Research And Implementation Of Switched-capacitor Multilevel Inverters

Compared to the traditional inverters with bulky inductance components,the switched-capacitor circuits which contain no magnetic components could effectively realize not only higher power density,smaller size and lighter weight,but also higher boost ratio.The grid-connected inverter system based on switched-capacitor circuits is a great significance to promote the development of grid-connected power system to miniaturization,integration and high efficiency.Chapter.2 describes the two-stage switched-capacitor converters.A bridge multilevel modular switched-capacitor DC-DC converter for the front end of a two-stage inverter is proposed firstly,which can output three levels voltage.Then the resonant bridge modular multilevel switched-capacitor topology is given,where the distributed inductance is treated as a resonant inductor.With the phase shift plus PWM control strategy,the resonant converter ensures that all switching devices are soft switching and the output voltage is adjustable.And the analysis of switched-capacitor ripple and output ripple is given.Then the paper proposed a seven-level inverter based on above switched-capacitor circuits with its superiority in conversion efficiency and power density.The topology is composed of DC-DC and DC-AC stages,aiming to improve system stability and simplify the control method.The DC-DC stage,which can be expanded to synthesize more levels,not only features multilevel voltage gain but also partially replaces the original bulk input capacitor and functions as an active energy buffer to enhance power decoupling ability between DC and AC sides.In DC-AC stage,the ladder wave used for inversion helps improve the quality of output waveform.Meanwhile,the multilevel voltage phase has been optimized to reduce the power loss further.A single-stage switched-capacitor five-level inverter topology is proposed in Chapter.3.And the analysis of topology and control strategy is given.The five-level inverter features the switched-capacitor module with DC-DC boost ability and the multilevel flying-capacitor-clamped inverter topology with flexible switching combination.Compared with traditional multilevel inverter,the component number of this topology is cut down due to the special composite structure.Moreover,the switching loss is reduced as a result of the part of the switches' operation under line voltage frequency.Then the carrier-based phase disposition PWM method is employed to help balance the capacitor voltage and achieve the output of the five levels.Based on the working modes and control method analysis,the relationship between modulation ratio and output voltage amplitude is calculated.At the end of the paper,the hardware and software based on DSP of the seven-level inverter and the five-level inverter are designed.The simulation model on Saber and experimental prototype is built to verify the effectiveness and advantages of above topologies and topology-based control strategies.