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可調電源模塊的選擇

2019/5/24 9:11:52??????點擊:
        LM317作為輸出電(dian)壓可變的(de)集(ji)成三端穩(wen)壓塊,是一(yi)種使(shi)用(yong)方便、應用(yong)廣泛(fan)的(de)集(ji)成穩(wen)壓塊。317系列穩(wen)壓塊的(de)型(xing)號很多:例如LM317HVH、W317L等。電(dian)子愛好(hao)者經(jing)常用(yong)317穩(wen)壓塊制作輸出電(dian)壓可變的(de)穩(wen)壓電(dian)源。
穩壓(ya)電(dian)源的(de)輸(shu)(shu)出電(dian)壓(ya)可(ke)用下式(shi)(shi)計(ji)算,Vo=1.25(1+R2/R1)。僅(jin)僅(jin)從公式(shi)(shi)本(ben)身(shen)看,R1、R2的(de)電(dian)阻值可(ke)以隨意設定。然而(er)作為穩壓(ya)電(dian)源的(de)輸(shu)(shu)出電(dian)壓(ya)計(ji)算公式(shi)(shi),R1和R2的(de)阻值是不能隨意設定的(de)。
首(shou)先317穩(wen)壓塊的(de)(de)輸(shu)出(chu)電壓變化范圍是Vo=1.25V—37V(高輸(shu)出(chu)電壓的(de)(de)317穩(wen)壓塊如LM317HVA、LM317HVK等,其輸(shu)出(chu)電壓變化范圍是Vo=1.25V—45V),所以R2/R1的(de)(de)比(bi)值范圍只(zhi)能是0—28.6。

其(qi)次(ci)是317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)(kuai)都有(you)一個(ge)最(zui)小(xiao)穩(wen)(wen)(wen)定(ding)工(gong)作(zuo)(zuo)電(dian)(dian)(dian)(dian)流(liu),有(you)的(de)(de)(de)(de)(de)資(zi)料稱為(wei)最(zui)小(xiao)輸出(chu)電(dian)(dian)(dian)(dian)流(liu),也有(you)的(de)(de)(de)(de)(de)資(zi)料稱為(wei)最(zui)小(xiao)泄放(fang)電(dian)(dian)(dian)(dian)流(liu)。最(zui)小(xiao)穩(wen)(wen)(wen)定(ding)工(gong)作(zuo)(zuo)電(dian)(dian)(dian)(dian)流(liu)的(de)(de)(de)(de)(de)值一般為(wei)1.5mA。由于(yu)317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)(kuai)的(de)(de)(de)(de)(de)生產廠家不(bu)(bu)同(tong)(tong)、型(xing)號不(bu)(bu)同(tong)(tong),其(qi)最(zui)小(xiao)穩(wen)(wen)(wen)定(ding)工(gong)作(zuo)(zuo)電(dian)(dian)(dian)(dian)流(liu)也不(bu)(bu)相同(tong)(tong),但(dan)一般不(bu)(bu)大于(yu)5mA。當317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)(kuai)的(de)(de)(de)(de)(de)輸出(chu)電(dian)(dian)(dian)(dian)流(liu)小(xiao)于(yu)其(qi)最(zui)小(xiao)穩(wen)(wen)(wen)定(ding)工(gong)作(zuo)(zuo)電(dian)(dian)(dian)(dian)流(liu)時,317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)(kuai)就不(bu)(bu)能正常工(gong)作(zuo)(zuo)。當317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)(kuai)的(de)(de)(de)(de)(de)輸出(chu)電(dian)(dian)(dian)(dian)流(liu)大于(yu)其(qi)最(zui)小(xiao)穩(wen)(wen)(wen)定(ding)工(gong)作(zuo)(zuo)電(dian)(dian)(dian)(dian)流(liu)時,317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)(kuai)就可以輸出(chu)穩(wen)(wen)(wen)定(ding)的(de)(de)(de)(de)(de)直流(liu)電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)(ya)。如果用(yong)317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)(kuai)制作(zuo)(zuo)穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)電(dian)(dian)(dian)(dian)源時,沒有(you)注意(yi)317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)(kuai)的(de)(de)(de)(de)(de)最(zui)小(xiao)穩(wen)(wen)(wen)定(ding)工(gong)作(zuo)(zuo)電(dian)(dian)(dian)(dian)流(liu),那(nei)么你制作(zuo)(zuo)的(de)(de)(de)(de)(de)可能(neng)會出(chu)現(xian)下(xia)述不正常現(xian)象(xiang):穩壓電(dian)源輸出(chu)的有載(zai)電(dian)壓和空載(zai)電(dian)壓差別(bie)較大(da)。

