開關電源是利(li)用(yong)(yong)現代電(dian)力電(dian)子技術(shu)，控制開關(guan)(guan)晶體管(guan)開通和關(guan)(guan)斷的(de)(de)時間(jian)比(bi)率，維持穩(wen)定輸出電(dian)壓(ya)的(de)(de)一(yi)種(zhong)電(dian)源(yuan)(yuan)。廣泛(fan)運用(yong)(yong)在工業、軍事、科研、通訊、醫療及多(duo)種(zhong)家用(yong)(yong)電(dian)器中。開關(guan)(guan)電(dian)源(yuan)(yuan)的(de)(de)發展與應(ying)用(yong)(yong)在節約能(neng)源(yuan)(yuan)、節約資(zi)源(yuan)(yuan)及保護(hu)環境方面都具(ju)有重要的(de)(de)意義。下面我們(men)就(jiu)來看(kan)看(kan)開關(guan)(guan)電(dian)源(yuan)(yuan)電(dian)路(lu)圖與維修技巧。
開(kai)關電(dian)(dian)(dian)源的主(zhu)要(yao)電(dian)(dian)(dian)路(lu)(lu)(lu)是由輸入電(dian)(dian)(dian)磁干擾濾(lv)波(bo)器(qi)（EMI）、整(zheng)(zheng)流(liu)濾(lv)波(bo)電(dian)(dian)(dian)路(lu)(lu)(lu)、功率變換(huan)電(dian)(dian)(dian)路(lu)(lu)(lu)、PWM控制(zhi)器(qi)電(dian)(dian)(dian)路(lu)(lu)(lu)、輸出(chu)整(zheng)(zheng)流(liu)濾(lv)波(bo)電(dian)(dian)(dian)路(lu)(lu)(lu)組成。輔助電(dian)(dian)(dian)路(lu)(lu)(lu)有輸入過欠壓保(bao)護電(dian)(dian)(dian)路(lu)(lu)(lu)、輸出(chu)過欠壓保(bao)護電(dian)(dian)(dian)路(lu)(lu)(lu)、輸出(chu)過流(liu)保(bao)護電(dian)(dian)(dian)路(lu)(lu)(lu)、輸出(chu)短路(lu)(lu)(lu)保(bao)護電(dian)(dian)(dian)路(lu)(lu)(lu)等。
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通電后觀察(cha)電源(yuan)是否有燒保險及個別元件冒煙(yan)等現象，若有要及時切斷供電進行檢修。
測量高壓濾波(bo)電(dian)容兩端有無300伏輸出(chu)，若無應(ying)重(zhong)點查(cha)整流二極管(guan)、濾波(bo)電(dian)容等。
測量高頻(pin)變壓器次級線圈有無輸出，若(ruo)無應(ying)重點(dian)查開關管是(shi)否(fou)損(sun)壞，是(shi)否(fou)起振，保護電路是(shi)否(fou)動作等，若(ruo)有則應(ying)重點(dian)檢查各(ge)輸出側(ce)的整(zheng)流二(er)極(ji)管、濾波電容、三(san)通(tong)穩壓管等。
如(ru)果(guo)電(dian)(dian)(dian)源(yuan)啟(qi)動一(yi)下(xia)就停止，則該電(dian)(dian)(dian)源(yuan)處于保(bao)(bao)護(hu)(hu)狀態(tai)下(xia)，可直接(jie)測量PWM芯片保(bao)(bao)護(hu)(hu)輸入腳的(de)電(dian)(dian)(dian)壓(ya)，如(ru)果(guo)電(dian)(dian)(dian)壓(ya)超出規定值，則說明(ming)電(dian)(dian)(dian)源(yuan)處于保(bao)(bao)護(hu)(hu)狀態(tai)下(xia)，應重(zhong)點檢(jian)查產生保(bao)(bao)護(hu)(hu)的(de)原因(yin)。
