Model-free Predictive Control Research Of Boost PFC Converter

The problem of input power factor and current harmonics of grid-tied equipment has become a primary power quality issue.To comply with the stringent requirements regarding the power factor and the total harmonic distortion(THD)of relevant standards,like IEC61000-3-2,power factor correction(PFC)is mandatorily used in power supplies for improving input power quality.Therefore,a PFC converter is usually inserted to a power supply as a pre-regulator to ensure a near unity power factor at the grid side input.The PFC converter is generally controlled by a dual-closed-loop average current mode control strategy,in which an inner current loop controls the input current of the converter to be sinusoidal and in phase with the input voltage.The output voltage of the converter,which is governed by the outer voltage loop,is regulated to be constant as predefined.To operate both control loops in closed-loop manner,PI controllers are designed to realize the input current tracking and output voltage regulation respectively in this dissertation.Firstly,assuming that the converter is operating in continuous conduction mode(CCM),and taking the effects of the digital control delay and the PWM modulator into consideration,the characteristics of the inner current loop in frequency domain is analyzed.At the same time,a mathematical model to describe the dynamics of the output voltage has been built from the perspective of energy.And then,the PI current controller and voltage controller are designed in frequency domain respectively,and the corresponding coefficients are determined.In order to verify the effectiveness of the designed dual-closed-loop PI control strategy combining simulation research with experiment research,simulation model and the experiment prototype of dual-closed-loop PI control of boost PFC converter system have been built.When PFC converter is operating in discontinuous conduction mode(DCM),the PFC converter of dual-closed-loop PI control has technical shortage of poor current tracking performance.Therefore,a unified ultra-local model that describes the dynamics of the inductor current in both CCM and DCM is studied and established in this dissertation,and then the model-free predictive current controller is designed,and the model-free predictive current control of boost PFC converter is proposed,and the performance comparison research with dual-closed-loop PI control of boost PFC converter is implemented.It aims to gain better current tracking performance,higher power factor and lower THD.Once unity power factor is achieved at the grid side of the converter,an input ripple power which is pulsating at twice the grid frequency appears.To compensate for the power mismatch at the input and output side of the converter,bulk electrolytic capacitors(ECs)are often paralleled at the dc output.But this leads to the EC-related reliability problems because of its limited lifetime.Moreover,a second harmonic ripple voltage is generated at the dc output due to the finite capacitance of these capacitors.It would create urgent technical problems,such as input current distortions and sluggish voltage control loop response in boost PFC converter of dual-closed-loop PI control.Therefore,the solution that a capacitive power decoupling circuit is paralleled to the dc output of the PFC converter is adopted in this dissertation.Based on the analysis of the circuit operation principle,the decoupling capacitor voltage model described by the second order differential equation is deduced,and the second order model-free predictive control of decoupling capacitor voltage is proposed.Through the system modeling and simulation research,it is verified that the proposed second model-free predictive control could effectively regulate the decoupling circuit to absorb the ripple power which exists in PFC converter.