Impact of Asymmetrical Leakage Inductance on a 380 V-12 V LLC Converter with Synchronous Rectifier for DCX Application

The 380 V-12 V DC-DC converter utilizing an LLC resonant topology is considered a promising option for future data center architectures. To achieve high power density, the planar transformer is commonly employed, using leakage inductance for resonance, and operating in DCX mode. Additionally, magnetizing inductance is adjusted by introducing an air gap. However, this approach may disrupt the balance of leakage inductance distribution in the center-tapped (CT) transformer, leading to asymmetrical operation, especially under light load conditions. This study analyzes the impacts of asymmetrical leakage inductance in a CT transformer for a 380 V-12 V LLC resonant converter by examining the magnetic field energy distribution with different winding and core assembly methods. Moreover, a 600-kHz, 250-W 380 V-to-12 V LLC converter prototype was developed for this purpose. Results indicate that, for applications where leakage inductance serves as the resonant inductance, only symmetrical cores (such as the EE core used in this study) can ensure equal leakage inductance in a CT transformer. Furthermore, integrated magnetic solutions are also suggested, offering valuable insights for future high-density converter designs.