Wireless power transfer has been gaining traction in recent years. A problem in wireless power transfer to moving objects is that the output power and power transmission efficiency fluctuate greatly with the coupling coefficient. The output power and power transmission efficiency can be kept constant with changes in the coupling coefficient using Parity–Time symmetry theory. This theory was previously applied to a series–parallel wireless power transfer system with external inductance, and bidirectional power transfer capability, improving system robustness. However, the output power and power transmission efficiency fluctuated with respect to changes in the coupling coefficient due to assumptions made in the equation during equivalent circuit conversion. Therefore, the current paper examines the use of frequency control based on Parity–Time symmetry theory to keep the output power and power transmission efficiency constant with respect to changes in the coupling coefficient. Whereas conventional frequency control methods exhibit multiple control convergence points, the proposed frequency control approach based on Parity–Time symmetry theory enables stable regulation toward the point of maximum power transmission efficiency. In a frequency control experiment, the output power and efficiency were kept constant using the proposed method at 982.7W and 87.9%, respectively, by controlling the secondary-side DC current to 3A.

With the rise of plug-in hybrid vehicles (PHVs), automotive DC-DC converters must be compact, lightweight, and efficient. Bidirectional isolated converters excel in EMI suppression and safety, supporting their use in EV fast-charging systems. However, high-frequency operation for miniaturization can introduce interference. This paper proposes a novel DAB-type DC-DC converter using a multi-phase two-level inverter without resonant circuits, avoiding issues with specific frequency components.