太陽能是為便攜式設(she)備(bei)供電的(de)有吸引力的(de)能源。一段時(shi)間以來(lai)，它一直(zhi)被廣泛(fan)地用于諸(zhu)如計算器和航天飛機這樣(yang)的(de)應用。最(zui)近，人們正考慮把太陽能用于包括移動電話充(chong)電器這樣(yang)的(de)范圍更寬廣的(de)消費電子(zi)應用。

　　然而，太陽能電池板所提供的功率高度依賴于工作環境。這包括諸如光密度、時間和位置之類的因素。因此，電池通常被用作能量存儲單元。當來自太陽能板的電能有余的時候，就可以對電池充電;當太陽能板提供的電能不足時，電池就可以為系統供電。我們如何設計鋰離子電池充電器以便(bian)從太(tai)陽(yang)能(neng)電(dian)(dian)池(chi)中獲(huo)取最多的功率(lv)并有效地對鋰電(dian)(dian)池(chi)充電(dian)(dian)呢?首(shou)先，我們(men)將討(tao)論太(tai)陽(yang)能(neng)電(dian)(dian)池(chi)的工作(zuo)原理和電(dian)(dian)氣輸出特(te)性(xing);然后(hou)，我們(men)將討(tao)論電(dian)(dian)池(chi)充電(dian)(dian)系統要求以及匹配太(tai)陽(yang)能(neng)電(dian)(dian)池(chi)特(te)性(xing)的系統解決方案(an)，以便(bian)從太(tai)陽(yang)能(neng)電(dian)(dian)池(chi)獲(huo)取最大的功率(lv)。

　　太(tai)陽能(neng)I-V特性(xing)

　　一般地說，太陽(yang)能電池由(you)p-n結(jie)(jie)構(gou)成，其中的(de)(de)(de)(de)光(guang)能(光(guang)子)引(yin)起電子和空(kong)穴(xue)的(de)(de)(de)(de)重新組合，產生電流。因為p-n結(jie)(jie)的(de)(de)(de)(de)特性類(lei)似(si)于(yu)二極管的(de)(de)(de)(de)特性，如圖1所示(shi)的(de)(de)(de)(de)電路通常(chang)被(bei)用于(yu)簡化(hua)太陽(yang)能電池的(de)(de)(de)(de)特性。

　　電(dian)(dian)流源IPH產生的(de)電(dian)(dian)流正比于落在(zai)(zai)太陽(yang)能電(dian)(dian)池(chi)上的(de)光量(liang)。在(zai)(zai)沒有(you)負(fu)載連接的(de)時候，幾乎所有(you)產生的(de)電(dian)(dian)流都流過二(er)極(ji)管(guan)D，其正向(xiang)電(dian)(dian)壓(ya)決定太陽(yang)能電(dian)(dian)池(chi)的(de)開路電(dian)(dian)壓(ya)(VOC)。該電(dian)(dian)壓(ya)的(de)變化嚴格地(di)取決于每一種類(lei)型(xing)的(de)太陽(yang)能電(dian)(dian)池(chi)。但是(shi)，對于大多數硅(gui)電(dian)(dian)池(chi)，其0.5V到0.8V之間的(de)電(dian)(dian)壓(ya)范圍恰好(hao)就是(shi)p-n結二(er)極(ji)管(guan)的(de)正向(xiang)電(dian)(dian)壓(ya)。

　　并(bing)聯電(dian)(dian)(dian)阻(zu)(RP)代(dai)(dai)表實際太陽(yang)能電(dian)(dian)(dian)池中出現的(de)微(wei)小(xiao)泄漏(lou)電(dian)(dian)(dian)流，Rs代(dai)(dai)表連(lian)接損(sun)耗。隨著負(fu)載(zai)電(dian)(dian)(dian)流增加(jia)，由(you)太陽(yang)能電(dian)(dian)(dian)池所產(chan)生的(de)大部分電(dian)(dian)(dian)流被分流到二極管并(bing)進入負(fu)載(zai)。對(dui)于(yu)大多負(fu)載(zai)電(dian)(dian)(dian)流的(de)數值，這只對(dui)輸(shu)出電(dian)(dian)(dian)壓(ya)有很小(xiao)的(de)影響(xiang)。

