太(tai)陽能是為(wei)便(bian)攜(xie)式設備(bei)供電(dian)的(de)有吸(xi)引力(li)的(de)能源(yuan)。一段時間以來(lai)，它一直被廣泛地用于(yu)諸如計算器和航天(tian)飛機這樣的(de)應(ying)用。最近，人(ren)們正考慮把(ba)太(tai)陽能用于(yu)包括移動電(dian)話充電(dian)器這樣的(de)范圍更寬廣的(de)消費電(dian)子(zi)應(ying)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從太(tai)陽能電(dian)(dian)(dian)池(chi)(chi)(chi)中獲取最多的功率并有效(xiao)地對(dui)鋰(li)電(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)電(dian)(dian)(dian)呢?首(shou)先(xian)，我(wo)們(men)將討(tao)論太(tai)陽能電(dian)(dian)(dian)池(chi)(chi)(chi)的工作原理和電(dian)(dian)(dian)氣輸出特性(xing);然后(hou)，我(wo)們(men)將討(tao)論電(dian)(dian)(dian)池(chi)(chi)(chi)充(chong)電(dian)(dian)(dian)系統要求以及匹(pi)配(pei)太(tai)陽能電(dian)(dian)(dian)池(chi)(chi)(chi)特性(xing)的系統解決方(fang)案，以便從太(tai)陽能電(dian)(dian)(dian)池(chi)(chi)(chi)獲取最大的功率。

　　太陽(yang)能I-V特性

　　一(yi)般地(di)說，太(tai)陽能(neng)電(dian)(dian)池由p-n結(jie)構成，其中的(de)(de)光能(neng)(光子)引(yin)起(qi)電(dian)(dian)子和空(kong)穴的(de)(de)重新組合，產生電(dian)(dian)流。因為p-n結(jie)的(de)(de)特(te)(te)性(xing)類似(si)于二極管的(de)(de)特(te)(te)性(xing)，如圖1所示的(de)(de)電(dian)(dian)路通常被用于簡化太(tai)陽能(neng)電(dian)(dian)池的(de)(de)特(te)(te)性(xing)。

　　電(dian)(dian)(dian)(dian)流源(yuan)IPH產生(sheng)的(de)(de)(de)電(dian)(dian)(dian)(dian)流正比(bi)于(yu)落在太(tai)陽(yang)能(neng)電(dian)(dian)(dian)(dian)池(chi)上(shang)的(de)(de)(de)光(guang)量。在沒(mei)有(you)負載(zai)連接(jie)的(de)(de)(de)時候，幾乎所有(you)產生(sheng)的(de)(de)(de)電(dian)(dian)(dian)(dian)流都(dou)流過二(er)極(ji)管D，其(qi)正向(xiang)電(dian)(dian)(dian)(dian)壓決定太(tai)陽(yang)能(neng)電(dian)(dian)(dian)(dian)池(chi)的(de)(de)(de)開路電(dian)(dian)(dian)(dian)壓(VOC)。該電(dian)(dian)(dian)(dian)壓的(de)(de)(de)變(bian)化(hua)嚴(yan)格(ge)地取決于(yu)每一種類型的(de)(de)(de)太(tai)陽(yang)能(neng)電(dian)(dian)(dian)(dian)池(chi)。但(dan)是(shi)，對于(yu)大多(duo)數(shu)硅(gui)電(dian)(dian)(dian)(dian)池(chi)，其(qi)0.5V到0.8V之間的(de)(de)(de)電(dian)(dian)(dian)(dian)壓范圍恰好就(jiu)是(shi)p-n結二(er)極(ji)管的(de)(de)(de)正向(xiang)電(dian)(dian)(dian)(dian)壓。

　　并聯電(dian)(dian)阻(RP)代(dai)(dai)表(biao)實際(ji)太(tai)陽能(neng)(neng)電(dian)(dian)池中出(chu)現的(de)(de)微小泄漏(lou)電(dian)(dian)流，Rs代(dai)(dai)表(biao)連接(jie)損耗。隨(sui)著負載電(dian)(dian)流增加，由太(tai)陽能(neng)(neng)電(dian)(dian)池所產(chan)生的(de)(de)大部分電(dian)(dian)流被(bei)分流到二極管并進入負載。對(dui)(dui)于大多(duo)負載電(dian)(dian)流的(de)(de)數值，這(zhe)只對(dui)(dui)輸(shu)出(chu)電(dian)(dian)壓(ya)有很小的(de)(de)影響。