總之，開(kai)關電(dian)源電(dian)路(lu)有(you)易有(you)難，功率有(you)大有(you)小，輸出(chu)電(dian)壓多種(zhong)多樣(yang)。只要按照(zhao)上述維修(xiu)技巧，多動(dong)手進行開(kai)關電(dian)源的維修(xiu)，就(jiu)能迅速(su)地排除(chu)開(kai)關電(dian)源故障，達到事半(ban)功倍的效果(guo)。
看：打開電(dian)源的(de)(de)外殼(ke)，檢查(cha)保險絲(si)是否熔斷，再觀(guan)察電(dian)源的(de)(de)內部情(qing)況，如果發現電(dian)源的(de)(de)PCB板上有燒焦(jiao)處或元件破裂，則(ze)應重點檢查(cha)此處元件及相(xiang)關電(dian)路(lu)元件。
聞：聞一(yi)下電源(yuan)內部是否(fou)有糊(hu)味(wei)，檢查是否(fou)有燒焦(jiao)的元器件。
問：問一下電源(yuan)損壞的經(jing)過(guo)，是否對(dui)電源(yuan)進(jin)行違規操作。
量(liang)：沒通電(dian)(dian)(dian)(dian)前(qian)，用萬(wan)用表(biao)量(liang)一下高(gao)壓(ya)(ya)(ya)電(dian)(dian)(dian)(dian)容兩端的電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)先(xian)。如果是開(kai)(kai)關電(dian)(dian)(dian)(dian)源(yuan)不(bu)起振或開(kai)(kai)關管開(kai)(kai)路引起的故(gu)障(zhang)，則大多數情況下，高(gao)壓(ya)(ya)(ya)濾波電(dian)(dian)(dian)(dian)容兩端的電(dian)(dian)(dian)(dian)壓(ya)(ya)(ya)未泄(xie)放(fang)(fang)悼，用萬(wan)用表(biao)測(ce)量(liang)AC電(dian)(dian)(dian)(dian)源(yuan)線兩端的正反向電(dian)(dian)(dian)(dian)阻(zu)(zu)(zu)及電(dian)(dian)(dian)(dian)容器充(chong)電(dian)(dian)(dian)(dian)情況，電(dian)(dian)(dian)(dian)阻(zu)(zu)(zu)值不(bu)應過低，否則電(dian)(dian)(dian)(dian)源(yuan)內(nei)部(bu)可能存在短路。電(dian)(dian)(dian)(dian)容器應能充(chong)放(fang)(fang)電(dian)(dian)(dian)(dian)。脫開(kai)(kai)負載，分別測(ce)量(liang)各組輸出(chu)端的對地電(dian)(dian)(dian)(dian)阻(zu)(zu)(zu)，正常時，表(biao)針應有電(dian)(dian)(dian)(dian)容器充(chong)放(fang)(fang)電(dian)(dian)(dian)(dian)擺動，最(zui)后(hou)指示(shi)的應為該路的泄(xie)放(fang)(fang)電(dian)(dian)(dian)(dian)阻(zu)(zu)(zu)的阻(zu)(zu)(zu)值。
一般情況下，保險絲熔斷說明開關電(dian)(dian)源的內(nei)部(bu)電(dian)(dian)路(lu)存(cun)在(zai)短路(lu)或過流的故障。由于開關電(dian)(dian)源工作(zuo)在(zai)高電(dian)(dian)壓(ya)，大電(dian)(dian)流的狀(zhuang)(zhuang)態(tai)下，直流濾(lv)(lv)波(bo)和變(bian)換振蕩電(dian)(dian)路(lu)在(zai)高壓(ya)狀(zhuang)(zhuang)態(tai)工作(zuo)時間(jian)(jian)太長，電(dian)(dian)壓(ya)變(bian)化相(xiang)對大。電(dian)(dian)網電(dian)(dian)壓(ya)的波(bo)動，浪涌都會(hui)引(yin)起電(dian)(dian)源內(nei)電(dian)(dian)流瞬間(jian)(jian)增大而(er)使(shi)保險絲熔斷。重點應(ying)檢查電(dian)(dian)源輸入端(duan)的整流二極管(guan)，高壓(ya)濾(lv)(lv)波(bo)電(dian)(dian)解電(dian)(dian)容，開關功率管(guan)，UC3842本身及外圍元器(qi)件等(deng)。檢查一下這(zhe)些元器(qi)件有無(wu)擊穿，開路(lu)，損壞(huai)，燒(shao)焦(jiao)，炸裂等(deng)現象。
