| In this paper,the adverse effects that input voltage of auxiliary converter varies greatly(1000-1800V)are analyzed.A three-level boost converter is introduced in front stage of the converter as a regulator circuit to raise the input voltage to 1800V.Combining the five switching modes of the converter,the continuous conduction mode and the discontinuous conduction mode are analyzed,then the output voltage transfer functions of the converter under different conduction modes are deduced.Operating characteristics of different conductive modes are compared with each other.In order to guarantee operations of the converter in continuous conduction mode with better performance,the critical conduction condition of the converter is analyzed.Focusing on the problem of inherent midpoint drift of the three-level boost converter,potential causes of the problem are analyzed,and a double-loop control method is proposed.According to different generation methods of the pulse,it can be divided into independent duty control strategy and pulse phase delay control strategy.Then small-signal models are set up respectively.Traditional modeling method is not applicable to the pulse phase delay control strategy because it ignores the current ripple.Therefore,a modeling method based on inductor current ripple is introduced,and a precise mathematical model is established.In order to reduce the coupling between the duty ratio signal and the pulse phase delay signal in dynamic regulation process,the enable module is added to the bias voltage loop.Based on this control mode,the output voltage loop and the bias voltage loop are respectively corrected and optimized.In order to carry out the simulation experiments more accurately and quickly,the steady-state and dynamic characteristics of the converters are verified based on the co-simulation of PSIM + Matlab.An experimental platform is built to verify the accuracy and feasibility of the theoretical analysis and simulation. |
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