三端稳压器 反向击穿

三端稳压器

三(san)端(duan)稳(wen)(wen)压(ya)(ya)(ya)器是(shi)一种(zhong)直到临(lin)界反向击(ji)穿电(dian)压(ya)(ya)(ya)前(qian)都具有很(hen)高电(dian)阻的(de)半导体器件。稳(wen)(wen)压(ya)(ya)(ya)管在(zai)反向击(ji)穿时，在(zai)一定(ding)的(de)电(dian)流范(fan)围内（或者(zhe)说在(zai)一定(ding)功率(lv)损耗范(fan)围内），端(duan)电(dian)压(ya)(ya)(ya)几乎不变(bian)，表现出稳(wen)(wen)压(ya)(ya)(ya)特性(xing)，因而广泛应(ying)用(yong)于稳(wen)(wen)压(ya)(ya)(ya)电(dian)源与(yu)限幅电(dian)路之中。

三端稳压管，主要有(you)两种(zhong)，一种(zhong)输(shu)(shu)出(chu)电(dian)压是固定(ding)的，称(cheng)为固定(ding)输(shu)(shu)出(chu)三端稳压管，另一种(zhong)输(shu)(shu)出(chu)电(dian)压是可(ke)调(diao)的，称(cheng)为可(ke)调(diao)输(shu)(shu)出(chu)三端稳压管。

三端稳压器的工作原理

因为(wei)固(gu)定(ding)三端稳压(ya)(ya)(ya)器属于串联型稳压(ya)(ya)(ya)电路，因此它的原理等同于串联型稳压(ya)(ya)(ya)电路。

其(qi)中(zhong)R1、Rp、R2组成的(de)(de)(de)分压(ya)(ya)(ya)(ya)器是(shi)(shi)取(qu)样电(dian)(dian)路(lu)(lu)，从输出端取(qu)出部(bu)分电(dian)(dian)压(ya)(ya)(ya)(ya)UB2作为(wei)取(qu)样电(dian)(dian)压(ya)(ya)(ya)(ya)加(jia)(jia)至(zhi)三(san)(san)(san)(san)极(ji)(ji)(ji)(ji)(ji)(ji)(ji)管(guan)(guan)(guan)(guan)(guan)T2的(de)(de)(de)基(ji)极(ji)(ji)(ji)(ji)(ji)(ji)(ji)。稳(wen)(wen)压(ya)(ya)(ya)(ya)管(guan)(guan)(guan)(guan)(guan)Dz以(yi)(yi)其(qi)稳(wen)(wen)定电(dian)(dian)压(ya)(ya)(ya)(ya)Uz作为(wei)基(ji)准电(dian)(dian)压(ya)(ya)(ya)(ya)，加(jia)(jia)在T2的(de)(de)(de)发(fa)射(she)极(ji)(ji)(ji)(ji)(ji)(ji)(ji)上。R3是(shi)(shi)稳(wen)(wen)压(ya)(ya)(ya)(ya)管(guan)(guan)(guan)(guan)(guan)的(de)(de)(de)限流电(dian)(dian)阻(zu)。三(san)(san)(san)(san)极(ji)(ji)(ji)(ji)(ji)(ji)(ji)管(guan)(guan)(guan)(guan)(guan)T2组成比较放大电(dian)(dian)路(lu)(lu)，它将(jiang)取(qu)样电(dian)(dian)压(ya)(ya)(ya)(ya)UB2与基(ji)准电(dian)(dian)压(ya)(ya)(ya)(ya)Uz加(jia)(jia)以(yi)(yi)比较和(he)放大，再(zai)去控制三(san)(san)(san)(san)极(ji)(ji)(ji)(ji)(ji)(ji)(ji)管(guan)(guan)(guan)(guan)(guan)T1的(de)(de)(de)基(ji)极(ji)(ji)(ji)(ji)(ji)(ji)(ji)电(dian)(dian)位。输入电(dian)(dian)压(ya)(ya)(ya)(ya)Ui加(jia)(jia)在三(san)(san)(san)(san)极(ji)(ji)(ji)(ji)(ji)(ji)(ji)管(guan)(guan)(guan)(guan)(guan)T1与负(fu)载RL相串(chuan)联的(de)(de)(de)电(dian)(dian)路(lu)(lu)上，因(yin)此(ci)，改变T1集(ji)电(dian)(dian)极(ji)(ji)(ji)(ji)(ji)(ji)(ji)间(jian)的(de)(de)(de)电(dian)(dian)压(ya)(ya)(ya)(ya)降UCE1便(bian)可调(diao)节(jie)RL两端的(de)(de)(de)电(dian)(dian)压(ya)(ya)(ya)(ya)Uo。也就是(shi)(shi)说，稳(wen)(wen)压(ya)(ya)(ya)(ya)电(dian)(dian)路(lu)(lu)的(de)(de)(de)输出电(dian)(dian)压(ya)(ya)(ya)(ya)Uo可以(yi)(yi)通过(guo)三(san)(san)(san)(san)极(ji)(ji)(ji)(ji)(ji)(ji)(ji)管(guan)(guan)(guan)(guan)(guan)T1加(jia)(jia)以(yi)(yi)调(diao)节(jie)，所(suo)以(yi)(yi)T1称(cheng)(cheng)为(wei)调(diao)整管(guan)(guan)(guan)(guan)(guan)。由于调(diao)整元件是(shi)(shi)晶体(ti)管(guan)(guan)(guan)(guan)(guan)管(guan)(guan)(guan)(guan)(guan)，而且在电(dian)(dian)路(lu)(lu)中(zhong)与负(fu)载相串(chuan)联，故称(cheng)(cheng)为(wei)晶体(ti)管(guan)(guan)(guan)(guan)(guan)串(chuan)联型(xing)稳(wen)(wen)压(ya)(ya)(ya)(ya)电(dian)(dian)路(lu)(lu)。电(dian)(dian)阻(zu)R4和(he)T1的(de)(de)(de)基(ji)极(ji)(ji)(ji)(ji)(ji)(ji)(ji)偏置电(dian)(dian)阻(zu)，也是(shi)(shi)T2的(de)(de)(de)集(ji)电(dian)(dian)极(ji)(ji)(ji)(ji)(ji)(ji)(ji)负(fu)载电(dian)(dian)阻(zu)。

