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Cutting the Last Wire


Reno Rossetti,
Director of the Integrated Circuits Group Strategy at Fairchild Semiconductor International.

Today’s much touted wireless gadgets could be more properly called wireless “communication” devices since, at some point, a wire is still needed to convey power from the wall to the appliance. On the other hand, hundreds of kilowatts (kW) of power have been successfully transmitted wirelessly across long distances for quite some time. While the focus of wireless power transmission has been so far in the megawatts (MW) , such as in solar energy harvesting or milliwatts (mW)as in RFID (Radio Frequency Identification), the time should be ripe for porting this technology into many different portable wireless applications.

The History of Wireless Power
The idea of beaming out power is not new. In 1899 Nikola Tesla’s wireless power transmission experiments at Wardenclyffe resulted in lighting lamps filled with gas (like neon) over 25 miles away without using wires.

The FCC, established in 1934, reserved the 2.4-2.5GHz BW for ISM (Industrial, Scientific and Medical), enabling significant scientific work in this arena. During World War II, the ability to convert energy to microwaves using a magnetron was developed. No method for converting microwaves back to electricity was known until 1964 when William C. Brown demonstrated a rectenna (rectifying antenna), which could convert microwave power to electricity.

In1968 Peter Glaser proposed the idea of using microwaves to transmit power to earth from solar powered satellites at power levels well below international safety standards; while on October 7, 1987, a SHARP (Stationary High Altitude Relay Platform) experiment was able to keep aloft a small airplane powered by an RF (Radio Frequency) beam. This flight was recognized as being the first of its kind by the Fédération Aéronautique Internationale.

Finally, in 1995, NASA set up a research, technology, and investment study for a 250MW Solar Powered System (SPS), while Japan’s goal is to build a low cost demonstration model by 2025.

Power Beaming Fundamentals
The beaming of RF power by means of a directive antenna is a relatively lossless process, yielding efficiencies of around 85%, rivaling that of any good switching regulator. The transmission of energy between two antennas is dictated by the size of the antennas, the wavelength of the radio frequency wave, and the distance between the two antennas (the intensity of the beam falls out of the equation if you assume for simplicity no transmission losses). Calling DT the diameter of the transmitting antenna, DR the diameter of the receiving antenna, λ the RF wavelength (λ=1/f with f the beat of the RF), H the distance and k a proportionality constant (typically 1.2), we have:

DT DR = 2kλH [1]

Naturally, concerns of power density need be accounted for as well. For example, the FDA enforces 5 milliwatts of microwave radiation per square centimeter at a distance of two inches from the microwave oven surface. Such safety requirements may set a limit to the miniaturization of the antennas beyond the dictates of equation (1).

Untethered Power
The need for untethered power is clear. The technology for implementation is mostly there, but it needs be ported from the large scale of megawatts––as in solar power transmission or at the other extreme the milliwatts of RFID applications––into the realm of portable computing, consumer and communications devices. By eliminating the signal wire, wireless data transmission makes the power wire stand out and alone in all of its ugliness. For example, since today’s flat panel TVs can literally be hung on a wall, it would be nice if they could get power without having to access a plug that has to be placed high on the wall.

The Future
Beaming power is the next logical step following the harvesting of stray power that is available in the surrounding environment. Power harvesting itself is a new concept from an industrial standpoint, but it is gaining acceptance in technologists’ minds. Actually, one could argue that at the current state of affairs, harvesting could be better described as “scavenging” since it relies on hunting and gathering whatever stray energy is floating in the air. While current harvesting power technology focuses on the receiving side of the power chain, beaming power, indeed harvesting at industrial scale, starts at the source and turns the paradigm on his head; from “power is where you can find it” to “power is where you put it –– wirelessly that is.”

About the author:
Reno Rossetti is currently Director of the Integrated Circuits Group Strategy at Fairchild Semiconductor. Reno has many years of experience in Design, Applications and Marketing in the Analog and Mixed Signal semiconductor industry. Reno’s whole career evolved around Analog, Power IC’s and Discrete Semiconductors. He holds several patents and has authored many articles in these areas. He worked at ST in spindle motor and voice coil drivers design. Later on he headed up National’s Power Management design group. At Fairchild Reno covered several positions in new product development and applications before moving into the current strategic role. Reno graduated in Electrical Engineering from Politecnico di Torino in his native Italy. He has also an MBA from the Bocconi University of Milan.

Reference
[1] Field and Waves in Communications Electronics Simon Ramo, John R. Whinnery, Theodore Van Duzer Wiley Editor Page 667, formula 12.29 (7) and others.

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