Natural balance of multicell converters: The general case

Abstract

This paper focuses on the development of the natural balancing theory for the -cell case. It describes the relationship between the models for different numbers of cells in a generic model for a -cell multicell converter. The model discussed is based on the same principles that were used to develop the two-cell model in [16], except that the mathematics is much more involved. The same conclusions that were found to be true for the two-cell case was also found to be true for the general case of cells. These conclusions include that the natural balancing mechanism of multicell converters depends on the overlap of the groups of harmonics of the switching function as well as on the load impedance. It will also be shown that the self-balancing mechanism ensures safe operation under most operating conditions where a high enough switching frequency is chosen and the load is not purely reactive. Two new aspects of the balancing theory were identified in the -cell case: 1) for fixed duty-cycle modulation there exists certain values of the duty-cycle that causes the natural balancing mechanism to fail and 2) for -cell converters the balance booster concept can be extended to a number of balance boosters tuned to multiples of the switching frequency. A “DesignTool” based on the balancing theory was developed to aid practicing engineers in designing multicell converters.