利盈娱乐(中国)创新平台有限公司

开(kai)关(guan)电源常用(yong)的(de)控制(zhi)方式有脉冲宽度调耐(PWM、脉冲频(pin)率(lv)调制(zhi)(PFM)和混合调制(zhi)三种。目前大多数开(kai)关(guan)电源都采用(yong)了PWM控制(zhi)。在髅开关电源中．如开关MOS管老是周期性地通(tong)／断，PWM控制(zhi)电路只是改(gai)变每个(ge)周期的脉冲宽度而已。跟着半(ban)导体(ti)技术的高速发(fa)展，开关电源(yuan)控制(zhi)电路的集(ji)成化水甲(jia)不断进步，外(wai)接电路越来越简(jian)朴，出(chu)(chu)产a益(yi)简(jian)化，成今日益(yi)降低．出(chu)(chu)产控制(zhi)驱动芯(xin)片的(de)厂家(jia)日(ri)益增多．其种类也日(ri)益多样化．本章将先容几种常用的(de)PWM集成控制芯(xin)片的(de)内(nei)部结(jie)构及其典型(xing)应用电路(lu)：自(zi)激式PWM控制电路(lu)

1、PWM控制(zhi)电(dian)路工作原理(li)

自(zi)渐式PWM控制电路具有(you)电路结(jie)构简(jian)朴、使用元器件少、本钱低等特点(dian)，广泛应用于50w以下的开关(guan)电源(yuan)中。自(zi)激式PWM控制电路的工(gong)作原(yuan)理如图7-l所示。

例(li)如在MOS管VT导通期(qi)间(tON，加在变(bian)压(ya)(ya)器(qi)低(di)级绕组(zu)(zu)Np两(liang)真(zhen)个(ge)电(dian)(dian)(dian)(dian)(dian)压(ya)(ya)为(wei)U，同时变(bian)压(ya)(ya)器(qi)T的(de)反馈绕组(zu)(zu)ND上感应出电(dian)(dian)(dian)(dian)(dian)压(ya)(ya)U，该电(dian)(dian)(dian)(dian)(dian)压(ya)(ya)为(wei)正反馈电(dian)(dian)(dian)(dian)(dian)压(ya)(ya)，加到VT基(ji)极上并使(shi)其进一(yi)步加速(su)导通，这时开关(guan)变(bian)压(ya)(ya)器(qi)T的(de)低(di)级绕组(zu)(zu)N。两(liang)真(zhen)个(ge)电(dian)(dian)(dian)(dian)(dian)压(ya)(ya)U＝u-

UcE。T的次级(ji)(ji)绕(rao)组(zu)NS上(shang)感(gan)(gan)应的电(dian)(dian)压(Ns/Np)U１对(dui)于整流(liu)碳化硅二极(ji)管VD来说为(wei)(wei)(wei)反向电(dian)(dian)压，因此，次级(ji)(ji)绕(rao)组(zu)中无电(dian)(dian)流(liu)。低级(ji)(ji)饶组(zu)电(dian)(dian)流(liu)为(wei)(wei)(wei)变(bian)压器(qi)的励(li)磁电(dian)(dian)流(liu)，设(she)低级(ji)(ji)绕(rao)组(zu)的电(dian)(dian)感(gan)(gan)为(wei)(wei)(wei)L、导(dao)通时(shi)间为(wei)(wei)(wei)L，则该励(li)磁电(dian)(dian)流(liu)为(wei)(wei)(wei)ut/Lr,，并随(sui)时(shi)间成比例增大(da)。VT的电(dian)(dian)流(liu)增大(da)，若其(qi)基极(ji)电(dian)(dian)流(liu)不能使其(qi)保持饱和状态，则VT脱离饱和而UcE随(sui)之增大(da)。因为(wei)(wei)(wei)UCE增加(jia)，所(suo)以(yi)变(bian)压器(qi)低级(ji)(ji)绕(rao)组(zu)的电(dian)(dian)压下降，基极(ji)电(dian)(dian)压UB随(sui)之下降，UCE进一(yi)步增加(jia)。因为(wei)(wei)(wei)正反馈作用，导(dao)致开关(guan)晶体(ti)场效应管讯速截止UT从导通到截(jie)止(zhi)瞬间(jian)(jian)，磁(ci)场的(de)(de)大小和方向都不变(bian)(bian)(bian)(bian)，保持安匝数相同，因(yin)此变(bian)(bian)(bian)(bian)压器(qi)次(ci)(ci)级(ji)(ji)绕(rao)(rao)组(zu)(zu)(zu)的(de)(de)感应(ying)(ying)电(dian)(dian)压为(wei)(wei)上正(zheng)下负(fu)，二(er)极(ji)(ji)管VD跨导通。这(zhei)时，若(ruo)输出电(dian)(dian)压为(wei)(wei)Uo，整流(liu)二(er)极(ji)(ji)管的(de)(de)压降(jiang)为(wei)(wei)UD，则(ze)变(bian)(bian)(bian)(bian)压器(qi)次(ci)(ci)级(ji)(ji)绕(rao)(rao)组(zu)(zu)(zu)电(dian)(dian)压U：=%-U。。若(ruo)次(ci)(ci)级(ji)(ji)绕(rao)(rao)组(zu)(zu)(zu)的(de)(de)电(dian)(dian)感为(wei)(wei)k，则(ze)流(liu)经(jing)二(er)VD的(de)(de)电(dian)(dian)流(liu)，。的(de)(de)波形(xing)如(ru)图7-2所示(shi)。电(dian)(dian)流(liu)ID的(de)(de)下降(jiang)速率(lv)为(wei)(wei)U：，／Ls．变(bian)(bian)(bian)(bian)压器(qi)低(di)级(ji)(ji)绕(rao)(rao)组(zu)(zu)(zu)存储的(de)(de)能(neng)量耦合到次(ci)(ci)级(ji)(ji)绕(rao)(rao)组(zu)(zu)(zu)，供应(ying)(ying)输出端负(fu)载。经(jing)由某一(yi)时间(jian)(jian)tOFF后(hou)，若(ruo)变(bian)(bian)(bian)(bian)压器(qi)低(di)级(ji)(ji)绕(rao)(rao)组(zu)(zu)(zu)中储存的(de)(de)能(neng)量都转移到输出侧(ce)，则(ze)二(er)极(ji)(ji)管VD截(jie)止(zhi)，变(bian)(bian)(bian)(bian)压器(qi)各绕(rao)(rao)组(zu)(zu)(zu)的(de)(de)电(dian)(dian)压瞬间(jian)(jian)为(wei)(wei)零。但启动绕(rao)(rao)组(zu)(zu)(zu)R。中的(de)(de)部门电(dian)(dian)流(liu)为(wei)(wei)VT的(de)(de)基(ji)极(ji)(ji)电(dian)(dian)流(liu)，VT重新(xin)导通，有集电(dian)(dian)极(ji)(ji)电(dian)(dian)流(liu)流(liu)过，并构成正(zheng)反馈，VT再次(ci)(ci)迅速导通，进(jin)入(ru)F工作(zuo)周(zhou)期，电(dian)路就持续(xu)工作(zuo)在(zai)自激振荡状态。

