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MOS管的(de)驱(qu)(qu)动(dong)(dong)(dong)对其工作(zuo)(zuo)效果起(qi)着决定性的(de)作(zuo)(zuo)用。设计(ji)师既要(yao)考(kao)虑减(jian)少(shao)开关损耗，又要(yao)求驱(qu)(qu)动(dong)(dong)(dong)波形较好即振荡小、过(guo)冲小、EMI小。这两方面往(wang)往(wang)是(shi)互相矛(mao)盾(dun)的(de)，需要(yao)寻求一(yi)个平衡点(dian)，即驱(qu)(qu)动(dong)(dong)(dong)电(dian)路的(de)优(you)化设计(ji)。驱(qu)(qu)动(dong)(dong)(dong)电(dian)路的(de)优(you)化设计(ji)包(bao)含两部(bu)分内容：一(yi)是(shi)最优(you)的(de)驱(qu)(qu)动(dong)(dong)(dong)电(dian)流、电(dian)压的(de)波形；二(er)是(shi)最优(you)的(de)驱(qu)(qu)动(dong)(dong)(dong)电(dian)压、电(dian)流的(de)大(da)小。在进(jin)行驱(qu)(qu)动(dong)(dong)(dong)电(dian)路优(you)化设计(ji)之前(qian)，必须先清楚(chu)MOS管的(de)模型、MOS管的(de)开关过(guo)程(cheng)、MOS管的(de)栅(zha)极(ji)电(dian)荷以及MOS管的(de)输入输出电(dian)容、跨接电(dian)容、等效电(dian)容等参数对驱(qu)(qu)动(dong)(dong)(dong)的(de)影(ying)响。

MOS管的模型

MOS管的等效电路模型及寄(ji)生参数(shu)如图1所示。图1中各部分(fen)的物理(li)意义为：

（1）LG和LG代表封装(zhuang)端到实际的栅极线路的电感和电阻。

（2）C1代表从栅极到源端(duan)N+间(jian)的电容，它(ta)的值(zhi)是(shi)由结构所固定的。

（3）C2+C4代表从栅(zha)极(ji)(ji)到(dao)(dao)源(yuan)极(ji)(ji)P区间(jian)的(de)电(dian)容。C2是(shi)电(dian)介质电(dian)容，共值是(shi)固定的(de)。而C4是(shi)由源(yuan)极(ji)(ji)到(dao)(dao)漏极(ji)(ji)的(de)耗尽区的(de)大小决定，并随(sui)栅(zha)极(ji)(ji)电(dian)压(ya)的(de)大小而改(gai)变。当栅(zha)极(ji)(ji)电(dian)压(ya)从0升到(dao)(dao)开启电(dian)压(ya)UGS（th）时(shi)，C4使整个栅(zha)源(yuan)电(dian)容增加(jia)10%～15%。

（4）C3+C5是由(you)一个固定大(da)小(xiao)的电介质电容和一个可变(bian)电容构成，当(dang)(dang)漏极电压改变(bian)极性时，其可变(bian)电容值(zhi)变(bian)得相当(dang)(dang)大(da)。

（5）C6是随漏极电压变换的(de)漏源电容。

MOS管(guan)输入电(dian)容（Ciss）、跨(kua)接电(dian)容(rong)(rong)（Crss）、输出(chu)电(dian)容(rong)(rong)（Coss）和栅(zha)源(yuan)电(dian)容(rong)(rong)、栅(zha)漏电(dian)容(rong)(rong)、漏源(yuan)电(dian)容(rong)(rong)间(jian)的(de)关系(xi)如下：

