A Family of High-Frequency Isolated Single-Phase Z-Source AC-AC Converters With Safe-Commutation Strategy

This paper extends the high-frequency transformer isolation concept to the Z-source (ZS) ac-ac converters and introduces a new family of high-frequency transformer isolated (HFTI) ZS ac-ac converters. The proposed HFTI-ZS converters retain all the benefits of their existing nonisolated counterparts, such as providing a larger range of output voltage with buck-boost functionality, reversing or maintaining the phase angle, reducing the in-rush and harmonic currents, and improving reliability. In addition to these benefits, the high-frequency (20 kHz) transformer (HFT) in the proposed ZS ac-ac converters provides the electrical isolation and safety with high power density as it eliminates the need for bulky and heavy line frequency (50 or 60 Hz) transformer for galvanic isolation, in applications such as dynamic voltage restorers, etc. The dc-blocking capacitor added in series with the HFT results in only ac voltage applied across its windings, which avoids its saturation. Various ZS-based HFTI ac-ac converters are proposed in this paper, and to verify their operation and advantages, example of quasi-ZS (qZS)-based isolated ac-ac converter is considered in detail. The commutation strategy is also developed to achieve the safe commutation, which avoids the current and voltage spikes without using any RC snubber. A 200-W laboratory prototype of HFTI-qZS ac-ac converter is fabricated and experiments are performed to validate the advantages of the proposed ac-ac converters.