鋰(li)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)必須考慮充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)、放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)時的安全(quan)性,以(yi)(yi)防止(zhi)特性劣(lie)化。但鋰(li)離子(zi)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)能(neng)量(liang)密度(du)(du)(du)高,難以(yi)(yi)確(que)保(bao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)的安全(quan)性,在過(guo)(guo)度(du)(du)(du)充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)狀態下(xia),電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)溫度(du)(du)(du)上(shang)升(sheng)后能(neng)量(liang)將過(guo)(guo)剩,于是電(dian)(dian)(dian)(dian)(dian)(dian)(dian)解(jie)(jie)液分(fen)(fen)解(jie)(jie)而產生氣體,容易使內(nei)壓上(shang)升(sheng)而產生自燃或破裂的危險;反(fan)之,在過(guo)(guo)度(du)(du)(du)放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)狀態下(xia),電(dian)(dian)(dian)(dian)(dian)(dian)(dian)解(jie)(jie)液因分(fen)(fen)解(jie)(jie)導(dao)致電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)特性及耐久(jiu)性劣(lie)化,降(jiang)低(di)可充電(dian)(dian)(dian)(dian)(dian)(dian)(dian)次數。因此鋰(li)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)的過(guo)(guo)充、過(guo)(guo)度(du)(du)(du)放電(dian)(dian)(dian)(dian)(dian)(dian)(dian)、過(guo)(guo)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)及短路保(bao)護很重(zhong)要(yao),所(suo)以(yi)(yi)通常都會(hui)在電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)包(bao)內(nei)設計保(bao)護線(xian)路,用以(yi)(yi)保(bao)護鋰(li)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)。

1 電路設計

1.1 電路概述

鋰離子電池保(bao)護(hu)電(dian)(dian)路包括過度充電(dian)(dian)保(bao)護(hu)、過電(dian)(dian)流(liu)/ 短路保(bao)護(hu)和過放電(dian)(dian)保(bao)護(hu)等,該電(dian)(dian)路就是要確保(bao)這(zhe)樣的過度充電(dian)(dian)及放電(dian)(dian)狀態(tai)時的安全,并防止特性劣(lie)化(hua)。它主要由集成(cheng)保(bao)護(hu)電(dian)(dian)路IC、貼(tie)片(pian)電(dian)(dian)阻、貼(tie)片(pian)電(dian)(dian)容、場效應管(MOSFET) 、有的還有熱敏電(dian)(dian)阻(NTC) 、識別電(dian)(dian)阻( ID) 、保(bao)險絲( FUSE) 等構(gou)成(cheng)。

其(qi)中集成保護電(dian)(dian)路(lu)IC 用(yong)來(lai)(lai)檢測(ce)保護電(dian)(dian)路(lu)當前的(de)電(dian)(dian)壓、電(dian)(dian)流、時(shi)間(jian)等參數以此來(lai)(lai)控制場(chang)效(xiao)應(ying)管(guan)的(de)開關(guan)(guan)狀態;場(chang)效(xiao)應(ying)管(guan)(MOSFET) 則根據保護IC 來(lai)(lai)控制回路(lu)中是否(fou)有需開或關(guan)(guan); 貼片(pian)電(dian)(dian)阻用(yong)作限(xian)流; 貼片(pian)電(dian)(dian)容作用(yong)為(wei)濾波、調節(jie)延遲時(shi)間(jian);熱敏電(dian)(dian)阻用(yong)來(lai)(lai)檢測(ce)電(dian)(dian)池塊內的(de)環境(jing)溫度; 保險絲防(fang)止流過電(dian)(dian)池的(de)電(dian)(dian)流過大,切斷(duan)電(dian)(dian)流回路(lu)。

1.2 電路(lu)原理及參數確定(ding)

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(wen)度(du)(du)上(shang)升(sheng)而(er)導致內壓上(shang)升(sheng),需(xu)終止(zhi)當前充電(dian)(dian)(dian)(dian)的(de)(de)狀態。此時(shi),集成保護電(dian)(dian)(dian)(dian)路IC 需(xu)檢測電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓,當到達(da)4.25V 時(shi)(假設電(dian)(dian)(dian)(dian)池(chi)過(guo)充電(dian)(dian)(dian)(dian)壓臨界點為4.25 V) 即激活(huo)過(guo)度(du)(du)充電(dian)(dian)(dian)(dian)保護,將功(gong)率MOS 由開(kai)轉為切斷,進而(er)截止(zhi)充電(dian)(dian)(dian)(dian)。另外,為防止(zhi)由于(yu)噪(zao)音所(suo)產生(sheng)的(de)(de)過(guo)度(du)(du)充電(dian)(dian)(dian)(dian)而(er)誤判為過(guo)充保護,因此需(xu)要設定延(yan)遲(chi)時(shi)間,并且延(yan)遲(chi)時(shi)間不(bu)能(neng)短于(yu)噪(zao)音的(de)(de)持續時(shi)間以免(mian)誤判。過(guo)充電(dian)(dian)(dian)(dian)保護延(yan)時(shi)時(shi)間tvdet1計算公式為:

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

式中(zhong): Vdd為保護N1 的過充電檢測(ce)電壓(ya)值。

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

如若C3 容值(zhi)為0.22 F ,則(ze)延時值(zhi)為:0. 22 /0. 01 ×77 = 1694 (ms)

1.2.2 過度放(fang)電(dian)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)池電(dian)(dian)壓高于過度(du)放電(dian)(dian)電(dian)(dian)壓時,過度(du)放電(dian)(dian)保護功能(neng)方可解(jie)除(chu)。另外,考慮到脈沖放電(dian)(dian)的情況,過放電(dian)(dian)檢測電(dian)(dian)路設有(you)延遲時間以避(bi)免產生誤(wu)動作。

1.2.3 過電流及短路電流保護

因為(wei)不明(ming)原因(放(fang)電(dian)(dian)時(shi)或(huo)(huo)正負極遭金(jin)屬物誤觸) 造(zao)成過(guo)(guo)(guo)電(dian)(dian)流或(huo)(huo)短(duan)路(lu),為(wei)確(que)保(bao)(bao)安全,必須使其立即(ji)停止放(fang)電(dian)(dian)。過(guo)(guo)(guo)電(dian)(dian)流保(bao)(bao)護(hu)IC 原理為(wei),當放(fang)電(dian)(dian)電(dian)(dian)流過(guo)(guo)(guo)大或(huo)(huo)短(duan)路(lu)情(qing)況產生時(shi),保(bao)(bao)護(hu)IC 將激活過(guo)(guo)(guo)(短(duan)路(lu)) 電(dian)(dian)流保(bao)(bao)護(hu),此時(shi)過(guo)(guo)(guo)電(dian)(dian)流的(de)檢測是將功率(lv)MOSFET 的(de)Rds (on) 當成感(gan)應阻抗用以監(jian)測其電(dian)(dian)壓的(de)下(xia)降情(qing)形,如(ru)果比所定的(de)過(guo)(guo)(guo)電(dian)(dian)流檢測電(dian)(dian)壓還高則停止放(fang)電(dian)(dian),運算公式(shi)為(wei):

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

同(tong)樣,過電流(liu)檢(jian)測也必須設有延遲時間以(yi)防有突發(fa)電流(liu)流(liu)入時產生(sheng)誤動(dong)作。通常(chang)(chang)在過電流(liu)產生(sheng)后,若能去除過電流(liu)因(yin)素(例如馬(ma)上(shang)與負載脫離) ,將會恢(hui)復(fu)其(qi)正常(chang)(chang)狀態,可以(yi)再進行正常(chang)(chang)的充(chong)放電動(dong)作。

2 結束語

在進行(xing)保護電(dian)(dian)路(lu)(lu)設(she)計時使電(dian)(dian)池(chi)充電(dian)(dian)到(dao)(dao)飽滿的(de)(de)狀態(tai)是(shi)使用者很關心的(de)(de)問題(ti),同時兼(jian)顧到(dao)(dao)安(an)全(quan)性問題(ti),因此需要(yao)在達到(dao)(dao)容許(xu)電(dian)(dian)壓(ya)時截止充電(dian)(dian)狀態(tai)。要(yao)同時符(fu)合(he)這(zhe)兩(liang)個條(tiao)件,必須有(you)高精(jing)密度的(de)(de)檢測器,目前檢測器的(de)(de)精(jing)密度為25 mV 。另(ling)外還(huan)必須考慮到(dao)(dao)集成(cheng)保護電(dian)(dian)路(lu)(lu)IC 功耗、耐高電(dian)(dian)壓(ya)問題(ti)。此外為了使功率(lv)MOSFET的(de)(de)Rds ( on) 在充電(dian)(dian)電(dian)(dian)流(liu)與放電(dian)(dian)電(dian)(dian)流(liu)時有(you)效應用, 需使該(gai)阻抗(kang)值盡量(liang)低, 目前該(gai)阻抗(kang)約(yue)為20～30 mΩ,這(zhe)樣過(guo)電(dian)(dian)流(liu)檢測電(dian)(dian)壓(ya)就可較低。