維(wei)修方(fang)法：首先仔細查看電(dian)(dian)(dian)(dian)(dian)路(lu)板(ban)上(shang)面(mian)的(de)(de)(de)(de)各個(ge)元件(jian)，看是(shi)(shi)(shi)(shi)否在這些元件(jian)的(de)(de)(de)(de)外表(biao)有(you)沒有(you)被燒糊,有(you)沒有(you)電(dian)(dian)(dian)(dian)(dian)解液溢(yi)出，聞一(yi)聞有(you)沒有(you)異味。經看，聞之后(hou)(hou)，再用(yong)萬(wan)用(yong)表(biao)進(jin)(jin)行(xing)(xing)檢查。首先測(ce)量(liang)(liang)(liang)一(yi)下(xia)(xia)電(dian)(dian)(dian)(dian)(dian)源(yuan)輸入端的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)阻(zu)(zu)值，若小于(yu)200K，則(ze)說(shuo)明(ming)后(hou)(hou)端有(you)局部短(duan)路(lu)現象，然后(hou)(hou)分(fen)別測(ce)量(liang)(liang)(liang)四只整流二極管正(zheng)，反向電(dian)(dian)(dian)(dian)(dian)阻(zu)(zu)和兩個(ge)限(xian)流電(dian)(dian)(dian)(dian)(dian)阻(zu)(zu)的(de)(de)(de)(de)阻(zu)(zu)值，看其有(you)無短(duan)路(lu)或燒壞(huai)(huai)；然后(hou)(hou)再測(ce)量(liang)(liang)(liang)一(yi)下(xia)(xia)電(dian)(dian)(dian)(dian)(dian)源(yuan)濾(lv)波電(dian)(dian)(dian)(dian)(dian)容(rong)是(shi)(shi)(shi)(shi)否能進(jin)(jin)行(xing)(xing)正(zheng)常(chang)充放電(dian)(dian)(dian)(dian)(dian)，再就測(ce)量(liang)(liang)(liang)一(yi)下(xia)(xia)開關功(gong)率管是(shi)(shi)(shi)(shi)否擊(ji)穿損壞(huai)(huai)，以及UC3842本身，及周圍元件(jian)是(shi)(shi)(shi)(shi)否擊(ji)穿，燒壞(huai)(huai)等。需要(yao)說(shuo)明(ming)的(de)(de)(de)(de)一(yi)點是(shi)(shi)(shi)(shi)：因(yin)是(shi)(shi)(shi)(shi)在路(lu)測(ce)量(liang)(liang)(liang)，有(you)可(ke)能會使測(ce)量(liang)(liang)(liang)結(jie)果(guo)有(you)誤，造成誤判。因(yin)此必要(yao)時(shi)可(ke)把(ba)元器(qi)件(jian)焊下(xia)(xia)來再進(jin)(jin)行(xing)(xing)測(ce)量(liang)(liang)(liang)。如果(guo)仍(reng)然沒有(you)上(shang)述情(qing)況(kuang)則(ze)測(ce)量(liang)(liang)(liang)一(yi)下(xia)(xia)輸入電(dian)(dian)(dian)(dian)(dian)源(yuan)線(xian)及輸出電(dian)(dian)(dian)(dian)(dian)源(yuan)線(xian)是(shi)(shi)(shi)(shi)否內部短(duan)路(lu)。