Investigation Of Conducted Electromagnetic Interference (EMI) Existed In Motor Drive System Based On Silicon Carbide (SiC) MOSFET

With the emerging of the next generation wide band-gap semiconductor device in recent years,such as silicon carbide(SiC)MOSFET,the performance of the motor drive system has been largely improved in terms of efficiency,power density,and reliability.As a result,the motor drive based on such novel and smart semiconductor device has been paid much more attentions,and gradually applied in modern power applications,such as aeronautics,railway transportation,and electric vehicles.However,compared with its silicon counterpart,the enhanced voltage/current slew rate and obvious switching ringings caused by SiC MOSFET during switching transient have amplified the high-frequency(HF)electromagnetic interference(EMI).Moreover,the higher switching frequency of the SiC MOSFET also intensifies the HF EMI dramatically.Meanwhile,the impedance of the motor is considered as an effective propagation path for these HF harmonics,and significantly impacts the level of EMI accordingly.Obviously,it is crucial to satisfy with the strict electromagnetic compatibility(EMC)standers to guarantee reliability of the motor drive in the designing stages.As a consequence,this dissertation has investigated switching characteristics of the SiC MOSFET and impedance characteristics of the motor in depth based on analysis,modeling,simulation,and experiment to reveal mechanism of conducted EMI existing in motor drive.The main concerns in this dissertation are summarized in the following aspects:(1)In order to effectively obtain the drain current of the SiC MOSFET during switching transient,a well designed current transformer(CT)is proposed,and fixed on drain of the MOSFET.The bandwidth of the CT is elaborated based on equivalent circuit model in low-and high-frequency ranges,and the saturation current of the CT is also calculated based on relative theories of magnetic components.The electric parameters of the CT are well designed to meet the requirements of sufficient bandwidth and saturation current for drain current measurement of the SiC MOSFET.The effectiveness of the proposed method is verified with experimental results based on double pulse test.In addition,impacts of placement of the CT is also discussed.(2)An analytical modeling based on finite state machine(FSM)is elaborated to comprehensively evaluate switching characteristics of the SiC MOSFET.According to operation principle of the SiC MOSFET,the switching transient can be divided into several sub-stages,which can be illustrated by equivalent circuit and represented by state equations.Then,interactions between these sub-stages are intuitively depicted by FSM,which can effectively describe switching behavior of the SiC MOSFET and solve these state equations.Simulation and experiment are carried out,and the results manifest that the model is capable of estimating switching characteristics of the SiC MOSFET in terms of switching speed,switching loss,and HF EMI noise.(3)In order to effectively mitigate the switching oscillation and alleviate HF EMI,the switching characteristics of the SiC MOSFET with RC snubber is investigated with an analytical model based on FSM.The accuracy of the model is verified through comparisons between calculated and measured waveforms during switching transition.In addition,the impacts of RC snubber on switching oscillation,switching loss and highfrequency(HF)EMI noise have been investigated based on the model,which shows that the added RC snubber can effectively avoid the switching oscillation,and reduce the level of HF EMI without additional switching loss.(4)In order to accurately represent the impedance characteristics of the motor,an improved HF model combined with distributed and lumped parameters is developed according to configuration of the motor winding.The parameters of model is determined based on the measured common mode(CM)and differential mode(DM)impedance characteristics in different frequency ranges.It can be observed that the model calculated impedances are in nice agreement with the measured ones,which verifies that proposed HF model can accurately depict both CM and DM impedances of the motor from 100Hz?10 MHz.(5)In order to comprehensively evaluate EMI in motor drive system,simulation and experiment investigations are carried out.Switching behavior of the SiC MOSFET as well as impedance characteristics of the motor are properly modeled with Simulink/Matlab.Meanwhile,the test bench of the motor drive based on SiC MOSFET is also built accordingly.Influences of motor impedance on EMI can be well demonstrated through frequency-domain analysis based on simulation results.In addition,impacts of switching characteristics of SiC MOSFET,such as switching response speed,switching frequency,and switching ringing,on EMI have also been well investigated according to relative experiment results.This dissertation has comprehensively investigated influences of switching characteristics of the SiC MOSFET and motor impedance on the level of conducted EMI based on analysis,modeling,simulation,and experiment,which can demonstrate mechanism of the EMI clearly.The results summarized in the dissertation are expected to provide much valuable instructions for engineers to deal with the EMI issues existing in motor drive system based on SiC MOSFET.