MOS管的开通过程

开(kai)(kai)(kai)关(guan)(guan)(guan)(guan)(guan)管(guan)(guan)(guan)(guan)的(de)(de)开(kai)(kai)(kai)关(guan)(guan)(guan)(guan)(guan)模式电(dian)(dian)(dian)(dian)(dian)(dian)路如图(tu)(tu)2所示，二(er)极(ji)管(guan)(guan)(guan)(guan)可是外接的(de)(de)或(huo)MOS管(guan)(guan)(guan)(guan)固(gu)有的(de)(de)。开(kai)(kai)(kai)关(guan)(guan)(guan)(guan)(guan)管(guan)(guan)(guan)(guan)在(zai)开(kai)(kai)(kai)通(tong)(tong)时(shi)(shi)的(de)(de)二(er)极(ji)管(guan)(guan)(guan)(guan)电(dian)(dian)(dian)(dian)(dian)(dian)压、电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)波(bo)形如图(tu)(tu)3所示。在(zai)图(tu)(tu)3的(de)(de)阶(jie)段(duan)1开(kai)(kai)(kai)关(guan)(guan)(guan)(guan)(guan)管(guan)(guan)(guan)(guan)关(guan)(guan)(guan)(guan)(guan)断，开(kai)(kai)(kai)关(guan)(guan)(guan)(guan)(guan)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)为零，此(ci)时(shi)(shi)二(er)极(ji)管(guan)(guan)(guan)(guan)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)和电(dian)(dian)(dian)(dian)(dian)(dian)感(gan)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)相(xiang)(xiang)等；在(zai)阶(jie)段(duan)2开(kai)(kai)(kai)关(guan)(guan)(guan)(guan)(guan)导通(tong)(tong)，开(kai)(kai)(kai)关(guan)(guan)(guan)(guan)(guan)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)上(shang)(shang)升(sheng)，同时(shi)(shi)二(er)极(ji)管(guan)(guan)(guan)(guan)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)下(xia)降。开(kai)(kai)(kai)关(guan)(guan)(guan)(guan)(guan)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)上(shang)(shang)升(sheng)的(de)(de)斜率(lv)和二(er)极(ji)管(guan)(guan)(guan)(guan)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)下(xia)降的(de)(de)斜率(lv)的(de)(de)绝(jue)对(dui)值(zhi)(zhi)相(xiang)(xiang)同，符(fu)号相(xiang)(xiang)反；在(zai)阶(jie)段(duan)3开(kai)(kai)(kai)关(guan)(guan)(guan)(guan)(guan)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)继续(xu)上(shang)(shang)升(sheng)，二(er)极(ji)管(guan)(guan)(guan)(guan)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)继续(xu)下(xia)降，并且二(er)极(ji)管(guan)(guan)(guan)(guan)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)符(fu)号改变，由正转到负；在(zai)阶(jie)段(duan)4，二(er)极(ji)管(guan)(guan)(guan)(guan)从负的(de)(de)反向(xiang)最大电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)IRRM开(kai)(kai)(kai)始(shi)减小，它们斜率(lv)的(de)(de)绝(jue)对(dui)值(zhi)(zhi)相(xiang)(xiang)等；在(zai)阶(jie)段(duan)5开(kai)(kai)(kai)关(guan)(guan)(guan)(guan)(guan)管(guan)(guan)(guan)(guan)完全开(kai)(kai)(kai)通(tong)(tong)，二(er)极(ji)管(guan)(guan)(guan)(guan)的(de)(de)反向(xiang)恢复完成，开(kai)(kai)(kai)关(guan)(guan)(guan)(guan)(guan)管(guan)(guan)(guan)(guan)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)等于(yu)电(dian)(dian)(dian)(dian)(dian)(dian)感(gan)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)。

图(tu)4是(shi)存储电(dian)荷高或(huo)低的(de)两种二极管(guan)(guan)电(dian)流(liu)、电(dian)压波形。从(cong)图(tu)中可以看(kan)出存储电(dian)荷少时(shi)，反向电(dian)压的(de)斜率大(da)，并且(qie)会产生有害的(de)振动。而前置电(dian)流(liu)低则存储电(dian)荷少，即在空载或(huo)轻载时(shi)是(shi)最坏条件。所以进行优化(hua)驱动电(dian)路(lu)设计时(shi)应着重考虑前置电(dian)流(liu)低的(de)情况(kuang)，即空载或(huo)轻载的(de)情况(kuang)，应使(shi)这(zhei)时(shi)二极管(guan)(guan)产生的(de)振动在可接受范围内。

