A novel modulation strategy for diode-assisted buck-boost VSI with maximum voltage gain and minimum switching frequency

Diode-assisted buck-boost voltage source inverter (VSI) boosts the dc source voltage by introducing a diode-capacitor network. With capacitive charging in parallel and discharging in series, it extends voltage regulation range and avoids extreme boost duty ratio. As for the unique structure, various pulse width modulation (PWM) strategies are provided with regarding to the chopped intermediate dc-link voltage. In order to maximize voltage gain, as well as to achieve the increased efficiency, this paper proposed a novel modulation strategy by reducing the switching frequency of power devices in the inverter bridge. The operation principle and relationship of switching device voltage stress versus voltage gain are analyzed in detail. Simulation and experiments are implemented to verify the theoretical analysis. Compared with existing modulation strategies, diode-assisted buck-boost VSI with new modulation scheme demonstrates good performances: less power device requirement and higher efficiency in high voltage gain application. Owing to improved performance, it is a more competitive topology for wide range dc/ac voltage regulation in renewable energy application, especially for ac output with relatively high fundamental frequency. Furthermore, the idea of proposed modulation scheme can also be extended to the control of other quasi two-stage inverter.