High Frequency Switching Characteristics Of GaN Devices Based On Power Half-Bridge Topology

The enhancement-mode GaN power devices are especially suitable for the power conversion system with high frequency,high efficiency and high power density because of the advantages of high electron mobility,high saturated electron velocity and high voltage resistance.Although GaN devices have broad application prospects in the field of power electronics,their special working principles and physical characteristics put forward new requirements for power system design and analysis.In order to ensure the optimal design of high frequency switching power supplies,it is necessary to have a thorough comprehension of GaN power devices.The previous studies on the switching characteristics of enhancement-mode GaN devices mostly focus on the device level and pulse switching conditions.In this paper,the power devices and power half-bridge topology are combined to analyze the high-frequency switching characteristics of enhancement-mode GaN devices.The research contents mainly include the following aspects:(1)Based on the theoretical knowledge of GaN power devices and synchronous Buck switching power supplies,the physical characteristics of enhancement-mode GaN devices are studied in depth.According to the mature converter design theory,the selection of switching devices,power inductor and driver integrated circuits are completed.The synchronous Buck converter based on GaN device is realized,and functional verification is carried out through simulation tool LTSpice XVII.(2)Based on the power half-bridge topology under the working conditions of high-frequency switches,the working current path of the circuit when GaN devices are in steady state and transient state respectively is studied.Focusing on the analysis of switching on transient electrical behavior.The defects such as high switching loss,undesired false turn-on,reliability threat and system EMI problem caused by junction capacitor discharge are revealed.For the power transient electrical behavior of enhancement-mode GaN device,a small signal model is established according to the physical structure of the device.Then the buffer layer charge transfer mechanism is proposed innovatively by virtue of band structure and charge distribution.It is revealed that the current/voltage surge phenomenon is caused by the transfer of electrons from the buffer layer to the two-dimensional electron gas channel in the miller platform stage.The above analysis results provide theoretical basis for optimizing the design of gate driver current.(3)The influence of circuit parameters on the electrical behavior of GaN devices in the turn-on transient process is studied.The changing rule of the high frequency switching characteristic of GaN devices with the system indicators is analyzed.The special driver scheme design of enhancement-mode GaN devices is optimized.The driver current determined by the circuit parameters and working environment is further deduced and improved to improve the safety and reliability and suppress EMI problems of the power system,so as to accelerate the development of the current mode driver scheme.(4)The captured or released electron behaviors of traps in GaN devices under different electrostress environment is explored.The dynamic change of conduction resistance in GaN devices under high-frequency operating conditions is revealed.And the influence of dynamic conduction resistance on topological performance of power half-bridge topology is analyzed.The final test results show that the trapped electrons are not fully released by the traps of GaN devices under high-frequency operating conditions.So there is a built-in negative potential in the buffer layer of GaN devices,leading to the decline of dynamic conduction resistance,affecting the hard switching loss of GaN devices,and thereby reducing the topological efficiency of power half-bridge.