A MHz LLC Converter Based Single-Stage Soft-Switching Isolated Inverter with Hybrid Modulation Method

The inverter that requires isolation typically consists of an isolated dc-dc and a pulse-width-modulated dc-ac stages. However, the component counts, control complexity, efficiency and power density tend to be suffered with such a two-stage configuration. In this paper, a single-stage soft-switching isolated inverter based on MHz LLC converter is introduced with a novel hybrid modulation method, which operates under full-bridge and half-bridge variable frequency conditions for high- and low-line outputs, respectively. A rectified sine wave can be generated at the output of LLC converter and isolated through a high-frequency transformer for additional voltage gains. A lossless line frequency unfolder circuit is added afterwards to convert the rectified sine wave to ac sine output. The optimization method for the resonant tank is also proposed to achieve even lower voltage gains at high frequency to improve the output ac waveform quality. Compared to the traditional two-stage method, this single-stage solution makes the circuit simpler, and the bulky dc bus capacitor can be saved. To verify the circuit design and the proposed hybrid modulation method, a GaN based 380 V dc – 600 V ac , 1.2 kW prototype is built and tested. Under the full load condition, the measured total harmonic distortion of output voltage is 2.67% with 98.30% efficiency for the overall inverter.