Research On High Frequency Digital LLC Resonant Converter Based On Simplified Optimal Trajectory Control

The energy issue has brought much attention to energy-saving and environmentally-friendly vehicles,and the design of power supply is a key factor in the current research of electric vehicles.Electric vehicle power supplies have strict requirements in power density,efficiency,reliability and dynamic characteristic,digital high-frequency high-power density switching power supply with new power devices is the current development trend.The LLC resonant topology has advantages of wide input voltage range,full-load range soft-switching and easy magnetic integration etc.which meets the development requirements of switching mode power supply.In this paper,the digital high-frequency LLC resonant converter based on characteristics of vehicle power supply is studied,the specific contents are introduced as follows:In this paper,working principle of LLC topology is analyzed in detail,the voltage gain characteristics in discontinuous mode(DCM)is analyzed to improve the accuracy of parameter design and the parameter design method is summarized.Soft-switching conditions of LLC topology in DCM are discussed in detail,and a novel method of chosing magnetic inductor is proposed to reduce system reactive current and turn-off loss.LLC resonant circuit is modeled and analyzed based on extended description function method(EDF).The system's transfer function and the distribution of zeros and poles are analyzed,a 4P2 Z compensator is designed through the digital controller method to satisfy the system requirements of steady-state and dynamic characteristics.Based on load mutations which always occur in electric vehicle,state plane trajectory is introduced to analyze LLC circuit.The pulse width of switches is calculated through the output current,and th e system can reach new steady state quickly to improve the dynamic characteristics.Aiming at the severe oscillation in resonant current during start-up stage,a start-up strategy is proposed based on SOTC.A dynamic limit to resonant current is set up to optimize the start-up performance of the converter.In order to reduce the cost of converter,the computational complexity of SOTC is simplified through look-up table and linear fitting method.The adverse effects of digital delay is reduced in high freque ncy converter,the digital closed-loop control system is realized in DSP.Finally an experimental prototype with full load power 300 W working frequency 500 k Hz was designed,and the experimental results are consistent with theoretical analysis and simulation results,which verifies the correctness of method proposed in this paper.The experimental prototype can realize ZVS in all working conditions and the maximum efficiency reaches 96% under half load condition.The results of load switching experiments show that dynamic response speed of the system is improved significantly when SOTC is used in digital controller.The resonant current is limited within the expected range during system start-up stage which proves the effectiveness of control method.