　　圖2所(suo)示為(wei)(wei)(wei)太(tai)陽能電(dian)(dian)(dian)(dian)池的(de)(de)輸出(chu)(chu)特(te)性(xing)(xing)，由于二(er)極(ji)管(guan)的(de)(de)I-V特(te)性(xing)(xing)存(cun)在微小的(de)(de)變化(hua)(hua)，串聯電(dian)(dian)(dian)(dian)阻(Rs)上的(de)(de)電(dian)(dian)(dian)(dian)壓降(jiang)(jiang)也(ye)存(cun)在微小的(de)(de)變化(hua)(hua)，但是(shi)，輸出(chu)(chu)電(dian)(dian)(dian)(dian)壓保持很(hen)大的(de)(de)恒定。然而，在一(yi)(yi)些點通(tong)過(guo)內部二(er)極(ji)管(guan)的(de)(de)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)是(shi)如(ru)(ru)此之小，以至于它(ta)變得(de)偏置不夠(gou)，并(bing)且，隨著負(fu)載(zai)(zai)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)的(de)(de)增(zeng)加(jia)，跨越它(ta)的(de)(de)電(dian)(dian)(dian)(dian)壓快速減少。最后(hou)，如(ru)(ru)果(guo)所(suo)有產(chan)生的(de)(de)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)流(liu)(liu)(liu)過(guo)負(fu)載(zai)(zai)并(bing)且不流(liu)(liu)(liu)過(guo)二(er)極(ji)管(guan)的(de)(de)話，輸出(chu)(chu)電(dian)(dian)(dian)(dian)壓就為(wei)(wei)(wei)零。該電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)被(bei)稱為(wei)(wei)(wei)太(tai)陽能電(dian)(dian)(dian)(dian)池的(de)(de)短路電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(ISC)，它(ta)與VOC一(yi)(yi)道(dao)是(shi)定義工(gong)作性(xing)(xing)能的(de)(de)主要參(can)數之一(yi)(yi)。因此，太(tai)陽能電(dian)(dian)(dian)(dian)池被(bei)認為(wei)(wei)(wei)是(shi)“電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)受(shou)限”的(de)(de)電(dian)(dian)(dian)(dian)源(yuan)。當輸出(chu)(chu)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)增(zeng)加(jia)的(de)(de)時候，其(qi)輸出(chu)(chu)電(dian)(dian)(dian)(dian)壓降(jiang)(jiang)低，直(zhi)到最終減少為(wei)(wei)(wei)零，如(ru)(ru)果(guo)負(fu)載(zai)(zai)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)達(da)到其(qi)短路電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)的(de)(de)話。

　　在大(da)多(duo)數(shu)應(ying)用中，人們期望(wang)從太(tai)陽(yang)能電(dian)(dian)(dian)池(chi)(chi)獲取盡可能多(duo)的(de)(de)(de)功率。因(yin)為(wei)輸(shu)(shu)(shu)出(chu)(chu)(chu)功率是(shi)輸(shu)(shu)(shu)出(chu)(chu)(chu)電(dian)(dian)(dian)壓和電(dian)(dian)(dian)流(liu)的(de)(de)(de)乘積，有必要確(que)定(ding)電(dian)(dian)(dian)池(chi)(chi)的(de)(de)(de)哪一部分的(de)(de)(de)工作區域(yu)產生的(de)(de)(de)輸(shu)(shu)(shu)出(chu)(chu)(chu)電(dian)(dian)(dian)壓和電(dian)(dian)(dian)流(liu)的(de)(de)(de)乘積的(de)(de)(de)數(shu)值最大(da)，這一點被稱為(wei)最大(da)功率點(MPP)。在一種極(ji)端情(qing)況下，輸(shu)(shu)(shu)出(chu)(chu)(chu)電(dian)(dian)(dian)壓為(wei)其最大(da)數(shu)值(VOC)，但是(shi)，輸(shu)(shu)(shu)出(chu)(chu)(chu)電(dian)(dian)(dian)流(liu)為(wei)零(ling);在其它極(ji)端情(qing)況下，輸(shu)(shu)(shu)出(chu)(chu)(chu)電(dian)(dian)(dian)流(liu)位(wei)其最大(da)值(ISC)，但是(shi)，輸(shu)(shu)(shu)出(chu)(chu)(chu)電(dian)(dian)(dian)壓為(wei)零(ling)。在兩種情(qing)況下，輸(shu)(shu)(shu)出(chu)(chu)(chu)電(dian)(dian)(dian)壓和電(dian)(dian)(dian)流(liu)的(de)(de)(de)乘積都是(shi)零(ling)。因(yin)此，MPP必須位(wei)于(yu)兩種極(ji)端情(qing)況之間(jian)的(de)(de)(de)某處。

