低电压大电流(liu)开(kai)关电路电流(liu)设计(ji)分(fen)析-大电流(liu)开(kai)关电源电路图大全-KIA MOS管
信息(xi)来源：本站 日期：2019-04-17
开关电源的设计
开(kai)(kai)关电(dian)(dian)源的基本结(jie)构主要由7部分(fen)组(zu)成：输(shu)入整(zheng)流滤波电(dian)(dian)路(lu)(lu)、高频开(kai)(kai)关变换器(qi)电(dian)(dian)路(lu)(lu)、整(zheng)流输(shu)出电(dian)(dian)路(lu)(lu)、控(kong)制电(dian)(dian)路(lu)(lu)、保护电(dian)(dian)路(lu)(lu)、辅助电(dian)(dian)源以及显示(shi)电(dian)(dian)路(lu)(lu)。
该设计的主电路拓扑结构如图(tu)1所示，输(shu)入市(shi)网220 V电压，通(tong)过RC滤波及整流桥整流、全桥逆变(bian)、高频变(bian)压器、输(shu)出整流以实(shi)现AC／DC／AC／DC的变(bian)换过程，最终(zhong)得到所需要的15 V直流稳压电源。
低电(dian)(dian)(dian)压大电(dian)(dian)(dian)流(liu)的(de)(de)(de)开关电(dian)(dian)(dian)源对高频干(gan)扰(rao)信号(hao)以及上电(dian)(dian)(dian)瞬(shun)间的(de)(de)(de)浪涌(yong)电(dian)(dian)(dian)流(liu)十分敏感，为(wei)了(le)保(bao)证电(dian)(dian)(dian)路(lu)稳定工(gong)作，消除(chu)来自电(dian)(dian)(dian)网的(de)(de)(de)各种干(gan)扰(rao)，输入的(de)(de)(de)220 V市电(dian)(dian)(dian)首先经RC滤(lv)(lv)波(bo)电(dian)(dian)(dian)路(lu)，对尖峰电(dian)(dian)(dian)压进行(xing)抑制。高频滤(lv)(lv)波(bo)后的(de)(de)(de)电(dian)(dian)(dian)压经整(zheng)(zheng)流(liu)电(dian)(dian)(dian)路(lu)整(zheng)(zheng)流(liu)，得到直流(liu)电(dian)(dian)(dian)压。桥(qiao)式整(zheng)(zheng)流(liu)电(dian)(dian)(dian)路(lu)后面的(de)(de)(de)滤(lv)(lv)波(bo)电(dian)(dian)(dian)具有充放电(dian)(dian)(dian)作用(yong)，滤(lv)(lv)除(chu)整(zheng)(zheng)流(liu)后的(de)(de)(de)交流(liu)成分。
它(ta)是(shi)开关(guan)(guan)电(dian)源的重要(yao)部(bu)分，逆变电(dian)路采(cai)用全桥(qiao)变换，由4个IGBT开关(guan)(guan)管(guan)(guan)组成桥(qiao)的四臂，每(mei)个IGBT并(bing)联1个高速功率二(er)极(ji)管(guan)(guan)，其钳位作用以减小开关(guan)(guan)管(guan)(guan)由导(dao)通(tong)转换为截止时，变压器(qi)产(chan)生的电(dian)压尖峰，以保(bao)护开关(guan)(guan)管(guan)(guan)不(bu)被击穿。IGBT，绝缘栅(zha)双极(ji)型晶体管(guan)(guan)，既具有输入阻(zu)抗高、速度快、热稳定(ding)性(xing)好和(he)驱动电(dian)路简单的优点，又(you)有通(tong)态(tai)电(dian)压低、耐压高的优点。开关(guan)(guan)管(guan)(guan)IGBT的栅(zha)极(ji)接收PWM信号，当门极(ji)加(jia)正压，MOSFET内形(xing)成沟道，并(bing)为PNP晶体管(guan)(guan)提供基(ji)极(ji)电(dian)流，从而使IGBT导(dao)通(tong)。此时从P区注入到(dao)N区进(jin)行电(dian)导(dao)调(diao)制，减小N一的电(dian)阻(zu)值，使高耐压的
