功率(lv)mos管(guan)(guan) 如何看懂功率(lv)mos管(guan)(guan)的(de)工作(zuo)原理,详解!
信息来源:本站(zhan) 日期:2017-10-11
功率放(fang)大电(dian)路(lu)是一(yi)种以(yi)输出较大功率为目(mu)的的放(fang)大电(dian)路(lu)。因此(ci),要(yao)求同时输出较大的电(dian)压(ya)和(he)电(dian)流(liu)。管子工作在接(jie)(jie)近极限状态。一(yi)般直接(jie)(jie)驱动负(fu)载(zai)(zai),带载(zai)(zai)能力要(yao)强。
功(gong)率MOSFET是(shi)(shi)(shi)较常使用的(de)一类(lei)功(gong)率器件(jian)(jian)。“MOSFET”是(shi)(shi)(shi)英文MetalOxideSemicoductorFieldEffectTransistor的(de)缩(suo)写,译(yi)成(cheng)中(zhong)文是(shi)(shi)(shi)“金属氧化物半(ban)导(dao)体场效(xiao)应(ying)管(guan)”。它(ta)是(shi)(shi)(shi)由金属、氧化物(SiO2或SiN)及半(ban)导(dao)体三种材料(liao)制成(cheng)的(de)器件(jian)(jian)。所谓功(gong)率MOSFET(PowerMOSFET)是(shi)(shi)(shi)指它(ta)能输出较大的(de)工作电流(liu)(几安到几十安),用于功(gong)率输出级的(de)器件(jian)(jian)。功(gong)率MOSFET可分为(wei)增强型(xing)和耗尽(jin)型(xing),按沟(gou)道(dao)分又可分为(wei)N沟(gou)道(dao)型(xing)和P沟(gou)道(dao)型(xing)。
做开(kai)关电(dian)源,常用功率MOSFET。一(yi)般(ban)而言,MOS管(guan)制造商(shang)采用RDS(ON)参数来定(ding)义导(dao)通阻(zu)抗;对(dui)(dui)ORing FET应用来说,RDS(ON)也(ye)是最重要的器件(jian)特性。数据手册(ce)定(ding)义RDS(ON)与栅(zha)极(或驱(qu)动)电(dian)压VGS以及流经开(kai)关的电(dian)流有关,但对(dui)(dui)于充分的栅(zha)极驱(qu)动,RDS(ON)是一(yi)个(ge)相对(dui)(dui)静态参数。
若(ruo)设(she)计人(ren)员试图开(kai)发尺寸(cun)最小、成(cheng)本最低(di)的(de)(de)(de)电(dian)(dian)源(yuan),低(di)导通阻抗(kang)更(geng)是(shi)加倍的(de)(de)(de)重要。在电(dian)(dian)源(yuan)设(she)计中(zhong)(zhong),每个电(dian)(dian)源(yuan)常常需(xu)要多个ORing MOS管(guan)并(bing)行工(gong)作,需(xu)要多个器件来把(ba)电(dian)(dian)流传送给负载。在许多情(qing)况下,设(she)计人(ren)员必须并(bing)联MOS管(guan),以有效降低(di)RDS(ON)。在DC电(dian)(dian)路中(zhong)(zhong),并(bing)联电(dian)(dian)阻性负载的(de)(de)(de)等(deng)效阻抗(kang)小于(yu)每个负载单(dan)独的(de)(de)(de)阻抗(kang)值。比(bi)如,两(liang)个并(bing)联的(de)(de)(de)2Ω电(dian)(dian)阻相当于(yu)一个1Ω的(de)(de)(de)电(dian)(dian)阻。因(yin)此,一般来说,一个低(di)RDS(ON)值的(de)(de)(de)MOS管(guan),具备大额定(ding)电(dian)(dian)流,就可以让设(she)计人(ren)员把(ba)电(dian)(dian)源(yuan)中(zhong)(zhong)所用MOS管(guan)的(de)(de)(de)数(shu)目(mu)减至最少。
