Sequential Waveform Synthesis for Multimodular SRC-Unfolding Inverter

Series resonant converter-unfolding inverter (SRCUI), a pseudo-dc-link inverter based on series resonant converter, is a promising topology featuring high efficiency and high step-up operation capabilities, which are important for many applications interfacing with renewable energy systems. Out of the existing modulation methods for SRCUI, variable-frequency modulation (VFM) tends to be the most efficient due to complete zero voltage switching (ZVS), but it suffers from limited modulation range. This limitation leads to distortion in the output waveform near zero crossing points. To extend the effective range of VFM and reduce the distortion, a technique termed sequential waveform synthesis (SWS) is proposed in this article. SWS is specifically created for input-parallel-output-series multimodular inverter based on SCRUIs. It activates/deactivates each module sequentially, so the remaining active modules have higher individual output voltage and power that assist in ZVS. The total output voltage waveform synthesized by individual output voltages has lower distortion and retains soft-switching function. The principle of SWS is explained in this article by using total harmonic distortion as a performance indicator. In addition, the modeling and controller design of variable-frequency-modulated SRCUI are also addressed since they were hardly studied before. The controller and the concept of SWS are implemented on a multimodular inverter prototype formed by three SRCUIs and verified by the experimental results.