A Low Profile Stacked Transformer for High-Efficiency High-Output-Current 380 V/12 V LLC Resonant Converters

This paper proposes a low profile transformer structure for high-efficiency and high-output-current LLC resonant converters, named stacked transformer. Besides minimization of termination losses by integrating the synchronous rectifiers (SRs) and output filter capacitors into the secondary windings, the vertically stacked structure can shrink the transformer footprint and significantly shorten the secondary current travelling loop at the output terminals. High efficiency and high power density are thus achieved, particularly for high-output-current applications. The close flux coupling and small leakage inductance between the secondary windings limit the voltage stress on the SR MOSFETs. Consistent and low voltage spikes on all of the SRs are ensured by uniform parameters due to the one-core set structure attained by flux cancellation. The proposed transformer and its performance are verified by finite element analysis simulation, experimental results and analysis on a 380 V/12 V 1.2 kW LLC resonant converter that achieves a peak efficiency of 98.2%.