Research On Model Predictive Control For Flying Capacitor Multilevel DC-DC Converter

Compared with the two-level DC-DC converter,the flying capacitor multi-level DC converter has lower device voltage stress and smaller inductor current ripple.Therefore,the flying capacitor multilevel DC-DC converter is very suitable for medium and high voltage fields and high efficiency and high-power density occasions.Different from the two-level DC-DC converter,the flying capacitor multilevel DC-DC converter not only needs to adjust the output voltage to a given value and limit the inductor current to avoid overcurrent,but also needs to balance the flying capacitor voltage.Since the flying capacitor multilevel DC-DC converter is a multivariable strong coupling nonlinear system with constraints,it is not easy to design a controller that can balance the flying capacitor voltage and quickly adjust the output voltage while meeting the system constraints.The finite control set model predictive control(FCS-MPC)that can provide fast dynamic response speed can be well applied to flying capacitor multilevel DC converters,but its switching frequency is not fixed and its steady-state performance is poor.This paper proposes a continuous control set model predictive control(CCS-MPC)for flying capacitor multilevel DC-DC converter,which can provide a fixed switching frequency and achieve excellent steady-state performance.In addition,this paper also proposes a flying capacitor voltage estimation method to realize sensorless active balancing control of flying capacitor voltage.First of all,this paper proposes CCS-MPC for three-level flying capacitor boost converter,including explicit model predictive control and simplified model predictive control.Compared with FCS-MPC,explicit model predictive control not only has fast dynamic response,but also has excellent steady-state performance.Simplified model predictive control is a simplification of explicit model predictive control,which can greatly reduce the computational burden and complexity while meeting the control requirements.The proposed CCS-MPC is used as an inner loop controller to control the inductor current and flying capacitor voltage.On this basis,an outer loop controller with a Luenberger observer is proposed,which can improve the dynamic performance and robustness of the output voltage under the current limiting condition.Compared with the previous control method,the proposed control method not only has good dynamic performance,but also has stronger ability to resist input voltage disturbance.Secondly,this article proposes a CCS-MPC for three-level flying capacitor buck converter,which is used to control flying capacitor voltage and inductor current.On this basis,an outer loop controller with Lomberg observer which can obtain fast output voltage dynamic response speed and good robustness is proposed.Compared with the previously proposed P controller,the proposed control method has better steady-state performance,and can avoid the use of load and input voltage sensors as much as possible.Compared with the improved P controller(namely IP controller),the proposed control method can obtain a faster dynamic response of the output voltage with a smaller peak inductor current.In order to avoid the use of flying capacitor voltage sensor,the estimation method of flying capacitor voltage is proposed to realize the active balance control of flying capacitor voltage without sensor,that is,the sensorless control method.Except that the steady-state performance and dynamic performance of the flying capacitor voltage are slightly worse,the rest of the sensorless control method is basically the same as the control method with sensor.Finally,this paper presents a flying capacitor voltage estimation method based on output voltage detection and inductor current measurement,which is suitable for general flying capacitor multilevel DC-DC converters.Then a CCS-MPC-based cascade control method suitable for flying capacitor multilevel DC-DC converters is proposed,also known as the sensored control method,which can well realize current limiting,output voltage control and active balance control of flying capacitor voltages.Compared with the previously proposed control method for four-level flying capacitor boost converter,the proposed sensored control method has a good dynamic response speed.The sensorless control method is combined with the flying capacitor voltages estimation method to form the sensorless control method,which can better realize the active balance control of flying capacitor voltages without using flying capacitor voltage sensors,thereby effectively avoiding natural oscillation and instability that may occur in natural equilibrium.