除了RDS(ON)之外,在(zai)MOS管(guan)的选(xuan)择过程中还有几个MOS管(guan)参数(shu)也对电(dian)(dian)源设计人(ren)员非常重要(yao)。许多(duo)情况下(xia),设计人(ren)员应该(gai)(gai)密切关注数(shu)据手册上的安全工作区(SOA)曲线(xian),该(gai)(gai)曲线(xian)同时描(miao)述了漏极电(dian)(dian)流(liu)(liu)和漏源电(dian)(dian)压的关系(xi)。基本上,SOA定义了MOSFET能够安全工作的电(dian)(dian)源电(dian)(dian)压和电(dian)(dian)流(liu)(liu)。在(zai)ORing FET应用中,首要(yao)问(wen)题(ti)是:在(zai)"完(wan)全导通状态"下(xia)FET的电(dian)(dian)流(liu)(liu)传送能力。实(shi)际上无需SOA曲线(xian)也可以获(huo)得(de)漏极电(dian)(dian)流(liu)(liu)值。
MOSFET在关(guan)断瞬间,会(hui)承(cheng)受到最(zui)(zui)大(da)的(de)(de)电(dian)(dian)(dian)(dian)压冲击(ji),这个最(zui)(zui)大(da)电(dian)(dian)(dian)(dian)压跟(gen)负(fu)(fu)载有很(hen)大(da)关(guan)系:如果(guo)是(shi)阻(zu)性(xing)负(fu)(fu)载,那就是(shi)来自(zi)VCC端的(de)(de)电(dian)(dian)(dian)(dian)压,但还需要(yao)考虑电(dian)(dian)(dian)(dian)源(yuan)本身的(de)(de)质量(liang),如果(guo)电(dian)(dian)(dian)(dian)源(yuan)质量(liang)不(bu)佳,需要(yao)在前级加(jia)(jia)些必(bi)要(yao)的(de)(de)保护措(cuo)施;如果(guo)是(shi)感(gan)性(xing)负(fu)(fu)载,那承(cheng)受的(de)(de)电(dian)(dian)(dian)(dian)压会(hui)大(da)不(bu)少,因为电(dian)(dian)(dian)(dian)感(gan)在关(guan)断瞬间会(hui)产生(sheng)感(gan)生(sheng)电(dian)(dian)(dian)(dian)动势(电(dian)(dian)(dian)(dian)磁(ci)感(gan)应定律),其方向与VCC方向相同(楞次定律),承(cheng)受的(de)(de)最(zui)(zui)大(da)电(dian)(dian)(dian)(dian)压为VCC与感(gan)生(sheng)电(dian)(dian)(dian)(dian)动势之和(he);如果(guo)是(shi)变压器负(fu)(fu)载的(de)(de)话,在感(gan)性(xing)负(fu)(fu)载基础上还需要(yao)再加(jia)(jia)上漏感(gan)引起(qi)的(de)(de)感(gan)应电(dian)(dian)(dian)(dian)动势。
对于以(yi)上几(ji)种负载情况,在(zai)计算出(或(huo)测出)最大(da)电(dian)压后,再(zai)留(liu)有(you)20%~30%的(de)裕量,就可(ke)(ke)以(yi)确定所需(xu)要(yao)的(de)MOSFET的(de)额定电(dian)压VDS值。在(zai)这里(li)需(xu)要(yao)说(shuo)的(de)是,为(wei)了更好的(de)成(cheng)本(ben)和更稳(wen)定的(de)性(xing)能,可(ke)(ke)以(yi)选(xuan)择在(zai)感性(xing)负载上并联续(xu)流(liu)二(er)极(ji)管与电(dian)感在(zai)关断(duan)时(shi)构成(cheng)续(xu)流(liu)回路,释放掉感生能量来保(bao)护MOSFET,如果必要(yao),还可(ke)(ke)以(yi)再(zai)加上RC缓冲电(dian)路(Snubber)来抑制电(dian)压尖峰(feng)。(注意二(er)极(ji)管方向不(bu)要(yao)接反。当然,你也可(ke)(ke)以(yi)直接选(xuan)择VDS足够大(da)的(de)MOSFET,前(qian)提是你不(bu)care成(cheng)本(ben)。)
