GaN-Based Wireless Power Enables Efficient, Seamless Multi-Device Charging
written by Daniel Costinett, Jie li, Jingjing Sun, and Peter Pham
submitted on 2020-06-25 16:36:24
Multi-device charging scenario which is the focus of this work
For consumer mobile electronics, wireless power transfer (WPT) promises to revolutionize the way users interact with their devices. Ubiquitous, seamless charging is expected to reduce onboard battery storage requirements, and untether mobile devices from periodic wired charging requirements.
At present, commercial implementations of wireless power transfer are largely limited to low power, low efficiency, and a charging paradigm where each device must be well aligned with a dedicated charger. Though the technology continues to progress, this approach fails to capitalize on the promise of spatial freedom and effortless, pervasive charging which makes wireless power transfer attractive.
Fig. 1 shows the target application and experimental demonstration loads used in this work. The goal is to design a workstation in which a single transmitter seamlessly charges electronic devices of varying power levels, simultaneously, when placed in arbitrary coplanar positions above a 0.5-m x 0.5-m charging area.
This work, in collaboration with Power America, leverages the capabilities of wide-bandgap semiconductors, particularly gallium nitride (GaN) transistors, to enable new design paradigms for WPT systems. In particular, the focus here is on achieving comparable power levels and efficiency to traditional wired charging, while allowing unobtrusive charging of many devices simultaneously.
This article begins by explaining the principles of operation behind popular WPT systems that use inductive power transfer. A simple model is presented and used to explain the constraints on power transfer efficiency, and the techniques that can be used to optimize efficiency in the case where a single load is being powered and where there are multiple loads. This leads to the proposal of an alternative approach, which achieves uniform coupling to multiple devices over a larger charging area for spatial freedom, while obtaining very high quality factor coils to allow high efficiency power transfer.
The article then delves into the details of designing such a single-transmitter, multi-receiver system for wireless charging. First, it describes the constraints on coil design and how these influence efficiency. The choice of operating frequency figures prominently here, and sets the stage for selection of a higher operating frequency (6.78 MHz) which is in line with the Air Fuel Alliance standard, as discussed.
Then a conventional end-to-end WPT system is presented to explain the impact of the various power stages on overall system efficiency. Because the conventional system only achieves about 65% overall efficiency, an alternative system structure is proposed that eliminates some of the cascaded stages, boosting efficiency to 85% or higher. This system structure is made feasible by the use of GaN transistors which excel at high frequency switching with large voltage bias. Requirements for implementing this alternative WPT system design are then described, followed by detailed discussion of the prototype design and presentation of experimental results.
The prototype system is designed to supply 100 W to five loads, while targeting 90% full load efficiency from ac-line to dc output and a transmitter power density greater than 25 W/in3. Two different receiver designs are presented. A high-power receiver meant to be integrated into the application is offered for loads >1 W, while a <1-W receiver is given as a retrofit solution for low-power applications. Finally, some ongoing research is discussed relating to transmission of wireless power to metal-body receivers as would be required in laptops.