Modelling and optimization of planar transformers for high power density step-down DC-DC converters

The optimization of planar transformers with integrated magnetic components is an important research focus when it comes to high efficiency and high power density step-down DC-DC converters. In this paper, to overcome the shortcomings of LLC topologies in variable frequency operation, a DC-DC converter of two-stage regulation with GaN (gallium nitride)-MOSFET is designed to achieve wide voltage range conversion. The planar transformer of the full-bridge LLC circuit is optimized at 12 V DC bus and high-power scenarios. An equivalent resonant inductance model based on the stray inductance and JMAG leakage inductance simulation is proposed, which results in only one magnetic component being used in the LLC. A method to control the resonant inductance and improve the efficiency by changing the PCB trace on the secondary side of the high-step-down-ratio (14:1:1) planar transformer is proposed. To verify the optimization model, two sets of converters with optimized planar transformers, using PQ-type ferrite magnetic cores, have been built. Experimental results show that the voltage conversion range is from 250-500 to 9-16 V, and the converter efficiency is increased to 94.39%@3 kW.