输(shu)入电(dian)源(yuan)U，一路通(tong)过(guo)开(kai)(kai)关变压器T的(de)低(di)级绕组连接到(dao)开(kai)(kai)关晶体管，VT的(de)集电(dian)檄(xi)，另一路通(tong)过(guo)启(qi)动(dong)电(dian)阻R加(jia)(jia)到(dao)wr的(de)基(ji)极。接通(tong)输(shu)入电(dian)源(yuan)U后(hou)，通(tong)过(guo)启(qi)动(dong)电(dian)阻R的(de)电(dian)流(liu)I（启(qi)动(dong)电(dian)流(liu))流(liu)经VT的(de)基(ji)及，VT导通(tong)，其(qi)集电(dian)极电(dian)流(liu)IP必定由(you)零开(kai)(kai)始逐渐增加(jia)(jia)。

在(zai)(zai)VT导通(tong)(tong)期(qi)间(jian)(jian)(oN)，变压器(qi)T的低级绕(rao)组从输(shu)入侧(ce)蓄积能(neng)(neng)量；在(zai)(zai)VT截(jie)止期(qi)间(jian)(jian)（tOFF），变压器(qi)T蓄积的能(neng)(neng)量通(tong)(tong)过次级绕(rao)组开(kai)释供应输(shu)出(chu)负载(zai)，此时低级绕(rao)组处于无电流畅通(tong)(tong)流畅的间(jian)(jian)歇工作方式(shi)。

在(zai)tON期间VT导的能量为

在tOFF期(qi)间，初级绕(rao)(rao)组(zu)侧无(wu)电流(liu)．ton期(qi)间压(ya)器T中蓄积的(de)能量通(tong)过次(ci)(ci)级绕(rao)(rao)组(zu)Ls释(shi)放(fang)。从ton转换(huan)到fOFF瞬间，初次(ci)(ci)缀绕(rao)(rao)组(zu)安匝(za)数相(xiang)等(deng)，因此，若变(bian)压(ya)器初级侧的(de)能量全部(bu)传递给次(ci)(ci)级侧，则(ze)有

式中，Ns为次级(ji)绕组匝(za)数(shu)；Is为绕组电流：

电L。与Lp比(bi)与绕(rao)组匝数的平方(fang)呈正比(bi)，即

振荡频率(lv)为(wei)f，则(ze)每秒提(ti)供的功率(lv)P=EF设变压器效率(lv)为(wei)'7，输出电(dian)压和电(dian)流分别(bie)为(wei)Uc，和，．，，则(ze)输出功率(lv)Pn为(wei)

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