鋰電(dian)(dian)(dian)池(chi)(chi)必須考慮充電(dian)(dian)(dian)、放(fang)電(dian)(dian)(dian)時的(de)安全性(xing),以(yi)(yi)防止特性(xing)劣化(hua)。但鋰離子電(dian)(dian)(dian)池(chi)(chi)能量密度高,難以(yi)(yi)確(que)保電(dian)(dian)(dian)池(chi)(chi)的(de)安全性(xing),在(zai)過度充電(dian)(dian)(dian)狀態(tai)下,電(dian)(dian)(dian)池(chi)(chi)溫度上(shang)升(sheng)后能量將過剩(sheng),于是電(dian)(dian)(dian)解液(ye)分解而產生氣體,容(rong)易(yi)使內壓上(shang)升(sheng)而產生自燃或(huo)破裂(lie)的(de)危險;反(fan)之,在(zai)過度放(fang)電(dian)(dian)(dian)狀態(tai)下,電(dian)(dian)(dian)解液(ye)因分解導致電(dian)(dian)(dian)池(chi)(chi)特性(xing)及耐久性(xing)劣化(hua),降低(di)可充電(dian)(dian)(dian)次數。因此鋰電(dian)(dian)(dian)池(chi)(chi)的(de)過充、過度放(fang)電(dian)(dian)(dian)、過電(dian)(dian)(dian)流及短路(lu)保護很重要,所以(yi)(yi)通(tong)常都(dou)會在(zai)電(dian)(dian)(dian)池(chi)(chi)包(bao)內設計保護線路(lu),用以(yi)(yi)保護鋰電(dian)(dian)(dian)池(chi)(chi)。

1 電路設計

1.1 電路概述

鋰離子電池保(bao)護電(dian)(dian)路包括過(guo)(guo)度充電(dian)(dian)保(bao)護、過(guo)(guo)電(dian)(dian)流/ 短路保(bao)護和(he)過(guo)(guo)放電(dian)(dian)保(bao)護等,該電(dian)(dian)路就是要確保(bao)這樣(yang)的(de)過(guo)(guo)度充電(dian)(dian)及放電(dian)(dian)狀態(tai)時的(de)安全,并(bing)防止特(te)性劣化。它主要由集成保(bao)護電(dian)(dian)路IC、貼(tie)片電(dian)(dian)阻、貼(tie)片電(dian)(dian)容(rong)、場(chang)效應管(MOSFET) 、有(you)的(de)還有(you)熱敏電(dian)(dian)阻(NTC) 、識別電(dian)(dian)阻( ID) 、保(bao)險絲(si)( FUSE) 等構成。

其中集成保(bao)(bao)護電(dian)(dian)路(lu)IC 用(yong)來(lai)檢(jian)測(ce)保(bao)(bao)護電(dian)(dian)路(lu)當前的電(dian)(dian)壓、電(dian)(dian)流(liu)、時(shi)間(jian)等(deng)參數(shu)以此來(lai)控(kong)制場(chang)效應(ying)管的開(kai)關狀態;場(chang)效應(ying)管(MOSFET) 則根(gen)據保(bao)(bao)護IC 來(lai)控(kong)制回路(lu)中是(shi)否有(you)需開(kai)或(huo)關; 貼(tie)片電(dian)(dian)阻(zu)用(yong)作限流(liu); 貼(tie)片電(dian)(dian)容作用(yong)為濾(lv)波、調節延遲時(shi)間(jian);熱敏電(dian)(dian)阻(zu)用(yong)來(lai)檢(jian)測(ce)電(dian)(dian)池塊內的環境溫度(du); 保(bao)(bao)險絲防止流(liu)過(guo)電(dian)(dian)池的電(dian)(dian)流(liu)過(guo)大,切(qie)斷電(dian)(dian)流(liu)回路(lu)。

1.2 電(dian)路原理及參數確定

1.2.1 過(guo)度充電保護(hu)

當充電器對鋰電池過度充電時,鋰電池會因溫度(du)上(shang)升(sheng)而(er)(er)導致內壓(ya)上(shang)升(sheng),需終(zhong)止(zhi)(zhi)當前充(chong)電(dian)(dian)(dian)的(de)狀態。此時(shi)(shi),集(ji)成(cheng)保(bao)護電(dian)(dian)(dian)路IC 需檢測電(dian)(dian)(dian)池電(dian)(dian)(dian)壓(ya),當到達4.25V 時(shi)(shi)(假設電(dian)(dian)(dian)池過(guo)(guo)充(chong)電(dian)(dian)(dian)壓(ya)臨界點為4.25 V) 即激(ji)活(huo)過(guo)(guo)度(du)充(chong)電(dian)(dian)(dian)保(bao)護,將功(gong)率MOS 由開(kai)轉為切(qie)斷,進而(er)(er)截止(zhi)(zhi)充(chong)電(dian)(dian)(dian)。另外,為防止(zhi)(zhi)由于噪音(yin)所產生的(de)過(guo)(guo)度(du)充(chong)電(dian)(dian)(dian)而(er)(er)誤判為過(guo)(guo)充(chong)保(bao)護,因此需要設定延(yan)(yan)遲時(shi)(shi)間(jian)(jian),并且延(yan)(yan)遲時(shi)(shi)間(jian)(jian)不能(neng)短于噪音(yin)的(de)持續時(shi)(shi)間(jian)(jian)以免誤判。過(guo)(guo)充(chong)電(dian)(dian)(dian)保(bao)護延(yan)(yan)時(shi)(shi)時(shi)(shi)間(jian)(jian)tvdet1計算(suan)公(gong)式(shi)為:

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

式中: Vdd為保(bao)護N1 的過充(chong)電檢(jian)測(ce)電壓值。

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

如若C3 容(rong)值為(wei)0.22 F ,則(ze)延時值為(wei):0. 22 /0. 01 ×77 = 1694 (ms)

1.2.2 過度(du)放(fang)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,且此時(shi)鋰電(dian)(dian)池電(dian)(dian)壓高于過度(du)放(fang)電(dian)(dian)電(dian)(dian)壓時(shi),過度(du)放(fang)電(dian)(dian)保護(hu)功(gong)能方可解除。另外,考慮到脈沖放(fang)電(dian)(dian)的情況,過放(fang)電(dian)(dian)檢測電(dian)(dian)路設有延遲時(shi)間以避免產生誤動作。

1.2.3 過(guo)電(dian)流及短路(lu)電(dian)流保護

因(yin)為(wei)不明原因(yin)(放電(dian)時或正負(fu)極遭金屬物誤觸) 造(zao)成(cheng)過(guo)電(dian)流或短(duan)(duan)路(lu),為(wei)確保(bao)安全,必(bi)須使其立即停止放電(dian)。過(guo)電(dian)流保(bao)護(hu)(hu)IC 原理為(wei),當放電(dian)電(dian)流過(guo)大或短(duan)(duan)路(lu)情(qing)況產生時,保(bao)護(hu)(hu)IC 將激活過(guo)(短(duan)(duan)路(lu)) 電(dian)流保(bao)護(hu)(hu),此時過(guo)電(dian)流的(de)檢測(ce)是將功率(lv)MOSFET 的(de)Rds (on) 當成(cheng)感應阻抗用(yong)以監測(ce)其電(dian)壓(ya)的(de)下(xia)降情(qing)形,如(ru)果比(bi)所(suo)定的(de)過(guo)電(dian)流檢測(ce)電(dian)壓(ya)還高則停止放電(dian),運算公式為(wei):

V_ = I ×Rds ( on) ×2 ( V_為(wei)(wei)過電流檢(jian)測(ce)電壓, I 為(wei)(wei)放電電流) (3)假設V_ = 0. 2V , Rds (on) = 25 mΩ,則保(bao)護(hu)電流的大(da)小為(wei)(wei)I = 4 A 。

同樣(yang),過電(dian)流(liu)(liu)(liu)檢測也必須設有延遲時間以防有突發電(dian)流(liu)(liu)(liu)流(liu)(liu)(liu)入時產(chan)生誤(wu)動作。通(tong)常在過電(dian)流(liu)(liu)(liu)產(chan)生后(hou),若能去(qu)除(chu)過電(dian)流(liu)(liu)(liu)因素(例如馬上與負載脫(tuo)離) ,將會恢復其正常狀態(tai),可以再進(jin)行正常的(de)充(chong)放電(dian)動作。

2 結束語

在進行(xing)保護(hu)電(dian)(dian)(dian)(dian)(dian)路設計時(shi)使電(dian)(dian)(dian)(dian)(dian)池充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)到(dao)飽滿的狀態是使用(yong)者很關心(xin)的問(wen)題,同時(shi)兼顧(gu)到(dao)安全性問(wen)題,因此需要(yao)(yao)在達到(dao)容(rong)許電(dian)(dian)(dian)(dian)(dian)壓時(shi)截止充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)狀態。要(yao)(yao)同時(shi)符合這(zhe)(zhe)兩個條件(jian),必須有(you)高精密度的檢測器(qi),目(mu)前(qian)(qian)檢測器(qi)的精密度為(wei)(wei)25 mV 。另外(wai)還必須考慮(lv)到(dao)集成保護(hu)電(dian)(dian)(dian)(dian)(dian)路IC 功耗(hao)、耐高電(dian)(dian)(dian)(dian)(dian)壓問(wen)題。此外(wai)為(wei)(wei)了使功率MOSFET的Rds ( on) 在充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流與放電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流時(shi)有(you)效應用(yong), 需使該阻抗(kang)值盡量低, 目(mu)前(qian)(qian)該阻抗(kang)約為(wei)(wei)20～30 mΩ,這(zhe)(zhe)樣(yang)過電(dian)(dian)(dian)(dian)(dian)流檢測電(dian)(dian)(dian)(dian)(dian)壓就可較低。