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SCT12A1 在输出硬短路下的负载断开保护功能介绍

发(fa)布时(shi)间(jian):2017-11-24 作(zuo)者:Silicon Content 阅读人数:8181


对于(yu)升(sheng)压(ya)(ya)型(xing)电(dian)源转(zhuan)化器(qi)(Boost DC/DC Converter,后文简(jian)称Boost),无(wu)论同(tong)(tong)步(bu)类型(xing)还是(shi)非同(tong)(tong)步(bu)类型(xing),输(shu)(shu)入(ru)端(duan)与输(shu)(shu)出(chu)(chu)之间(jian)由于(yu)转(zhuan)化器(qi)架(jia)构(gou)的(de)(de)制约,始终存在(zai)(zai)一条(tiao)(tiao)由输(shu)(shu)入(ru)端(duan)、经电(dian)感、再(zai)经二(er)(er)极管(guan)(非同(tong)(tong)步(bu)升(sheng)压(ya)(ya)转(zhuan)换器(qi))或开关(guan)MOSFET(同(tong)(tong)步(bu)升(sheng)压(ya)(ya)转(zhuan)换器(qi))的(de)(de)体二(er)(er)极管(guan)、至输(shu)(shu)出(chu)(chu)的(de)(de)电(dian)流通(tong)路(lu)。由于(yu)这(zhei)条(tiao)(tiao)电(dian)流通(tong)路(lu)的(de)(de)存在(zai)(zai),即使设备处于(yu)关(guan)闭状(zhuang)态,输(shu)(shu)出(chu)(chu)端(duan)始终有电(dian)压(ya)(ya)输(shu)(shu)出(chu)(chu)。这(zhei)种在(zai)(zai)系(xi)统(tong)启动(dong)前不(bu)期望的(de)(de)输(shu)(shu)出(chu)(chu)电(dian)压(ya)(ya),不(bu)仅会(hui)导致(zhi)系(xi)统(tong)闭锁或故障,而且会(hui)增加系(xi)统(tong)的(de)(de)待机功率损(sun)(sun)耗。同(tong)(tong)时,在(zai)(zai)此种情况(kuang)下,如果输(shu)(shu)出(chu)(chu)不(bu)慎接地短(duan)路(lu),升(sheng)压(ya)(ya)转(zhuan)换器(qi)输(shu)(shu)入(ru)与地形(xing)成(cheng)通(tong)路(lu)导致(zhi)瞬间(jian)电(dian)流过大,进而损(sun)(sun)坏器(qi)件,并在(zai)(zai)负(fu)载电(dian)路(lu)端(duan)造成(cheng)灾难性故障。针对上述问题,本文将详(xiang)细介绍SCT12A1负(fu)载断开功能(neng)(芯洲专利(li)设计)在(zai)(zai)输(shu)(shu)出(chu)(chu)硬(ying)(ying)短(duan)路(lu)(Hard short)状(zhuang)况(kuang)下的(de)(de)硬(ying)(ying)短(duan)路(lu)保护功能(neng),有效提高升(sheng)压(ya)(ya)电(dian)源转(zhuan)化器(qi)的(de)(de)可靠性。

图1为典型(xing)开(kai)关(guan)型(xing)DCDC 升压转换器(qi)电路,包括开(kai)关(guan)管Q1, 续流管Q2,储(chu)能器(qi)件(jian)电感L,输出电容Cout。