当电(dian)(dian)(dian)(dian)(dian)网电(dian)(dian)(dian)(dian)(dian)压降低或负载(zai)电(dian)(dian)(dian)(dian)(dian)阻减小而使输(shu)出端电(dian)(dian)(dian)(dian)(dian)压有(you)所(suo)下降时(shi)，其取样电(dian)(dian)(dian)(dian)(dian)压UB2相应减小，T2基极(ji)电(dian)(dian)(dian)(dian)(dian)位下降。但因T2发射(she)极(ji)电(dian)(dian)(dian)(dian)(dian)位既稳(wen)压管(guan)(guan)的稳(wen)定(ding)Uz保持(chi)不变，所(suo)以发射(she)极(ji)电(dian)(dian)(dian)(dian)(dian)压UBE2减小，导(dao)致(zhi)T2集电(dian)(dian)(dian)(dian)(dian)极(ji)电(dian)(dian)(dian)(dian)(dian)流减小而集电(dian)(dian)(dian)(dian)(dian)极(ji)电(dian)(dian)(dian)(dian)(dian)位Uc2升高(gao)。由于(yu)放大(da)管(guan)(guan)T2的集电(dian)(dian)(dian)(dian)(dian)极(ji)与调整(zheng)管(guan)(guan)T1的基极(ji)接在(zai)一起，故T1基极(ji)电(dian)(dian)(dian)(dian)(dian)位升高(gao)，导(dao)致(zhi)集电(dian)(dian)(dian)(dian)(dian)极(ji)电(dian)(dian)(dian)(dian)(dian)流增大(da)而管(guan)(guan)压降UCE1减小。因为T1与RL串联，所(suo)以，输(shu)出电(dian)(dian)(dian)(dian)(dian)压Uo基本不变。

同(tong)理,当电(dian)网(wang)电(dian)压或负(fu)载发生变化(hua)引起输(shu)出电(dian)压Uo增大(da)时，通过(guo)取样、比较(jiao)放大(da)、调整(zheng)等(deng)过(guo)程，将使调整(zheng)调整(zheng)管的(de)管压降UCE1增加，结(jie)果抑制了输(shu)出端电(dian)压的(de)增大(da)，输(shu)出电(dian)压仍基(ji)本保持不变。

调(diao)节电位器Rp，可对输出电压进(jin)行微调(diao)。调(diao)整管T1与(yu)负(fu)载电阻RL组成的是(shi)射极(ji)输出电路，所(suo)以具(ju)有(you)稳定输出电压的特点。

