A Magnetic Integration Half-Turn Planar Transformer and its Analysis for LLC Resonant DC-DC Converters

A high frequency planar transformer with integrated leakage inductance and half-turn secondary windings is proposed for LLC isolated DC-DC converters. The primary windings are wound on both the center leg and the sides leg of an EI planar ferrite core to integrate the leakage inductance. With the proposed method, the magnetizing inductance is constant while adjusting the leakage inductance, thus the design of the magnetizing inductance and the leakage inductance can be decoupled. To adapt the high voltage transform ratio from 380V to 12V, the number of turns of the secondary windings is designed to be 0.5. The output voltage ripple of the LLC converter equipped with the proposed transformer is analyzed and its output filter is designed. The characteristics of the proposed transformer are simulated by ANSYS Maxwell and Simplorer. Finally, a 360V~400V input, regulated 12V output, 750W LLC resonant converter with silicon carbide (SiC) devices was built with the proposed transformer. The evaluated converter realized a power density of 23.1W/cm3 and a peak efficiency of 97% with all the auxiliary power and control circuit included.