Junction Temperature Correction And Monitoring Method Considering Parasitic Effect In SiC MOSFETs

Silicon Carbide（SiC）power devices are widely used in power systems,rail transportation and electric vehicles due to their material properties of high thermal conductivity,high breakdown voltage strength and low loss.In general,power devices operating under severe operating conditions for a long time will fail due to power cycle or thermal stress.The thermal reliability of SiC Metal-Oxide-Semiconductor Field-Effect Transistorha（MOSFET）become increasingly prominent.Temperature Sensitive Electrical Parameter（TSEP）is a commonly used junction temperature monitoring method in industry.However,the fast switching characteristics of SiC MOSFETs make them very sensitive to parasitic parameters.Therefore,the application of TSEPs needs to consider parasitic effects.In this paper,the Buck converter composed of SiC MOSFETs is taken as the research object,and the related research work is mainly carried out from the relationship between the spectral characteristics of drain source voltage at the turn off time and the parasitic parameters in the circuit and the drain current turn-on conversion rate di _DS（on）/dt.The specific research contents are as follows:（1）First,the transfer characteristics and output characteristics of SiC MOSFET at different junction temperatures are analyzed.Then,the switching losses in each stage of the turn-on process are analyzed in blocks,and the generation mechanism of the displacement current in the parasitic capacitor of SiC MOSFET modules is obtained.Finally,the influence of parasitic parameters of SiC MOSFET chip on switching process is introduced in detail.（2）A method to modify the temperature sensitive characteristics of di _DS（on）/dt based on the frequency of drain source voltage turn-off oscillation is proposed.By analyzing the dynamic characteristics of channel current and drain current,it is concluded that the inductance of power loop affects the size of di _DS（on）/dt without changing its temperature sensitivity.Based on the R-L-C equivalent circuit model,the linear expressions of di _DS（on）/dt and drain source voltage turn off oscillation frequency are derived.The experimental test platform is built.The oscillation frequency measured by oscilloscope and board-PC can reflect the change of power loop inductance.The experimental results show that the proposed method can greatly reduce the error of junction temperature monitoring.（3）A junction temperature correction model considering the influence of parasitic parameters is established.Firstly,according to the mathematical analysis formula of di _DS（on）/dt,it is explained that the driving resistance affects the size of di _DS（on）/dt and its temperature sensitivity at the same time.Secondly,the relationship between the oscillation characteristics and the parasitic parameters in the circuit is studied.It is shown that the peak value of the frequency spectrum at the turn off time of the drain source voltage is the intermediate value between the temperature sensitive characteristics of the di _DS（on）/dt and the parasitic parameters.Finally,based on the experimental measurement data,a junction temperature correction model is established,which can monitor the junction temperature more accurately through di _DS（on）/dt after the parasitic parameters of the external circuit change.In practical application,the input of the modified model can be read through the FFT voltage spectrum of the oscilloscope.