Control Strategy Of Isolated Bi-Directional Half-Bridge Three-Level CLLLC DC-DC Converter

With the addition of distributed renewable energy in power grid,cleanliness and intelligence have become the development directions of modern power grid.Power electronic transformers are widely concerned by experts and scholars.Due to the ability to connect different voltage levels and distributed power like photovoltaic and wind power,bi-directional isolated DC-DC converter stage can maximize the utilization of renewable energy.Therefore,it is a key point to affect the overall efficiency and power density of PET,and it is the hit of PET.In this paper,a bi-directional half-bridge three-level CLLLC converter which can be used in PET is studied.It combines the functions of voltage conversion,isolation and bi-directional power flow,and integrates the advantages of the half-bridge three-level structure and the CLLLC resonant converter.Firstly,the operating modes of bi-directional half-bridge three-level CLLLC converter in sub-resonance,resonance and over-resonance are analyzed.The equivalent model of the circuit is established by using the fundamental approximation equivalent method.According to the model,the resonant frequency characteristics and gain characteristics of the circuit are analyzed in detail,which bulid up some theoretical basis for the set of resonant parameters.Then,the effective value of the primary-secondary resonant current in bi-directional halfbridge three-level CLLLC converter is derived in detail when the dead-time is considered.Based on the three-level ZVS soft-switching condition,the design method of excitation inductor and other resonant network parameters are determined.Meanwhile,high frequency transformer and resonant inductor are designed and the design and test of GaN MOSFET driver board is complished.Finally,a prototype of 500W/400V-110 V bi-directional isolated half-bridge three-level CLLLC DC-DC converter based on GaN MOSFET is designed.A digital controller suitable for this topology is designed.The functions include constant frequency duty cycle soft start output voltage hysteresis Brust_PFM control and secondary side synchronous rectifier.The 110 V forward and 400 V reverse bidirectional experimental test are completed in different voltage levels under full load condition.The experiment results reveal that the efficiency of the system reaches maximum near the resonance point while decrease on upper and lower resonance regions.