IGBT也具有(you)低(di)的(de)(de)(de)通(tong)态(tai)压降(jiang)。在门极上加负(fu)电压时，MOSFET内的(de)(de)(de)沟道消失。PNP晶体(ti)管的(de)(de)(de)基极电流(liu)(liu)被切(qie)断，IGBT关断。T1、T4与，I2、 轮流(liu)(liu)高(gao)频(pin)通(tong)断，将直(zhi)流(liu)(liu)电压变成交(jiao)流(liu)(liu)电压，再经高(gao)频(pin)隔离变压器变成所需的(de)(de)(de)隔离输出(chu)交(jiao)流(liu)(liu)电压。此处的(de)(de)(de)高(gao)频(pin)变压器采(cai)用铁(tie)基铌铜纳米(mi)晶环形铁(tie)芯高(gao)频(pin)变压器，这(zhei)种变压器具有(you)损(sun)耗小(xiao)、漏感(gan)低(di)、体(ti)积小(xiao)等特(te)点。
控制电(dian)路是开关电(dian)源稳定(ding)工作的(de)重要保证，选取UC3825作为控制芯片(pian)，它由振荡器(qi)、PWM比(bi)较器(qi)、PWM锁存(cun)器(qi)、输出驱动器(qi)、限流比(bi)较器(qi)、过流比(bi)较器(qi)、基(ji)准电(dian)压源、故(gu)障锁存(cun)器(qi)、软启(qi)动电(dian)路、欠压锁定(ding)等组成。UC3825最高开关频率可达到(dao)1 MHz，输出脉冲最大传输延
迟时间为(wei)50 ns，具(ju)有软启动控制和欠压锁定等功(gong)能，应用UC3825的(de)(de)(de)功(gong)能设(she)计开关(guan)电源的(de)(de)(de)控制电路(lu)，UC3825的(de)(de)(de)2个脉冲(chong)输(shu)出端(duan)为(wei)开关(guan)管(guan)IGBT提供PWM驱(qu)动信号，交替(ti)输(shu)出脉冲(chong)。因此(ci)，每个输(shu)出端(duan)输(shu)出脉冲(chong)的(de)(de)(de)频率(lv)(lv)是振荡(dang)器(qi)频率(lv)(lv)的(de)(de)(de)1／2，振荡(dang)器(qi)的(de)(de)(de)频率(lv)(lv)为(wei)200 kHz，所以(yi)输(shu)出PWM脉冲(chong)的(de)(de)(de)频率(lv)(lv)为(wei)100 kHz，输(shu)出脉冲(chong)占空比在0％ ～50％以(yi)内调整，为(wei)了避免桥臂短路(lu)，常设(she)置死区(qu)时间，因此(ci)实际应用中一般达不到50％ 。UC3825的(de)(de)(de)4、6、11、14脚的(de)(de)(de)波形如(ru)图2所示(shi)，11、14脚波形经驱(qu)动电路(lu)输(shu)出耦合至IGBT的(de)(de)(de)栅极(ji)，控制IGBT的(de)(de)(de)开通和关(guan)断。开关(guan)管(guan)的(de)(de)(de)驱(qu)动电路(lu)如(ru)图3所示(shi)。
在实(shi)际应用(yong)中，IGBT的(de)驱动(dong)(dong)(dong)电(dian)路(lu)(lu)对IGBT的(de)工作好坏(huai)具(ju)(ju)有相当(dang)(dang)大的(de)影响，因(yin)此，驱动(dong)(dong)(dong)电(dian)路(lu)(lu)要具(ju)(ju)有几点要求：能(neng)(neng)提供(gong)合适的(de)正反向电(dian)压，保(bao)证管子可(ke)靠的(de)开通和(he)(he)关(guan)断；动(dong)(dong)(dong)态驱动(dong)(dong)(dong)能(neng)(neng)力强；信(xin)号传(chuan)输基本(ben)无延(yan)时(shi)，提高(gao)工作速度；当(dang)(dang)出现(xian)异常情况时(shi)，IGBT能(neng)(neng)实(shi)现(xian)软关(guan)断，对管子进行过(guo)(guo)压、过(guo)(guo)流保(bao)护。使用(yong)的(de)驱动(dong)(dong)(dong)电(dian)路(lu)(lu)能(neng)(neng)够提供(gong)+15 V和(he)(he)一15 V的(de)正向和(he)(he)反向电(dian)压，开通和(he)(he)关(guan)断延(yan)时(shi)控制在1 s内，且具(ju)(ju)有过(guo)(guo)压和(he)(he)短(duan)路(lu)(lu)保(bao)护，另(ling)外(wai)，具(ju)(ju)有故障软关(guan)断功能(neng)(neng)。