　　可(ke)以容(rong)易地證(zheng)明：在(zai)(zai)任何應用中(zhong)，MPP實(shi)際(ji)上出(chu)(chu)現在(zai)(zai)太(tai)陽(yang)(yang)能電(dian)池(chi)的(de)輸(shu)出(chu)(chu)特性(xing)(見圖3)下(xia)(xia)半部的(de)某(mou)個位置。實(shi)際(ji)上，問題在(zai)(zai)于太(tai)陽(yang)(yang)能電(dian)池(chi)的(de)MPP的(de)嚴格位置會(hui)根(gen)據光線和(he)環境溫度(du)變(bian)化。因此，所設計的(de)系(xi)統要產生(sheng)最大的(de)太(tai)陽(yang)(yang)能，就必須動態(tai)地調節太(tai)陽(yang)(yang)能電(dian)池(chi)輸(shu)出(chu)(chu)的(de)電(dian)流，以便它(ta)在(zai)(zai)實(shi)際(ji)工作條件下(xia)(xia)位于或(huo)接近(jin)MPP工作。

　　優化充(chong)電器設計以從太(tai)陽能(neng)板獲(huo)得(de)最大的功率。

　　跟(gen)蹤太陽能(neng)板系(xi)統的(de)MPP的(de)途(tu)徑有(you)多種(zhong)，這些(xie)常常相(xiang)當(dang)復(fu)雜，特別是(shi)在(zai)諸(zhu)如衛星通信這樣的(de)重要(yao)任(ren)務系(xi)統中(zhong)(zhong)。然而(er)，在(zai)許多對成(cheng)本(ben)敏感(gan)的(de)應用中(zhong)(zhong)，極其精確的(de)MPP跟(gen)蹤方(fang)案卻(que)是(shi)不必要(yao)的(de)。所有(you)的(de)要(yao)求就(jiu)是(shi)以簡(jian)單(dan)、低(di)成(cheng)本(ben)的(de)解決方(fang)案儲存大(da)約90%的(de)可用能(neng)量。充電(dian)控制(zhi)系(xi)統如何使太陽能(neng)電(dian)池(chi)以接近MPP的(de)方(fang)式工作(zuo)呢?

　　動(dong)態功(gong)率路(lu)徑管理(DPPM)技術可(ke)以滿足跟(gen)蹤MPP所面臨的(de)這(zhe)種挑(tiao)戰。圖(tu)4顯示(shi)了(le)從太陽能(neng)板(ban)獲得最大功(gong)率的(de)鋰(li)離子(zi)(zi)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)(dian)(dian)應用電(dian)(dian)(dian)(dian)(dian)(dian)路(lu)，其中，MOSFET Q2被用于調(diao)節(jie)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)、充電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)壓或系統總線(xian)電(dian)(dian)(dian)(dian)(dian)(dian)壓。太陽能(neng)板(ban)被用做為(wei)單顆(ke)鋰(li)離子(zi)(zi)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)(dian)(dian)(dian)的(de)電(dian)(dian)(dian)(dian)(dian)(dian)源。太陽能(neng)板(ban)由(you)若干(gan)串(chuan)在一(yi)起的(de)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)組成，每(mei)一(yi)串(chuan)具有11個串(chuan)聯(lian)的(de)硅電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)，其行為(wei)就像電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)受到限(xian)制(zhi)的(de)電(dian)(dian)(dian)(dian)(dian)(dian)壓源，其中，電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)限(xian)度由(you)太陽能(neng)板(ban)的(de)大小(xiao)以及照射在上面的(de)光通量來確(que)定(ding)。

