Research On Controlling Integrated Buck-boost+LLC Cascaded Converter

High-voltage batteries charge the low-voltage auxiliary batteries by the aid of a DC/DC converter in electric vehicles.The integrated Buck-Boost + LLC cascaded converter can satisfy the requirement of wide input voltage range and can work in constant-voltage or constant-current states,which is siutable for the applications in electrical vehicles.In this paper,a new phase-shift selection scheme is proposed to improve the existing PWM+Phase-shift control strategy.Under the improved control strategy,the conduction losses of the converter is reduced,the efficiency is improved,and the ZVS region of the converter is also widened.At the same time,the improved control strategy can achieve continuous phase shift angle required by ZVS of all switches when the input voltage and transmission power is changed continuously,which overcomes the sudden change of phase shift angle in the original control strategy,simplifies control of the system,and improves the reliability of the converter.This paper first introduces the topology of the integrated Buck-Boost + LLC cascaded converter,and the division of the operation modes,the gain characteristics and the power characteristics of the PWM+Phase-shift control strategy,then points out the existing problem in the strategy that the selected phase shift angle is discrete and results in high conduction losses.In order to reduce the conduction loss,the ZVS condition of the converter is analyzed and the range of the phase shift angle which can ensure ZVS at the specific power is achieved,then the optimal phase shift angle selection strategy is proposed,the ZVS range and its influencing factor under the improved control strategy are also pointed out.Then the improved strategy is compared with the original one,and the improved strategy proves to reduce the conduction losses,expand the ZVS region and improve the stability of the system.Through the modeling of the system,the transfer function of the system is calculated,a close-loop regulator is designed to reduce the effect on the output voltage and the output current when the load is changed,and input voltage feed-forward control strategy is also designed to improve dynamic performance when input voltage changes,which are all verified by simulation,and the control strategy of the converter is given.Finally,hardware design methods including the main switches,the synchronous rectifiers,inductance and transformer is given,and a 200~450V input and 14V/71.5A output converter prototype is designed to verify the theoretical analysis.