在串联型(xing)稳(wen)压电(dian)源电(dian)路的工作过(guo)程中，要求调整管(guan)始(shi)终处在放大状态。通过(guo)调整管(guan)的电(dian)流(liu)等(deng)(deng)于负载(zai)(zai)电(dian)流(liu)，因此必须(xu)选用适当的大功率管(guan)作调整管(guan)，并按规定安装(zhuang)散热装(zhuang)置。为了防止短(duan)路或(huo)长期过(guo)载(zai)(zai)烧坏调整管(guan)，在直流(liu)稳(wen)压器中一般还设有短(duan)路保护和(he)过(guo)载(zai)(zai)保护等(deng)(deng)电(dian)路。

三端稳压器反向击穿

三端稳压器反向击(ji)穿(chuan)，集成(cheng)稳压器‘应用测试过程’中，任意两个管(guan)脚之(zhi)间严格禁止出现瞬态的‘反偏(pian)’状态（包括mS级的瞬态反偏(pian)），否(fou)则电路(lu)有可能在(zai)瞬间被反偏(pian)击(ji)穿(chuan)烧毁(hui)。

电路(lu)有可能出现反(fan)偏击穿的外部(bu)状态：GND→IN、OUT→IN、GND→OUT、ADJ→OUT，应(ying)用中要(yao)求：GND →IN≤|-1V|,其(qi)他管脚间(jian)为≤|-5V|。

【以(yi)上几种现象(xiang)损坏的稳压器(qi)、可以(yi)通(tong)过电路解剖观察芯片(pian)有(you)不同(tong)的烧毁痕迹(ji)。】

在测(ce)试或装配过(guo)程中‘人为疏(shu)忽’造成‘反接’状态，是“没有什么措施可(ke)以(yi)(yi)保护”的(de)。如：发(fa)生管教倒(dao)置，输入电压极性接反、样品规格(ge)与使用规格(ge)不符等情况，必须(xu)操作中细心(xin)的(de)加以(yi)(yi)防止(zhi)。

在(zai)特(te)定情(qing)况(kuang)下(xia)有(you)应用电路(lu)产生的(de)‘瞬态’的(de)反偏状态，可以参考(kao)推荐(jian)保护(hu)措(cuo)施增加相(xiang)应保护(hu)元件还是可以避免的(de)，当缺少相(xiang)应的(de)保护(hu)元件时‘偶发’条(tiao)件下(xia)稳压器(qi)电路(lu)的(de)随机损(sun)坏的(de)情(qing)况(kuang)将不能避免这种情(qing)况(kuang)引起的(de)电路(lu)失效不属(shu)于电路(lu)产品质(zhi)量问题。

（一）OUT→IN端反偏

1、产生来源：

①负载时(shi)(shi)对电(dian)(dian)(dian)池类的充电(dian)(dian)(dian)应(ying)用(yong)，断电(dian)(dian)(dian)时(shi)(shi)电(dian)(dian)(dian)池并未脱离电(dian)(dian)(dian)路输(shu)出(chu)端。

②电路输入存(cun)在有(you)大(da)的(de)负向脉冲(chong)，如果电路输入同时存(cun)在较大(da)的(de)感性负荷的(de)反(fan)电动势(shi)。

③电路(lu)输出(chu)端接入大的(de)滤波电解电容。（发生较(jiao)多的(de)反接击(ji)穿情况）

保(bao)护(hu)措施：在电(dian)路I-O端之(zhi)间增(zeng)加(jia)保(bao)护(hu)二极管Di，将I-O反偏电(dian)压(ya)钳(qian)位在1V以(yi)内。

2、机理(li)分析(xi)：

只要不(bu)是稳(wen)压(ya)电(dian)(dian)路输(shu)出应用环境造(zao)成的工作条件，应用中在(zai)电(dian)(dian)路输(shu)出端不(bu)应接(jie)(jie)(jie)入大的滤(lv)波(bo)电(dian)(dian)解(jie)电(dian)(dian)容。接(jie)(jie)(jie)入这(zhei)个电(dian)(dian)解(jie)有(you)可(ke)能(neng)造(zao)成一个电(dian)(dian)路的OUT→IN端之间的‘反接(jie)(jie)(jie)’机会。

