An Investigation on Fully Zero-Voltage-Switching Condition for High-Frequency GaN Based LLC Converter in Solid-State-Transformer Application

LLC resonant converter can help achieve soft switching for both primary side and secondary side devices, which makes it a great candidate for today’s high-frequency, high-efficiency and high power density solid state transformer (SST) applications. For cases to achieve a constant DC output voltage, it is desired that the LLC converter works in the zero-voltage-switching (ZVS) region that the switching frequency (fs) is smaller than the resonant frequency (fr). However, when a much wider frequency modulation range with ZVS guaranteed is needed to achieve a wide output voltage range and a variable load, the region that fs is larger than fr is the better choice. This region, which is rarely analyzed in the literature, is investigated in this paper. Detailed analysis for charging/discharging process during deadtime is shown. A simple but accurate model to determine fully ZVS condition in this region is proposed, which can also be used for deadtime optimization. Moreover, a 1 MHz/2 kW GaN based LLC converter prototype is built and presented for verification.