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Second Harmonic Current Reduction for Cascaded Inverter with Pre-regulator + LLC Converter as Front-End DC–DC Converter

In the cascaded inverter, the second-harmonic current (SHC) in the input of inverter penetrates into the front-end dc-dc converter, leading to a poor performance. In this paper, the pre-regulator+LLC converter, which shows superior performance in the applications with wide input voltage range, is employed as the front-end stage. Firstly, the boost converter is taken as the pre-regulator and the model of boost+LLC converter is proposed. Based on that, three approaches for SHC reduction are proposed from a point of output impedance, which are: 1) Reducing the filter capacitor of boost converter; 2) Introducing a notch filter in cascade with voltage regulator, 3) Adding a virtual impedance in series with the output rectifier of LLC resonant converter. Finally, a 6-kVA cascaded inverter is tested and experimental results are presented to verify the validity of the proposed methods.