理论上，只要电(dian)(dian)(dian)源(yuan)的输入滤(lv)波(bo)电(dian)(dian)(dian)解足(zu)(zu)够大(da)，可以保证即使在稳压(ya)电(dian)(dian)(dian)路输入电(dian)(dian)(dian)压(ya)处(chu)于纹波(bo)‘谷点’时也可以满(man)足(zu)(zu)电(dian)(dian)(dian)路稳压(ya)必须(xu)的最小输入压(ya)差的特定要求。电(dian)(dian)(dian)路自身(shen)具有(you)很强的纹波(bo)抑制能力，根本没有(you)必要在输出端(duan)接滤(lv)波(bo)电(dian)(dian)(dian)解电(dian)(dian)(dian)容。

接输出端(duan)的滤波电(dian)(dian)容(rong)对于电(dian)(dian)源滤波所起的作用很小，这个电(dian)(dian)容(rong)有(you)时反而(er)会造成意外损坏稳(wen)压器电(dian)(dian)路的特定(ding)条件。

不推(tui)荐(jian)在输出端(duan)接滤(lv)波电(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)(rong)式由于(yu)电(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)(rong)的(de)(de)‘储能(neng)作用’，电(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)(rong)充电(dian)(dian)(dian)(dian)(dian)(dian)后可等(deng)效为一电(dian)(dian)(dian)(dian)(dian)(dian)池，在特定的(de)(de)情况下，如输入电(dian)(dian)(dian)(dian)(dian)(dian)压(ya)在瞬(shun)间的(de)(de)短路(lu)，感性元器件产生的(de)(de)负脉冲或关(guan)机后输入电(dian)(dian)(dian)(dian)(dian)(dian)压(ya)下降(jiang)更(geng)快(kuai)时，有(you)可能(neng)造成电(dian)(dian)(dian)(dian)(dian)(dian)路(lu)VO＞VI的(de)(de)电(dian)(dian)(dian)(dian)(dian)(dian)位反偏(pian)状(zhuang)态。在输出端(duan)接滤(lv)波电(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)(rong)＞20μF以(yi)后，电(dian)(dian)(dian)(dian)(dian)(dian)路(lu)调整管(guan)的(de)(de)反偏(pian)-5V左右。输出电(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)(rong)的(de)(de)放电(dian)(dian)(dian)(dian)(dian)(dian)过程可能(neng)在mS量(liang)级的(de)(de)瞬(shun)态反向击穿或损伤(shang)电(dian)(dian)(dian)(dian)(dian)(dian)路(lu)调整管(guan)，输出电(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)(rong)容(rong)(rong)(rong)量(liang)越大，反偏(pian)电(dian)(dian)(dian)(dian)(dian)(dian)压(ya)越高(gao)，对调整管(guan)损伤(shang)越严重(zhong)直至烧毁。

3、附带的建议(yi)：

50Hz市电(dian)整流(liu)滤波(bo)应(ying)用时，CD值为每(mei)1A负载(zai)电(dian)流(liu)取2000μf范围(wei)，否则输(shu)入纹波(bo)可(ke)(ke)能较大。在(zai)稳(wen)压电(dian)路(lu)的(de)输(shu)入电(dian)压已经很低时，就可(ke)(ke)能造成(cheng)（Vi-Vo）低于(yu)电(dian)路(lu)工作(zuo)时允许的(de)最小压差（Vi-Vo）min，出(chu)现(xian)电(dian)路(lu)瞬(shun)间脱(tuo)离稳(wen)压控制(zhi)的(de)状态。

电(dian)(dian)(dian)路(lu)(lu)脱离(li)稳(wen)(wen)(wen)压(ya)状(zhuang)态时输(shu)出(chu)不等比(bi)跟踪输(shu)入变化。在(zai)(Vi-Vo)≥(Vi-Vo)min时，输(shu)出(chu)电(dian)(dian)(dian)压(ya)稳(wen)(wen)(wen)定(ding)在(zai)途中(zhong)的(de)(de)直(zhi)线Vo段，在(zai)(Vi-Vo)＜(Vi-Vo)min时，由(you)于这时电(dian)(dian)(dian)路(lu)(lu)脱离(li)稳(wen)(wen)(wen)压(ya)状(zhuang)态，输(shu)入的(de)(de)纹波(bo)电(dian)(dian)(dian)压(ya)几乎完全镜象(xiang)到电(dian)(dian)(dian)路(lu)(lu)输(shu)出(chu)，这时稳(wen)(wen)(wen)压(ya)电(dian)(dian)(dian)路(lu)(lu)的(de)(de)纹波(bo)抑制比(bi)功能此时不会起作用，对外表(biao)现电(dian)(dian)(dian)路(lu)(lu)稳(wen)(wen)(wen)压(ya)特(te)性(xing)很(hen)差或电(dian)(dian)(dian)路(lu)(lu)不能稳(wen)(wen)(wen)压(ya)。如下图中(zhong)Vo为(wei)稳(wen)(wen)(wen)压(ya)时的(de)(de)值、Vo'为(wei)用直(zhi)流电(dian)(dian)(dian)压(ya)表(biao)的(de)(de)测量的(de)(de)平均值、Vd为(wei)镜像到输(shu)出(chu)的(de)(de)输(shu)入纹波(bo)谷点(dian)电(dian)(dian)(dian)压(ya)。