额定(ding)电(dian)压确定(ding)后(hou),电(dian)流就可以(yi)计算出来了。但这里(li)需要考虑(lv)两个参数:一个是(shi)连续工作电(dian)流值(zhi)(zhi)和脉冲(chong)电(dian)流尖峰值(zhi)(zhi)(Spike和Surge),这两个参数决定(ding)你应该(gai)选多(duo)大(da)的额定(ding)电(dian)流值(zhi)(zhi)。
场(chang)(chang)效(xiao)应(ying)(ying)(ying)管是根(gen)据(ju)三极(ji)(ji)管的(de)原理开(kai)发出的(de)新一代放大(da)元件(jian)(jian),功(gong)(gong)率(lv)(lv)(lv)(lv)MOSFET场(chang)(chang)效(xiao)应(ying)(ying)(ying)管具(ju)有负(fu)的(de)电(dian)流(liu)温度系数,可(ke)以避免(mian)它工作(zuo)的(de)热不稳定性(xing)和二次(ci)击穿(chuan),适合于大(da)功(gong)(gong)率(lv)(lv)(lv)(lv)和大(da)电(dian)流(liu)工作(zuo)条件(jian)(jian)下(xia)的(de)应(ying)(ying)(ying)用(yong)。功(gong)(gong)率(lv)(lv)(lv)(lv)MOSFET场(chang)(chang)效(xiao)应(ying)(ying)(ying)管从(cong)驱(qu)动模式上看,属于电(dian)压型(xing)(xing)驱(qu)动控制元件(jian)(jian),驱(qu)动电(dian)路的(de)设计比较(jiao)简单,所(suo)需(xu)驱(qu)动功(gong)(gong)率(lv)(lv)(lv)(lv)很小。采用(yong)功(gong)(gong)率(lv)(lv)(lv)(lv)MOSFET场(chang)(chang)效(xiao)应(ying)(ying)(ying)作(zuo)为开(kai)关电(dian)源中(zhong)的(de)功(gong)(gong)率(lv)(lv)(lv)(lv)开(kai)关,在启动或稳态工作(zuo)条件(jian)(jian)下(xia),功(gong)(gong)率(lv)(lv)(lv)(lv)MOSFET场(chang)(chang)效(xiao)应(ying)(ying)(ying)管的(de)峰值电(dian)流(liu)要(yao)比采用(yong)双极(ji)(ji)型(xing)(xing)功(gong)(gong)率(lv)(lv)(lv)(lv)晶体管小得多(duo)。功(gong)(gong)率(lv)(lv)(lv)(lv)场(chang)(chang)效(xiao)应(ying)(ying)(ying)管与双极(ji)(ji)型(xing)(xing)功(gong)(gong)率(lv)(lv)(lv)(lv)晶体管之间的(de)特性(xing)比较(jiao)如下(xia):
1. 驱(qu)(qu)动(dong)(dong)方式:场效应(ying)管(guan)是(shi)电压(ya)驱(qu)(qu)动(dong)(dong),电路设计比(bi)较简(jian)单,驱(qu)(qu)动(dong)(dong)功(gong)率小(xiao);功(gong)率晶体管(guan)是(shi)电流(liu)驱(qu)(qu)动(dong)(dong),设计较复杂,驱(qu)(qu)动(dong)(dong)条件(jian)选择困难,驱(qu)(qu)动(dong)(dong)条件(jian)会(hui)影(ying)响开关(guan)速度。
2. 开关速度:场(chang)效(xiao)应(ying)管无少(shao)数载(zai)流子存(cun)储效(xiao)应(ying),温(wen)度影响小,开关工(gong)作(zuo)频(pin)率(lv)可达150KHz以上;功(gong)率(lv)晶体管有少(shao)数载(zai)流子存(cun)储时间限制其开关速度,工(gong)作(zuo)频(pin)率(lv)一般不超过50KHz。
3. 安全工作区(qu):功率场效应管无(wu)二次击穿(chuan),安全工作区(qu)宽;功率晶体(ti)管存(cun)在二次击穿(chuan)现象,限制了安全工作区(qu)。
