The WPT dilemma — High K or high Q?

Today, adopters of high power wireless chargers are faced with a dilemma: is the appropriate figure of merit kQ or kω? Wireless Power Transfer, WPT, throughput power is generally taken as the product of the high frequency inverter kVA rating multiplied by a function of kQ, with emphasis on high Q. On the other hand, an optimal matching condition relies on kω, which provides a guide to matching to the secondary loaded time constant. In this paper important insights into WPT operation in terms of k and ra are developed through investigation of the resonant tank network associated with the coupler. The inverse network of the inductive “T” is the capacitive “pi” both of which have the same power throughput given an appropriate choice of the inversion constant, Ro. When coupling co efficient, k, is varied over its full range the outcome is inescapable, one cannot compensate for low k with higher ω, or consequently with higher Q. Comparative simulations are run that support this. A commercial high power WPT system based on S-S compensation and high k is shown for reference.