为了得(de)到(dao)稳定(ding)的(de)输(shu)(shu)出，在(zai)输(shu)(shu)出端(duan)进(jin)行(xing)实时采样(yang)。如图(tu)4所(suo)示(shi)，采样(yang)电(dian)(dian)流流经采样(yang)电(dian)(dian)阻会(hui)产生(sheng)一(yi)个压(ya)降，将(jiang)该(gai)压(ya)降作为反馈信号(hao)(hao)，输(shu)(shu)入到(dao)电(dian)(dian)压(ya)比(bi)(bi)较器(qi)，与给(ji)定(ding)的(de)基准电(dian)(dian)压(ya)进(jin)行(xing)比(bi)(bi)较，产生(sheng)一(yi)个差(cha)(cha)值(zhi)，通过误差(cha)(cha)放(fang)大(da)(da)器(qi)比(bi)(bi)较放(fang)大(da)(da)后，输(shu)(shu)出的(de)差(cha)(cha)值(zhi)信号(hao)(hao)和锯齿波(bo)(或三角波(bo))比(bi)(bi)较，从而改(gai)变输(shu)(shu)出脉冲的(de)宽度，当(dang)输(shu)(shu)出大(da)(da)于
考虑到(dao)开(kai)(kai)关(guan)电(dian)(dian)(dian)源(yuan)的(de)特(te)(te)点和实际的(de)电(dian)(dian)(dian)气特(te)(te)性，为了(le)使其能(neng)在恶劣(lie)环境以(yi)及突发故(gu)障情况下(xia)安全(quan)可靠地工作(zuo)，在实际的(de)制(zhi)作(zuo)中(zhong)充(chong)分(fen)利用UC3825的(de)特(te)(te)性，设计了(le)电(dian)(dian)(dian)压(ya)(ya)电(dian)(dian)(dian)流(liu)保护(hu)电(dian)(dian)(dian)路以(yi)及软(ruan)(ruan)启(qi)动保护(hu)等保护(hu)电(dian)(dian)(dian)路。软(ruan)(ruan)启(qi)动是通过软(ruan)(ruan)启(qi)动(SOFT、START)脚(jiao)的(de)外接电(dian)(dian)(dian)容实现(xian)的(de)。接通电(dian)(dian)(dian)源(yuan)后，软(ruan)(ruan)启(qi)动脚(jiao)外接电(dian)(dian)(dian)容放(fang)电(dian)(dian)(dian)，该(gai)脚(jiao)处于低(di)电(dian)(dian)(dian)平(ping)，误差(cha)放(fang)大(da)器输(shu)出低(di)电(dian)(dian)(dian)平(ping)，开(kai)(kai)关(guan)电(dian)(dian)(dian)源(yuan)无(wu)输(shu)出电(dian)(dian)(dian)压(ya)(ya)。当内部电(dian)(dian)(dian)流(liu)源(yuan)给软(ruan)(ruan)启(qi)动脚(jiao)外接电(dian)(dian)(dian)容充(chong)电(dian)(dian)(dian)时(shi)，误差(cha)放(fang)大(da)器输(shu)出电(dian)(dian)(dian)压(ya)(ya)逐渐升(sheng)(sheng)高，直到(dao)闭环调节功能(neng)开(kai)(kai)始工作(zuo)，开(kai)(kai)关(guan)电(dian)(dian)(dian)源(yuan)输(shu)出电(dian)(dian)(dian)压(ya)(ya)逐渐升(sheng)(sheng)高到(dao)额定值(zhi)。
一(yi)旦限流(liu)(liu)(ILIM)脚的电(dian)(dian)(dian)(dian)平(ping)(ping)超过(guo)(guo)1．2 V，故(gu)=障锁存器(qi)置位，输(shu)出(chu)脚变为(wei)低电(dian)(dian)(dian)(dian)平(ping)(ping)；同时(shi)(shi)，软(ruan)启动(dong)(dong)脚外接(jie)电(dian)(dian)(dian)(dian)容(rong)以250 txA的电(dian)(dian)(dian)(dian)流(liu)(liu)放电(dian)(dian)(dian)(dian)。在软(ruan)启动(dong)(dong)电(dian)(dian)(dian)(dian)容(rong)放完(wan)电(dian)(dian)(dian)(dian)后，限流(liu)(liu)脚电(dian)(dian)(dian)(dian)平(ping)(ping)降到1．2 V以下时(shi)(shi)，故(gu)障锁存器(qi)就不输(shu)出(chu)脉冲。这时(shi)(shi)，故(gu)障锁存器(qi)复位，芯片开始(shi)软(ruan)启动(dong)(dong)过(guo)(guo)程。过(guo)(guo)流(liu)(liu)保(bao)(bao)护和过(guo)(guo)压保(bao)(bao)护是在主电(dian)(dian)(dian)(dian)路中(zhong)采样输(shu)入到保(bao)(bao)护电(dian)(dian)(dian)(dian)路中(zhong)，对电(dian)(dian)(dian)(dian)源起到保(bao)(bao)护作用(yong)。保(bao)(bao)护电(dian)(dian)(dian)(dian)路如图5、图6所示。
