Multilevel Converter And Its Control Strategy

Multilevel(ML) converters have been attracting more and more attention because they have the advantages such as low voltage stresses for power devices and reduced harmonic content of the voltage waveform. They are very suitable for high voltage and high power conversion applications.In this paper, the basic commutation cell in multilevel converters is proposed, based on which a series of multilevel converters with flying capacitors are derived.The operation principle of Buck ML converter and its output characteristics are analyzed in details in this paper. The Buck ML converter splits voltage constraints among series-connected switches through flying capacitors. An effective control of flying-capacitor voltage is required to guarantee voltage balancing across semiconductors. A novel control strategy for the flying capacitor voltage is proposed in this paper. With this control strategy, decoupling control of the flying capacitor voltage is obtained, which has no influence on the output voltage. So the closed-loop control of flying capacitor voltage and output voltage can be designed independently. The effectiveness of the proposed control strategy is verified experimentally by a three-level and a four-level Buck converter. This control strategy is simpler and more feasible compared with the method presented before.