一(yi)般情(qing)況(kuang)下(xia)(xia)，熔(rong)斷器(qi)熔(rong)斷故障，整流二極管，電(dian)(dian)(dian)(dian)(dian)源(yuan)濾(lv)波電(dian)(dian)(dian)(dian)(dian)容(rong)，開關功(gong)率管，UC3842是(shi)(shi)(shi)(shi)易損件(jian)，損壞(huai)(huai)的(de)(de)(de)(de)概率可(ke)達95%以上(shang)，一(yi)般著(zhu)重檢查一(yi)下(xia)(xia)這些元器(qi)件(jian)，就可(ke)很容(rong)易排除此類故障。
如果保(bao)險絲是(shi)完好的(de)，在有(you)負(fu)載(zai)的(de)情(qing)況(kuang)下(xia)，各級直流電(dian)(dian)(dian)壓(ya)無輸出(chu)。這種情(qing)況(kuang)主要是(shi)以下(xia)原因造成的(de)：電(dian)(dian)(dian)源中出(chu)現(xian)開路(lu)，短路(lu)現(xian)象，過壓(ya)，過流保(bao)護電(dian)(dian)(dian)路(lu)出(chu)現(xian)故障，振蕩(dang)電(dian)(dian)(dian)路(lu)沒有(you)工作，電(dian)(dian)(dian)源負(fu)載(zai)過重，高頻整流濾(lv)波電(dian)(dian)(dian)路(lu)中整流二(er)極管被(bei)擊(ji)穿，濾(lv)波電(dian)(dian)(dian)容漏電(dian)(dian)(dian)等。
維修(xiu)方法：首先，用(yong)萬用(yong)表(biao)測量(liang)一下高頻變壓(ya)(ya)器(qi)次(ci)級的(de)各個元器(qi)件是否有損壞(huai)(huai)。在(zai)排除了高頻整流二(er)極(ji)管(guan)擊穿(chuan)、負載短路(lu)(lu)的(de)情況后(hou)，然后(hou)在(zai)測量(liang)各輸出(chu)端的(de)直流電(dian)(dian)壓(ya)(ya)，如(ru)果這(zhe)時輸出(chu)仍為零，則(ze)可以(yi)肯定(ding)是電(dian)(dian)源的(de)控制電(dian)(dian)路(lu)(lu)出(chu)了故(gu)障(zhang)。控制電(dian)(dian)路(lu)(lu)的(de)兩部分(fen)是集成(cheng)開(kai)關電(dian)(dian)源控制器(qi)和過壓(ya)(ya)保護電(dian)(dian)路(lu)(lu)。最后(hou)用(yong)萬用(yong)表(biao)靜態測量(liang)高頻濾波電(dian)(dian)路(lu)(lu)中整流二(er)極(ji)管(guan)及低壓(ya)(ya)濾波電(dian)(dian)容是否損壞(huai)(huai)。如(ru)果確實(shi)相(xiang)關的(de)元件損壞(huai)(huai)，在(zai)更換好新的(de)完好的(de)元件后(hou)，開(kai)機測試，一般故(gu)障(zhang)即可排除。需要說明(ming)的(de)是：電(dian)(dian)源輸出(chu)線斷線或(huo)開(kai)焊(han)，虛焊(han)也會(hui)造成(cheng)這(zhe)種故(gu)障(zhang)。在(zai)維修(xiu)時應注意這(zhe)一點。
電源負載能力(li)差是(shi)一(yi)個(ge)常見的(de)(de)故障，一(yi)般都是(shi)出(chu)現在老式(shi)或是(shi)工作時(shi)間長的(de)(de)電源中，主要原(yuan)因是(shi)各元器件老化，開關管的(de)(de)工作不(bu)穩(wen)定，沒有及(ji)時(shi)進行散熱(re)等(deng)。此外還有穩(wen)壓二(er)(er)極管發熱(re)漏(lou)電，整(zheng)流二(er)(er)極管損壞等(deng)。
