An LLC Resonant Single-Stage Inverter With High-Frequency Link and Soft-Switching

Abstract

High-frequency-link converters play a significant role in the new generation of smart grids and power systems. Usually, high-frequency-link inverters adopt voltage-fed topologies. However, in dc converters, the widely used <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> topology is a current-fed structure, which has the advantages of soft-switching and high efficiency. The disadvantage of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> topology is that the operating range is narrow in the traditional operating mode. With the development of topologies and control strategies, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> converters are also well employed in wide input or output range occasions. Based on this, this article proposes a single-stage inverter with the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> topology. The proposed inverter employs a topology-morphing of full-bridge (FB) to half-bridge (HB) and a hybrid control strategy of the pulse frequency modulation (PFM) combined with the pulsewidth modulation (PWM). The operating area is divided into three regions according to the output voltage. The FB-PFM, HB-PFM, and HB-PWM are adopted in different regions, so as to achieve a narrow frequency range and full soft-switching. Operation principles and characteristics of the proposed inverter are analyzed in detail, and the key parameters of the inverter are designed. To verify the proposed scheme, a 2 kW prototype of 480 V input and 220 V/50 Hz output was built and tested with a maximum efficiency of 97.6%.