4. 导(dao)体电(dian)压:功(gong)率(lv)场效(xiao)应(ying)管(guan)属于高(gao)电(dian)压型,导(dao)通(tong)电(dian)压较(jiao)高(gao),有(you)(you)正温度系数(shu);功(gong)率(lv)晶(jing)体管(guan)无论耐电(dian)压的(de)高(gao)低,导(dao)体电(dian)压均(jun)较(jiao)低,具有(you)(you)负温度系数(shu)。
5. 峰值电(dian)流(liu):功率(lv)场效应(ying)管在开关电(dian)源中用做开关时(shi)(shi)(shi),在启(qi)动和稳态工(gong)作(zuo)时(shi)(shi)(shi),峰值电(dian)流(liu)较低;而功率(lv)晶体(ti)管在启(qi)动和稳态工(gong)作(zuo)时(shi)(shi)(shi),峰值电(dian)流(liu)较高。
6. 产品(pin)成本(ben):功率场(chang)效应管(guan)的成本(ben)略高(gao);功率晶体管(guan)的成本(ben)稍低。
7. 热(re)击(ji)穿(chuan)(chuan)(chuan)效应:功率(lv)场(chang)效应管无热(re)击(ji)穿(chuan)(chuan)(chuan)效应;功率(lv)晶体管有热(re)击(ji)穿(chuan)(chuan)(chuan)效应。
8. 开(kai)关(guan)(guan)损耗:场效应管的(de)开(kai)关(guan)(guan)损耗很小;功率晶(jing)体管的(de)开(kai)关(guan)(guan)损耗比较大。
另(ling)外(wai),功率MOSFET场(chang)效应(ying)管(guan)(guan)(guan)(guan)大(da)多(duo)(duo)集成(cheng)有阻(zu)尼(ni)二(er)(er)(er)(er)极(ji)管(guan)(guan)(guan)(guan),而双(shuang)极(ji)型(xing)功率晶体管(guan)(guan)(guan)(guan)大(da)多(duo)(duo)没有集成(cheng)阻(zu)尼(ni)二(er)(er)(er)(er)极(ji)管(guan)(guan)(guan)(guan)。场(chang)效应(ying)管(guan)(guan)(guan)(guan)内的阻(zu)尼(ni)二(er)(er)(er)(er)极(ji)管(guan)(guan)(guan)(guan)可以(yi)为开(kai)(kai)关(guan)电(dian)源(yuan)感性(xing)线圈提(ti)供无功电(dian)流(liu)通路(lu)。所以(yi),当场(chang)效应(ying)管(guan)(guan)(guan)(guan)的源(yuan)极(ji)电(dian)位(wei)高于漏(lou)极(ji)时,这个(ge)阻(zu)尼(ni)二(er)(er)(er)(er)极(ji)管(guan)(guan)(guan)(guan)导(dao)通,但在(zai)开(kai)(kai)关(guan)电(dian)源(yuan)中(zhong)不能使用这个(ge)阻(zu)尼(ni)二(er)(er)(er)(er)极(ji)管(guan)(guan)(guan)(guan),需要另(ling)外(wai)并联超(chao)快速二(er)(er)(er)(er)极(ji)管(guan)(guan)(guan)(guan)。场(chang)效应(ying)管(guan)(guan)(guan)(guan)内的阻(zu)尼(ni)二(er)(er)(er)(er)极(ji)管(guan)(guan)(guan)(guan)在(zai)关(guan)断过程中(zhong)与一般二(er)(er)(er)(er)极(ji)管(guan)(guan)(guan)(guan)一样存在(zai)反(fan)向恢(hui)复电(dian)流(liu)。此(ci)时二(er)(er)(er)(er)极(ji)管(guan)(guan)(guan)(guan)一方面(mian)承受着漏(lou)-源(yuan)极(ji)之间急剧上(shang)升的电(dian)压,另(ling)一方面(mian)又有反(fan)向恢(hui)复电(dian)流(liu)流(liu)过。
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