　　DPPM監測因電流受限電源引起的系統總線電壓(VOUT)降。連接到系統總線上的電容(CO)開始放電，一旦系統所需要的電流和電池充電器的電(dian)(dian)流大于太陽(yang)能板所(suo)提供的電(dian)(dian)流，就會造成系(xi)統(tong)(tong)的總(zong)(zong)線(xian)電(dian)(dian)壓開(kai)始下降。當系(xi)統(tong)(tong)總(zong)(zong)線(xian)電(dian)(dian)壓跌落到(dao)預(yu)設(she)的DPPM閥值的時候，電(dian)(dian)池充電(dian)(dian)控制系(xi)統(tong)(tong)就把系(xi)統(tong)(tong)總(zong)(zong)線(xian)電(dian)(dian)壓調節到(dao)DPMM閥值。

　　從這個太(tai)陽(yang)(yang)能(neng)板獲得的最大(da)輸(shu)出(chu)(chu)電(dian)(dian)(dian)(dian)壓(ya)(VOC)通常在5.5V到6V之間。因為該電(dian)(dian)(dian)(dian)壓(ya)低于(yu)預設的6V輸(shu)出(chu)(chu)調節電(dian)(dian)(dian)(dian)壓(ya)，MOSFET Q1被完全關閉(bi)。如果系(xi)統和電(dian)(dian)(dian)(dian)池(chi)充(chong)電(dian)(dian)(dian)(dian)器(qi)所(suo)需要的總電(dian)(dian)(dian)(dian)流(liu)(liu)超過太(tai)陽(yang)(yang)能(neng)電(dian)(dian)(dian)(dian)池(chi)的輸(shu)出(chu)(chu)電(dian)(dian)(dian)(dian)流(liu)(liu)―取決于(yu)光線強弱(ruo)―能(neng)力，太(tai)陽(yang)(yang)能(neng)板的輸(shu)出(chu)(chu)電(dian)(dian)(dian)(dian)壓(ya)將下跌，從而(er)使輸(shu)出(chu)(chu)電(dian)(dian)(dian)(dian)壓(ya)(VOUT)下降。當(dang)VOUT下降到VDPPM―也是太(tai)陽(yang)(yang)能(neng)板的輸(shu)出(chu)(chu)電(dian)(dian)(dian)(dian)壓(ya)―的時候，充(chong)電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)就(jiu)下降了。太(tai)陽(yang)(yang)能(neng)板現在將在接近(jin)其(qi)MPP的狀態下工作，如果VDPPM被設置為接近(jin)MPP的話。通過恰當(dang)地把RDPPM編(bian)程到一個電(dian)(dian)(dian)(dian)平(ping)，就(jiu)容許VOUT保持在最小的4.5V，從而(er)實現這一點。這個VDPPM數值就(jiu)被人們所(suo)采用，因為它相當(dang)符合太(tai)陽(yang)(yang)能(neng)板的MPP。

