鋰電(dian)(dian)池(chi)(chi)必須考(kao)慮充(chong)電(dian)(dian)、放電(dian)(dian)時的安全性,以防止特性劣化。但鋰離子電(dian)(dian)池(chi)(chi)能量(liang)密度(du)高(gao),難以確保(bao)(bao)電(dian)(dian)池(chi)(chi)的安全性,在過(guo)(guo)度(du)充(chong)電(dian)(dian)狀態(tai)下,電(dian)(dian)池(chi)(chi)溫度(du)上(shang)升(sheng)后能量(liang)將過(guo)(guo)剩,于是(shi)電(dian)(dian)解(jie)液(ye)分解(jie)而產(chan)生(sheng)氣(qi)體,容易使內(nei)壓(ya)上(shang)升(sheng)而產(chan)生(sheng)自燃(ran)或破裂的危險;反之,在過(guo)(guo)度(du)放電(dian)(dian)狀態(tai)下,電(dian)(dian)解(jie)液(ye)因分解(jie)導致電(dian)(dian)池(chi)(chi)特性及耐久性劣化,降低可充(chong)電(dian)(dian)次數(shu)。因此鋰電(dian)(dian)池(chi)(chi)的過(guo)(guo)充(chong)、過(guo)(guo)度(du)放電(dian)(dian)、過(guo)(guo)電(dian)(dian)流(liu)及短路保(bao)(bao)護(hu)很重要,所以通常都會在電(dian)(dian)池(chi)(chi)包內(nei)設計保(bao)(bao)護(hu)線(xian)路,用以保(bao)(bao)護(hu)鋰電(dian)(dian)池(chi)(chi)。

1 電路設計

1.1 電路概述

鋰離子電池保(bao)(bao)護電路(lu)(lu)包括過(guo)度充電保(bao)(bao)護、過(guo)電流/ 短(duan)路(lu)(lu)保(bao)(bao)護和(he)過(guo)放(fang)電保(bao)(bao)護等,該(gai)電路(lu)(lu)就是(shi)要確保(bao)(bao)這樣的(de)過(guo)度充電及放(fang)電狀(zhuang)態時的(de)安(an)全,并防止(zhi)特性劣化。它主要由(you)集(ji)成保(bao)(bao)護電路(lu)(lu)IC、貼片(pian)(pian)電阻、貼片(pian)(pian)電容(rong)、場效(xiao)應管(guan)(MOSFET) 、有的(de)還有熱敏(min)電阻(NTC) 、識別(bie)電阻( ID) 、保(bao)(bao)險絲( FUSE) 等構成。

其中(zhong)集成保(bao)(bao)護(hu)電(dian)路(lu)IC 用(yong)(yong)來(lai)(lai)檢測保(bao)(bao)護(hu)電(dian)路(lu)當前的(de)(de)電(dian)壓、電(dian)流(liu)、時間等(deng)參(can)數以(yi)此來(lai)(lai)控制(zhi)場(chang)效應管的(de)(de)開關狀(zhuang)態;場(chang)效應管(MOSFET) 則(ze)根據(ju)保(bao)(bao)護(hu)IC 來(lai)(lai)控制(zhi)回路(lu)中(zhong)是否有需開或關; 貼片(pian)(pian)電(dian)阻(zu)(zu)用(yong)(yong)作(zuo)限流(liu); 貼片(pian)(pian)電(dian)容作(zuo)用(yong)(yong)為(wei)濾(lv)波(bo)、調節延遲時間;熱(re)敏電(dian)阻(zu)(zu)用(yong)(yong)來(lai)(lai)檢測電(dian)池塊內(nei)的(de)(de)環(huan)境(jing)溫(wen)度(du); 保(bao)(bao)險絲防止流(liu)過電(dian)池的(de)(de)電(dian)流(liu)過大,切(qie)斷電(dian)流(liu)回路(lu)。

1.2 電路(lu)原理及參數(shu)確定

1.2.1 過度充電保護

當充電器對鋰電池過度充電時,鋰電池會因溫(wen)度上升(sheng)而導致內(nei)壓上升(sheng),需終(zhong)止(zhi)(zhi)當前充(chong)電(dian)(dian)的狀(zhuang)態。此(ci)時,集成保護電(dian)(dian)路IC 需檢測電(dian)(dian)池電(dian)(dian)壓,當到達(da)4.25V 時(假設電(dian)(dian)池過(guo)(guo)充(chong)電(dian)(dian)壓臨界(jie)點為(wei)4.25 V) 即激活過(guo)(guo)度充(chong)電(dian)(dian)保護,將功率MOS 由開轉為(wei)切斷,進而截(jie)止(zhi)(zhi)充(chong)電(dian)(dian)。另外,為(wei)防(fang)止(zhi)(zhi)由于(yu)噪音(yin)所(suo)產生的過(guo)(guo)度充(chong)電(dian)(dian)而誤(wu)(wu)判(pan)為(wei)過(guo)(guo)充(chong)保護,因此(ci)需要(yao)設定(ding)延遲(chi)時間(jian),并且延遲(chi)時間(jian)不(bu)能短于(yu)噪音(yin)的持續(xu)時間(jian)以(yi)免誤(wu)(wu)判(pan)。過(guo)(guo)充(chong)電(dian)(dian)保護延時時間(jian)tvdet1計(ji)算公式(shi)為(wei):

