Research On Control Strategy Of Bidirectional Half-bridge Three-Level LLC Resonant Converter

Bidirectional DC/DC converters have both bidirectional power conversion and energy management functions.In recent years,they have been used as key interface devices in new energy power generation and energy storage systems,electric vehicles,and uninterruptible power systems.They have received more and more attention and application.LLC resonant converter can achieve zero voltage turn-on of the switching tubes in the full load range,also can achieve high switching frequency and magnetic integration technology,so it has the characteristics of high efficiency and high power density.In the half-bridge three-level structure,the voltage stress of the switch tubes is very small,only half of the DC bus voltage,which is very suitable for applications where the voltage level is relatively high.Therefore,this paper takes bidirectional half-bridge three-level LLC resonant converter as the research object,and studies the various control strategies adopted in detail.First,the basic working principle and equivalent circuit diagram of the converter when using unilateral control are analyzed in detail.First harmonic approximation method is used to model and analyze the circuit,derive the AC voltage gain formula of the converter,and calculate the soft switching conditions of the converter in different working modes.When the converter uses a synchronous control strategy.and the switching frequency is less than the resonance frequency,there is a phase difference between the secondary voltage and current.Based on this,the expression of the equivalent load is re-derived,and a more accurate gain formula for the converter is obtained at this time.Secondly,based on the synchronous control strategy,an improved synchronous control strategy is adopted.This control strategy can maintain a wide range of voltage gain,and only need to control the switching frequency,it can realize the automatic forward and backward mode transition of converter.The basic working principle and equivalent circuit of three different modes of the converter when Gac≥1 are analyzed in detail.Using first harmonic approximation method,the equivalent load and AC voltage gain formula of the converter in different modes are obtained,and the conditions for the soft switching of the switch tubes in each mode are analyzed.Thirdly,the soft switching problem in the improved synchronous control strategy is analyzed.In order to solve the problem,a hybrid control strategy is proposed,which can expand the soft switching range and improve efficiency of the converter.The hybrid control strategy is universal and does not require specific parameter design.According to different implementation methods,hybrid control strategies can be divided into variable frequency hybrid control and phase shift hybrid control.The realization methods of the two hybrid control strategies are analyzed in detail,including the mode switching conditions and the specific control flow.Finally,according to the relevant electrical parameter indexes of the converter,the parameter design was carried out and a 12 KW experimental platform was built.The experimental results verify the correctness of the theoretical analysis and the feasibility of various control strategies.