栅极电荷QG和驱动效果的关系

栅(zha)极电(dian)(dian)荷QG是(shi)使栅(zha)极电(dian)(dian)压从(cong)0升到10V所(suo)需的栅(zha)极电(dian)(dian)荷，它可(ke)以表示为(wei)驱(qu)动电(dian)(dian)流值(zhi)与开通时间之(zhi)积或栅(zha)极电(dian)(dian)容值(zhi)与栅(zha)极电(dian)(dian)压之(zhi)积。现在大部分MOS管的栅(zha)极电(dian)(dian)荷QG值(zhi)从(cong)几十纳(na)库(ku)仑到一(yi)、两百纳(na)库(ku)仑。

栅(zha)(zha)(zha)极(ji)(ji)电荷(he)QG包含了两个部分(fen)：栅(zha)(zha)(zha)极(ji)(ji)到源极(ji)(ji)电荷(he)QGS；栅(zha)(zha)(zha)极(ji)(ji)到漏极(ji)(ji)电荷(he)QGD—即“Miller”电荷(he)。QGS是(shi)使栅(zha)(zha)(zha)极(ji)(ji)电压(ya)从(cong)0升(sheng)到门限值(zhi)（约3V）所需(xu)(xu)电荷(he)；QGD是(shi)漏极(ji)(ji)电压(ya)下降时(shi)克服“Miller”效应所需(xu)(xu)电荷(he)，这(zhei)存在(zai)于UGS曲线比较平坦的第二段（如(ru)图5所示），此时(shi)栅(zha)(zha)(zha)极(ji)(ji)电压(ya)不(bu)变、栅(zha)(zha)(zha)极(ji)(ji)电荷(he)积聚而漏极(ji)(ji)电压(ya)急聚下降，也就是(shi)在(zai)这(zhei)时(shi)候需(xu)(xu)要(yao)驱动尖(jian)峰(feng)电流限制，这(zhei)由芯片内部完(wan)成或(huo)外接电阻完(wan)成。实(shi)际的QG还可以(yi)略(lve)大，以(yi)减小等(deng)效RON，但是(shi)太大也无益，所以(yi)10V到12V的驱动电压(ya)是(shi)比较合理的。这(zhei)还包含一个重要(yao)的事实(shi)：需(xu)(xu)要(yao)一个高的尖(jian)峰(feng)电流以(yi)减小MOS管损耗和转(zhuan)换时(shi)间。

重要是的对于IC来说，MOS管的平均电容负荷并不是MOS管的输入电容Ciss,而是等效输入电容Ceff(Ceff=QG/UGS)，即整个0

漏(lou)极电流在QG波(bo)(bo)形(xing)的(de)(de)QGD阶段(duan)出现，该段(duan)漏(lou)极电压依然很(hen)高，MOS管的(de)(de)损耗该段(duan)最大，并随(sui)UDS的(de)(de)减小(xiao)而减小(xiao)。QGD的(de)(de)大部分(fen)用来减小(xiao)UDS从(cong)关断电压到(dao)UGS(th)产生的(de)(de)“Miller”效(xiao)应。QG波(bo)(bo)形(xing)第三段(duan)的(de)(de)等效(xiao)负载电容是：

优化栅极驱动设计

在大多数的(de)(de)开关功率应(ying)用电(dian)路中(zhong)(zhong)，当栅极(ji)被驱动(dong)，开关导通时(shi)漏(lou)极(ji)电(dian)流(liu)上(shang)升(sheng)的(de)(de)速度是漏(lou)极(ji)电(dian)压下降(jiang)速度的(de)(de)几倍，这将造成功率损耗增加。为了(le)解决问(wen)题(ti)(ti)可(ke)以增加栅极(ji)驱动(dong)电(dian)流(liu)，但增加栅极(ji)驱动(dong)上(shang)升(sheng)斜率又将带来过冲、振(zhen)荡、EMI等(deng)问(wen)题(ti)(ti)。优化(hua)栅极(ji)驱动(dong)设(she)计，正(zheng)是在互相矛盾(dun)的(de)(de)要(yao)求中(zhong)(zhong)寻求一(yi)个(ge)平(ping)(ping)衡点，而这个(ge)平(ping)(ping)衡点就(jiu)是开关导通时(shi)漏(lou)极(ji)电(dian)流(liu)上(shang)升(sheng)的(de)(de)速度和(he)漏(lou)极(ji)电(dian)压下降(jiang)速度相等(deng)这样(yang)一(yi)种(zhong)波形(xing)，理想的(de)(de)驱动(dong)波形(xing)如(ru)图6所示(shi)。

