Study On Wide Gain Variable Mode Dual Half-bridge LLC Resonant Converter

Electric vehicles have the characteristics of zero carbon emission and more environmental protection than traditional fuel vehicles,which can provide an important contribution to the realization of the "double carbon" strategy in China.On-board Charging(OBC)is a key technology to solve the range anxiety of electric vehicles and has become a hot research topic for many scholars in recent years.In an OBC application,the DC-DC converter stage usually adopts LLC resonant structure with excellent soft switching and low EMI characteristics.Generally,the charging voltage of the power battery varies greatly with time during the constant current charging stage,requiring the LLC DC converter stage to have a wide-output-gain characteristic.However,the conventional LLC resonant converter needs to rely on frequency conversion to adjust the voltage output gain,with the large frequency conversion making the converter normalized output gain affected by the load quality factor,which is easy to cause system instability and hard switching of power devices.Therefore,these disadvantages limit the normalized output gain range of the LLC resonant converter making it difficult to adapt to wide gain applications.To this end,this paper takes a wide-gain LLC resonant dc converter as the research object,derives a double half-bridge based variable-mode LLC resonant converter based on the conventional full-bridge LLC resonant converter structure,and then designs a variable-mode control strategy to achieve 0～2x normalized output gain of the LLC converter stage.The main research of this paper is as follows:Firstly,the circuit topology is designed,and a double half-bridge parallel structure is adopted in the primary side of the converter,which effectively lowers the voltage and current stresses of the switching tubes and resonant devices;a transformer series structure is adopted in the secondary side,and a reconfigurable voltage doubling rectifier unit is introduced,and its principle and power path are analyzed.Then,the operating mechanism of the dual half-bridge LLC resonant converter(DHB-LLC)circuit is derived,obtaining the high gain(HGM)and low gain(LGM)operating modes of the circuit.Moreover,the transition gain mode(TGM)with a narrow range of frequency conversion is designed to make a smooth transition between the gain intervals.Next,the submodes of the circuit are analyzed.The relationships between the normalized gain and the phase shift angle,duty cycle and switching frequency in each mode are calculated and analyzed with simulations.A hybrid modulation strategy of PSM-PWM-PFM is designed based on the gain characteristics.After that,a variable-mode control strategy is designed for switching between the three operating modes,so that the DHB-LLC can achieve a normalized gain of 0～2x in the narrow frequency range.Finally,an experimental prototype with a power of 400 W is designed,and its resonance parameters and hardware circuit are designed in detail.The soft switching characteristics of the DHB-LLC in each operating mode and the effectiveness of the variable-mode control strategy are verified through physical experiments.And the result verified that the DHB-LLC can adapt to the wide gain output application scenario of vehicle charging.