LLC Converter Main Transformer Losses: Eliminating Air Gaps and Integrating Parallel External Inductors

Magnetic components are critical in power electronics design, particularly in high-efficiency power conversion applications. Although the introduction of an air gap can effectively control inductance and achieve zero-voltage switching (ZVS), it also causes fringing effects, resulting in additional losses, increased AC resistance, inconsistent coupling between the primary and secondary windings, uneven current distribution, and hotspot formation. While distributed air gaps have been developed to alleviate these issues, different circuits require varying numbers of distributed gaps. Additionally, this design faces manufacturing challenges, such as increased complexity in winding layout to avoid air gaps and the need to increase the magnetic column height to meet design requirements. To address these challenges, this paper proposes using an external inductor to replace traditional air gap magnetizing inductance, enabling air-gap-free energy transfer in the main transformer. A 380 V/12 V input, 500 W output LLC resonant converter serves as the validation platform. Simulation and experimental results show that this approach reduces air gap losses by 7.2 W and effectively mitigates the issue of uneven winding current distribution. Furthermore, copper losses related to the air gap are reduced by 49.6 %, demonstrating a significant improvement in energy efficiency. This paper provides a solution for designing high-power density converters, significantly enhancing system efficiency and reliability, and offering valuable insights for future power electronics applications.