　　圖2所(suo)示為(wei)太陽(yang)能電(dian)(dian)(dian)(dian)(dian)(dian)池的輸(shu)出(chu)(chu)特(te)性(xing)，由于二極(ji)管的I-V特(te)性(xing)存(cun)在(zai)微小的變化(hua)，串聯電(dian)(dian)(dian)(dian)(dian)(dian)阻(Rs)上的電(dian)(dian)(dian)(dian)(dian)(dian)壓降也(ye)存(cun)在(zai)微小的變化(hua)，但是(shi)(shi)，輸(shu)出(chu)(chu)電(dian)(dian)(dian)(dian)(dian)(dian)壓保持很(hen)大的恒定。然而，在(zai)一些點通過內部二極(ji)管的電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)是(shi)(shi)如此之(zhi)小，以至于它(ta)(ta)變得(de)偏置不(bu)夠，并(bing)且，隨著負(fu)載(zai)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)的增加，跨(kua)越它(ta)(ta)的電(dian)(dian)(dian)(dian)(dian)(dian)壓快速(su)減(jian)少(shao)。最(zui)(zui)后，如果所(suo)有產生的電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)流(liu)(liu)(liu)(liu)過負(fu)載(zai)并(bing)且不(bu)流(liu)(liu)(liu)(liu)過二極(ji)管的話，輸(shu)出(chu)(chu)電(dian)(dian)(dian)(dian)(dian)(dian)壓就為(wei)零。該電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)被(bei)(bei)稱(cheng)為(wei)太陽(yang)能電(dian)(dian)(dian)(dian)(dian)(dian)池的短(duan)路電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)(ISC)，它(ta)(ta)與VOC一道是(shi)(shi)定義工作性(xing)能的主要參數之(zhi)一。因(yin)此，太陽(yang)能電(dian)(dian)(dian)(dian)(dian)(dian)池被(bei)(bei)認為(wei)是(shi)(shi)“電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)受限”的電(dian)(dian)(dian)(dian)(dian)(dian)源。當輸(shu)出(chu)(chu)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)增加的時候(hou)，其輸(shu)出(chu)(chu)電(dian)(dian)(dian)(dian)(dian)(dian)壓降低，直到最(zui)(zui)終減(jian)少(shao)為(wei)零，如果負(fu)載(zai)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)達到其短(duan)路電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)的話。

　　在(zai)大(da)(da)多數應用(yong)中(zhong)，人們(men)期(qi)望從太陽能電(dian)(dian)池獲取盡可能多的(de)(de)功(gong)率。因(yin)為(wei)輸(shu)出(chu)功(gong)率是(shi)輸(shu)出(chu)電(dian)(dian)壓(ya)和(he)電(dian)(dian)流(liu)的(de)(de)乘(cheng)積(ji)，有必要(yao)確(que)定電(dian)(dian)池的(de)(de)哪一(yi)部分的(de)(de)工作區(qu)域產(chan)生的(de)(de)輸(shu)出(chu)電(dian)(dian)壓(ya)和(he)電(dian)(dian)流(liu)的(de)(de)乘(cheng)積(ji)的(de)(de)數值最大(da)(da)，這一(yi)點被稱為(wei)最大(da)(da)功(gong)率點(MPP)。在(zai)一(yi)種極端情(qing)(qing)況(kuang)(kuang)下(xia)，輸(shu)出(chu)電(dian)(dian)壓(ya)為(wei)其(qi)最大(da)(da)數值(VOC)，但(dan)是(shi)，輸(shu)出(chu)電(dian)(dian)流(liu)為(wei)零;在(zai)其(qi)它極端情(qing)(qing)況(kuang)(kuang)下(xia)，輸(shu)出(chu)電(dian)(dian)流(liu)位其(qi)最大(da)(da)值(ISC)，但(dan)是(shi)，輸(shu)出(chu)電(dian)(dian)壓(ya)為(wei)零。在(zai)兩種情(qing)(qing)況(kuang)(kuang)下(xia)，輸(shu)出(chu)電(dian)(dian)壓(ya)和(he)電(dian)(dian)流(liu)的(de)(de)乘(cheng)積(ji)都是(shi)零。因(yin)此，MPP必須位于(yu)兩種極端情(qing)(qing)況(kuang)(kuang)之間的(de)(de)某處。

　　可以(yi)容易地證明：在(zai)(zai)任(ren)何應用中，MPP實(shi)際上出現在(zai)(zai)太陽(yang)(yang)能(neng)(neng)電池的(de)輸(shu)出特性(見(jian)圖3)下半(ban)部的(de)某個位置。實(shi)際上，問題在(zai)(zai)于(yu)太陽(yang)(yang)能(neng)(neng)電池的(de)MPP的(de)嚴(yan)格位置會根據光線和(he)環(huan)境溫度變化。因此，所設(she)計的(de)系統要產生(sheng)最(zui)大的(de)太陽(yang)(yang)能(neng)(neng)，就必(bi)須動態(tai)地調(diao)節太陽(yang)(yang)能(neng)(neng)電池輸(shu)出的(de)電流(liu)，以(yi)便它在(zai)(zai)實(shi)際工作條(tiao)件下位于(yu)或接近MPP工作。

