)成感應阻抗用以監測(ce)(ce)其電(dian)(dian)(dian)壓(ya)的(de)下(xia)降(jiang)情形,如果比(bi)所定(ding)的(de)過(guo)(guo)(guo)電(dian)(dian)(dian)流(liu)(liu)檢測(ce)(ce)電(dian)(dian)(dian)壓(ya)還(huan)高則停止(zhi)放(fang)電(dian)(dian)(dian),運算公式為:

V_ = I ×Rds ( on) ×2 ( V_為過電(dian)流(liu)(liu)檢(jian)測電(dian)壓, I 為放電(dian)電(dian)流(liu)(liu)) (3)假設(she)V_ = 0. 2V , Rds (on) = 25 mΩ,則(ze)保護(hu)電(dian)流(liu)(liu)的(de)大小為I = 4 A 。

同(tong)樣,過(guo)電(dian)(dian)流(liu)檢測也(ye)必須(xu)設有延遲(chi)時(shi)(shi)間以(yi)防有突(tu)發電(dian)(dian)流(liu)流(liu)入時(shi)(shi)產生(sheng)誤動作。通常(chang)在過(guo)電(dian)(dian)流(liu)產生(sheng)后,若能去除過(guo)電(dian)(dian)流(liu)因素(su)(例如馬(ma)上與負載脫離(li)) ,將會恢復其正常(chang)狀態,可(ke)以(yi)再進行正常(chang)的充(chong)放電(dian)(dian)動作。

2 結束語

在(zai)進(jin)行保護(hu)電(dian)路設(she)計時(shi)(shi)使電(dian)池充電(dian)到(dao)飽(bao)滿的(de)狀(zhuang)態是(shi)使用者很(hen)關心的(de)問題,同(tong)時(shi)(shi)兼顧到(dao)安(an)全性問題,因此(ci)需(xu)要在(zai)達(da)到(dao)容許電(dian)壓時(shi)(shi)截止充電(dian)狀(zhuang)態。要同(tong)時(shi)(shi)符合(he)這兩個條(tiao)件,必須(xu)有(you)高精(jing)密度的(de)檢測器,目前檢測器的(de)精(jing)密度為25 mV 。另外還必須(xu)考慮到(dao)集成保護(hu)電(dian)路IC 功(gong)耗、耐高電(dian)壓問題。此(ci)外為了使功(gong)率MOSFET的(de)Rds ( on) 在(zai)充電(dian)電(dian)流與放(fang)電(dian)電(dian)流時(shi)(shi)有(you)效應用, 需(xu)使該(gai)阻抗值盡量低, 目前該(gai)阻抗約為20～30 mΩ,這樣過電(dian)流檢測電(dian)壓就(jiu)可較低。