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激光器产生波长为1 064 nm的激光,脉宽为25 ps。经聚焦单元聚焦后,辐照在被测器件(Device Under Test,DUT)的激光光斑直径为2.5 μm。脉冲激光试验设备如图1所示。
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试验器件为星用电源模块上用的高速脉宽调制器(TI PWM UC1825A),其测试电路如图2所示。
试验中被检测参数如表1所示。
表 1 PWM UC1825A检测信号参数
Table 1. Parameters of PWM 1825A for test
Signal pin Performance description Vcc Device power supply SS Soft start port Aout PWM clock output Bout PWM clock output 由于脉冲激光在介质中的射程很短,DUT的晶圆外层是塑封材料,且晶圆正面覆有金属导线层,脉冲激光无法穿透这些介质层而到达内部有源区,因此试验前对样品进行了开盖处理[8]。
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试验中,利用设计的测试系统,实时动态检测被测UC1825A芯片的电源引脚、PWM时钟输出引起的信号。UC1825A芯片的内部原理框图如图3所示。
通过其原理框图可以发现,在UC1825A芯片内部,存在RESTART DELAY LATCH锁存器,此锁存器为自动重启延迟锁存器,为数字逻辑电路,其作用体现在软启动过程中,在采样软启动SS端电容电压进行置位或复位,当SS端电容超过5 V时,如果FAULT LATCH锁存器存在Q端高电平跳变,则RESTART DELAY LATCH锁存器输出置位,内部恒流源以250 μA电流对SS端电容进行放电;当SS端电容电压低于0.2 V时,内部复位电路会使FAULT LATCH锁存器的Q´端输出高电平跳变,此时,RESTART DELAY LATCH锁存器会复位,内部恒流源以9 μA电流对SS端电容进行充电。
当电源完成软启动正常工作时,其SS端电容电压为高于5 V的电压,此时,如果FAULT LATCH锁存器的Q´端输出高电平跳变,则SS端电容将从5 V电压进行放电至0.2 V后充电,完成重启动工作。根据芯片的设计,FAULT LATCH锁存器Q端输出跳高的原因是9引脚ILIM端电压跳高超过1.2 V,此时发生过流保护,导致电源进行软启动工作。
在设计中,软启动电容为100 nF,软启动时间为55 ms,由于电源芯片UC1825A的供电采用变换器输出自持供电,所以软启动SS端电容放电重启会导致芯片的自持供电低于芯片的欠压保护阈值,导致电源整机关机。
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由于器件正面覆盖金属层,激光无法直接穿透金属层,导致正面辐照时激光很难到达有源区,无法有效触发单粒子效应。图4所示为激光从器件衬底入射(背面辐照)方法的示意图,可使激光穿透器件背面的硅衬底而到达器件内部有源区[9-10]。
在脉冲激光单粒子效应背辐照试实验中,一般使激光聚焦在被测器件衬底表面,但由于硅材料的折射作用,激光经过衬底到达器件有源区时束斑面积将显著增大。束斑尺寸可由下式计算:
$$ {\omega _z} = {\omega _0}\sqrt {1 + {{({z / {{z_0}}})}^2}} $$ (1) $$ {z_0} = \pi n\omega _0^2/\lambda $$ (2) 式中:ωz为到达器件有源区后的束斑半径;ω0为入射激光束斑半径;z为激光传播距离,即衬底厚度;n为硅的折射率,取3.51;z0为共焦长度;λ为激光波长。文中实验所用脉冲激光波长为1 064 nm,光斑直径2.5 μm,待测芯片衬底经减薄后剩余约100 μm,通过计算可得到达有源区的束斑半径约为4.6 μm。束斑尺寸的增大将显著降低脉冲激光沉积在器件敏感节点的能量,因此,试验中通过提高待测器件衬底表面在z轴上的位置,使脉冲激光经过衬底折射后再聚焦于被测器件的有源区,如图4所示。
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脉宽调制器单粒子效应失效模式主要包括输出频率异常和输出占空比异常。发生输出频率异常的主要原因有:(1) 单个高能粒子入射到脉宽调制器的参考电压模块,引起参考电压发生波动,使得输出脉冲出现长时间的丢失;(2) 单个高能粒子入射到脉宽调制器的软启动模块,其电容发生异常充电,导致脉宽调制器的输出脉冲发生长时间的丢失。发生输出占空比异常的主要原因有:(1) 单个高能粒子入射到脉宽调制器输出信号的组合电路和锁存电路,导致占空比发生异常;(2) 单个高能粒子入射到脉宽调制器的时钟电路,导致时钟频率发生异常,进而影响占空比[11-13]。
单粒子效应评估表征是对器件敏感性的评价,单粒子效应截面是表征器件单粒子敏感性的主要参数。对于脉冲激光单粒子效应模拟试验来说,获取效应截面与脉冲激光能量之间的关系是模拟试验的关键。效应截面的计算公式如下:
$$\sigma = N/{N_{\rm{pulse \;laser}}}$$ (3) 式中:σ为效应截面;N为单粒子效应错误数;Npulse laser为脉冲激光入射总次数。
选取不同能量的脉冲激光进行单粒子效应辐照试验,并计算相应的效应截面,之后一脉冲激光能量为横坐标,效应截面为纵坐标,即可得到效应截面与脉冲激光能量之间的相互关系。
Simulation test study of single event transient effect for high speed PWM with pulse laser
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摘要: 利用脉冲激光验证高速脉宽调制控制器(Pulse Width Modulator,PWM)单粒子瞬态效应的敏感性和防护设计。试验中,通过改变脉冲激光能量,逐步扫描PWM控制器电路,确定了诱发单粒子瞬态效应的激光能量阈值和敏感区域。通过改变PWM控制器软启动配置电路设计,验证了防护电路设计的合理性,为卫星电源子系统的单粒子瞬态效应防护设计提供技术参考。Abstract: Sensibility and protective design of single event transient effect for Pulse Width Modulator(PWM) was validated by pulse laser. During test, by changing the energy of the pulse laser and scanning the PWM circuit step by step, the energy threshold of the pulse laser and the sensitive region of PWM induced single event transient effect were determined. By changing the soft start configuration circuit design of PWM, the rationality of the protection circuit design was verified, which provided a technical reference for the single event transient protection design of satellite power subsystems.
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表 1 PWM UC1825A检测信号参数
Table 1. Parameters of PWM 1825A for test
Signal pin Performance description Vcc Device power supply SS Soft start port Aout PWM clock output Bout PWM clock output -
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