在开(kai)关(guan)变换电路和高频变压器之间加上(shang)一个检(jian)测(ce)电流的互感器，将检(jian)测(ce)量输入到(dao)UC3825的9脚限流端，当检(jian)测(ce)到(dao)开(kai)关(guan)
电(dian)(dian)流(liu)(liu)达到上(shang)限(xian)电(dian)(dian)流(liu)(liu)时，上(shang)限(xian)电(dian)(dian)流(liu)(liu)比较(jiao)器(qi)输(shu)(shu)出(chu)为(wei)(wei)高电(dian)(dian)平，比较(jiao)器(qi)的(de)(de)输(shu)(shu)出(chu)就是上(shang)限(xian)电(dian)(dian)流(liu)(liu)触(chu)(chu)发器(qi)的(de)(de)S端，当S为(wei)(wei)高电(dian)(dian)平时，触(chu)(chu)发器(qi)输(shu)(shu)出(chu)为(wei)(wei)高，即触(chu)(chu)发器(qi)输(shu)(shu)出(chu)连接的(de)(de)NPN管子基极变高，NPN管子导通，FB被(bei)拉低，从(cong)而(er)保护电(dian)(dian)路的(de)(de)过(guo)流(liu)(liu)问题。
在主电(dian)(dian)(dian)(dian)路(lu)输出端(duan)直接(jie)实时采(cai)样电(dian)(dian)(dian)(dian)压，与给定电(dian)(dian)(dian)(dian)压比(bi)较后，把(ba)信号放(fang)大，经过(guo)(guo)隔(ge)离后输入(ru)到(dao)(dao)(dao)UC3825的(de)(de)(de)2脚，控制(zhi)(zhi)PWM信号的(de)(de)(de)占(zhan)空(kong)比(bi)，从而控制(zhi)(zhi)主电(dian)(dian)(dian)(dian)路(lu)输出电(dian)(dian)(dian)(dian)压的(de)(de)(de)变化(hua)。过(guo)(guo)热(re)(re)保护是通(tong)(tong)(tong)过(guo)(guo)检验系统中(zhong)的(de)(de)(de)热(re)(re)继(ji)电(dian)(dian)(dian)(dian)器通(tong)(tong)(tong)断(duan)来(lai)具体实现的(de)(de)(de)，当开关电(dian)(dian)(dian)(dian)源正常(chang)工(gong)作(zuo)时，热(re)(re)继(ji)电(dian)(dian)(dian)(dian)器处于常(chang)通(tong)(tong)(tong)状态，一旦温(wen)度过(guo)(guo)热(re)(re)，超过(guo)(guo)额定值，热(re)(re)继(ji)电(dian)(dian)(dian)(dian)器开关会断(duan)开，通(tong)(tong)(tong)断(duan)的(de)(de)(de)信号会反馈到(dao)(dao)(dao)控制(zhi)(zhi)电(dian)(dian)(dian)(dian)路(lu)中(zhong)，达到(dao)(dao)(dao)过(guo)(guo)热(re)(re)保护的(de)(de)(de)作(zuo)用。
以(yi)太网接(jie)口则是(shi)实现将(jiang)数据远传调度中心功能(neng) 。该设计(ji)采(cai)用的嵌入式微(wei)处(chu)理器是(shi)以(yi)ARM7TDMI为内核的一款16／32位(wei)RISC处(chu)理器S3C44BOX，该处(chu)理器通(tong)过提供全(quan)面(mian)的、通(tong)用的片上外(wai)设，减(jian)少了外(wai)围(wei)的元器件配置，从(cong)而使(shi)系(xi)统(tong)成(cheng)本大(da)为降低。该系(xi)统(tong)采(cai)用杭(hang)州立宇泰(tai)公司的开发板(ban)设计(ji)，其(qi)主要结构如图5所示。
L296大电(dian)流开关电(dian)源芯(xin)片组成(cheng)的稳压电(dian)源电(dian)路图(tu)
