Research On Short Circuit Characteristic And Detection Of SiC MOSFET

As a typical representative of advanced power semiconductor devices,SiC devices are capable of significantly improving the power density and environmental adaptability of power electronic equipment by virtue of high-frequency characteristics,low loss,high-temperature and high-pressure endurance.In a series of SiC power devices,SiC MOSFET has been a research hotspot because of the high-coincidence with Si IGBT in the application.The short-circuit robustness of SiC MOSFET is the key of the reliability of the device which is an inevitable problem in the replacement.Taking it as an entry point,this thesis aims to study short-circuit characteristics,reliability and fault detection of SiC MOSFET.Involving the theory of SiC MOSFET short-circuit behavior,this thesis highlighted various types of short-circuit faults including changes of external parameters and internal micro-mechanism at different stages of the short-circuit process,and discussed novel issues such as the uneven voltage distribution in the bridge-arm short-circuit.The short-circuit failure mechanism of the device was briefly analyzed from perspectives of failure precursor,failure mode,etc.In view of the high voltage and large current throughout the short-circuit,considering the safety and controllability,this thesis has designed and built a experiment platform that can simulate various short-circuit modes of SiC MOSFET.Based on the platform,the effects of factors including bus voltage,drive parameters,and etc on the short-circuit characteristics of SiC MOSFETs were evaluated by Hard Switching Fault and Fault Under Loading.Combining experimental datas and related simulation,the influence of parameters including bus voltage,load current and driving voltage on the voltage division of upper and lower devices has been discussed during the bridge-arm short-circuit.Considering temperature-dependence parameters of SiC MOSFET,a short-circuit thermoelectric model was established following the principle of unsteady heat conduction,to describe the chip internal temperature distribution.Based on the model,this thesis horizontally compared the short-circuit reliability of different SiC MOSFETs,and also confirmed that the positive feedback of junction-temperature difference further exacerbated uneven voltage distribution during the bridge-arm short-circuit,which seriously weakened the short-circuit capability of the SiC MOSFET power module.On account of the shortcoming of the blanking time mechanism hindering the detection speed in the traditional desaturation method,an improved scheme,that is V_dsdetection of SiC MOSFET based on variable blanking time,has been proposed to avoid the possibility that blanking time is enough small to falsely triggering protection and speed up the detection process.By means of related simulation and experiment,it is further verified that the blanking time in this detection scheme can be self-adaptive with the change of turn-on speed,which can effectively shorten the response time of short-circuit protection and reduce the short-circuit thermal impact on the device.There are 90 pictures,8tables and 89 references in this thesis.