Design and development of a fractional-turn transformer for high power density LLC resonant converters

This study proposes a 48 V – 6 V isolated direct current DC-DC converter using an LLC series resonant converter with wide bandgap GaN components under a 1-MHz switching frequency. A loss analysis was performed on different primary circuit structures for the converter, and a full-bridge structure was selected. In addition, a quarter-turn planar transformer structure was adopted. With the voltage conversion ratio of 8:1 unchanged, the adopted transformer required fewer primary and secondary winding turns, with lower alternating current copper losses, compared with an average transformer. The study also explored and optimized parallel arrangements of transformer windings, adopted parametric design of transformer dimensions, and determined the optimal balance between core losses and copper losses in a limited circuit area. The magnetic simulation software Ansys Maxwell was used to verify the magnetic flux distribution of the quarter-turn planar transformer. An experiment confirmed that the proposed LLC series resonant converter has a switching frequency, input voltage, output voltage, output power, power density, and peak efficiency of 1 MHz, 48 V, 6 V, 900 W, 63 W/cm
³
, and 98.2%, respectively.