Study On Topological Structures And Control Strategies Of Multilevel Boost Converters

In wind, solar and fuel cell renewable power generation applications, multilevel Boost converters are noted by people depending on the merits of high voltage gians, low component stresses and small ripples. Pointing to three applications including multi-input sources, high-input-high-output voltage, low-input-high-output voltage, three different kinds of multilevel Boost converter topologies are derived in the dissertation. And corresponding closed-loop control strategies are researched.In the existing multi-input application, a topology composed of several two-level Boost converters in series at the output terminals, has limited boosting capacity. Therefore, an active-type switched-capacitor network and a reactive-type switchedcapacitor network are proposed. And a parallel-type switched-capacitor group and a series-type switched-capacitor group are subsequently derived. Then, multilevel Boost converters based on the parallel-type switched-capacitor group and multilevel Boost converters based on the series-type switched-capacitor group are proposed. The former ones can make all capacitor voltages balanced by self-balanced function of capacitor voltages and achieve the goal of boosting voltage by the accumulation of some capacitor voltages in series. Capacitor voltages are self-balanced in every switched-capacitor network and all capacitor voltages are in series to achieve the goal of boosting voltage in the latter ones. The advantages and disadvantages of the two ones are: they both have simultaneous supplying power function and individual supplying power function; as the number of switched-capacitor networks increases, the voltage gain increases; as no interleaved scheme is adopted, the modulation strategy is simple but the input current ripple and current stresses across switches are high. Single voltage loop has been applied on the proposed topology to do experimental researches, and experimental results verify the correctness and feasibility of the proposed topologies and control strategy.In the high input voltage and high output voltage application, the existing input-series-output-series(ISOS) multilevel Boost converters have limited boosting capacity. Thus, an ISOS multilevel Boost converter with switched-capacitor network is proposed by combining the series-interleaved structure and the reactive switchedcapacitor network. And the ISOS five-level Boost converter with switched-capacitor network is taken as an example to do analysis of operating principle and performances. The analysis results declare that the input current ripple is small and ripple frequency is two times the switching frequency since the series-interleaved structure is adopted. Five output levels are achieved, which helps reduce the component voltage stress to be a quarter of the output voltage. Compared with the ISOS five-level Boost converter, the ISOS five-level Boost converter with switched-capacitor network saves half number of the switches, which could reduce cost of components and driver circuits. Though two more capacitors are necessary, the voltage gain of the proposed converter increases by one time. Furthermore, the neutral-point potential balance control could be much simpler due to the reactive switched-capacitor network. A three-loop control strategy composed of outer voltage-loop, inner current-loop and voltage-balance loop has been researched and the neutral-point potential imbalance issue caused by inconsistency of component parameters can be depressed well. Simulation and experimental researches have been done and results verify the correctness and feasibility of the proposed topology and control strategy.In the low input voltage and high output voltage application, an input-parallel-output-series(IPOS) three-level Boost converter is proposed by combining parallel-interleaved structure and active switched-capacitor network, which makes the IPOS multi-level Boost converter composed of nonisolated Boost converters be possible. From the point of power conservation principle, the relation between input current-balance and output voltage-balance is analyzed, which indicates input current-balance can ensure output voltage-balance when the two two-level Boost converters have the same component parameters. The input current can be balanced due to the flying capacitor and then the output voltage could be self-balanced. However, in practical application, the component parameters of the two Boost converters are impossible to be the same, which affects the neutral-point potential. Thus, output split capacitors, input split inductors, voltage drops of switch components and parasitic resistances having influence on the neutral-point potential are analyzed in details. The analysis results show that the former three factors have small influences on the neutral-point potential, which could be neglected. However, the parasitic resistances having influence on the neutral-point potential could not be neglected. Therefore, based on the double-loop control strategy I and the double-loop control strategy II that are researched respectively for the converter, three-loop control strategy I and three-loop control strategy II with voltage-balance control have also been respectively researched. Moreover, the uniform topology structure and three-loop control strategy of the IPOS multilevel Boost converter have been derived. Then the uniform circuit structure and control strategy of the IPOS multi-input multilevel Boost converter are also derived. Simulation and experimental research have been studied and results verify the correctness and feasibility of the theoretical analysis.