Research On Full-bridge Three-level LLC Converter Based On Variable Frequency And Double Phase-shift Control

In recent years, accompanied by prominent energy crisis and environmental pollution, the government vigorously concerns the development of electric vehicles. High power DC/DC converter as an important part of the electric vehicle charger, with the advantage of high efficiency and low cost, has aroused people’s widespread attention. With the increase of power, the large-power DC/DC converter is faced with wide input voltage range.This subject chose the Full-bridge Three-level LLC Resonant converter as the main circuit, and put forward the variable frequency and double phase-shift control strategy.The Full-bridge Three-level topology under double phase-shift modulation, working in the three-level mode, with the switching voltage stress halved, suitable for the high-voltage and high-power.The LLC resonant network, known as the high efficiency; Frequency modulation and high DC gain ratio, low input with high output, suitable for wide input voltage application.Meanwhile, along with the soft switching technology, to further enhance the efficiency.Therefore, it’s of great significance to develop the DC/DC converter with high efficiency and wide input voltage range, which is suitable for mass demand.Firstly, this article briefly describes the high-power DC/DC converter development trends, through analysis and comparison, a full-bridge three-level LLC resonant converter chosed as the main circuit topology. Based on the fixed-frequency double phase-shift control and variable-frequency control characteristics, a variable frequency and double phase-shift control is proposed, as with its working principle and design process interpreted in detail.Secondly, through the conversion between the two-level model and three-level model, build a three-level small signal model, derive a closed-loop control system transfer function, and design the compensator Gc(s).Finally, to design the hardware circuit of the system and software algorithms, to build 700V/15kw experimental prototype whose core controller is TMS320F28035 is realized. To test the experiments under different switching frequency with the input voltage of 200V-600V, output voltage of 700V, and to compare and analysis the 5K.w pure resistire load experiment under overload, fullload, and lightload.The explained results show that under the double phase-shift modulation with the three-level mode, the voltage stress of the switch halves and ZVS achieved for the entire working range.Meanwhile, combined with the variable frequency modulation, the DC Gain rise to 3, high output with low input, and the efficiency is about 94%.