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Research on Current Backflow of Asymmetric CHB Multi-Level Inverter

For the traditional asymmetric cascaded H-bridge multi-level inverters, the conventional hybrid modulation method has the problem of current backflow in a certain modulation index range. Although the monopolar modulation method effectively solves this problem, the high-voltage unit participates in the high-frequency operation in part of the range, which limits the improvement of the switching frequency of the whole system. The hybrid frequency modulation method can reduce the switching frequency of the high voltage unit to a certain extent, but the harmonic characteristics of the output voltage will be affected. In order to solve the above problems, a double frequency modulation method based on level-shifted PWM (LS-PWM) is proposed. On the one hand, it solves the inherent power back filling problem of the traditional hybrid modulation method, on the other hand, it ensures that the output voltage of the inverter has good harmonic characteristics when the switching frequency of the high voltage unit is low. The results of simulation and experiment prove the correctness of the theoretical analysis.