Study On Switching Transient Characteristics Of High-voltage Silicon Carbide Bipolar Power Devices And Methods Of Regulation And Control

In recent years,with the development of science and technology,the application field of power electronics is tending to be higher temperature,higher voltage and higher power.The third-generation semiconductor materials,which is represented by Silicon Carbide(SiC),have many obvious advantages in material characteristics.With the increasing maturity of the related theories and fabrication technologies,the trend of replacing Si-based devices with SiC power electronic devices is becoming increasingly clear.In all of the SiC power semiconductor switches,compared with SiC MOSFET,there is conductivity modulated effect for SiC IGBT and SiC GTO during their forward conduction mode,therefore,there two devices have obvious advantages in the high voltage and high power applications.The highest blocking voltage of the experimentally reported SiC IGBT and GTO has already exceeded 20 k V.However,due to the particularity of SiC materials and its device design,there are still some problems in the switching performances of both SiC IGBT and SiC GTO,which have restricted the further improvements in devices’ characteristics,and also affected their wide applications in power conversion systems.Concentrating on the problems in the switching transients of both SiC IGBT and SiC GTO,this thesis has conducted a series of theoretical research work.The research focus and achievements are mainly reflected in the following aspects:(1)This thesis proposes a physical model which mathematically describes the voltage rise rate dv/dt and power dissipation in the turn-off transient of SiC IGBT,and on the basis of this model,a design method of optimizing the trade-off between turnoff dv/dt and turn-off power loss is also proposed accordingly.A TCAD simulation model is firstly built to reproduce the turn-off voltage punch-through phenomenon and the resulting high dv/dt,and the variation of the excess carrier is also analyzed.Based on the mathematical relationship between the concentration of the excess carrier and its extraction current during turn-off transient,a physical analytical model describing the voltage rise rate dv/dt and power dissipation in the turn-off transient of SiC IGBT is proposed and established,and the accuracy of this model is verified by TCAD simulation results within a wide range of design parameters.During this process,the limiting design parameters of dv/dt and power dissipation in the turn-off transient are identified,also a design method of optimizing the trade-off between turn-off dv/dt and turn-off power loss is also proposed.The optimum design result by simulation indicates that the turn-off power dissipation is reduced by 60% and the dv/dt is also cut down by around 80% simultaneously without sacrificing the forward voltage drop.(2)This thesis proposes a soft punch-through design method which features a step-doped N buffer layer,by means of which the dv/dt during the turn-off transient of SiC IGBT is reduced and the advantage from the conventional punch-through design that turn-off power loss is insensitive to the dc-link voltage increasing is also maintained.Firstly,the main reason that accounts for high dv/dt after punch-through is revealed to be the fact that the amount of the removed charge in the N buffer layer is rather small.An analysis is made theoretically on the relationship between the amount of the removed charge in the N buffer layer and the design parameters,and a soft punchthrough design method is proposed accordingly.This method divides the charge dose needed in N buffer during turn-off transient from the total dose amount,and thus the regional doping concentration can be adjusted individually.That is to say,the N buffer is composed of two step-doped layers.In one case study of the soft punch-through design,the charge amount of the excess carrier in the buffer layer can be increased by around 15 times,and the turn-off dv/dt is reduced by 36%,while the turn-off power loss is reduced by about 30%,also the disadvantage of being more sensitive to the increasing dc-link voltage is eliminated.(3)This thesis proposes a physical model describing the turn-on delay period and the current rise transient of SiC GTO thyristor,and key parameters dominating both the turn-on delay time and the current rise rate are revealed.In view of the slow current rise phenomenon in the pulsed power current of SiC GTO,a mixed-mode simulation circuit is built firstly in order to observe the turn-on current in the resistive switching circuit.Based on the turning point of the initial turn-on current waveform and combining the observed current conduction dynamics of the parasitic transistors within SiC GTO,the turn-on transient phase is divided into the turn-on delay transient and the transistors’ coupling transient,also the turn-on delay transient can be divided into three sub-stages,namely the ramping transient of the gate driving signal,the charging transient of the parasitic PN junction and the PNP conduction transient.Based on the above analysis,the current components within GTO during each sub-stage is analyzed,and then a physical model describing the current rise process during the dynamic turnon transient is proposed,the accuracy of which is verified by TCAD simulation results within a wide range of design parameters.At the same time,the dominant physical mechanisms for the time length of both the turn-on delay transient and the fast current rise transient are identified,and the main design parameters involved in each physical mechanism are analyzed in detail,also,design methods of reducing the turn-on delay time and accelerating the current rise during the transistors’ coupling transient are discussed extensively.(4)The research on the integrated multi-cell of SiC GTO is conducted,which gives a deeper physical insight into the dynamic turn-on transient of SiC GTO.Firstly,the integrated multi-cell mixed-mode simulation platform is built,and both the interactions between adjacent cells and the parasitic resistance caused by electrodes metallization have been taken into consideration.The rise tendency of each Anode contact’s current is observed,and the specific cause for the current inhomogeneity is analyzed.The rise tendency of each Anode contact’s voltage,each Gate contact’s voltage and the voltage between each Anode and Gate contact is also observed,and the direct reasons for each voltage’s rising with the increasing turn-on time are analyzed.Then the charge amount which is calculated by the integration of each Anode contact current during each PNP conduction transient is obtained,and its rise tendency with cell numbering increasing is analyzed,thus the interaction mechanism between adjoining cells is revealed.What’s more,on the basis of the integrated multi-cell simulation results,the design method of enhancing the current uniformity and homogeneity among all the Anode contacts during the dynamic turn-on transient is proposed,and the validity of the proposed method has been verified by integrated multi-cell mixed-mode simulation results.(5)A comprehensive comparative study on the electrical characteristics between SiC IGBT and SiC GTO is conducted in this thesis.By means of TCAD simulation model combining fundamental physical modeling,the electrical characteristics of both devices with the same voltage rating are compared.Firstly,the IV characteristics at different temperatures and the strength of the conductivity modulation under the same current density are compared,and then comparison on the forward voltage drop is made.The turn-on di/dt characteristic is compared at different temperatures in a resistive load switching circuit,and the turn-off dv/dt characteristic of both devices are studied in a clamped inductive load switching circuit.Further comparison on the trade-off between the forward voltage drop and turn-off power loss is made,and the maximum operation frequency under varied load current densities is evaluated,then the total power loss of both devices at different operation frequencies is compared.Moreover,the RBSOA of both devices is theoretically analyzed by means of physical modeling,and the strength of weakness of both devices is further identified.