（二）GND→IN端反偏

1、产生来源(yuan)：

在(zai)汽车(che)类(lei)电子领域的电路应(ying)用(yong)中发生(sheng)较多(duo)，如部(bu)分(fen)车(che)型的汽车(che)马达(da)、点火系统等感(gan)性设备(bei)产生(sheng)的约50-200mS脉(mai)宽范(fan)围(wei)的瞬态微分(fen)峰值电压可(ke)(ke)达(da)+70V、-80V范(fan)围(wei)，5-20mS脉(mai)宽范(fan)围(wei)的可(ke)(ke)能达(da)到+110V、-250V范(fan)围(wei)。其中的负向脉(mai)冲造成了这种(zhong)反偏。（正想(xiang)脉(mai)冲可(ke)(ke)能造成电路输出出现‘瞬态’过压击穿烧毁）

2、保护措施：将(jiang)反偏电压(ya)钳位在1V以内，对于负向脉冲增加(jia)Df进行(xing)抑制。

（三）GND→OUT端反偏

1、产生来源：

①来自正负输出配对应用，当正负输出的(de)公共负载发生(sheng)瞬(shun)态的(de)短路(lu)时。

②电(dian)路输出(chu)存在有(you)大(da)的感性负荷(he)，较大(da)的反向电(dian)动势产生的负向脉冲。

③为获得高于稳压器标称(cheng)值(zhi)的(de)电路(lu)(lu)输出，输出电压被(bei)Dz抬起一个固定(ding)电压值(zhi)（如(ru)下图），没有保护二极管D的(de)情况下，输出存在瞬态(tai)的(de)短(duan)路(lu)(lu)情况。

由于(yu)设(she)计算短稳压器电(dian)(dian)(dian)路(lu)正(zheng)常工作时(shi)，电(dian)(dian)(dian)路(lu)的(de)公共端(duan)的(de)电(dian)(dian)(dian)位(wei)应是最低的(de)（绝(jue)对(dui)值），无论任(ren)何原因造成电(dian)(dian)(dian)路(lu)GND端(duan)电(dian)(dian)(dian)位(wei)高于(yu)其他两端(duan)电(dian)(dian)(dian)位(wei)的(de)情况，即属于(yu)电(dian)(dian)(dian)路(lu)的(de)‘反(fan)偏’状态(tai)，反(fan)偏电(dian)(dian)(dian)位(wei)的(de)绝(jue)对(dui)值达到5V以上(shang)时(shi)，mS量级脉宽的(de)瞬态(tai)‘反(fan)偏’就(jiu)会造成内部电(dian)(dian)(dian)路(lu)的(de)相关区(qu)域受损或烧毁(hui)。这(zhei)种击穿是随机发生的(de)现象，不同(tong)批次产品(pin)或同(tong)批次产品(pin)发生概率可能不同(tong)。

2、保护措施：应将反偏(pian)电压钳位在1V以内。增加途中二极管D。

（四）Adj→OUT端反偏

可调三端稳压器典(dian)型应用电(dian)路

1、产生来源：

从可(ke)调(diao)系列电(dian)路(lu)的典型应(ying)用电(dian)路(lu)图可(ke)以看出，应(ying)用中调(diao)整(zheng)端(duan)Adj的电(dian)位(wei)应(ying)永远(yuan)低(di)于(yu)输出端(duan)OUT的电(dian)位(wei)，参(can)考(kao)OUT端(duan)的电(dian)位(wei)而言(yan)Vadj=VREF=-1.25V。

