An Isolated Bi-directional Soft-switched High- frequency-AC Link DC-AC Converter Using SiC MOSFETs Mengqi Wang, Suxuan Guo, Qingyun Huang, Wensong Yu and Alex Q. Huang Department of Electrical and Computer Engineering North Carolina State University Raleigh, USA mwang7@ncsu.edu Abstract—An isolated bi-directional soft-switched DC-AC converter with high-frequency-AC (HFAC) link using Silicon Carbide (SiC) MOSFETs is presented in this paper. A unipolar-SPWM oriented modulation technique is proposed to enable the full-bridge (FB) stage to realize zero-voltage- switching (ZVS) and the cycloconverter stage to realize zero- current-switching (ZCS). Furthermore, the proposed modulation technique allows half of the switches in cycloconverter to work at line frequency (LF) instead of switching frequency, which significantly reduces switching loss. Because of SiC MOSFET’s low on-resistance and great switching performance under high frequency conditions, they are used for all the switches such to further reduce the switching loss and conduction loss. Thus the switching frequency can be pushed to a much higher level, i.e. 50-100 kHz, which largely reduces the profile of the transformer and inductor. Therefore, the power-density of the converter is highly improved. The advantages of utilizing SiC MOSFETs are validated by simulation and experimental results. Keywords—HFAC link, DC-AC converter, soft-switching, SiC MOSFET I. INTRODUCTION Wide-Bandgap (WBG) devices, such as Silicon Carbide (SiC) MOSFETs, have emerged as a promising alternative that pushes the limits of the power semiconductors [1]. The SiC material has superior performances in electrical breakdown field, thermal conductivity, electron saturated drift velocity, and irradiation tolerance [2]. Those remarkable advantages enable the SiC devices to work at higher voltage, frequency and temperature. Therefore, SiC device-based converters is expected to achieve high efficiency and high power-density as well [3], [4]. In present isolated AC grid-fed UPS, distributed renewable energy systems and propulsion systems, when low profile and weight are required, the power conversion is usually realized by a two-stage DC-AC converter: full-bridge (FB) stage convert DC to HFAC power, and cycloconverter enables the HFAC power to fed to utility grid through a HFAC transformer link [5], [6]. The power conversion is usually realized by full-bridge (FB) stage cascaded by cycloconverter stage through a HFAC transformer link [7]. Most common devices used for the AC switches in cycloconverter are Silicon-controlled rectifier (SCR) and Insulated-gate bipolar transistor (IGBT). SCR and IGBT switches are only suitable for medium frequency, i.e., 5 kHz, because of high switching losses and conduction losses. Si MOSFETs should be a better option to operate under high frequency, but for high voltage applications, the reverse recovery issue is still severe thus the switching frequency is quite limited [8]. In order to mitigate the reverse recovery loss of the Si MOSFET body diode, people suggest paralleling Schottky diode to the device. However, problem still exists even with Schottky diode. Section II will address this issue in detail. Thus we proposed to use SiC MOSFETs for the DC-AC converter. Voltage spikes introduced by the HF transformer leakage inductance is another important issue. Adding an RC snubber branch to the HFAC side of cycloconverter is a straightforward and widely used approach, which is easy to carry out. But its effectiveness depends on different conditions. Moreover, the power dissipation on the snubber resistor lowers the efficiency. Sree and Mohan proposed adding an energy recovery circuit to clamp the voltage spikes [9]. The energy is recovered to the DC source by a transformer plus a diode rectifier bridge. This approach comprises additional four diodes and a transformer which leads to power loss due to the energy circulating in the system. In reference [10], the authors proposed an active clamp approach. A pair of bidirectional AC switch with a snubber capacitor are adopted to absorb the energy stored in the leakage inductance. However, additional AC switch and driver circuit result in circulating energy and increase the 88 978-1-4799-5493-3/14/$31.00 ©2014 IEEE