图1.   Boost基本结构

在一(yi)(yi)个(ge)开(kai)关(guan)(guan)周期里,当开(kai)关(guan)(guan)闭合的(de)(de)(de)(de)时候,续(xu)(xu)(xu)流(liu)管Q2关(guan)(guan)断(duan)(duan),输(shu)(shu)出(chu)电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)容Cout存(cun)(cun)的(de)(de)(de)(de)电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)荷(he)与(yu)(yu)前级(ji)电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)感(gan)L和开(kai)关(guan)(guan)管Q1断(duan)(duan)开(kai)(此时Cout与(yu)(yu)电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)感(gan)L通(tong)(tong)过(guo)续(xu)(xu)(xu)流(liu)管Q2的(de)(de)(de)(de)寄生(sheng)体二(er)极管电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)气连接,由(you)于(yu)(yu)(yu)该体二(er)极管导通(tong)(tong)能力非常弱,故(gu)在介(jie)绍(shao)Boost型DCDC转(zhuan)换(huan)器(qi)时候简化为此时Q1与(yu)(yu)L断(duan)(duan)开(kai)),对后级(ji)电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)放电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)。电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)感(gan)L, 开(kai)关(guan)(guan)管Q1与(yu)(yu)输(shu)(shu)出(chu)形成独立(li)回路(lu)。电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源对电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)感(gan)充电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian),电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)感(gan)L储能。开(kai)关(guan)(guan)断(duan)(duan)开(kai),由(you)于(yu)(yu)(yu)电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)感(gan)的(de)(de)(de)(de)电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)方向不能突(tu)变(bian),电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)继(ji)续(xu)(xu)(xu)从(cong)电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)感(gan)近输(shu)(shu)入端(duan)(duan)一(yi)(yi)侧流(liu)向输(shu)(shu)出(chu)端(duan)(duan)侧,续(xu)(xu)(xu)流(liu)管导通(tong)(tong),此时,电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)感(gan)和电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源的(de)(de)(de)(de)极性一(yi)(yi)致,给电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)容充电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)。在开(kai)关(guan)(guan)型DCDC升(sheng)压转(zhuan)换(huan)器(qi)停止(zhi)工(gong)作(zuo)的(de)(de)(de)(de)状(zhuang)态(tai)下,一(yi)(yi)旦存(cun)(cun)在输(shu)(shu)入电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)压,从(cong)输(shu)(shu)入端(duan)(duan)经(jing)(jing)电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)感(gan)L,再(zai)经(jing)(jing)续(xu)(xu)(xu)流(liu)管Q2体直至输(shu)(shu)出(chu)端(duan)(duan),从(cong)在一(yi)(yi)个(ge)不可避免(mian)的(de)(de)(de)(de)电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)通(tong)(tong)路(lu)。由(you)于(yu)(yu)(yu)这个(ge)通(tong)(tong)路(lu)的(de)(de)(de)(de)存(cun)(cun)在,即便(bian)转(zhuan)换(huan)器(qi)在停止(zhi)工(gong)作(zuo)状(zhuang)态(tai)下,输(shu)(shu)出(chu)端(duan)(duan)依(yi)旧有电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)压存(cun)(cun)在,既输(shu)(shu)入电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)压。也(ye)就是(shi)常说(shuo)的(de)(de)(de)(de)负(fu)(fu)载(zai)未完全断(duan)(duan)开(kai)。在使用升(sheng)压型DCDC转(zhuan)换(huan)器(qi)的(de)(de)(de)(de)电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)子器(qi)件(jian)中,负(fu)(fu)载(zai)的(de)(de)(de)(de)未完全断(duan)(duan)开(kai),会增加整个(ge)系(xi)统关(guan)(guan)机(ji)状(zhuang)态(tai)下的(de)(de)(de)(de)功(gong)耗。对于(yu)(yu)(yu)现在越来越多的(de)(de)(de)(de)电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池供电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)的(de)(de)(de)(de)移动设备,关(guan)(guan)机(ji)状(zhuang)态(tai)下的(de)(de)(de)(de)功(gong)耗会一(yi)(yi)次充电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)下电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池的(de)(de)(de)(de)续(xu)(xu)(xu)航时间(jian),也(ye)就是(shi)终端(duan)(duan)消费(fei)者口中的(de)(de)(de)(de)“电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池漏电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)”现象。同时,由(you)于(yu)(yu)(yu)输(shu)(shu)出(chu)端(duan)(duan)在关(guan)(guan)机(ji)状(zhuang)态(tai)下始终有电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)压存(cun)(cun)在,一(yi)(yi)旦发生(sheng)输(shu)(shu)出(chu)端(duan)(duan)短路(lu),输(shu)(shu)入端(duan)(duan)与(yu)(yu)地之(zhi)间(jian)形成直接通(tong)(tong)路(lu),瞬间(jian)产生(sheng)巨大电(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu),但又(you)不能通(tong)(tong)过(guo)关(guan)(guan)机(ji)的(de)(de)(de)(de)方式来保护整个(ge)系(xi)统,进而带(dai)来损(sun)坏转(zhuan)换(huan)器(qi)有源器(qi)件(jian)和整个(ge)负(fu)(fu)载(zai)系(xi)统的(de)(de)(de)(de)风险。  

