Research On High-speed Switching Characteristics And Application Of Cascode GaN HEMT

The power semiconductor device is the core of the power electronic converter,so the performance of the device often determines the performance of the entire converter.Compared with traditional Si devices,the new wide bandgap GaN material devices have better performance.The gradual replacement of traditional Si devices through GaN devices will further promote the development of power electronic converters towards high frequency,high efficiency,and high power density.Cascode Gallium Nitride High Electron Mobility Transistor(Cascode GaN HEMT)is a representative GaN device.It uses a unique structure to combine low-voltage Si MOSFET and highvoltage depletion-mode GaN HEMT connected together,it has been widely used in medium and high voltage occasions.Under this background,in order to give full play to the advantages of Cascode GaN HEMT,this thesis has carried out some researches on its high-speed switching characteristics and application.Firstly,the static characteristics of Cascode GaN HEMT are compared with traditional Si devices.For its special structures,three operating modes,forward blocking,forward conduction and reverse conduction are analyzed.Under the application background of Cascode GaN HEMT high-speed switch,the influence of parasitic parameters in the circuit can no longer be ignored.Based on the double-pulse test circuit with parasitic parameters,the specific switching process of the device is analyzed,and accurate switching process waveforms of Cascode GaN HEMT were obtained through experiment.Secondly,based on the analysis of the working process of the Cascode GaN HEMT,the main factors affecting its switching characteristics are summarized,and three types of factors such as driving circuit parameters,circuit operating conditions,and parasitic inductance are further analyzed through theoretical derivation and simulation.The change rule of the device switching characteristics with different parameters is obtained,and the reference range of the drive circuit parameters is given.It is proposed that the parasitic inductance should be optimally designed according to two different requirements for high efficiency and reliability in practical applications.In view of the problem that the spikes and oscillations during the high-speed switching of the device may bring hidden dangers to the reliability and stability of the circuit,the causes are analyzed from the active and passive tubes in the bridge circuit.Four suppression measures were studied for the spike and oscillation problems of the active tube and their effectiveness was verified.The influencing factors and parameter optimization strategies for the crosstalk problem of the passive tube were also studied.This provides a basis for the design of high-speed switching applications for Cascode GaN HEMT.Finally,the Cascode GaN HEMT is applied to the LLC resonant converter.The output capacitance of the Cascode GaN HEMT is smaller than that of the traditional Si device.So the dead time is optimized and the conduction loss of the primary side switch tube and secondary side rectification tube are reduced.At the same time,the drive circuit is optimized for the high-speed switching of Cascode GaN HEMT,which improves the stability of the circuit.By building a 240 W LLC resonant converter prototype,the superior performance of the Cascode GaN HEMT is verified.The thesis has 97 figures,5 tables and 80 references.