L296大电(dian)流开关电(dian)源芯片(pian)(pian)组成的稳(wen)压(ya)电(dian)源电(dian)路图如图a、b、c所示为(wei)由L296单片(pian)(pian)大电(dian)流开关电(dian)源芯片(pian)(pian)组成的5～15V、4A稳(wen)压(ya)电(dian)源。
L296单片(pian)大电流(liu)开关电源芯片(pian)特点(dian)是：
（1）保(bao)护(hu)功(gong)能完善(shan)。内(nei)设有(you)软启动、过(guo)流、过(guo)热、过(guo)压保(bao)护(hu)。
（2）最(zui)大输出(chu)电流(liu)为(wei)4A，功率为(wei)160W，输出(chu)电压在5.1～40V之间可调(diao)。
（3）设(she)(she)有特殊(shu)功能：工作禁止控(kong)制、同步控(kong)制（在几(ji)片(pian)作多路(lu)(lu)(lu)输出时，保(bao)证工作频率一致）、重置电(dian)(dian)路(lu)(lu)(lu)（可提(ti)供电(dian)(dian)源工作状态(tai)的检测信号）、撬棍过(guo)压保(bao)护电(dian)(dian)路(lu)(lu)(lu)（当输出电(dian)(dian)压超过(guo)预设(she)(she)额定电(dian)(dian)压20％时，产生(sheng)一个100mA的驱(qu)动信号，用来触发外部保(bao)护电(dian)(dian)路(lu)(lu)(lu)动作）。如(ru)图c所示(shi)为电(dian)(dian)流扩展电(dian)(dian)路(lu)(lu)(lu)形式(shi)。
电(dian)(dian)(dian)路(lu)如图所(suo)示(shi)，该电(dian)(dian)(dian)路(lu)可以获(huo)得更大的(de)(de)输(shu)(shu)出功率(lv)，只需更改部(bu)分(fen)器(qi)件。图中(zhong)左(zuo)边(bian)的(de)(de)电(dian)(dian)(dian)路(lu)R1，L1，D1，C1至C7是(shi)(shi)常规的(de)(de)共模滤波(bo)和(he)整流电(dian)(dian)(dian)路(lu)，获(huo)取约300V的(de)(de)直流电(dian)(dian)(dian)压(ya)供DC-DC变(bian)换电(dian)(dian)(dian)路(lu)使(shi)用(yong)；最右边(bian)电(dian)(dian)(dian)路(lu)L5，C11等是(shi)(shi)普通的(de)(de)LC滤波(bo)电(dian)(dian)(dian)路(lu)；IC2，D8，R9，R10组成电(dian)(dian)(dian)压(ya)反馈电(dian)(dian)(dian)路(lu)，形成闭(bi)环结构，稳定电(dian)(dian)(dian)源输(shu)(shu)出电(dian)(dian)(dian)压(ya)；中(zhong)间部(bu)分(fen)是(shi)(shi)DC-DC变(bian)换器(qi)，降噪声的(de)(de)关键是(shi)(shi)对这一部(bu)分(fen)的(de)(de)电(dian)(dian)(dian)路(lu)进行(xing)适当处理(li)。
本文(wen)介绍一种型号为L4960的(de)DC-DC变(bian)换的(de)开(kai)关电(dian)(dian)源，如图(tu)所示。从图(tu)中可以(yi)看出电(dian)(dian)路(lu)十分简单(dan)，它(ta)的(de)使(shi)用(yong)方法与LM317非常相似，先使(shi)用(yong)50Hz电(dian)(dian)源变(bian)压(ya)(ya)器进行(xing)AC-AC变(bian)换，将(jiang)~220V降至~34V，然后由VD1和C1整流滤波(bo)为46V左右的(de)直流电(dian)(dian)压(ya)(ya)输入给L4960，再由L4960进行(xing)DC-DC变(bian)换，这时输出电(dian)(dian)压(ya)(ya)的(de)变(bian)化(hua)范围下可调至5V，上(shang)调至40V，最大输出电(dian)(dian)流可达2.5A（还可以(yi)接大功率开(kai)关管进行(xing)扩流），并且(qie)内设完(wan)善的(de)保护(hu)(hu)功能，如过流保护(hu)(hu)、过热保护(hu)(hu)等。
