太陽能(neng)是為便攜(xie)式設備供電(dian)的有吸引力的能(neng)源。一段時間以來，它(ta)一直(zhi)被廣(guang)泛地用(yong)(yong)于諸如(ru)計算器和航天飛機這(zhe)樣的應用(yong)(yong)。最近，人們正(zheng)考慮把太陽能(neng)用(yong)(yong)于包括移動(dong)電(dian)話充電(dian)器這(zhe)樣的范圍(wei)更(geng)寬廣(guang)的消費電(dian)子(zi)應用(yong)(yong)。

　　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(yi)便(bian)從太陽(yang)能電(dian)(dian)池(chi)中獲(huo)取最(zui)(zui)多的功率并有效地對鋰(li)電(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)呢?首先，我們將(jiang)討(tao)論太陽(yang)能電(dian)(dian)池(chi)的工(gong)作原(yuan)理和電(dian)(dian)氣輸出特性(xing)(xing);然后(hou)，我們將(jiang)討(tao)論電(dian)(dian)池(chi)充(chong)(chong)電(dian)(dian)系(xi)統要求(qiu)以(yi)及匹配太陽(yang)能電(dian)(dian)池(chi)特性(xing)(xing)的系(xi)統解決方案，以(yi)便(bian)從太陽(yang)能電(dian)(dian)池(chi)獲(huo)取最(zui)(zui)大的功率。

　　太陽(yang)能I-V特性(xing)

　　一(yi)般地說，太陽(yang)能(neng)(neng)電(dian)池由p-n結構成，其中的(de)光能(neng)(neng)(光子(zi))引起電(dian)子(zi)和空穴的(de)重新(xin)組合，產生電(dian)流。因為p-n結的(de)特性(xing)(xing)類似于(yu)(yu)二極管(guan)的(de)特性(xing)(xing)，如圖1所示的(de)電(dian)路通(tong)常被用于(yu)(yu)簡化太陽(yang)能(neng)(neng)電(dian)池的(de)特性(xing)(xing)。

　　電(dian)(dian)(dian)(dian)流源(yuan)IPH產生(sheng)的(de)電(dian)(dian)(dian)(dian)流正(zheng)比于(yu)落在(zai)太陽能電(dian)(dian)(dian)(dian)池上的(de)光量。在(zai)沒(mei)有負載連(lian)接的(de)時候(hou)，幾乎所有產生(sheng)的(de)電(dian)(dian)(dian)(dian)流都流過二極(ji)管D，其(qi)正(zheng)向電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)決定太陽能電(dian)(dian)(dian)(dian)池的(de)開路電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)(VOC)。該電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)的(de)變(bian)化(hua)嚴格地取決于(yu)每(mei)一種類型的(de)太陽能電(dian)(dian)(dian)(dian)池。但是，對于(yu)大多數硅電(dian)(dian)(dian)(dian)池，其(qi)0.5V到0.8V之間(jian)的(de)電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)范圍恰好就是p-n結二極(ji)管的(de)正(zheng)向電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)。

　　并(bing)聯電(dian)(dian)(dian)阻(RP)代(dai)表實際(ji)太陽能(neng)電(dian)(dian)(dian)池中出(chu)現的微小泄漏電(dian)(dian)(dian)流(liu)，Rs代(dai)表連(lian)接損(sun)耗。隨著負載(zai)電(dian)(dian)(dian)流(liu)增加，由太陽能(neng)電(dian)(dian)(dian)池所產(chan)生的大部分電(dian)(dian)(dian)流(liu)被分流(liu)到二極(ji)管并(bing)進入負載(zai)。對于大多負載(zai)電(dian)(dian)(dian)流(liu)的數值，這只對輸出(chu)電(dian)(dian)(dian)壓(ya)有很小的影響。

