Power Stage and Feedback Loop Design for LLC Resonant Converter in High-Switching-Frequency Operation

As converter switching frequencies are moving toward megahertz frequencies for high power density, secondary leakage parasitics that were previously negligible have to be considered in mathematical modeling for LLC resonant converters. At high-switching-frequency operation, the power stage design must take secondary leakage inductance into account because it can affect the input-output voltage gain. In addition, the feedback loop design should consider the effect of the time delay caused by the performance limitation of a digital controller to improve the small-signal model accuracy of the converter. Using the proposed power stage and feedback control loop design considerations, the LLC resonant converter can achieve high power conversion efficiency and stability enhancement at high switching frequencies. All the proposed methods are experimentally verified using a 240-W prototype LLC resonant converter operating at 1-MHz switching frequency.