Design and Implementation of a Planar Transformer With Fractional Turns for High Power Density <i>LLC</i> Resonant Converters
Yu‐Chen Liu, Chen Chen, Kai-De Chen, Yong-Long Syu, De-Jia Lu, Katherine A. Kim, Huang‐Jen Chiu
Abstract
This article proposes an isolated LLC resonant converter with MHz-level switching frequency that can achieve zero-voltage switching in the full-load range. In the proposed converter, conventional silicon devices are replaced with wide band-gap gallium nitride devices to reduce switching losses in the power device. A planar transformer structure with reduced winding length on the secondary side is proposed, which utilizes flux cancellation and reduces core loss while maintaining a low core volume. The path of the secondary winding is a fraction of a full turn, which reduces secondary-side losses while maintaining a turns ratio of 16:1. This planar transformer achieves the goals of decreasing core loss and volume while increasing power density. Within a limited circuit layout area, optimal points for the efficiency of core loss and copper loss were calculated. The magnetic simulation software ANSYS Maxwell was employed to verify whether the actual circuit behavior of this planar transformer conformed to the design theory. Finally, a resonant converter was achieved with a switching frequency operating at 1 MHz, input voltage of 380 V, output voltage of 12 V, output power of 800 W, power density of 55 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , and maximum efficiency of 96.5%.