維修(xiu)方法：用萬用表著重(zhong)檢查一(yi)(yi)下穩壓(ya)二極(ji)管，高壓(ya)濾波(bo)電容(rong)，限流電阻(zu)有無變(bian)質(zhi)等(deng)再(zai)仔細檢查一(yi)(yi)下電路板上的(de)所有焊(han)(han)點是(shi)否開(kai)焊(han)(han)，虛接等(deng)。把開(kai)焊(han)(han)的(de)焊(han)(han)點重(zhong)新焊(han)(han)牢，更(geng)換變(bian)質(zhi)的(de)元(yuan)器件，一(yi)(yi)般故障(zhang)即可(ke)排除(chu)。
這種(zhong)現象說明開關電源未工作(zuo)，或(huo)者工作(zuo)后進入了保護(hu)狀(zhuang)態。
維修方法：首先應判(pan)斷一下開(kai)(kai)關(guan)電(dian)源的(de)(de)(de)主控芯(xin)片(pian)UC3842是(shi)否處在工作狀態或已經(jing)損(sun)壞(huai)。判(pan)斷方法是(shi)這樣的(de)(de)(de)：加電(dian)測UC3842的(de)(de)(de)第7腳(jiao)(jiao)(jiao)對地(di)電(dian)壓(ya)，若(ruo)(ruo)測第8腳(jiao)(jiao)(jiao)有+5V電(dian)壓(ya)，1，2，4，6腳(jiao)(jiao)(jiao)也有不(bu)同的(de)(de)(de)電(dian)壓(ya)，則(ze)說明電(dian)路已起振，UC3842基(ji)本(ben)(ben)正(zheng)常；若(ruo)(ruo)7腳(jiao)(jiao)(jiao)電(dian)壓(ya)低(di)，其余管(guan)腳(jiao)(jiao)(jiao)無(wu)電(dian)壓(ya)或不(bu)波動(dong)，則(ze)UC3842已損(sun)壞(huai)。UC3842芯(xin)片(pian)損(sun)壞(huai)最常見(jian)的(de)(de)(de)是(shi)6，7腳(jiao)(jiao)(jiao)對地(di)擊穿，5，7腳(jiao)(jiao)(jiao)對地(di)擊穿和(he)1，7腳(jiao)(jiao)(jiao)對地(di)擊穿。如(ru)果(guo)這幾只腳(jiao)(jiao)(jiao)都為擊穿，而(er)開(kai)(kai)關(guan)電(dian)源還是(shi)不(bu)能(neng)正(zheng)常啟動(dong)，則(ze)UC3842必壞(huai)，應直接(jie)更換。若(ruo)(ruo)判(pan)斷芯(xin)片(pian)未壞(huai)，則(ze)就著重檢查開(kai)(kai)關(guan)功率管(guan)的(de)(de)(de)柵極(ji)（G極(ji)）的(de)(de)(de)限(xian)流電(dian)阻是(shi)否開(kai)(kai)焊，虛(xu)接(jie)，變值，變質(zhi)以及開(kai)(kai)關(guan)功率管(guan)本(ben)(ben)身是(shi)否性能(neng)不(bu)良(liang)。除此之(zhi)外，電(dian)源輸出線也有可能(neng)斷線或接(jie)觸不(bu)良(liang)也會造(zao)成(cheng)這種故(gu)障。因此在維修時(shi)也應注意(yi)檢查一下。
這種(zhong)故障(zhang)往往來自于(yu)穩壓取樣(yang)和(he)穩壓控制(zhi)電(dian)(dian)路(lu)出(chu)現(xian)故障(zhang)所致。在(zai)開關電(dian)(dian)源中(zhong)，直流(liu)輸出(chu)、取樣(yang)電(dian)(dian)阻、誤差(cha)取樣(yang)放大器(qi)(qi)（如(ru)LM324，LM358等）、光耦合(he)器(qi)(qi)（PC817）、電(dian)(dian)源控制(zhi)芯(xin)片（UC3842）等電(dian)(dian)路(lu)共同構(gou)成了一(yi)個閉合(he)的控制(zhi)環路(lu)，任何(he)一(yi)處(chu)出(chu)問題都會導(dao)致輸出(chu)電(dian)(dian)壓升高。
