In-Depth Design and Multiobjective Optimization of an Integrated Transformer for Five-Phase LLC Resonant Converters

High-power isolated dc–dc converters are preferred in the medium-voltage cascaded H-bridge (CHB) photovoltaic (PV) generation systems due to high power density and galvanic isolation. However, high-power transformers in dc–dc converters can result in high power losses and poor heat dissipation due to thick isolation materials. The three-phase LLC (3P-LLC) converter has been reported suitable for high power applications. Comparing to the single-phase counterpart, the 3P-LLC converter employs three discrete transformers, leading to a larger core area for better heat dissipation. To further increase the power density and the heat dissipation capability of resonant converters, increasing the phase number and integrating magnetic components are considered. This work proposes a five-phase LLC (5P-LLC) resonant converter for the CHB PV application. To increase the power density, a five-phase magnetic integrated transformer (5P-MIT) is proposed and designed with an optimal limb structure to avoid local flux saturation. To further increase the power density, reduce the transformer losses, and enhance the heat dissipation capability, multiobjective optimization is developed to meet multiple design requirements simultaneously. A design case is studied to show the advances of the proposed 5P-MIT. To further investigate the proposed 5P-LLC, a down-scaled prototype is developed with a 5P-MIT. Experiments are conducted to verify the analyses and the functionality.