　　圖2所示為太陽能電(dian)(dian)(dian)(dian)池(chi)(chi)的(de)(de)(de)輸(shu)(shu)(shu)出(chu)特性(xing)，由于(yu)二極管的(de)(de)(de)I-V特性(xing)存(cun)在(zai)微(wei)小的(de)(de)(de)變(bian)化，串(chuan)聯(lian)電(dian)(dian)(dian)(dian)阻(Rs)上的(de)(de)(de)電(dian)(dian)(dian)(dian)壓降也存(cun)在(zai)微(wei)小的(de)(de)(de)變(bian)化，但是(shi)，輸(shu)(shu)(shu)出(chu)電(dian)(dian)(dian)(dian)壓保持很(hen)大(da)的(de)(de)(de)恒定。然而，在(zai)一些點通過(guo)內部二極管的(de)(de)(de)電(dian)(dian)(dian)(dian)流(liu)(liu)是(shi)如此之(zhi)小，以至于(yu)它變(bian)得偏置不夠，并(bing)且，隨著負載電(dian)(dian)(dian)(dian)流(liu)(liu)的(de)(de)(de)增加，跨越(yue)它的(de)(de)(de)電(dian)(dian)(dian)(dian)壓快速減少(shao)。最后，如果(guo)所有(you)產生的(de)(de)(de)電(dian)(dian)(dian)(dian)流(liu)(liu)流(liu)(liu)過(guo)負載并(bing)且不流(liu)(liu)過(guo)二極管的(de)(de)(de)話，輸(shu)(shu)(shu)出(chu)電(dian)(dian)(dian)(dian)壓就為零(ling)。該(gai)電(dian)(dian)(dian)(dian)流(liu)(liu)被稱為太陽能電(dian)(dian)(dian)(dian)池(chi)(chi)的(de)(de)(de)短路電(dian)(dian)(dian)(dian)流(liu)(liu)(ISC)，它與VOC一道(dao)是(shi)定義工作(zuo)性(xing)能的(de)(de)(de)主(zhu)要參數(shu)之(zhi)一。因此，太陽能電(dian)(dian)(dian)(dian)池(chi)(chi)被認為是(shi)“電(dian)(dian)(dian)(dian)流(liu)(liu)受限”的(de)(de)(de)電(dian)(dian)(dian)(dian)源。當輸(shu)(shu)(shu)出(chu)電(dian)(dian)(dian)(dian)流(liu)(liu)增加的(de)(de)(de)時候，其(qi)輸(shu)(shu)(shu)出(chu)電(dian)(dian)(dian)(dian)壓降低(di)，直到(dao)最終減少(shao)為零(ling)，如果(guo)負載電(dian)(dian)(dian)(dian)流(liu)(liu)達(da)到(dao)其(qi)短路電(dian)(dian)(dian)(dian)流(liu)(liu)的(de)(de)(de)話。

　　在(zai)大多數(shu)應用(yong)中，人們期望從太陽能電(dian)(dian)(dian)池(chi)獲取盡可能多的(de)(de)功率(lv)。因為輸(shu)出(chu)(chu)功率(lv)是(shi)輸(shu)出(chu)(chu)電(dian)(dian)(dian)壓(ya)和(he)(he)電(dian)(dian)(dian)流的(de)(de)乘積，有必(bi)要確定電(dian)(dian)(dian)池(chi)的(de)(de)哪一(yi)(yi)部(bu)分(fen)的(de)(de)工作區域產生的(de)(de)輸(shu)出(chu)(chu)電(dian)(dian)(dian)壓(ya)和(he)(he)電(dian)(dian)(dian)流的(de)(de)乘積的(de)(de)數(shu)值(zhi)(zhi)最(zui)(zui)大，這一(yi)(yi)點被稱為最(zui)(zui)大功率(lv)點(MPP)。在(zai)一(yi)(yi)種(zhong)極(ji)端情(qing)況(kuang)(kuang)下，輸(shu)出(chu)(chu)電(dian)(dian)(dian)壓(ya)為其(qi)最(zui)(zui)大數(shu)值(zhi)(zhi)(VOC)，但是(shi)，輸(shu)出(chu)(chu)電(dian)(dian)(dian)流為零;在(zai)其(qi)它極(ji)端情(qing)況(kuang)(kuang)下，輸(shu)出(chu)(chu)電(dian)(dian)(dian)流位(wei)其(qi)最(zui)(zui)大值(zhi)(zhi)(ISC)，但是(shi)，輸(shu)出(chu)(chu)電(dian)(dian)(dian)壓(ya)為零。在(zai)兩種(zhong)情(qing)況(kuang)(kuang)下，輸(shu)出(chu)(chu)電(dian)(dian)(dian)壓(ya)和(he)(he)電(dian)(dian)(dian)流的(de)(de)乘積都是(shi)零。因此，MPP必(bi)須位(wei)于兩種(zhong)極(ji)端情(qing)況(kuang)(kuang)之間的(de)(de)某處。

　　可以容易地證明：在(zai)任何應用中，MPP實際(ji)上出現(xian)在(zai)太陽(yang)能電池的輸出特性(見圖(tu)3)下半(ban)部的某個位(wei)(wei)置。實際(ji)上，問題在(zai)于太陽(yang)能電池的MPP的嚴格位(wei)(wei)置會根據光(guang)線和環境溫度變(bian)化。因此，所設計的系統要產(chan)生最大(da)的太陽(yang)能，就必須動態地調節太陽(yang)能電池輸出的電流，以便(bian)它在(zai)實際(ji)工作條(tiao)件下位(wei)(wei)于或(huo)接近(jin)MPP工作。