　　優化充電器設計以從太陽能板獲得最大的功率。

　　跟(gen)蹤太陽能(neng)板系(xi)統的(de)MPP的(de)途徑有多種，這(zhe)些常常相當復雜，特別是在(zai)諸如(ru)衛星通信這(zhe)樣(yang)的(de)重要(yao)任務系(xi)統中。然(ran)而(er)，在(zai)許多對成本敏感的(de)應用中，極其精確的(de)MPP跟(gen)蹤方案卻是不必要(yao)的(de)。所(suo)有的(de)要(yao)求就(jiu)是以簡單、低成本的(de)解(jie)決方案儲存大約90%的(de)可用能(neng)量。充電(dian)控制系(xi)統如(ru)何(he)使(shi)太陽能(neng)電(dian)池以接近MPP的(de)方式工作(zuo)呢?

　　動態功率(lv)(lv)路徑管理(DPPM)技術(shu)可以滿足跟蹤MPP所面臨(lin)的(de)(de)這(zhe)種挑戰。圖(tu)4顯示了從(cong)太陽能板獲得(de)最大(da)功率(lv)(lv)的(de)(de)鋰離子電(dian)(dian)(dian)(dian)池充電(dian)(dian)(dian)(dian)應用電(dian)(dian)(dian)(dian)路，其中(zhong)，MOSFET Q2被(bei)用于(yu)調節電(dian)(dian)(dian)(dian)池充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流、充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)壓或系(xi)統總線電(dian)(dian)(dian)(dian)壓。太陽能板被(bei)用做為(wei)單顆鋰離子電(dian)(dian)(dian)(dian)池充電(dian)(dian)(dian)(dian)的(de)(de)電(dian)(dian)(dian)(dian)源。太陽能板由(you)若干(gan)串在一(yi)起的(de)(de)電(dian)(dian)(dian)(dian)池組成，每一(yi)串具(ju)有(you)11個串聯的(de)(de)硅電(dian)(dian)(dian)(dian)池，其行為(wei)就像電(dian)(dian)(dian)(dian)流受到限制的(de)(de)電(dian)(dian)(dian)(dian)壓源，其中(zhong)，電(dian)(dian)(dian)(dian)流限度由(you)太陽能板的(de)(de)大(da)小以及(ji)照射在上面的(de)(de)光(guang)通量來確(que)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)(de)電(dian)流大于太陽能板(ban)所(suo)提供(gong)的(de)(de)電(dian)流，就(jiu)會造成系(xi)(xi)統的(de)(de)總(zong)線(xian)電(dian)壓(ya)開(kai)始下(xia)降。當系(xi)(xi)統總(zong)線(xian)電(dian)壓(ya)跌落到(dao)預設的(de)(de)DPPM閥值(zhi)(zhi)的(de)(de)時候，電(dian)池充(chong)電(dian)控制系(xi)(xi)統就(jiu)把系(xi)(xi)統總(zong)線(xian)電(dian)壓(ya)調節到(dao)DPMM閥值(zhi)(zhi)。