　　假設跨越MOSFET Q1的電壓降為300mV，那么，跨越每一個電池的電壓將等于436mV，從而把太陽能板的功率輸出最大化。如果VOUT大于4.5V，DPPM功能毫無作用―要把太陽能板從其MPP移開。但是，這只能發生在如果系統及電池充電器所需要的(de)(de)(de)功(gong)率(lv)(lv)小于太(tai)陽(yang)能板(ban)能夠提供(gong)(gong)(gong)的(de)(de)(de)功(gong)率(lv)(lv)的(de)(de)(de)情況(kuang)下。在(zai)這(zhe)種情況(kuang)下，降低效(xiao)率(lv)(lv)不(bu)(bu)(bu)是那么重要。如圖3所示，隨著輸出功(gong)率(lv)(lv)逼近MPP，輸出功(gong)率(lv)(lv)曲線變得(de)十分平坦，然后，突然急劇(ju)下降。因此，把(ba)VDPPM設置(zhi)得(de)稍高比設置(zhi)得(de)稍低要好(hao)。這(zhe)樣(yang)做將(jiang)把(ba)不(bu)(bu)(bu)正確(que)的(de)(de)(de)工(gong)作(zuo)點對(dui)輸出功(gong)率(lv)(lv)的(de)(de)(de)影響(xiang)最(zui)小化(hua)。如果(guo)太(tai)陽(yang)能板(ban)提供(gong)(gong)(gong)的(de)(de)(de)功(gong)率(lv)(lv)不(bu)(bu)(bu)足以(yi)為系統供(gong)(gong)(gong)電(dian)(dian)，甚至(zhi)當電(dian)(dian)池充(chong)電(dian)(dian)電(dian)(dian)流已經被降低到(dao)零的(de)(de)(de)時(shi)候(hou)，MOSFET Q2就導通，VOUT下降到(dao)剛好(hao)低于電(dian)(dian)池電(dian)(dian)壓VBAT，并且(qie)電(dian)(dian)池提供(gong)(gong)(gong)太(tai)陽(yang)能板(ban)所不(bu)(bu)(bu)能提供(gong)(gong)(gong)的(de)(de)(de)電(dian)(dian)流。

　　如果充電器工作(zuo)在DPPM狀態，內部安(an)全定時(shi)器就(jiu)自動地延長時(shi)間。因此，當(dang)考(kao)慮諸(zhu)如低(di)光線或無(wu)光條件之類的特殊(shu)工作(zuo)條件時(shi)，電(dian)池(chi)充(chong)電(dian)就(jiu)非常低(di)，或電(dian)池(chi)可(ke)能(neng)(neng)甚至工作(zuo)在放電(dian)模式。要設置覆蓋所有應用的合適的充(chong)電(dian)安(an)全定時(shi)器幾(ji)乎是(shi)不(bu)可(ke)能(neng)(neng)的。否則，就(jiu)可(ke)能(neng)(neng)產生一個(ge)(ge)虛假的安(an)全定時(shi)器錯誤。因此，解決這個(ge)(ge)問題的一個(ge)(ge)選項就(jiu)是(shi)禁止(zhi)安(an)全定時(shi)器工作(zuo)。

　　太陽(yang)(yang)能(neng)(neng)板所(suo)提供的(de)電(dian)(dian)源被認為是(shi)(shi)“電(dian)(dian)流受限”的(de)電(dian)(dian)壓源。太陽(yang)(yang)能(neng)(neng)板對鋰(li)電(dian)(dian)池(chi)的(de)最大充電(dian)(dian)功率(lv)的(de)實現途徑是(shi)(shi)：當系(xi)統(tong)和電(dian)(dian)池(chi)充電(dian)(dian)所(suo)需(xu)要的(de)總電(dian)(dian)流超過太陽(yang)(yang)能(neng)(neng)板的(de)輸出電(dian)(dian)流能(neng)(neng)力(li)時，要通過降低充電(dian)(dian)電(dian)(dian)流來(lai)調節(jie)MPP附(fu)近的(de)系(xi)統(tong)總線電(dian)(dian)壓。對于設計一個可能(neng)(neng)的(de)太陽(yang)(yang)能(neng)(neng)板供電(dian)(dian)的(de)系(xi)統(tong)來(lai)說(shuo)，關(guan)鍵的(de)元素就是(shi)(shi)系(xi)統(tong)功率(lv)和電(dian)(dian)池(chi)充電(dian)(dian)功率(lv)控制架構。