Study On Nonlinear Characteristics Of High-power-density GaN Resonant Step-up Converter

With the rapid development of semiconductor materials,the Gallium Nitride(Ga N)high electron mobility transistor(HEMT)is considered as the representative of the third generation wide-band gap power semiconductor devices.Due to the characteristic of high power density,high switching frequency and high stability,Ga N HEMT has been widely adopted in applications of spacecraft electric propulsion,new energy,energy storage,electric vehicles etc.However,the fast switching characteristic of Ga N HEMT meets the nonlinear parameters of the circuit could bring new application issues,such as the increase of output voltage ripple and the reduction of transient response of the power system.On the one hand,the interaction between high switching frequency and parasitic parameters will deteriorate the switching process,which beyonds the ability of traditional analysis methods.On the other hand,the complex structure of the high-frequency converter could introduce self-sustaining quasiperiodic oscillation(Dean and Hamill(DH)phenomenon).It could also provide propagation paths for high-frequency harmonics which increases the intensity of electromagnetic interference(EMI).These issues deteriorate the stability and reliability of the system and increase the design difficulty of the converter.To overcome that,this dissertation studies the effect of nonlinear characteristics on Ga N HEMT resonant step-up converter in aspect of system reliability and converter stability.Contributions and innovations are summarized as follows:1.In order to overcome the shortcomings of traditional methods which cannot effectively estimate the switching loss of high-power-density Ga N HEMT.A postprocessingtechnique based switching loss estimation method is proposed based on the fully investigation on the impact of nonlinear parameters on the device.The method is made up of three processes which are the wavelet denoising process,the voltage and current(V-I)alignment process,and the linear interpolation process.Firstly,based on the fully consideration of the measurement setup and the understanding of the switching behavior of the device,the wavelet shrinkage method is adopted in the wavelet denoising process to effectively minimize the interference of background noise,and the soft threshold is selected to achieve better filtering effect.Then,for the voltage and current alignment process,the device characteristic expression in the classic SPICE model is improved based on the deterioration mechanism of the Ga N HEMT switching process,and the quantitative effect of parasitic parameters on switching loss and system EMI is evaluated.Therefore,the reference point of switching waveforms is established and the propagation delay of probe in the process of V-I alignment is eliminated.Finally,with the automatic selection method of the interpolation step,the linear interpolation process improves the alignment resolution and reduces the measuring error from the sampling rate limitation.Experimental results show that the proposed method can estimate the switching loss of Ga N HEMT with high accuracy and improve the designing effectiveness of the power supply system.2.In order to study the mechanism of DH phenomenon and improve the circuit stability,a simplified analytical model of Ga N HEMT resonant step-up converter is established and an improved method including the designing boundary is proposed.Since the reverse recovery process of the diodes in the bipolar Cockcroft-Walton Voltage Multiplier(Bi CWVM)is the source of DH phenomenon,based on the configuration of the resonant tank and the gain transferring characteristics,the simplified analytical model of DH phenomenon is established by modeling the input-side of Bi CWVM as a voltageinductor circuit.Moreover,a piecewise linear diode model is considered to characterize the diode reverse recovery process.Therefore,the voltage and current relationships in the multilevel Bi CWVM is derived and gives the conditions for DH phenomenon in the simplified circuit and Ga N HEMT resonant step-up converter.What’s more,the relationship between the resonance frequency drifting problem and DH phenomenon during the light load is discussed.The mitigation methods including parameter design,hybrid or full silicon carbide(Si C)solutions are proposed,and the applicable conditions of each method under different switching frequencies are determined.Experimental results show that the simplified analytical model can accurately reflect the generation mechanism of DH phenomenon,and the proposed method can effectively minimizing the occurrence of DH phenomenon which providing a necessary theoretical reference for optimizing the design of Ga N HEMT resonant step-up converter.3.In order to study the EMI issues in Ga N HEMT resonant step-up converter,the effects of nonlinear parameters on EMI generation mechanism and Ga N HEMT switch characteristics,propagation path and high-voltage tank characteristics,suppression strategy and system characteristics are systematically analyzed through simulation and experiment.Based on the consideration of the basic test principle of conducted EMI,the Ga N HEMT switching process and its oscillation phenomenon are simplified as a model of the combination of the trapezoidal wave and sinusoidal exponential attenuation wave.Through the simplified model,the influence of Ga N HEMT and its effect on EMI in the converter is evaluated with the combination of the soft switching characteristics.Moreover,with taking the high-frequency characteristics of passive components into account,three capacitors simplified model of the transformer and the equivalent capacitance model of Bi CWVM are established,respectively.Based on the basic principle of impedance measurement,the current conduction directions of differential mode and common mode are obtained and the EMI coupling path is evaluated.With Si fast recovery diodes as counterpart,the influence of Si C Schottky barrier diodes of Bi CWVM on system EMI is evaluated and the application conditions are given.What’s more,The filtering effect of EMI filter is discussed and the influence of winding capacitances of common mode choke on EMI is obtained.The results show the analysis can accurately reflect the influence of nonlinear parameters on the EMI generation mechanism,propagation path and suppression strategy of the converter,and provide a method for the reliability design of Ga N HEMT resonant step-up converter.In this dissertation,the generation mechanism and mitigation method of nonlinear characteristics of high-power-density Ga N HEMT resonant step-up converter are studied from three parts: the device,the component and the system through theoretical analysis,models,simulations and experiments.The dissertation provides theoretical and engineering guidances for the optimization design of high-ratio resonant step-up converter.