A novel type of combined multilevel converter topologies

A novel type of combined multilevel converters is discussed, combining the conventional diode clamped or flying capacitor multilevel converters with two-level bridge legs. Compared with the conventional multilevel converters, this novel type requires much fewer clamping diodes and flying capacitors at the same number of output voltage levels. So the reliability of the circuit is improved and costs are reduced. The construction rule and modulation method of these topologies are presented in the paper. To confirm the validity of the new topologies, a combined diode clamped seven-level half-bridge inverter and a combined flying capacitor five-level half-bridge inverter are taken as examples for simulations and experiments, and the results proved the validity.

14 Introduction

Because multilevel converters can synthesize multi-staircase output voltage by combining several DC link voltages, they have better harmonic spectra and lower device voltage stress. Thus they are suitable for high-voltage and high-power applications. Having more voltage levels, the synthesized staircase waveform is much closer to sinusoidal waveform and the total harmonic distortion is much lower. However, the conventional multilevel converters need more clamping diodes or flying capacitors to increase the number of voltage levels, which inhibits their applications in circuit layout. In this paper, a novel type of combined multilevel converters is discussed, combining the conventional diode clamped or flying capacitor multilevel converters with two level bridge legs. The proposed topologies require much fewer clamping devices than the conventional multilevel converters at the same output voltage levels.