LLC Resonant Converter Based Single-stage Inverter with Multi-resonant Branches using Variable Frequency Modulation

This paper presents a novel single-stage inverter based on multi-resonant LLC converter along with its design optimization. A single-stage dc-ac inverter reduces the component count and is favorable for the unity power factor output load. To achieve single-stage dc to ac, one can apply a resonant converter with varying frequency to adjust the voltage gain. However, given a finite switching frequency, a conventional LLC converter has a limited range of the voltage gain, which will have difficulties of producing a half sinusoidal for unfolding into ac sinusoidal output. The proposed novelty is to add a paralleled high-order harmonic based L-C resonant branch to obtain a zero-gain voltage point within a limited switching frequency while achieving a wide range of voltage gains. The proposed circuit features straightforward control, high power density, and high efficiency with less circulating energy. A 1.2 kW prototype that converts 380 Vdc input to 600 Vac output has been built to show the performance of the proposed single-stage inverter and achieves 97.3% efficiency and 2.17% THD.