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 a diode-assisted capacitor network. The new topology extends voltage regulation range and avoids extreme duty ratio of switching devices in front boost circuit. As for the unique structure, various pulse-width modulation (PWM) strategies are provided to obtain the maximum linear voltage gain with regard to the chopped intermediate dc-link voltage of the inverter bridge in one switching time period. However, when the inverter supplies inductive or capacitive load with small load power factor, the uni-directional current flow of diode may limit the application of the converter utilizing small voltage vectors for ac output voltage extension. This new operation condition is not expected and has not been discussed before. This paper makes a detailed circuit analysis of the relationship between the undesired operation condition and load power factor. All the theoretical findings are finally verified by simulations and experiments.