Analysis On Closed-loop Control Strategys Of Buck Three Level DC Converter

The traditional two level power converters are not suitable for the high voltage output environment,because the power transistor will be easily damaged in the high voltage output condition.So the multi-level converters are attracting more and more attentions for the high voltage output environment.The Buck three level DC converter which derived on the basis of the multi-level converters is widely researched in high voltage output environment.The power transistor of Buck three level DC converters can withstand lower voltage stress,but the voltage of flying capacitor should be kept on stable,which is half of the input voltage.Aim at problem of the stability of the flying capacitor voltage,two kinds control strategies of the flying capacitor are introduced,that is,voltage decoupling control strategy and valley current control strategy.The inductor current slew rate of traditional Buck three level DC converter is less than the traditional two-level converters during a load step-up change,which affects the performance of the dynamic characteristics.In addition,when valley current is in control,as the lowness of desired output voltage,inductance current will be in harmonic oscillation,which influences the stability of the converter.This thesis makes a detailed study on the dynamic response of the Buck three level DC converter when the load is changing and the elimination of harmonic oscillation of inductance current when the valley current is in control.(1)Sliding mode control has good robustness and dynamic performance,in this thesis,a double closed loop controller is introduced to Buck three level DC converter according to the improved sliding surface.The effectiveness and advantages of the controller are validated by simulation.The parameters of the sliding surface are obtained after a number of tests,so the best dynamic response can't be confirmed during a load change.In order to get a better dynamic response,the circuit structure of the Buck three level DC converters is changed so as to increase the inductor current slew rate during a load change.A control method is introduced in this thesis which utilizes the concept of capacitor charge balance to achieve optimal dynamic response for the Buck three level DC converter.Since the inductor current slew rate of steady state and transient period are different,the inductance current waveform is not symmetric.So,a transient controller is designed by calculating the proportion of transient time through output capacitance current.The rationality of the controller is verified by simulation.(2)With valley current mode controlled Buck three level DC converter,this thesis studies the theory and condition of harmonic oscillation produced by inductance current.Regarding to the traditional slope compensation will increase the valley value of the steady-state inductor current,a zero-perturbation dynamic slope compensation scheme is introduced.The simulation results show that the zero-perturbation dynamic slope compensation suppresses the fast scale instabilities of the system effectively.With the application of the Jacobian matrix solution the stable working range of the ramp parameter of the slope compensation is derived,which can guide the selection of the control parameter.(3)Through previous analysis,it's easy to notice that the ramp of slop compensation is affected by the converter parameters.When the converter parameters change,the ramp of slope compensation is unable to satisfy requirements.A novel and exact adaptive slope compensation circuit is proposed in this thesis,which introduces the on-resistance of the power transistor.The integral control scheme of the Buck three level DC converter is given.The validity and feasibility of the circuit is verified on the simulation software.