尽管L4960的(de)使(shi)用方法与(yu)LM317差不多，但开关电源的(de)L4960与(yu)线性(xing)电源的(de)LM317相(xiang)比，效率(lv)不可同曰而语，L4960最(zui)(zui)大可输出(chu)100W的(de)功率(lv)（Pmax=40V*2.5A=100W），但本身(shen)最(zui)(zui)多只消耗7W，所以散(san)热器很(hen)小，制(zhi)作容易。与(yu)L4960类似的(de)还(hai)有L296，其基(ji)本参数与(yu)L4960相(xiang)同，只是最(zui)(zui)大输出(chu)电流可高达4A，且具有更(geng)多的(de)保护功能(neng)，封(feng)装(zhuang)形式也(ye)不一样。
所示为触(chu)(chu)摸(mo)(mo)延时(shi)开关电(dian)(dian)源电(dian)(dian)路。手触(chu)(chu)动触(chu)(chu)摸(mo)(mo)片时(shi)，相当于给VT1基极一个触(chu)(chu)发(fa)信号使(shi)之(zhi)(zhi)导通，于是VI2也导通，电(dian)(dian)源经VT2给电(dian)(dian)容器cz充(chong)(chong)电(dian)(dian)，当充(chong)(chong)电(dian)(dian)电(dian)(dian)压足够高时(shi)，VT2截止(zhi)，VT3导通，VT4也随之(zhi)(zhi)导通，灯泡(pao)发(fa)光、照明(ming)。当C2上的电(dian)(dian)荷慢(man)慢(man)放掉后vr3截止(zhi)。VT4也截止(zhi)使(shi)灯泡(pao)熄灭。
触摸延(yan)时开关电源电路(lu)
开关电源(yuan)电路(lu)(lu)图原理图如图所示(shi)。虽然稳压精度不高，但能满足一般要求，且电路(lu)(lu)简洁，采用常规元件，成(cheng)本极低，输(shu)出允许开路(lu)(lu)和短路(lu)(lu)。
开关电源电路图
市电(dian)(dian)(dian)经D1整流(liu)及C1滤波后得到约(yue)300V的(de)(de)直流(liu)电(dian)(dian)(dian)压(ya)加在(zai)变压(ya)器(qi)的(de)(de)①脚（L1的(de)(de)上端(duan)），同(tong)时此(ci)电(dian)(dian)(dian)压(ya)经R1给V1加上偏(pian)置后后使(shi)其微(wei)微(wei)导通(tong)，有(you)电(dian)(dian)(dian)流(liu)流(liu)过L1，同(tong)时反馈线圈L2的(de)(de)上端(duan)（变压(ya)器(qi)的(de)(de)③脚）形(xing)(xing)成正电(dian)(dian)(dian)压(ya)，此(ci)电(dian)(dian)(dian)压(ya)经C4、R3反馈给V1，使(shi)其更导通(tong)，乃至饱和，最(zui)后随(sui)反馈电(dian)(dian)(dian)流(liu)的(de)(de)减小，V1迅速退出饱和并截止(zhi)，如此(ci)循(xun)环形(xing)(xing)成振荡，在(zai)次(ci)级线圈L3上感应(ying)出所需(xu)的(de)(de)输出电(dian)(dian)(dian)压(ya)。
L2是反馈(kui)线圈，同(tong)(tong)时也与D4、D3、C3一起组成稳压(ya)(ya)电路(lu)。当线圈L3经D6整流后(hou)在C5上(shang)的(de)电压(ya)(ya)升高后(hou)，同(tong)(tong)时也表现为L2经D4整流后(hou)在C3负(fu)极上(shang)的(de)电压(ya)(ya)更低(di)，当低(di)至约为稳压(ya)(ya)管(guan)D3（9V）的(de)稳压(ya)(ya)值时D3导通，使(shi)V1有基(ji)极短路(lu)到地(di)，关断V1，最终使(shi)输出电压(ya)(ya)降低(di)。电路(lu)中R4、D5、V2组成过流保(bao)护电路(lu)。
当某些(xie)原(yuan)因(yin)引(yin)起V1的(de)(de)工作电(dian)(dian)流大(da)太时，R4上产生的(de)(de)电(dian)(dian)压(ya)互感器(qi)经D5加至V2基极，V2导(dao)通，V1基极电(dian)(dian)压(ya)下(xia)降，使V1电(dian)(dian)流减(jian)小(xiao)。D3的(de)(de)稳压(ya)值(zhi)理论为9V+0.5~0.7V，在实(shi)际应用时，若要改(gai)变输出电(dian)(dian)压(ya)，只要更换(huan)不同稳压(ya)值(zhi)的(de)(de)D3即可，稳压(ya)值(zhi)越小(xiao)，输出电(dian)(dian)压(ya)越低，反之则越高(gao)。
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