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

式中: Vdd為保護N1 的過充(chong)電(dian)(dian)檢測電(dian)(dian)壓值(zhi)。

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

如(ru)若C3 容值(zhi)為0.22 F ,則延時值(zhi)為:0. 22 /0. 01 ×77 = 1694 (ms)

1.2.2 過度放電保(bao)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(li)電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓高于(yu)過(guo)度放(fang)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)壓時(shi),過(guo)度放(fang)電(dian)(dian)(dian)(dian)保護功能方可解除。另外,考(kao)慮到(dao)脈沖(chong)放(fang)電(dian)(dian)(dian)(dian)的情況,過(guo)放(fang)電(dian)(dian)(dian)(dian)檢測電(dian)(dian)(dian)(dian)路設有延遲(chi)時(shi)間以(yi)避免產生誤動作(zuo)。

1.2.3 過(guo)電(dian)流及短路電(dian)流保(bao)護

因為(wei)不明原(yuan)(yuan)因(放(fang)電(dian)時(shi)或正(zheng)負極遭金(jin)屬物誤觸) 造成過(guo)電(dian)流(liu)或短路(lu)(lu),為(wei)確保(bao)安全,必須使其立(li)即停止(zhi)放(fang)電(dian)。過(guo)電(dian)流(liu)保(bao)護(hu)IC 原(yuan)(yuan)理為(wei),當(dang)放(fang)電(dian)電(dian)流(liu)過(guo)大或短路(lu)(lu)情況(kuang)產生時(shi),保(bao)護(hu)IC 將激活過(guo)(短路(lu)(lu)) 電(dian)流(liu)保(bao)護(hu),此(ci)時(shi)過(guo)電(dian)流(liu)的(de)檢測(ce)是將功率MOSFET 的(de)Rds (on) 當(dang)成感(gan)應阻抗用以監測(ce)其電(dian)壓的(de)下降情形,如(ru)果比(bi)所定的(de)過(guo)電(dian)流(liu)檢測(ce)電(dian)壓還高(gao)則停止(zhi)放(fang)電(dian),運算公式為(wei):

V_ = I ×Rds ( on) ×2 ( V_為過電流(liu)檢測電壓, I 為放電電流(liu)) (3)假(jia)設V_ = 0. 2V , Rds (on) = 25 mΩ,則保護(hu)電流(liu)的大小為I = 4 A 。

同樣,過(guo)電(dian)流檢測也必須設有延遲(chi)時(shi)間以防(fang)有突發電(dian)流流入時(shi)產生誤動作。通常(chang)在(zai)過(guo)電(dian)流產生后(hou),若能去除過(guo)電(dian)流因素(su)(例如馬上與(yu)負載脫離(li)) ,將會恢復其正常(chang)狀態,可以再進行正常(chang)的充放電(dian)動作。

2 結束語

在進(jin)行保護電(dian)路設計(ji)時使電(dian)池(chi)充電(dian)到(dao)(dao)飽滿的(de)狀態是(shi)使用者很關心(xin)的(de)問題(ti)(ti),同時兼顧到(dao)(dao)安全性問題(ti)(ti),因此(ci)需(xu)要(yao)在達(da)到(dao)(dao)容許電(dian)壓(ya)時截(jie)止充電(dian)狀態。要(yao)同時符(fu)合這兩個條件(jian),必須(xu)有高(gao)精密度的(de)檢(jian)(jian)(jian)測(ce)器,目前檢(jian)(jian)(jian)測(ce)器的(de)精密度為25 mV 。另外還必須(xu)考慮到(dao)(dao)集成保護電(dian)路IC 功(gong)耗、耐高(gao)電(dian)壓(ya)問題(ti)(ti)。此(ci)外為了使功(gong)率(lv)MOSFET的(de)Rds ( on) 在充電(dian)電(dian)流(liu)與(yu)放電(dian)電(dian)流(liu)時有效應用, 需(xu)使該阻抗(kang)值盡量低, 目前該阻抗(kang)約為20～30 mΩ,這樣(yang)過電(dian)流(liu)檢(jian)(jian)(jian)測(ce)電(dian)壓(ya)就可較低。