Research On Characteristics And Application Of Cascode GaN HEMT

After decades of development,the performance of Si-based power switching devices is approaching its material limits.The power electronic converter is limited to further development in the direction of high frequency,high efficiency,and high power density.As an outstanding representative of the third generation of wide bandgap semiconductor devices,Cascode GaN HEMT utilizes a cascode structure to achieve the normally-off nature of GaN devices,with unmatched steady-state and dynamic performance of Si-based devices.In order to promote its replacement of Si devices and give full play to the performance advantages of Cascode GaN HEMTs,this thesis studies its characteristics and applications in hard switching and soft switching states.Firstly,for the special structure of Cascode GaN HEMT,the parameters related to its output characteristics,transfer characteristics,driving characteristics and switching characteristics are compared with Si MOSFETs under the same voltage and current level,and it is clear that it is better than Si MOSFET.In order to obtain accurate switching loss and clarify the mechanism of the influence of various parameters on the switching process,this thesis establishes a practical segmentation analysis model for the dynamic process of Cascode GaN HEMT turn-on and turn-off,which is verified by the dual-pulse test hardware platform.The prediction of the process voltage and current waveform is accurate,and the switching loss error is within 10%.Secondly,in order to deal with the problem of voltage and current spikes and oscillations and high losses caused by parasitic parameters of Cascode GaN HEMT in high-frequency and high-speed hard switching applications,thus limiting the switching frequency rise.Based on the specific analysis and comprehensive consideration of the influence of driving resistance,parasitic inductance and parasitic capacitance on the switching process,this thesis proposes a parasitic inductance optimization strategy for different design goals.The optimization method of hardware design for Cascode GaN HEMT high-speed application is given.The switching process under different driving resistors and drain current was tested.At the same time,a 1 MHz hard-switching Buck converter is manufactured to verify the stability of Cascode GaN HEMT in highfrequency and high-speed hard-switching topology.Finally,Cascode GaN HEMT is applied to the soft-switching topology of LLC resonant converter.The relationship between the output capacitance of the switching device and the dead-time and the RMS of the first and second side currents is analyzed.Taking advantage of the small output capacitance of Cascode GaN HEMT,the circulating current of the converter is reduced,the conduction loss and transformer loss are further reduced,and the efficiency of the converter is improved.A LLC converter with 97% maximum efficiency and 96.2% full load efficiency was built.