        在(zai)應(ying)用(yong)中(zhong),為了電(dian)(dian)(dian)路的穩定工作,在(zai)一般情況下(xia),還需要接二極管作為保護電(dian)(dian)(dian)路,防止電(dian)(dian)(dian)路中(zhong)的電(dian)(dian)(dian)容(rong)放電(dian)(dian)(dian)時的高壓把(ba)317燒壞。
1.要(yao)解決317穩(wen)(wen)壓(ya)(ya)塊最(zui)小(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)(gong)作(zuo)(zuo)電流(liu)的問題(ti),可以通(tong)過設定(ding)(ding)R1和R2阻值的大小(xiao)(xiao),而(er)使(shi)317穩(wen)(wen)壓(ya)(ya)塊空載(zai)(zai)時(shi)(shi)(shi)輸出的電流(liu)大于(yu)或等于(yu)其最(zui)小(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)(gong)作(zuo)(zuo)電流(liu),從(cong)而(er)保(bao)(bao)證(zheng)317穩(wen)(wen)壓(ya)(ya)塊在(zai)(zai)(zai)空載(zai)(zai)時(shi)(shi)(shi)能(neng)(neng)夠(gou)穩(wen)(wen)定(ding)(ding)地(di)工(gong)(gong)作(zuo)(zuo)。此(ci)時(shi)(shi)(shi),只(zhi)要(yao)保(bao)(bao)證(zheng)Vo/(R1+R2)≥1.5mA,就可以保(bao)(bao)證(zheng)317穩(wen)(wen)壓(ya)(ya)塊在(zai)(zai)(zai)空載(zai)(zai)時(shi)(shi)(shi)能(neng)(neng)夠(gou)穩(wen)(wen)定(ding)(ding)地(di)工(gong)(gong)作(zuo)(zuo)。上(shang)式中的1.5mA為317穩(wen)(wen)壓(ya)(ya)塊的最(zui)小(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)(gong)作(zuo)(zuo)電流(liu)。當然,只(zhi)要(yao)能(neng)(neng)保(bao)(bao)證(zheng)317穩(wen)(wen)壓(ya)(ya)塊在(zai)(zai)(zai)空載(zai)(zai)時(shi)(shi)(shi)能(neng)(neng)夠(gou)穩(wen)(wen)定(ding)(ding)地(di)工(gong)(gong)作(zuo)(zuo),Vo/(R1+R2)的值也可以設定(ding)(ding)為大于(yu)1.5mA的任(ren)意(yi)值。
2.經計算可(ke)知R1的(de)最(zui)大取值(zhi)為(wei)R1≈0.83KΩ。又因(yin)為(wei)R2/R1的(de)最(zui)大值(zhi)為(wei)28.6。所以R2的(de)最(zui)大取值(zhi)為(wei)R2≈23.74KΩ。在使用317穩壓(ya)塊的(de)輸出電(dian)壓(ya)計算公式計算其輸出電(dian)壓(ya)時(shi),必須保證R1≥0.83KΩ,R2≤23.74KΩ兩(liang)個不等式同時(shi)成立(li),才(cai)能保證317穩壓(ya)塊在空載(zai)時(shi)能夠(gou)穩定(ding)地工(gong)作。

3.當(dang)然(ran)在317穩壓(ya)(ya)塊(kuai)的(de)輸出端并聯(lian)泄(xie)流(liu)(liu)電(dian)阻(zu)R也可以為(wei)(wei)317穩壓(ya)(ya)塊(kuai)提(ti)供最小穩定(ding)工(gong)作(zuo)電(dian)流(liu)(liu)。但是,由于并聯(lian)的(de)泄(xie)流(liu)(liu)電(dian)阻(zu)不(bu)能隨輸出電(dian)壓(ya)(ya)的(de)變化而(er)(er)變化,如果要保證(zheng)317穩壓(ya)(ya)塊(kuai)在輸出電(dian)壓(ya)(ya)為(wei)(wei)1.25V時,其輸出電(dian)流(liu)(liu)大于其最小穩定(ding)工(gong)作(zuo)電(dian)流(liu)(liu),則在317穩壓(ya)(ya)塊(kuai)的(de)輸出電(dian)壓(ya)(ya)為(wei)(wei)37V時,流(liu)(liu)過泄(xie)流(liu)(liu)電(dian)阻(zu)的(de)電(dian)流(liu)(liu)就太大了(le)(le),這樣(yang)不(bu)僅浪費了(le)(le)電(dian)能,而(er)(er)且增加了(le)(le)317穩壓(ya)(ya)塊(kuai)的(de)負擔,不(bu)是一(yi)種妥當(dang)的(de)辦法(fa)。

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