在输出(chu)电压取样(yang)回(hui)路中，VR2=Vadj=VO-VREF=VO-1.25V≈VO，可(ke)见在可(ke)调系列电路读书处取样(yang)回(hui)路中，电压几乎(hu)是全部的降(jiang)在可(ke)调电阻(zu)R2上(shang)。因(yin)此、VC3=VR2≈VO。

产生VO-ADJ端‘反接’的(de)原因：应用中在(zai)可调系列电(dian)(dian)(dian)(dian)路时，在(zai)输出下(xia)取样电(dian)(dian)(dian)(dian)阻的(de)两端并联一支10μ~47μ的(de)输出纹波抑制电(dian)(dian)(dian)(dian)解C3，这(zhei)个(ge)C3确实(shi)可起到减小输出纹波电(dian)(dian)(dian)(dian)压(ya)的(de)作用（注(zhu)意：纹波参(can)数测试(shi)时，对C3的(de)容量也是有(you)具体要求(qiu)的(de)）。但因有(you)VC3=VR2≈VO的(de)存在(zai)，接入这(zhei)个(ge)电(dian)(dian)(dian)(dian)解也创造了(le)一个(ge)ADJ→VO端之间的(de)电(dian)(dian)(dian)(dian)路‘反接’机(ji)会。

可调三端内部电路简图(tu)

从内部电(dian)路的局部电(dian)路图中可(ke)以看出，在VO-Adj端的反偏(pian)足够大时(shi)(shi)，Q17的EB结(jie)就可(ke)能因(yin)为(wei)电(dian)压反偏(pian)被击(ji)穿，但(dan)足可(ke)以烧毁与电(dian)阻R14相关的电(dian)阻体、10~20μm宽(kuan)的铝引线(xian)层、引线(xian)孔等局部电(dian)路部分。当外部表征为(wei)器件的输出电(dian)压不(bu)能进行调整时(shi)(shi)，这时(shi)(shi)电(dian)路已经失(shi)效了。

造(zao)成这类击穿(chuan)由于三(san)极管(guan)的(de)(de)(de)BVEBO是很低的(de)(de)(de)（不(bu)妨用(yong)分(fen)立器件(jian)的(de)(de)(de)三(san)极管(guan)实测一(yi)下）。从(cong)图中(zhong)可以(yi)看出(chu)(chu)、接于Adj端的(de)(de)(de)C3是通(tong)过调(diao)整电(dian)(dian)流Iadj进行(xing)充电(dian)(dian)的(de)(de)(de)，但C3被充电(dian)(dian)后自身是没有‘低阻的(de)(de)(de)’放电(dian)(dian)通(tong)路(lu)的(de)(de)(de)。如果(guo)应用(yong)中(zhong)设(she)点(dian)VO较(jiao)高(gao)，由于VC3≈VO，必然有C3两端的(de)(de)(de)电(dian)(dian)压VC3也很高(gao)。如这时出(chu)(chu)现VO≈0时的(de)(de)(de)瞬(shun)间‘短路(lu)’状(zhuang)态，必然造(zao)成Adj→VO端的(de)(de)(de)反偏击穿(chuan)，C3储存的(de)(de)(de)能量通(tong)过调(diao)整端内部电(dian)(dian)路(lu)进行(xing)逆向(xiang)放电(dian)(dian)（放电(dian)(dian)通(tong)路(lu)用(yong)虚线表示）。

在C3＞10μF、VC3＞5V以后，仅需(xu)mS量(liang)(liang)级的(de)瞬(shun)态，就有可能(neng)烧伤(shang)稳压(ya)器调整端内部的(de)局部电路，由于烧毁(hui)所(suo)需(xu)能(neng)量(liang)(liang)很小(xiao)，因(yin)此显微镜下(xia)观察(cha)这种(zhong)造成电路失效(xiao)的(de)烧痕最小(xiao)，一般(ban)仅在20μm×20μm范围以内。