　　優化充(chong)電器設計以從太陽能板獲得(de)最大(da)的功率。

　　跟蹤太陽能(neng)板系(xi)(xi)統(tong)的(de)(de)MPP的(de)(de)途徑有多種，這些常常相當復雜(za)，特別是在(zai)(zai)諸如衛星通信這樣的(de)(de)重要任務系(xi)(xi)統(tong)中(zhong)(zhong)。然而，在(zai)(zai)許多對成本敏感的(de)(de)應用中(zhong)(zhong)，極(ji)其精(jing)確的(de)(de)MPP跟蹤方(fang)案卻(que)是不必要的(de)(de)。所(suo)有的(de)(de)要求就(jiu)是以(yi)簡單、低成本的(de)(de)解決方(fang)案儲存(cun)大(da)約90%的(de)(de)可用能(neng)量。充電(dian)控制系(xi)(xi)統(tong)如何使太陽能(neng)電(dian)池以(yi)接近(jin)MPP的(de)(de)方(fang)式工(gong)作呢?

　　動態功率路(lu)徑管理(DPPM)技術可以滿(man)足(zu)跟蹤MPP所面臨的(de)(de)(de)這種挑戰(zhan)。圖4顯(xian)示了從(cong)太陽(yang)(yang)能板獲得最大功率的(de)(de)(de)鋰(li)離子電池(chi)(chi)充(chong)電應用(yong)電路(lu)，其(qi)中，MOSFET Q2被(bei)用(yong)于調(diao)節電池(chi)(chi)充(chong)電電流、充(chong)電電壓(ya)或系統總線電壓(ya)。太陽(yang)(yang)能板被(bei)用(yong)做為(wei)單顆鋰(li)離子電池(chi)(chi)充(chong)電的(de)(de)(de)電源。太陽(yang)(yang)能板由若干串(chuan)在一起(qi)的(de)(de)(de)電池(chi)(chi)組成，每一串(chuan)具有(you)11個串(chuan)聯的(de)(de)(de)硅電池(chi)(chi)，其(qi)行(xing)為(wei)就像電流受到(dao)限制(zhi)的(de)(de)(de)電壓(ya)源，其(qi)中，電流限度由太陽(yang)(yang)能板的(de)(de)(de)大小(xiao)以及(ji)照射在上面的(de)(de)(de)光通量來(lai)確定。

　　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(de)電流(liu)大于太陽(yang)能板所(suo)提供的(de)電流(liu)，就會造成(cheng)系(xi)統(tong)的(de)總線電壓開始下降。當系(xi)統(tong)總線電壓跌(die)落到(dao)預設的(de)DPPM閥(fa)值(zhi)的(de)時(shi)候，電池充電控制(zhi)系(xi)統(tong)就把系(xi)統(tong)總線電壓調節到(dao)DPMM閥(fa)值(zhi)。

