Variable DC voltage as a solution to improve output voltage quality in multilevel converters

As the multilevel converters have many advantageous features over the conventional two-level converters, they have been widely investigated and applied in industry. These include ability to work with higher voltage/power levels with low-voltage semiconductor devices and improved output voltage quality and hence decreased need for filter elements. However, the multilevel converters when operating in lower modulation indices do not show their quality-related advantages as the number of voltage levels in low modulation indices is substantially lower than that of maximum possible number. In this paper, in order to overcome the problem, the multilevel converter with variable dc voltage is proposed. The key idea is to adjust the dc voltages so that even in lower modulation indices maximum number of voltage levels is generated. This makes it possible to exploit multilevel converters in a more effective way. The proposed approach is supported with simulation results.