Single-Stage Isolated Multiresonant <i>LLC</i> Inverter With Over 98% Efficiency and Reduced Backflow Power

This paper introduces an innovative single-stage isolated inverter based on a multi-resonant LLC topology that integrates a third-order harmonic LC branch in a conventional LLC tank. By introducing a zero-gain point in the resonant network, multi-resonant LLC enables a wide output range under a limited switching frequency span and remains low total harmonic distortion (THD) under different load conditions. Moreover, the injection of third-order harmonic effectively reduces backflow power, leading to lower conduction losses and enhancing the efficiency over traditional LLC designs. A 2-kW hardware prototype demonstrates the practical viability of this approach, achieving a peak efficiency of 98.14% at half load. Experimental results also confirm the soft-switching operation and keep THD of the output voltage below 2% over a wide load range, making the proposed inverter a compelling solution for high-performance isolated single-stage dc-ac applications.