維修方法(fa)：由于開關電(dian)(dian)(dian)源中(zhong)有(you)過(guo)(guo)壓(ya)(ya)(ya)保護(hu)(hu)電(dian)(dian)(dian)路，輸出電(dian)(dian)(dian)壓(ya)(ya)(ya)過(guo)(guo)高(gao)首(shou)先會使過(guo)(guo)壓(ya)(ya)(ya)保護(hu)(hu)電(dian)(dian)(dian)路動作(zuo)(zuo)。因(yin)此對于這種故(gu)障的(de)(de)維修，我們(men)可(ke)以(yi)通過(guo)(guo)斷開過(guo)(guo)壓(ya)(ya)(ya)保護(hu)(hu)電(dian)(dian)(dian)路，使過(guo)(guo)壓(ya)(ya)(ya)保護(hu)(hu)電(dian)(dian)(dian)路不起作(zuo)(zuo)用(yong)(yong)，在這時，測量(liang)(liang)開機瞬間(jian)的(de)(de)電(dian)(dian)(dian)源主電(dian)(dian)(dian)壓(ya)(ya)(ya)。如果測量(liang)(liang)值比(bi)正(zheng)常值高(gao)出IV以(yi)上，說明(ming)輸出電(dian)(dian)(dian)壓(ya)(ya)(ya)過(guo)(guo)高(gao)。我們(men)應(ying)著重(zhong)檢查取樣電(dian)(dian)(dian)阻(zu)是否(fou)變值或(huo)(huo)損(sun)壞(huai)(huai)，精(jing)密穩壓(ya)(ya)(ya)放大器（TL431）或(huo)(huo)光耦合器（PC817）性能不良，變質(zhi)或(huo)(huo)損(sun)壞(huai)(huai)；其(qi)中(zhong)精(jing)密穩壓(ya)(ya)(ya)放大器（TL431）極(ji)易(yi)損(sun)壞(huai)(huai)，我們(men)可(ke)用(yong)(yong)下述方法(fa)對精(jing)密穩壓(ya)(ya)(ya)放大器（TL431）作(zuo)(zuo)出好(hao)壞(huai)(huai)的(de)(de)判別：將(jiang)TL431的(de)(de)參考端（Ref）與它的(de)(de)陰極(ji)（Cathode）相連，串10k的(de)(de)電(dian)(dian)(dian)阻(zu)，接入5V電(dian)(dian)(dian)壓(ya)(ya)(ya)，若(ruo)陽極(ji)（Anode）與陰極(ji)之(zhi)間(jian)為(wei)(wei)2、5V，并且等(deng)待片(pian)刻還仍然為(wei)(wei)2、5V，則為(wei)(wei)好(hao)管，否(fou)則為(wei)(wei)壞(huai)(huai)管。
對于這種故(gu)障現象，根據維修經驗(yan)可知，除穩壓(ya)(ya)控制電(dian)路(lu)會引起(qi)輸出電(dian)壓(ya)(ya)過(guo)低外(wai)，還有一些原因會引起(qi)輸出電(dian)壓(ya)(ya)過(guo)低，主(zhu)要有以下幾點(dian):
① 開(kai)(kai)關(guan)電(dian)(dian)源負(fu)載有短(duan)路(lu)(lu)故(gu)障(zhang)。此(ci)時，應斷開(kai)(kai)開(kai)(kai)關(guan)電(dian)(dian)源電(dian)(dian)路(lu)(lu)的所(suo)有負(fu)載，以(yi)區分是(shi)開(kai)(kai)關(guan)電(dian)(dian)源電(dian)(dian)路(lu)(lu)還是(shi)負(fu)載電(dian)(dian)路(lu)(lu)有故(gu)障(zhang)。若斷開(kai)(kai)負(fu)載電(dian)(dian)路(lu)(lu)電(dian)(dian)壓輸出正常(chang)，說(shuo)明(ming)是(shi)負(fu)載過重；若仍不正常(chang)，說(shuo)明(ming)開(kai)(kai)關(guan)電(dian)(dian)源電(dian)(dian)路(lu)(lu)有故(gu)障(zhang)。
