Multi-loop controller design for diode-assisted buck-boost voltage source inverter

Diode-assisted buck-boost voltage source inverter extends voltage regulation range and avoids the extreme boost duty ratio of switching devices in front boost circuit by introducing diode-capacitor network. It provides an attractive DC-AC power conversion with enhanced voltage buck-boost capability, low cost and high efficiency in high voltage gain application. However, due to the chopped intermediate dc-link voltage, the front boost circuit and inverter bridge have to be controlled coordinately, which increases the complexity of controller design. This paper presents model analysis and reveals that the converter can be equivalent to conventional dc-dc boosted voltage source inverter with modified modulation index. Therefore, the DC and AC-side regulator are designed separately. Like boost-type dc-dc converter, the intermediate DC-link voltage has a non-minimum-phase system transient response. This undesired influence may be transferred to the ac-side, resulting in initial voltage undershoot and oscillatory. Therefore, the close-loop control bandwidth of the inverter stage is relatively much larger than that of the front boost circuit to minimize such an influence on the output ac-side. Finally, all the theoretical findings and proposed control algorithms are verified by simulation and experiment results.