针(zhen)对Boost存在不可控电(dian)流路径的(de)(de)(de)问题(ti),SCT12A1提(ti)供了如下解决方案,在转(zhuan)换器输出(chu)和(he)负载(zai)端插(cha)入P沟道(dao)型MOSFET来断开负载(zai)并(bing)提(ti)供输出(chu)端短路保护。P型MOSFET的(de)(de)(de)源极(ji)连接(jie)到(dao)SCT12A1的(de)(de)(de)VOUT引(yin)(yin)脚。在VOUT引(yin)(yin)脚和(he)P型MOSFET的(de)(de)(de)源极(ji)需(xu)要接(jie)输出(chu)电(dian)容,以维持控制回路的(de)(de)(de)稳定性。

SCT12A1硬短路保(bao)护功能的(de)打(da)嗝(ge)时(shi)间(jian)可(ke)以(yi)(yi)通过(guo)SS引脚与(yu)地(di)之间(jian)电(dian)(dian)容(rong)(rong)容(rong)(rong)值不同进行调节。当(dang)硬短路保(bao)护被触发(fa)之后,SS引脚复位对地(di)放电(dian)(dian),内部5μA的(de)电(dian)(dian)流源对SS引脚外(wai)接的(de)电(dian)(dian)容(rong)(rong)进行充电(dian)(dian)。当(dang)SS电(dian)(dian)压超过(guo)1.2V后,再(zai)一次启动打(da)嗝(ge)。打(da)嗝(ge)时(shi)间(jian)可(ke)以(yi)(yi)通过(guo)公式1计算得到。

t_SS=(C_SS*  V_REF)/I_SS (1)

其中(zhong):

tSS :软启动时间

VREF: 内部参(can)考电压(ya) 1.2V

CSS:SS引(yin)脚的(de)对地电容

ISS:内部对SS引脚充电(dian)电(dian)流(liu) 5uA

外部P-FET选择

P-FET置于Boost输出端(duan)和负载端(duan)之间(jian)。发(fa)生短(duan)路时, VOUT_LOAD短(duan)路,而VOUT依旧正常输出。


图2.   短(duan)路时P-FET两(liang)端(duan)电气特性

硬短路保(bao)护(hu)时(shi),Boost输出端在P-FET导(dao)通(tong)时(shi)候可以直接等效计(ji)算:

V_DS=V_OUT

I_(DS_DC)=I_OUT

外置(zhi)P-FET的安(an)全(quan)工作(zuo)区(Safe Operation Area, SOA)需要谨慎遵守。短路保护过程(cheng)中(zhong),产生(sheng)的热可以这样计算:

Q_FET=1/2×V_OUT×I_SHORT×t_SHORT

例如,VOUT=12V, ISHORT=15A,tSHORT=60us,这(zhei)样这(zhei)样短路(lu)过程产生的热QFET为5.4mJ。

选择外(wai)置(zhi)P-FET选择外(wai)置(zhi)P-FET时候(hou),需要流出足够的(de)裕(yu)度(du),以保证P-FET在短路事件中不会被毁坏。接着上面(mian)假设的(de)应用条件,在12V-VOUT/1A-IOUT的(de)条件下,P-FET的(de)选择应该(gai)需要满足这样(yang)的(de)要求:

V_DS≥12 V

I_(DS_DC)≥1A

SOA>3.6 mJ


外部(bu)FET选(xuan)择示例

FDMC612PZ -20VDS,-14A Id, 10nC Qg Fairchild

CSD25404Q3 -20VDS, -18A Id, 10.9nC Qg Texas Instruments

图 3.   Fairchild FDMC612PZ SOA


图 4.   TI CSD25404Q3


SCT12A1硬短路保护应用波形(xing)


图 5.   0A负载下(xia)输(shu)出硬短路(PFM)

VIN=3.6V, VOUT=9V, Ta=27ºC


图6.   3A负(fu)载下输出硬短路(lu)(PFM)

VIN=3.6V, VOUT=9V, Ta=27ºC


参考设计(ji)文(wen)献:

SCT12A1硬短路(Hard Short)保护  

SCT12A1产品介绍

SCT12A0产品介绍


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