Improving Efficiency of Beyond-10-MHz LLC Resonant DC-DC Converters Using Spiral Transformers with Magnetic Flux Path Control Technology

Transformers used in switching power supplies can be reduced in size by increasing the frequency. However, the loss in transformers increases at higher drive frequency, which hinders higher efficiency. Transformers have transitioned from a planar type in which windings are stacked on top to a spiral type in which windings are wound in a spiral shape as a more compact structure. Therefore, herein, the authors proposed the magnetic flux path control (MPC) technology to reduce the loss of the spiral transformer. We revealed by magnetic field simulation and actual measurement that the ac resistance can be reduced by applying MPC to the spiral transformer. By applying MPC to the spiral transformer, the ac resistance at 15 MHz was reduced by 27% from 656 to 478 $\text{m}\Omega $ for the actual measurement by the impedance analyzer. Also, the efficiency improvement effect of power supplies by the proposed transformer has been demonstrated in a beyond-10 MHz 48 V/6 V 18 W LLC resonant dc–dc converter prototype with gallium nitride (GaN) devices. By applying the proposed transformer to the prototype, the full-load efficiency improved by 4.0%, from 65.2% to 69.2%. These results show that the application of MPC can reduce the ac resistance of the spiral transformer and increase the efficiency of the LLC resonant dc–dc converter.