图6的(de)(de)(de)UGS波(bo)形(xing)(xing)包括了(le)这样几部分：UGS第(di)一段(duan)是(shi)快(kuai)(kuai)速上(shang)升(sheng)到门限电(dian)(dian)(dian)(dian)压(ya)；UGS第(di)二段(duan)是(shi)比较缓(huan)的(de)(de)(de)上(shang)升(sheng)速度(du)以减慢漏极电(dian)(dian)(dian)(dian)流(liu)(liu)的(de)(de)(de)上(shang)升(sheng)速度(du)，但(dan)此时的(de)(de)(de)UGS也必须满(man)足所(suo)需的(de)(de)(de)漏极电(dian)(dian)(dian)(dian)流(liu)(liu)值；UGS第(di)四段(duan)快(kuai)(kuai)速上(shang)升(sheng)使漏极电(dian)(dian)(dian)(dian)压(ya)快(kuai)(kuai)速下降；UGS第(di)五段(duan)是(shi)充电(dian)(dian)(dian)(dian)到最后的(de)(de)(de)值。当(dang)然(ran)，要得到完全一样的(de)(de)(de)驱(qu)动(dong)波(bo)形(xing)(xing)是(shi)很困难的(de)(de)(de)，但(dan)是(shi)可以得到一个大概的(de)(de)(de)驱(qu)动(dong)电(dian)(dian)(dian)(dian)流(liu)(liu)波(bo)形(xing)(xing)，其(qi)上(shang)升(sheng)时间(jian)(jian)等于理想的(de)(de)(de)漏极电(dian)(dian)(dian)(dian)压(ya)下降时间(jian)(jian)或(huo)漏极电(dian)(dian)(dian)(dian)流(liu)(liu)上(shang)升(sheng)的(de)(de)(de)时间(jian)(jian)，并(bing)且(qie)具有足够(gou)的(de)(de)(de)尖(jian)峰值来充电(dian)(dian)(dian)(dian)开关期间(jian)(jian)的(de)(de)(de)较大等效电(dian)(dian)(dian)(dian)容。该栅(zha)极尖(jian)峰电(dian)(dian)(dian)(dian)流(liu)(liu)IP的(de)(de)(de)计(ji)算是(shi)：电(dian)(dian)(dian)(dian)荷必须完全满(man)足开关时期的(de)(de)(de)寄(ji)生电(dian)(dian)(dian)(dian)容所(suo)需。

6 应用实例

在笔者设计的48V50A电(dian)路(lu)中采用双晶(jing)体管正(zheng)激(ji)式(shi)变换电(dian)路(lu)，其开关管采用IXFH24N50，其参数为(wei)：

根(gen)据(ju)如前所述，驱动电(dian)压(ya)、电(dian)流(liu)的理想(xiang)波(bo)形(xing)不应该是一条(tiao)直线，而应该是如图6所示的波(bo)形(xing)。实验波(bo)形(xing)见图7。

7 结论

本文详细介绍(shao)了MOS管的(de)电(dian)路(lu)模型、开关过(guo)程、输入输出电(dian)容、等(deng)效电(dian)容、电(dian)荷(he)存储等(deng)对MOS管驱动波(bo)形的(de)影响，及根据这些参数对驱动波(bo)形的(de)影响进行的(de)驱动波(bo)形的(de)优化设计实例，取得了较好的(de)实际效果。