2、保护措施：增加典型(xing)应(ying)用电路图中(zhong)的反(fan)向(xiang)钳位二(er)极管DO。

防(fang)止(zhi)测试中电路样品异常损坏(huai)：

针对正压可(ke)调系(xi)列电(dian)路，测(ce)试仪器使用前应明确必须有反向钳位(wei)二极管DO存在。

其理(li)由(you)很(hen)简单(dan)：对(dui)于正可调(diao)(diao)系列(lie)金(jin)属封装的(de)(de)(de)(de)(de)电(dian)(dian)(dian)路(lu)，金(jin)属壳(qiao)体(ti)的(de)(de)(de)(de)(de)装配孔为第三引出(chu)端(duan)、并被定(ding)义为电(dian)(dian)(dian)路(lu)的(de)(de)(de)(de)(de)输出(chu)端(duan)。对(dui)样(yang)品(pin)进行测试时、先接触插(cha)座的(de)(de)(de)(de)(de)肯(ken)定(ding)是插(cha)入(ru)孔中的(de)(de)(de)(de)(de)1脚(jiao)、2脚(jiao)，即输入(ru)端(duan)和调(diao)(diao)整(zheng)(zheng)端(duan)，一旦1脚(jiao)、2脚(jiao)插(cha)入(ru)后、电(dian)(dian)(dian)源(yuan)立刻通(tong)过调(diao)(diao)整(zheng)(zheng)电(dian)(dian)(dian)流Iadj对(dui)接在(zai)C3充电(dian)(dian)(dian)。如(ru)设定(ding)的(de)(de)(de)(de)(de)VC3较高(gao)、最后接触插(cha)座的(de)(de)(de)(de)(de)输出(chu)端(duan)可能(neng)存在(zai)短路(lu)的(de)(de)(de)(de)(de)条件时（可参(can)考下方C-V图(tu)的(de)(de)(de)(de)(de)△t瞬间），就(jiu)有(you)可能(neng)在(zai)测试中烧伤稳压器(qi)调(diao)(diao)整(zheng)(zheng)内部的(de)(de)(de)(de)(de)局部电(dian)(dian)(dian)路(lu)。声明：由(you)这种情况造成测试后电(dian)(dian)(dian)路(lu)样(yang)品(pin)失效不是个案。

三端稳压器的引脚识别与好坏判断方法

(1)引脚识(shi)别

三(san)端(duan)(duan)(duan)稳压(ya)器(qi)的封(feng)(feng)(feng)装(zhuang)有金属封(feng)(feng)(feng)装(zhuang)和(he)塑料(liao)封(feng)(feng)(feng)装(zhuang)两种，外(wai)形如(ru)同一只大功率晶体管，引脚的排列(lie)如(ru)图9-30所(suo)示(shi)。不同系列(lie)的稳压(ya)器(qi)，其各脚的作用(yong)不同。其中(zhong)最常(chang)用(yong)的W78××系列(lie)稳压(ya)器(qi)，为(wei)(wei)(wei)输入(ru)端(duan)(duan)(duan)(I)，为(wei)(wei)(wei)输出端(duan)(duan)(duan)(O)，为(wei)(wei)(wei)公共端(duan)(duan)(duan)(COM);W79××系列(lie)则(ze)是(shi)为(wei)(wei)(wei)公共端(duan)(duan)(duan)，为(wei)(wei)(wei)输出端(duan)(duan)(duan)，为(wei)(wei)(wei)输入(ru)端(duan)(duan)(duan);常(chang)用(yong)的可调三(san)端(duan)(duan)(duan)稳压(ya)器(qi)LM317T，外(wai)形如(ru)W78××，其为(wei)(wei)(wei)可调端(duan)(duan)(duan)，为(wei)(wei)(wei)输入(ru)端(duan)(duan)(duan)，为(wei)(wei)(wei)输出端(duan)(duan)(duan)。端(duan)(duan)(duan)输出电(dian)压(ya)值由(you)端(duan)(duan)(duan)电(dian)压(ya)变(bian)化调节。

(2)性能(neng)鉴别

对78××和(he)79××系列三端稳压(ya)器(qi)，鉴别其好坏可(ke)使(shi)用(yong)万(wan)用(yong)表(biao) R × 100挡，分别检测其输入端与输出(chu)端的(de)正(zheng)、反向电(dian)阻(zu)值。正(zheng)常时(shi)，阻(zu)值相差在数千(qian)欧以上;若(ruo)阻(zu)值相差很小或近似为(wei)零，说明其已损坏。表(biao)9-14、表(biao)9-15为(wei)最(zui)常用(yong)的(de)78××和(he)79××21种(zhong)三端稳压(ya)器(qi)的(de)实测各(ge)引(yin)脚非在路电(dian)阻(zu)值，供检测时(shi)参(can)考。

表下表 78××三端稳压器各引脚(jiao)非(fei)在路电阻(zu)值(kΩ )

下表(biao) 79××三(san)端稳压器(qi)各引(yin)脚非在路电(dian)阻(zu)值(kΩ )

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