② 輸出電壓端整流二(er)極(ji)管、濾波電容(rong)失效(xiao)等(deng)，可以通過(guo)代換法進行判斷。
③ 開關功率管的(de)性能(neng)下降(jiang)(jiang)，必然導(dao)致開關管不能(neng)正常導(dao)通(tong)，使電(dian)源的(de)內阻增加，帶負載能(neng)力下降(jiang)(jiang)。
④ 開關功率管的源極（S極），通常接一(yi)個阻(zu)值(zhi)很(hen)小，但功率很(hen)大的電(dian)(dian)阻(zu)，作為(wei)過流(liu)保(bao)護檢測電(dian)(dian)阻(zu)，此電(dian)(dian)阻(zu)的阻(zu)值(zhi)一(yi)般在0.2到0.8之間(jian)。此電(dian)(dian)阻(zu)如(ru)變值(zhi)或開焊，接觸不良也會(hui)造成輸出電(dian)(dian)壓過低的故障。
高頻變壓器(qi)不(bu)良，不(bu)但造成(cheng)(cheng)輸出電壓下降(jiang)，還會造成(cheng)(cheng)開(kai)關功率管激勵不(bu)足從而屢損開(kai)關管。
⑤ 高(gao)壓(ya)直流濾波電(dian)(dian)容不(bu)良，造成電(dian)(dian)源帶負(fu)(fu)載能力差，一接負(fu)(fu)載輸出電(dian)(dian)壓(ya)便下降。
⑥ 電源輸出線接觸不良，有一定的接觸電阻，造成(cheng)輸出電壓過低。
⑦ 電(dian)網(wang)電(dian)壓(ya)是否過低。雖然(ran)(ran)開(kai)關電(dian)源在低壓(ya)下(xia)仍然(ran)(ran)可以輸(shu)(shu)出額定(ding)的(de)電(dian)壓(ya)值，但當電(dian)網(wang)電(dian)壓(ya)低于開(kai)關電(dian)源的(de)最低電(dian)壓(ya)限(xian)定(ding)值時，也會使輸(shu)(shu)出電(dian)壓(ya)過低。
維(wei)修方法(fa)：對于(yu)這(zhe)種故障我(wo)們可以(yi)(yi)(yi)根據(ju)(ju)以(yi)(yi)(yi)上故障原(yuan)因(yin)，來逐(zhu)(zhu)一(yi)進(jin)行排查(cha)(cha)。但在(zai)實(shi)(shi)際(ji)(ji)維(wei)修時，可根據(ju)(ju)實(shi)(shi)際(ji)(ji)情況(kuang)來進(jin)行排查(cha)(cha)，不(bu)一(yi)定要逐(zhu)(zhu)一(yi)排查(cha)(cha)。首(shou)先用(yong)萬用(yong)表檢(jian)查(cha)(cha)一(yi)下高(gao)壓(ya)直流(liu)濾波(bo)電(dian)(dian)容是否(fou)(fou)(fou)變(bian)質，容量是否(fou)(fou)(fou)下降，能否(fou)(fou)(fou)正常(chang)充放(fang)電(dian)(dian)。如無以(yi)(yi)(yi)上現(xian)象，則測量一(yi)下開(kai)關功率(lv)管的(de)(de)柵極(ji)(ji)（G極(ji)(ji)）的(de)(de)限(xian)流(liu)電(dian)(dian)阻以(yi)(yi)(yi)及源極(ji)(ji)（S極(ji)(ji)）的(de)(de)過(guo)(guo)流(liu)保(bao)(bao)護檢(jian)測電(dian)(dian)阻是否(fou)(fou)(fou)變(bian)值(zhi)，變(bian)質或(huo)開(kai)焊，接(jie)觸(chu)不(bu)良。經判別后，若無問題，我(wo)們就檢(jian)查(cha)(cha)一(yi)下高(gao)頻(pin)變(bian)壓(ya)器的(de)(de)鐵芯是否(fou)(fou)(fou)完好無損(sun)。因(yin)在(zai)日常(chang)生活使(shi)(shi)用(yong)中，不(bu)可避免(mian)的(de)(de)重摔或(huo)重幢，使(shi)(shi)高(gao)頻(pin)變(bian)壓(ya)器的(de)(de)鐵芯損(sun)壞(huai)。