　　從這(zhe)個(ge)(ge)太(tai)(tai)(tai)(tai)陽(yang)(yang)能(neng)板(ban)(ban)獲(huo)得的最(zui)大輸(shu)出(chu)電(dian)(dian)壓(ya)(VOC)通常在(zai)5.5V到(dao)6V之間。因(yin)為該電(dian)(dian)壓(ya)低(di)于(yu)預設的6V輸(shu)出(chu)調節電(dian)(dian)壓(ya)，MOSFET Q1被(bei)完全關閉。如果系統和電(dian)(dian)池(chi)充電(dian)(dian)器(qi)所(suo)需要的總電(dian)(dian)流超過(guo)太(tai)(tai)(tai)(tai)陽(yang)(yang)能(neng)電(dian)(dian)池(chi)的輸(shu)出(chu)電(dian)(dian)流―取決(jue)于(yu)光線(xian)強弱―能(neng)力，太(tai)(tai)(tai)(tai)陽(yang)(yang)能(neng)板(ban)(ban)的輸(shu)出(chu)電(dian)(dian)壓(ya)將下跌，從而使輸(shu)出(chu)電(dian)(dian)壓(ya)(VOUT)下降(jiang)。當VOUT下降(jiang)到(dao)VDPPM―也是(shi)太(tai)(tai)(tai)(tai)陽(yang)(yang)能(neng)板(ban)(ban)的輸(shu)出(chu)電(dian)(dian)壓(ya)―的時候，充電(dian)(dian)電(dian)(dian)流就(jiu)下降(jiang)了(le)。太(tai)(tai)(tai)(tai)陽(yang)(yang)能(neng)板(ban)(ban)現在(zai)將在(zai)接(jie)近(jin)其MPP的狀態下工作，如果VDPPM被(bei)設置(zhi)為接(jie)近(jin)MPP的話。通過(guo)恰(qia)當地把RDPPM編(bian)程到(dao)一個(ge)(ge)電(dian)(dian)平，就(jiu)容許VOUT保持在(zai)最(zui)小(xiao)的4.5V，從而實現這(zhe)一點(dian)。這(zhe)個(ge)(ge)VDPPM數值就(jiu)被(bei)人們所(suo)采用，因(yin)為它(ta)相當符合(he)太(tai)(tai)(tai)(tai)陽(yang)(yang)能(neng)板(ban)(ban)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(suo)需要的(de)(de)(de)功率(lv)(lv)小于(yu)太(tai)(tai)陽(yang)能(neng)板能(neng)夠提供的(de)(de)(de)功率(lv)(lv)的(de)(de)(de)情(qing)況下。在這種情(qing)況下，降低(di)(di)效率(lv)(lv)不是那么(me)重要。如圖3所(suo)示，隨著輸出(chu)(chu)功率(lv)(lv)逼近MPP，輸出(chu)(chu)功率(lv)(lv)曲線變得(de)十(shi)分平坦，然后，突然急劇(ju)下降。因此(ci)，把VDPPM設置得(de)稍(shao)(shao)高比設置得(de)稍(shao)(shao)低(di)(di)要好。這樣做將把不正(zheng)確(que)的(de)(de)(de)工作點(dian)對(dui)輸出(chu)(chu)功率(lv)(lv)的(de)(de)(de)影響最小化。如果太(tai)(tai)陽(yang)能(neng)板提供的(de)(de)(de)功率(lv)(lv)不足(zu)以為系統供電(dian)，甚至當電(dian)池(chi)(chi)(chi)充電(dian)電(dian)流(liu)已(yi)經被降低(di)(di)到零的(de)(de)(de)時候，MOSFET Q2就導通，VOUT下降到剛(gang)好低(di)(di)于(yu)電(dian)池(chi)(chi)(chi)電(dian)壓VBAT，并且電(dian)池(chi)(chi)(chi)提供太(tai)(tai)陽(yang)能(neng)板所(suo)不能(neng)提供的(de)(de)(de)電(dian)流(liu)。

　　如果充電器工作在(zai)DPPM狀態(tai)，內部安全(quan)(quan)(quan)定時器就自(zi)動地延長時間。因(yin)此，當(dang)考(kao)慮(lv)諸如低光線或無(wu)光條(tiao)件之(zhi)類的(de)(de)特殊工作條(tiao)件時，電池(chi)充(chong)電就非常低，或電池(chi)可能(neng)(neng)甚至工作在(zai)放電模式。要設置覆蓋所(suo)有應(ying)用的(de)(de)合適的(de)(de)充(chong)電安全(quan)(quan)(quan)定時器幾乎是(shi)(shi)不可能(neng)(neng)的(de)(de)。否(fou)則(ze)，就可能(neng)(neng)產生一個虛假的(de)(de)安全(quan)(quan)(quan)定時器錯(cuo)誤。因(yin)此，解決這個問題的(de)(de)一個選項(xiang)就是(shi)(shi)禁止安全(quan)(quan)(quan)定時器工作。

　　太陽(yang)(yang)能(neng)板所提供(gong)的(de)(de)電(dian)(dian)(dian)源被認為(wei)是(shi)“電(dian)(dian)(dian)流(liu)受限”的(de)(de)電(dian)(dian)(dian)壓源。太陽(yang)(yang)能(neng)板對鋰(li)電(dian)(dian)(dian)池的(de)(de)最大(da)充電(dian)(dian)(dian)功率(lv)的(de)(de)實現途徑(jing)是(shi)：當系(xi)統(tong)和電(dian)(dian)(dian)池充電(dian)(dian)(dian)所需要的(de)(de)總(zong)電(dian)(dian)(dian)流(liu)超過太陽(yang)(yang)能(neng)板的(de)(de)輸出電(dian)(dian)(dian)流(liu)能(neng)力時，要通過降低充電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)來調節MPP附近的(de)(de)系(xi)統(tong)總(zong)線電(dian)(dian)(dian)壓。對于(yu)設計一(yi)個可能(neng)的(de)(de)太陽(yang)(yang)能(neng)板供(gong)電(dian)(dian)(dian)的(de)(de)系(xi)統(tong)來說，關鍵的(de)(de)元素就是(shi)系(xi)統(tong)功率(lv)和電(dian)(dian)(dian)池充電(dian)(dian)(dian)功率(lv)控制架構。