Maximizing the efficiency of wireless power transfer with a receiver-side switching voltage regulator

Abstract

Output voltage regulation is an essential technology for achieving stable wireless power supply. A receiver-side switching voltage regulator is useful for realizing output voltage regulation. However, this paper shows that the switching voltage regulator degrades the transfer efficiency to below 50% in a wireless power transfer system that consists of a class-D power inverter and series-resonant transmitting and receiving resonators. Such efficiency degradation is caused by the instability of an operating point where the efficiency is >50%. The input resistance value of the switching voltage regulator at a stable operating point is much higher than the optimum value for maximizing the efficiency. To stabilize the high-efficiency operating points, this paper formulates a stability condition and derives its sufficient condition. The sufficient condition facilitates a system design method using a K-impedance inverter that allows for the optimum input resistance value to lie in the range of allowable input resistance values. In addition, we introduce an input-voltage-based efficiency maximization method for the system with the receiver-side switching voltage regulator. By combining these two methods, efficiency maximization is realized with the receiver-side switching voltage regulator. The proposed methods were verified by both simulations and measurements.

Alvus

“…The factors that underlies in efficiency enhancement for general WPT systems are multi-fold, however in on our Alvus concept, there are two core factors that need to be considered: (a) maximum efficiency point tracking and (b) optimum route generation. As for (a) in "single" receiver cases, methods and implementations found in previous literature can be easily applied [28]. These studies can also be naively applied to "multiple" receiver systems by applying time-division multiplexing and charging each receiver one by one.…”

“…The receiver module is placed on the n-th relay (or the transmitter when n = 0) and power is transferred via n + 1 hops. Using the measured S-parameters, we calculated the maximum transfer efficiency when the real part of load impedance can be varied [28]. We used this figure of merit since it is known that the real part of load impedance can be adjusted through the voltage conversion ratio of DC/DC converters, whereas the tuning of the imaginary part of load impedance is still an challenge unless bulky capacitor banks are used.…”

“…The DC-to-DC efficiency was evaluated by measuring the input DC power and the DC power consumption at the load. Since the transfer efficiency depends on the DC voltage powering the class-D amplifier, we manually swept the input voltage and measured the value when the efficiency was maximized [28]. Although our current implementation of Alvus does not automate this sweeping function, it can be integrated by adding monitoring circuits for input voltage/current and using programmable power sources [see section 6.2.1].…”

A Cuttable Wireless Power Transfer Sheet

“…To calculate the power transfer efficiency from §5.3.1 to §5.3.4, we used the formulation of Zargham et al that derives the upper limit of power transfer efficiency (i.e., the efficiency with perfect impedance matching) from the scattering parameters (S-parameters) [51], considering that various maximum efficiency point tracking methods are studied in the literature [27]. Lastly, we investigated the total DC-to-DC power transfer efficiency that our system can achieve in § 5.3.5; note that the optimization of the AC-DC/DC-AC conversion is not the intention of this study and this is just to give a reference for future system design.…”

Load optimization factors for analyzing the efficiency of wireless power transfer systems using two-port network parameters

“…The difficulty in maintaining the optimal impedance is further increased by the fact that the receiver does not necessarily have mechanisms to adjust the values of load resistance and load reactance, and because the adjustment resolution of such mechanisms-even when they do exist-has limits. It is known that the load resistance can be adjusted by a DC/DC converter connected to a subsequent rectifier circuit stage [20,21], and methods to follow the optimal value have been actively studied [22][23][24][25]. However, even with a DC/DC converter, the load resistance cannot be completely controlled to the optimal resistance value.…”