Multilevel Single-Phase PWM Converters with Shared Legs and Cascaded Transformers

In this work, a family of multilevel single-phase converters is proposed. This family is based on cascaded cells, each composed by K two level legs connected to the primary windings of K - 1 transformers, while their secondary windings are cascaded to generate a multilevel voltage. Two kinds of cells are considered: the shared leg (SL), in which one leg is connected to all transformers, and the multiple shared legs (MSL), which has various legs each shared between two transformers. The turns ratios of the transformers can be calculated in order to give each pole voltage a different voltage gain in the secondary side. This causes the legs and transformers to be asymmetrical, increasing the levels-per-switch generated. The generalized model, turns ratio calculation and PWM design are provided for proposed converters composed by M SL or MSL cells. By means of simulation and experimental results, two topologies within proposed family are analyzed, one composed by one SL cell, and the other by one MSL cell, both with four legs. They are compared to the conventional cascaded h-bridge (CHB) with two bridges and corresponding isolation transformers. Various cases of asymmetry are considered, and results are analyzed to show that the configurations within the proposed family can provide better performance than the CHB in terms of switching stress and harmonic distortion.