Study Of Design And Digital Control Of GaN-based AC-DC Converter

Gallium nitride high electron mobility transistors(GaN HEMTs)are gradually recognized by the market owing to the excellent electrical characteristics,and have been first applied in consumer electronics.Due to the high-speed switching characteristics of enhancement mode GaN HEMT,the GaN-based AC-DC converter has the potentials of high frequency and miniaturization,and its application research has become a hot spot in academic and industrial fields.Focused on GaN-based two-stage AC-DC converter,this thesis aims at the optimization of discontinuous conduction mode(DCM)boost power factor correction(PFC)converters and LLC resonant converters in the two-stage AC-DC converters,where the current distortion compensation control strategy,efficiency optimization control for DCM boost PFC converter,and the high-precision DC gain model and zero voltage switching(ZVS)implementation time for LLC resonant converter are studied respectively.First,based on the basic electrical characteristics of the enhancement mode GaN HEMT,the volt-ampere characteristics,switching speed,and the influence of the high dv/dt during the switching process on the driving circuit are analyzed respectively.In order to better apply the GaN HEMT,its key parameters are analyzed,and the linear charge equivalent model is proposed for the nonlinear characteristics of the parasitic capacitance.Furthermore,aiming at the reliability problems caused by high dv/dt and high di/dt in practical engineering applications,suppression methods are summarized from the perspective of PCB layout and routing,which lays the foundation for the application of GaN-based boost PFC converter and GaN-based LLC resonant converter.Aiming at the problems of high current distortion and low power factor for GaN-based DCM boost PFC converter,a sensorless average current mode(SACM)control is studied.With consideration of resistive parasitics in the converter circuit,an accurate inductor current mode is established,where the effect of the parasitics on the power factor and input current for PFC converter controlled under SACM control is analyzed quantitatively.Furthermore,the SACM control with duty cycle compensation gain is proposed.This compensation control strategy reshapes the input current to sine wave,which improves the power factor of the converter over a large power range,and effectively reduces the current harmonic distortion.Aiming at the efficiency optimization for GaN-based DCM boost PFC converter,the comprehensive charge rate model is established by deriving the total input and output charge of the converter,which is based on the parasitics in the circuit and the switching process of GaN HEMT.Based on comprehensive charge rate model,the efficiency function is derived with the input voltage,output voltage and GaN HEMT switching ON-time.Since there is a specific switching ON-time that maximizes the efficiency at each operation point,the maximum efficiency average current mode(MEAC)control is proposed,where the converter loss under the MEAC control is analyzed.By adjusting the switching ON-time,the MEAC controller maximizes the efficiency of the PFC converter,besides,the input current is regulated as sinusoidal by the adjusted switching cycle,which improves the power factor and current harmonic distortion.Finally,aiming at the large error caused by the traditional fundamental harmonic approximation model for GaN-based LLC resonant converter,an extended harmonic approximation(EHA)model for DC gain is proposed with consideration of the secondary side leakage inductance.In order to solve the driving design problems in GaN-based LLC resonant converter,the influence of high dv/dt and high di/dt during the current transition on the driving circuit is analyzed in detail,based on which the driving solution of GaN-based LLC resonant converter is analyzed and summarized.Furthermore,with the EHA model,the ZVS realization for LLC resonant converter under DCM operation during the dead time is analyzed considering the secondary side parasitic capacitance,where the time to achieve ZVS transition is derived.A prototype is built to verify the accuracy of the model and analyses,which provides an important design reference for accurately adjusting the dead time of GaNbased LLC resonant converter.