影响(xiang)MOSFET开关速度除了其本身固有Tr,Tf外,还有一个重要的(de)(de)参数(shu):Qg (栅极总静电(dian)荷(he)容(rong)量(liang)).该参数(shu)与栅极驱动电(dian)路的(de)(de)输出(chu)内(nei)阻(zu)共同(tong)构成(cheng)了一个时间参数(shu),影响(xiang)着(zhe)MOSFET的(de)(de)性能(你(ni)主板的(de)(de)MOSFET的(de)(de)栅极驱动电(dian)路就(jiu)集成(cheng)在IRU3055这块PWM控制芯片内(nei)); r6 @0 k" S/ l3 }4 u, r/ W

厂(chang)家给出的(de)(de)Tr,Tf值,是(shi)在栅(zha)极驱(qu)动内(nei)(nei)阻(zu)(zu)小到(dao)可以忽略的(de)(de)情况(kuang)下(xia)测出的(de)(de),实际应用中就不一样(yang)了,特(te)别是(shi)栅(zha)极驱(qu)动集成在PWM芯片(pian)中的(de)(de)电(dian)路,从PWM到(dao)MOSFET栅(zha)极的(de)(de)布线(xian)的(de)(de)宽度,长(zhang)度,都会深刻影响MOSFET的(de)(de)性能(neng).如果PWM的(de)(de)输(shu)出内(nei)(nei)阻(zu)(zu)本来就不低,加(jia)上MOS管的(de)(de)Qg又大(da),那么(me)不论其Tr,Tf如何优秀,都可能(neng)会大(da)大(da)增加(jia)上升和下(xia)降(jiang)的(de)(de)时间

偶(ou)认为,BUCK同步变换(huan)器中,高侧MOS管(guan)的(de)(de)Qg比RDS等(deng)其(qi)他(ta)参数(shu)更重(zhong)要,另外,栅极(ji)驱动内阻与Qg的(de)(de)配合也很(hen)重(zhong)要,一(yi)定 程(cheng)度(du)(du)上(shang)就是由它的(de)(de)充电时间决定高侧MOSFET的(de)(de)开关速度(du)(du)和损耗(hao).

看从(cong)哪个角度出(chu)(chu)发。电(dian)(dian)(dian)荷泻放慢(man)，说(shuo)明(ming)时间(jian)(jian)常(chang)数(shu)大(da)(da)。时间(jian)(jian)常(chang)数(shu)是Ciss与Rgs的(de)乘积。栅源(yuan)极绝缘电(dian)(dian)(dian)阻(zu)大(da)(da)，说(shuo)明(ming)制(zhi)造(zao)工艺控制(zhi)较好，材(cai)料、芯片和管壳封(feng)装(zhuang)的(de)表(biao)面杂质(zhi)少(shao)，漏电(dian)(dian)(dian)少(shao)。时间(jian)(jian)常(chang)数(shu)大(da)(da)，栅源(yuan)极等效(xiao)输入电(dian)(dian)(dian)容也大(da)(da)。栅源(yuan)极等效(xiao)输入电(dian)(dian)(dian)容，与管芯尺(chi)寸(cun)成(cheng)正(zheng)比(bi)并(bing)与管芯设计(ji)有(you)关(guan)。通(tong)常(chang)，管芯尺(chi)寸(cun)大(da)(da)，Ron(导通(tong)电(dian)(dian)(dian)阻(zu))小、跨导（增益）大(da)(da)。栅源(yuan)极等效(xiao)电(dian)(dian)(dian)容大(da)(da)，会(hui)增加开(kai)关(guan)时间(jian)(jian)、降低开(kai)关(guan)性(xing)能、降低工作速度、增加功率损耗(hao)。Ciss与电(dian)(dian)(dian)荷注入率成(cheng)正(zheng)比(bi)，可(ke)能还与外(wai)加电(dian)(dian)(dian)压有(you)关(guan)并(bing)具(ju)有(you)非(fei)线性(xing)等。以上，均是在相同条(tiao)件(jian)下的(de)对比(bi)。从(cong)应用角度出(chu)(chu)发，同等价格，多数(shu)设计(ji)希望选用3个等效(xiao)电(dian)(dian)(dian)容（包括(kuo)Ciss）小的(de)器(qi)件(jian)。Ciss=Cgd+Cgs，充放电(dian)(dian)(dian)时间(jian)(jian)上也有(you)先后，先是Cgs充满，然(ran)后是Cgd.。

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