Impact of Asymmetrical Leakage Inductance on a 380 V–12 V LLC Converter with Synchronous Rectifier for DC Transformer Application

High-density 380 V–12 V LLC resonant converters typically employ planar transformers with integrated leakage inductance. To achieve Zero-Voltage Switching (ZVS), an air gap is introduced to adjust the magnetizing inductance (Lm). However, this gap alters the internal magnetic field (H) distribution. In Center-Tapped (CT) structures, this alteration leads to asymmetric leakage inductances between the positive and negative half-cycles, causing resonant frequency mismatch and performance degradation, particularly under light-load conditions. In this work, the asymmetrical leakage inductance effect in a CT transformer for a 380 V–12 V LLC resonant converter is systematically investigated. A mathematical model is derived to quantify the leakage inductance distribution, revealing that the relative position between the air gap and the windings significantly affects the symmetry. Based on this modeling analysis, the centralized assembly method is identified as the optimal solution to ensure impedance symmetry. The accuracy of the proposed model and the effectiveness of this structure are validated through Finite Element Analysis (FEA) simulations and a hardware prototype of a 250-W, 600-kHz LLC converter. Results demonstrate that this method eliminates the approximately 11% leakage inductance discrepancy (1.8 μH vs. 1.6 μH), ensuring stable operation across the full load range.