Research On Several Key Issues Of High Efficiency LLC Converters

More electric aircraft technology is a new technology and revolution in the development history of aviation power supply.In the existing avionic power system,the wide-variable frequency AC(WFAC)and high voltage DC(HVDC)power systems are the mainstream of the applications.Thereinto,for the DC power system the coexists in the form of high voltage 270 V DC and low voltage 28 V DC bus provide power for DC loads of different needs.Voltage conversion between 270 V and 28 V is very necessary.Since various types of power electronic converters occupy most of the weight of the power system in the aircraft,conversion efficiency and reliability between them have become the focus of ongoing discussions and research.In this paper,the high-reliability,high-efficiency,high-power-density power conversion requirements of 270V/28 V DC/DC conversion for more electric aircraft are studied.Some key issues of the resonant LLC converter in aviation applications are studied.The problems of the LLC resonant converter under the wide output conditions are discussed.Firstly,for the problem that it is difficult to provide accurate and intuitive modal analysis for resonant converters based on traditional fundamental analysis and time domain analysis,this paper studies the method of state plane analysis applied to resonant LLC converter.The trajectory analysis is carried out under the characteristics of different load conditions,and the detailed trajectory is derived on the conditions that the ZVS/ZCS soft switching of the LLC converter should be satisfied in the state plane space.The state plane analysis lays the foundation for the study of the full text.Secondly,for the wide range of output voltage gain range,the strong nonlinear characteristics of the resonant LLC converter make it difficult to solve the transmission characteristics of the converter.This paper proposes a numerical model based on the phase plane method to solve the transfer gain.The transfer gain of the converter under a wide range of conditions is derived based on this model.According to the loss analysis in the numerical model,the efficiency optimization strategy of the converter under a wide range of output voltage is proposed.The efficiency of the converter is optimized by controlling the phase shift angle under a wide range of output conditions.This part of the research provides a theoretical reference for the nonlinear resonant converter gain characteristics analysis.Thirdly,for the problem that the output current of the LLC converter is difficult to control under the short circuit condition,where large short circuit current under aeronautical applications is required,this paper derives the working trajectory and corresponding control method of the converter under short circuit based on the state plane analysis.This control strategy can reduce the static loss of the converter under short circuit.For the transient overshoot problem existing in the short circuit,this paper proposes a transient control strategy based on the level detection control of the short circuit process.The method reduces the current surge of the short circuit transient.The short-circuit steady-state and transient control strategies proposed in this paper improve the reliability of the converter.This part of the research work provides a theoretical reference for the static and dynamic characteristics analysis of resonant converters.Finally,for the soft start of the converter in the bidirectional application of the LLC converter and the Buck/Boost cascade structure,this paper proposes a control method and circuit based on the duty cycle presetting.The control method and the control circuit are started,which greatly reduces the current inrush of the converter during the startup process and has a faster starting speed.This paper focuses on the reliability problems and efficiency optimization strategies of resonant LLC converters used in 270V/28 V DC conversion in aviation applications.The work provides a theoretical reference and technical solutions for the current aviation power DC power conversion technology.