Wireless Power Charger Makes Efficient Flooring
Laura Peters, Senior Editor -- Semiconductor International, 2/1/2007
As workspaces and living rooms become increasingly populated with more and more battery-consuming electronic devices, it has occurred to some researchers that there may be a better way to supply power. How about on an as-needed basis? And let’s build the power supplies into the environment, making them less intrusive.This is the idea behind smart power transmission sheets, which selectively feed power to the electronic devices placed on them. Takayasu Sakurai, professor of the Center for Collaborative Research at the University of Tokyo, and Takao Someya, associate professor of the university’s Quantum Phase Electronics Center, demonstrated these power transmission sheets, made of printed plastic MEMS switches and organic field-effect transistors (FETs), at the recent International Electron Devices Meeting (IEDM) .
The concept represents a step toward ubiquitous electronics, where multiple electronic systems, scattered over desks, floors and walls, can be directly powered (Fig. 1 ). The wireless transmission system may directly drive electronic objects and/or charge a rechargeable battery in the objects without requiring a connector, thereby simplifying recharging procedures.
Printing technologies were used to manufacture the wireless power transmission sheet on a plastic film. The 1-mm-thick sheet (Fig. 2 ) contains a 2-D array of 8 × 8 cells containing position sensing and power transmission units. The effective power transmission area is 21 × 21 cm2. The flexible sheet weighs 50 g.
Electromagnetic coupling using an organic active transistor matrix senses the position of an object. Power is fed to it inductively by an array of copper coils driven by a printed plastic MEMS-switching matrix. Because the power transmission only occurs selectively when an object is sensed, net power-coupling efficiency is 62.3%. Power levels as high as 29.3 W have been transferred in this fashion.
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| 2. The large-area wireless power transmission sheet is peeled back and shows only parts of each of the underlying three sheets. The peeled layer is the power transmission coil sheet, with the MEMS-switching sheet (brown), position-sensing coil sheet (octagonal coils) and organic FET sheet (active matrix tile pattern) underneath. The device assembly, comprised of a transmission system driven by an 8 × 8 MEMS-switching matrix and a positioning sensing system driven by an 8 × 8 organic FET active matrix, has a transmission area of 21 × 21 cm2. (Source: University of Tokyo) |
The system integrates the position sensing and power transmission sheets. In the position-sensing sheet, the organic FET active matrix was fabricated on a polyimide film. Inkjet printing was used to pattern silver gate electrodes and polyimide gate dielectric layers. A pentacene channel layer and gold source/drain electrodes were deposited in vacuum. Channel length and width were 13 µm and 48 mm, respectively. The position-sensing coil array was manufactured by screen printing, and uses a resonance frequency of 2.95 MHz. Copper lines with a space and width (S/W) of 100 µm were wrapped 38 times to form the coils with a 10 mm inner diameter (ID). The inductance and resistance are 20 µH and 17 Ω.
The power transmission sheet contains a power transmission coil array over a MEMS-switching matrix. The copper coils (10 mm ID, 300 µm S/W) were turned 13 times to give an inductance of 3 µH and resistance of 1 Ω. The MEMS-switching matrix, formed by inkjet and screen printing, used a 25-µm-thick polyimide membrane to form the electrodes for power transmission and electrostatic attraction.