使(shi)(shi)高(gao)頻(pin)變(bian)壓(ya)器的(de)(de)磁(ci)通量，磁(ci)感應強(qiang)度，以(yi)(yi)(yi)及磁(ci)路等(deng)都會(hui)受到(dao)很大的(de)(de)影(ying)響(xiang)，造成傳輸(shu)的(de)(de)效(xiao)率(lv)，能量將會(hui)大打(da)折(zhe)扣。由于(yu)高(gao)頻(pin)變(bian)壓(ya)器為了(le)減小(xiao)渦流(liu)，增大高(gao)頻(pin)交流(liu)電(dian)(dian)的(de)(de)傳輸(shu)效(xiao)率(lv)，它(ta)的(de)(de)鐵芯是用(yong)軟(ruan)磁(ci)鐵氧體制作而成的(de)(de)。這(zhe)種磁(ci)性材(cai)料具有高(gao)的(de)(de)導磁(ci)率(lv)，但質脆，易碎。因(yin)此(ci)它(ta)的(de)(de)損(sun)壞(huai)率(lv)也是很高(gao)的(de)(de)。因(yin)此(ci)在(zai)維(wei)修時千萬不(bu)要忘了(le)檢(jian)查(cha)(cha)此(ci)處(chu)，以(yi)(yi)(yi)免(mian)走彎路。除此(ci)之外(wai)還(huan)有可能就是輸(shu)出(chu)濾波(bo)電(dian)(dian)容容量降低(di)，甚至失容或(huo)開(kai)焊，虛接(jie)；電(dian)(dian)源輸(shu)出(chu)限(xian)流(liu)電(dian)(dian)阻變(bian)值(zhi)或(huo)虛接(jie)，電(dian)(dian)源輸(shu)出(chu)線虛接(jie)等(deng)。在(zai)實(shi)(shi)際(ji)(ji)維(wei)修時，這(zhe)些(xie)因(yin)素都不(bu)要放(fang)過(guo)(guo)，都應檢(jian)查(cha)(cha)一(yi)下，以(yi)(yi)(yi)保(bao)(bao)證萬無一(yi)失。
這(zhe)種(zhong)故障原因主要是由于控(kong)制風(feng)扇的(de)三極管（8550或8050）損(sun)壞，或者(zhe)風(feng)扇本身損(sun)壞或風(feng)葉(xie)被雜(za)物(wu)卡住。但有些開關電(dian)源(yuan)中(zhong)采(cai)用的(de)是智能(neng)散熱，對(dui)于采(cai)用這(zhe)種(zhong)方式散熱的(de)開關電(dian)源(yuan)，熱敏電(dian)阻損(sun)壞的(de)概(gai)率是很大的(de)。
維修方法：首先用(yong)萬(wan)用(yong)表(biao)測(ce)量一下控(kong)制(zhi)風扇的(de)三極管是否損(sun)壞，若測(ce)得此(ci)管未損(sun)壞那就有(you)可(ke)能是風扇本身損(sun)壞。可(ke)以把(ba)風扇從電(dian)路板上(shang)拔下來(lai)，另外(wai)(wai)接上(shang)一個12V的(de)直(zhi)流電(dian)（注(zhu)(zhu)意正負極），看是否轉動(dong)，并看有(you)無(wu)異(yi)物卡住。若擺動(dong)幾下風扇的(de)電(dian)線，風扇就轉動(dong)，則說(shuo)明電(dian)線內部有(you)斷(duan)線或接頭接觸不良(liang)。若仍(reng)不轉動(dong)，則風扇必(bi)壞。對于采用(yong)智(zhi)能散熱(re)(re)的(de)開(kai)關電(dian)源來(lai)說(shuo)，除按上(shang)述檢查外(wai)(wai)，還(huan)應檢查一下熱(re)(re)敏(min)(min)電(dian)阻是否不良(liang)或損(sun)壞，開(kai)焊(han)等(deng)。但要注(zhu)(zhu)意此(ci)熱(re)(re)敏(min)(min)電(dian)阻為負溫度系數的(de)熱(re)(re)敏(min)(min)電(dian)阻，更換(huan)時應注(zhu)(zhu)意。
