Operation Modes Analysis and Limitation for Diode-Assisted Buck–Boost Voltage Source Inverter With Small Voltage Vector

Diode-assisted buck-boost voltage source inverter (VSI) boosts the dc source voltage by introducing diode-assisted capacitor network. With parallel capacitive charging and series capacitive discharging, the new topology extends voltage regulation range and avoids extreme duty ratio of switching devices in the front boost circuit. As for the unique structure, various novel pulse-width modulation strategies are provided to obtain the linear voltage gain with regard to the chopped intermediate dc-link voltage of the inverter bridge in one switching time period. It provides a competitive solution for high voltage gain applications. However, when the inverter supplies inductive or capacitive load with extreme low load power factor, the unidirectional current flow of the diode may limit the application of the converter utilizing small voltage vectors for ac output voltage extension. As a result, diode-assisted buck-boost VSI exhibits new operation mode that has not been discussed before. This paper makes a detailed circuit analysis of the relationship between the undesired operation condition and load power factor. Then, a simple approach to totally eliminate the undesired limitation by replacing the diodes with controlled switching devices is provided. Finally, the related simulations and experiments are implemented to verify all the theoretical findings and solutions.