Voltage Balancing Strategy for a Five-level Flying Capacitor Converter Using Phase Disposition PWM with Sawtooth-Shaped Carriers

Abstract- The flying capacitor (FC) multilevel converter has attracted a great deal of interest in the recent years because of its easier extension to converters of a higher number of levels (n>3), as compared to its counterpart the diode-clamped converter (DCC). This is due to the difficulties of achieving capacitor voltage balance for the DCC. On the other hand, the voltages in the flying capacitors of the FC converter can be controlled thanks to the availability of redundant switching states. The main focus of this paper is to develop a voltage balancing scheme for a five-level FC converter based on phase disposition pulse-width modulation (PD-PWM). Since there are multiple states that produce the same output voltage at the leg of the converter, such a redundancy is used to regulate the FC voltages at their desired levels. The selection of the optimal states is performed by minimizing a cost function. A drawback observed when using standard symmetrical triangular carriers for the PD-PWM, is the additional switching events that are produced due to transitions within the same voltage level. Nevertheless, this fact can be avoided by using sawtooth carrier waveforms instead. Simulation results for the sawtooth carriers show a reduction of the switching frequency of about 20 % as compared to the use of standard triangular carriers while maintaining the FC voltage balance. Moreover, by analyzing the results, it can be seen that the proposed PD-PWM voltage balancing scheme is robust to static and dynamic loading conditions. I