High-Frequency Transformer Design for LLC Resonant Converter with High Insulation Capability

Due to high efficiency and simple structure, LLC resonant converter is a promising topology for developing solid-state transformers. The heart of an LLC converter is the high frequency transformer that provides necessary insulation and step-up/down between primary and secondary sides. In addition, the efficiency of the transformer is a critical factor as it processes the entire power. This becomes even more challenging when circuit switching frequency is raised for shrinking passive component sizes, as skin effect and other non-ideal effects start to kick in. In solid-state transformer applications, size, efficiency and insulation have stringent specifications and meeting all requirements at once is not a simple task. Therefore, this paper presents multiple perspectives in terms of transformer design in an LLC converter and demonstrates with a 3-kW transformer capable of more than 15 kV insulation. Testing results show peak converter efficiency of 98.9% with transformer power density of 480 W/in3.