Structure Optimization Of Cascode-type GaN HEMT And Its Application In PFC Converter

With the increasingly serious problems of energy crisis,global warming and the development of new energy industry,high efficiency and miniaturization have become the development trend of power electronics technology.As a core part of power electronics technology,the characteristics of semiconductor power transistors directly affect the performance of the system.With the changes in demand,Si-based devices can no longer meet the needs of power electronic technology development in some applications.Power transistors based on third-generation semiconductor materials,gallium nitride,have received widespread attention for their excellent performance since their appearance.In particular,gallium nitride high electron mobility transistors(GaN HEMT)have shown strong potential in the low-voltage consumer-level field due to their higher switching frequency and no reverse recovery loss.However,the depletion-mode characteristics of the device limit its application range.Based on the key industrial projects of 600 V enhancement-mode GaN HEMT,this paper conducts related research.First,the theoretical analysis of the working principle of the depletion mode GaN HEMT was carried out,and the advantages and disadvantages of the current four types of enhancement mode GaN HEMT(Cascode type,P-GaN type,trench gate type,and ion implantation type)were discussed.Relevant researches on P-GaN type and Cascode type devices were carried out respectively.For P-GaN devices,the influencing factors of the threshold voltage of the device were determined through finite element simulation,and then an enhanced mode GaN HEMT with a mixed conductive channel was proposed,with a threshold voltage of more than 3V and a saturation current capability more than 0.4A /mm,but the breakdown characteristic of the device will be restricted by the existing technological level.For Cascode-type devices,the working principle was clarified,and the substrate and packaging structure of the device were optimized from the perspective of reducing parasitic parameters and improving heat dissipation capabilities.Based on the above research work,from the perspective of process difficulty and cost,Cascode technology was finally used to prepare 600 V enhanced mode GaN HEMT.Secondly,in order to test the switching characteristics of the Cascode device,a double pulse test was performed on the device.The turn-on and turn-off characteristics of the Cascode device were analyzed,and on this basis,the high-frequency drive circuit of the device was designed,and the corresponding double-pulse test platform was built to test the device under different drain-source voltages and different saturation currents.After testing,the turn-on time of the device was less than 100 ns,and the turn-off time was less than 200 ns.The device works normally under the test conditions of 200V/7A,300V/6A,400V/4A and below.Current collapse will occur if the above conditions were exceeded,and the leakage current at 400V/4A was 300 m A,which was higher than the same specification GaN HEMT Device.Finally,in order to test the performance of the above-mentioned Cascode device in practical applications,based on the normal operating range of the device determined in the device test of switching characteristic,a totem pole bridgeless PFC converter prototype with a rated power of 200 W was built,switching frequency was 200 k Hz.Based on the average current control method,a closed-loop controller was designed for the converter,then simulation verification and software and hardware design were performed.The test results show that the efficiency of the prototype under rated conditions was 96.5%,and the power factor was greater than 0.99.The experimental results show that the performance of the converter meets the design requirements,and the device can work normally under this working condition.