On the Limitations of the Coupled Mode Theory and Parity-Time Symmetry for Near-Field Wireless Power Transfer Research

The coupled mode theory (CMT) and parity-time (PT) symmetry were recently used in studying wireless power transfer (WPT) systems. This paper re-examines these approaches. The limitations of CMT and PT symmetry are identified and highlighted so that WPT researchers can avoid their pitfalls. Electric circuit theory (ECT) is based on laws of physics and offers exact solutions. CMT is actually derived from ECT through two simplification stages, and it only gives approximate solutions. CMT involves assumptions that neglect high-frequency components in the WPT systems. Therefore, CMT cannot offer more information and better accuracy than ECT for WPT studies. The PT symmetry leads to the zero-phase-angle condition that was well known from previous WPT research based on ECT for minimizing reactive power flow and maximizing efficiency. The limitations of these two approaches are explained in detail and compared with ECT with practical examples. Both theoretical and practical results are included in the comparison. These results show that CMT and PT symmetry have shortcomings arising from their respective underlying assumptions. They do not offer new explanations and insights that ECT cannot offer. ECT is still the most appropriate analytical tool to study near-field WPT systems on the conditions that the dimensions of the circuit components are much smaller than the wavelengths of the operating frequencies.