　　從這(zhe)個太陽(yang)能(neng)(neng)板(ban)獲(huo)得(de)的(de)(de)最(zui)大輸(shu)(shu)出電(dian)(dian)壓(ya)(VOC)通常在5.5V到(dao)(dao)6V之間。因為(wei)該電(dian)(dian)壓(ya)低(di)于預設的(de)(de)6V輸(shu)(shu)出調節(jie)電(dian)(dian)壓(ya)，MOSFET Q1被完全關閉。如(ru)果(guo)系統和電(dian)(dian)池充電(dian)(dian)器所需要的(de)(de)總電(dian)(dian)流(liu)(liu)超過太陽(yang)能(neng)(neng)電(dian)(dian)池的(de)(de)輸(shu)(shu)出電(dian)(dian)流(liu)(liu)―取決于光線強弱―能(neng)(neng)力，太陽(yang)能(neng)(neng)板(ban)的(de)(de)輸(shu)(shu)出電(dian)(dian)壓(ya)將下跌，從而(er)使輸(shu)(shu)出電(dian)(dian)壓(ya)(VOUT)下降。當(dang)VOUT下降到(dao)(dao)VDPPM―也是太陽(yang)能(neng)(neng)板(ban)的(de)(de)輸(shu)(shu)出電(dian)(dian)壓(ya)―的(de)(de)時候，充電(dian)(dian)電(dian)(dian)流(liu)(liu)就(jiu)下降了。太陽(yang)能(neng)(neng)板(ban)現在將在接近(jin)(jin)其MPP的(de)(de)狀態下工作，如(ru)果(guo)VDPPM被設置(zhi)為(wei)接近(jin)(jin)MPP的(de)(de)話(hua)。通過恰當(dang)地把RDPPM編程(cheng)到(dao)(dao)一個電(dian)(dian)平，就(jiu)容(rong)許VOUT保持在最(zui)小(xiao)的(de)(de)4.5V，從而(er)實(shi)現這(zhe)一點。這(zhe)個VDPPM數值就(jiu)被人們所采用(yong)，因為(wei)它相當(dang)符合太陽(yang)能(neng)(neng)板(ban)的(de)(de)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(suo)需要(yao)(yao)的(de)功(gong)(gong)(gong)(gong)率(lv)小于(yu)太(tai)(tai)陽能板(ban)(ban)能夠(gou)提(ti)(ti)供(gong)(gong)的(de)功(gong)(gong)(gong)(gong)率(lv)的(de)情況下(xia)。在這(zhe)種情況下(xia)，降(jiang)低(di)(di)效率(lv)不(bu)(bu)(bu)是那么(me)重要(yao)(yao)。如圖3所(suo)示(shi)，隨(sui)著輸出(chu)功(gong)(gong)(gong)(gong)率(lv)逼近MPP，輸出(chu)功(gong)(gong)(gong)(gong)率(lv)曲線變得十分(fen)平坦，然(ran)(ran)后，突然(ran)(ran)急劇下(xia)降(jiang)。因此(ci)，把VDPPM設置得稍高比設置得稍低(di)(di)要(yao)(yao)好(hao)。這(zhe)樣(yang)做將(jiang)把不(bu)(bu)(bu)正確的(de)工作(zuo)點對(dui)輸出(chu)功(gong)(gong)(gong)(gong)率(lv)的(de)影響最小化(hua)。如果太(tai)(tai)陽能板(ban)(ban)提(ti)(ti)供(gong)(gong)的(de)功(gong)(gong)(gong)(gong)率(lv)不(bu)(bu)(bu)足以為系統供(gong)(gong)電(dian)，甚至(zhi)當電(dian)池(chi)充電(dian)電(dian)流已經(jing)被降(jiang)低(di)(di)到(dao)零的(de)時候，MOSFET Q2就(jiu)導通(tong)，VOUT下(xia)降(jiang)到(dao)剛好(hao)低(di)(di)于(yu)電(dian)池(chi)電(dian)壓VBAT，并且(qie)電(dian)池(chi)提(ti)(ti)供(gong)(gong)太(tai)(tai)陽能板(ban)(ban)所(suo)不(bu)(bu)(bu)能提(ti)(ti)供(gong)(gong)的(de)電(dian)流。

　　如果充電器工作(zuo)在DPPM狀態，內部安全(quan)定(ding)(ding)時(shi)(shi)(shi)器就自(zi)動地延長(chang)時(shi)(shi)(shi)間。因此，當考(kao)慮諸如低光線或(huo)無光條(tiao)件(jian)之(zhi)類的特殊工作(zuo)條(tiao)件(jian)時(shi)(shi)(shi)，電(dian)(dian)(dian)池(chi)充電(dian)(dian)(dian)就非常低，或(huo)電(dian)(dian)(dian)池(chi)可(ke)能甚至工作(zuo)在放電(dian)(dian)(dian)模式。要設(she)置覆蓋(gai)所有應用(yong)的合適(shi)的充電(dian)(dian)(dian)安全(quan)定(ding)(ding)時(shi)(shi)(shi)器幾乎是(shi)(shi)不可(ke)能的。否(fou)則，就可(ke)能產(chan)生一個(ge)虛假的安全(quan)定(ding)(ding)時(shi)(shi)(shi)器錯(cuo)誤。因此，解決這個(ge)問題的一個(ge)選項就是(shi)(shi)禁止安全(quan)定(ding)(ding)時(shi)(shi)(shi)器工作(zuo)。

　　太(tai)(tai)陽(yang)能(neng)(neng)板(ban)(ban)(ban)所提供的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)源被認為是(shi)“電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)受限”的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)源。太(tai)(tai)陽(yang)能(neng)(neng)板(ban)(ban)(ban)對(dui)鋰電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)的(de)最大(da)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)功率(lv)的(de)實現途(tu)徑是(shi)：當系統和電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)所需要的(de)總電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)超過太(tai)(tai)陽(yang)能(neng)(neng)板(ban)(ban)(ban)的(de)輸出電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)能(neng)(neng)力時(shi)，要通(tong)過降低充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)來調(diao)節MPP附近的(de)系統總線(xian)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)。對(dui)于設(she)計(ji)一個可(ke)能(neng)(neng)的(de)太(tai)(tai)陽(yang)能(neng)(neng)板(ban)(ban)(ban)供電(dian)(dian)(dian)(dian)(dian)(dian)(dian)的(de)系統來說(shuo)，關鍵的(de)元素就(jiu)是(shi)系統功率(lv)和電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)功率(lv)控制架構。