Analysis and Design of LLC Resonant Converters for Wide, Low-input, High-output DC Bus Applications

Today\u27s large-scale utility applications with microinverter and batteries require DC/DC converters with a wide voltage range capabilities in order to fulfil the wide voltage system requirements. It has been shown that the LLC resonant converter is a good solution for wide voltage range applications because it is typically controlled by frequency-modulation. However, to achieve a wide voltage range, the LLC converter needs to operate in a wide switching-frequency range. This leads to increased switching losses and increased circulating current. Moreover, a small inductor ratio or/and low-quality factor are required to increase the voltage gain. Therefore, the small magnetizing inductance causes a high magnetizing current with high conduction loss, making it hard to design magnetic components. Several resonant converters for wide voltage range applications have been proposed in the open literature to improve efficiency. In first part, a novel LLC converter with a reconfigurable rectifier structure is proposed to regulate the wide voltage range photovoltaic (PV) panel. The proposed converter can operate in three operation modes that leads to a narrow switching-frequency range close to the resonant-frequency resulting in increased converter performance efficiency. The benefits of this topology include improved efficiency and narrow switching-frequency range while achieving soft-switching in all MOSFETs and diodes. In second part, a new three-port LLC converter for a PV microinverter with high-DC bus applications is proposed. Two control modulations are adopted to regulate the power flow. On the primary side, two switches are implemented to reduce the conduction losses. On the secondary side, two rectifiers are employed in one structure to make the proposed converter operate close to the resonant frequency and to boost the voltage with a moderate transformer. The proposed converter can achieve soft-switching for all MOSFETs and diodes, resulting in improved efficiency and realizing a narrow switching-frequency range