Single-Phase Direct PWM Bipolar Buck–Boost AC–AC Converter Without Commutation Problem

The direct pulsewidth modulation (PWM) ac-ac converters are seeing rapid development due to their single-stage power conversion with reduced footprints, due to the elimination of intermediate dc-link capacitor. However, they only provide unipolar voltage gain and fixed frequency operation and use the bidirectional voltage-blocking devices without natural inductor current free-wheeling path, restricting their practical applications. This article proposes a direct PWM bipolar and step-variable frequency buck-boost ac-ac converter. The proposed converter is fabricated by connecting input-output$LC$filters and a small value film capacitor to six unidirectional voltage-blocking devices, which are arranged as a three-leg inverter module. It can provide noninverting and inverting buck-boost voltage operations with a symmetric bipolar voltage gain of± d/(1-d)and can also change the frequency of output voltage in discrete steps, same as that of single-phase matrix converters (MCs). It has no open-circuit risk of inductor due to the presence of natural current free-wheeling path during PWM dead time. Therefore, it is unaffected by the commutation problem of conventional direct ac-ac converters. The proposed converter can realize high-quality input and output due to the presence of second-order input-output filters with continuous input-output currents. A detailed analysis is provided based on proposed switch modulation strategies and confirmed through extensive experimental tests on a laboratory-fabricated hardware.