RF Wireless Energy Harvesting
Wireless monitoring networks are included in a vast array of applications ranging from medical devices to supply chain management [1-3]. Often, these devices will use rechargeable batteries to supply power to a monitoring or sensing network. Over time, these batteries will need to be replaced. Our research focuses on improving the longevity of rechargeable batteries in wireless sensing networks through far-field wireless energy harvesting. By coupling an ambient energy transducer, such as a solar cell, with a power converter, we can increase the voltage produced by the transducer in order to recharge a battery. Through converter control, we can increase the longevity of the battery while continuously monitoring the state of the harvesting system.
RF energy harvesting systems have been implemented using Silicon devices, and show efficiencies of 60% at 300 μW [4-6]. Our research explores the benefits of Gallium-Nitride (GaN) on system efficiency and power density in an RF energy harvesting system.
The goal of our research is to establish a method for optimizing low power boost converters for use in far-field energy harvesting systems. By using a database of manufacturer-provided device characteristics, we are able to construct Figures of Merit (FOMs) capable of predicting component power loss based on boost converter parameters. A loss model based on a resistor emulation approach shown in  is constructed. Using this loss model, we were able to predict device power losses and system efficiency over a wide range of operating points.
It has been well-established through research that GaN devices have significantly fewer parasitic charges than silicon devices [7,8]. Because switching losses are proportional to both parasitic gate charges and frequency, lower gate charges allow for potential higher frequency operation while maintaining low power loss. One benefit of higher frequency operation is the potential to reduce inductor size while maintaining a constant current ripple. By reducing inductor size, we can increase the power density of the energy harvesting system. The creation of the FOMs allow us to understand the trade-offs which come with higher switching frequency and higher power density.
In order to illustrate the use of this system in a real-world application, we are constructing a system which uses an energy transducer called a rectenna, which transforms RF energy to electrical energy, to charge a battery through an optimized converter. A microcontroller controls the converter operation while monitoring the voltage at the output of the rectenna and the battery voltage. A transceiver then transmits this information to a receiver which displays the information to a user.
How WBG Can Help
After completing the optimization process, a GaN device was shown to have the most favorable FOM. A converter using the GaNFET was built and evaluated over a wide range of operating points. The efficiency of the converter was shown to be 74% at 300 μW. This is a significant improvement as compared to the previous benchmark of 60% at 300 μW. Further research is being conducted on the potential improvements to power density as a result of higher switching frequency.
- Doug Bouler
- Jared Baxter
 N. K. Hoang, J. S. Lee and S. G. Lee, "Maximum power transfer considering limited available input power in ultrasonic wireless power transfer for implanted medical devices," 2014 IEEE Fourth International Conference on Consumer Electronics Berlin (ICCE-Berlin), Berlin, 2014, pp. 431-432.
 R. Lakshmanan, K. H. Keat and R. Sinnadurai, "Wireless power transfer for small scale application," Research and Development (SCOReD), 2013 IEEE Student Conference on, Putrajaya, 2013, pp. 31-36.
 F. Iannello, O. Simeone and U. Spagnolini, "Energy Management Policies for Passive RFID Sensors with RF-Energy Harvesting," Communications (ICC), 2010 IEEE International Conference on, Cape Town, 2010, pp. 1-6.
 E. Falkenstein, D. Costinett, R. Zane and Z. Popovic, "Far-Field RF-Powered Variable Duty Cycle Wireless Sensor Platform," in IEEE Transactions on Circuits and Systems II: Express Briefs, vol. 58, no. 12, pp. 822-826, Dec. 2011.
 D. Costinett, E. Falkenstein, R. Zane and Z. Popovic, "RF-powered variable duty cycle wireless sensor," Microwave Conference (EuMC), 2010 European, Paris, 2010, pp. 41-44.
 "Resistor Emulation Approach to Low-Power Energy Harvesting," Power Electronics Specialists Conference, 2006. PESC '06. 37th IEEE, Jeju, 2006, pp. 1-7.
 M. Acanski, J. Popovic-Gerber and J. A. Ferreira, "Comparison of Si and GaN power devices used in PV module integrated converters," 2011 IEEE Energy Conversion Congress and Exposition, Phoenix, AZ, 2011, pp. 1217-1223.
 W. Zhang et al., "Evaluation and comparison of silicon and gallium nitride power transistors in LLC resonant converter," 2012 IEEE Energy Conversion Congress and Exposition (ECCE), Raleigh, NC, 2012, pp. 1362-1366.