04-05-2011, 12:02 PM
Abstract
This paper describes an electronic system forwireless energy transmission to supply low-power electronicdevices like wireless sensors. It presents the first receiverASICs for this kind of application. Two prototypes with supplycurrents of 4 and 100 milli Amperes serve as proof-of-concept.Furthermore, several implementations of transmitters andreceivers with off-the-shelf components are discussed, showingdifferent system options.
I. MOTIVATION
Low-power devices commonly have an energy storageelement like a battery or a capacitor to provide energy fortheir different electronic circuits. In order to be recharged,the energy storage must be cable-connected to a voltageregulator and the mains power supply. Regulator, wiring andconnectors as well as the energy storage itself, especially incase of a battery, are often a source of error. Moreover,regulators and connectors are not always compatible withdifferent application devices, so that each device type needsits own custom-tailored charging solution. The need forwiring, connectors or even batteries in electronic equipmentcan be eliminated by wireless energy transmission. It is theideal solution when little board space is available and lowcosts are of paramount importance. Besides mobile terminalsand wireless sensors application areas cover totallyencapsulated or isolated devices and devices in remote orinaccessible locations. Depending on the application and theemployed components, i.e. antennas and capacitors, differentfrequencies can be used for power transfer. Furthermore, itcan be combined with data communication for softwareupdates, read-out of wireless sensors and the like. The entirefunctionality can be implemented on a single integratedcircuit, with only a few external components [1][2].
II. OVERVIEW
Different physical principles can be used for wirelessenergy transmission. For distances smaller than thewavelength inductive coupling is employed. Well-knownexample for this effect is the transformer in form of twocoupled inductors. The basic relations will be presented here.The magnetic inductance B of one coil of the transformer iswhere μ0 is the magnetic field constant, N1 is the numberof windings on the primary inductor of the transformer, r isthe radius of this coil, x is the distance from the center thecoil and I1 is the current through this inductor. The mutualinductance or coupling coefficient M defines the flux fromthe primary winding, which is enclosed by the secondarywinding. It is calculated as where N2 and A2 are the number of windings and the areaof the secondary coil, respectively. The induced voltage U2in the secondary coil is the frequency, the resonance frequency and L2and R2 are the inductance and the resistance of the secondarycoil, respectively. In a wireless energy transmission system,the primary coil will be the antenna o f the transmitter andthe secondary coil will be the antenna of the receiver at theapplication device side [3]. For larger distances, electro-magnetic coupling is theeffect to be used for energy transmission. The receive energyis calculated as follows where Pt is the transmit power, R the distance betweentransmitter and receiver and is the wavelength [4].The system architecture is the same for both principles,whereas the electro-magnetic components like the antennahave to be adapted and optimized for the various physicalprinciples
.III. IMPLEMENTED SYSTEM
A wireless energy transmission system consists of thetransmitter and the receiver. The transmitter generates theelectro-magnetic field and comprises an oscillator, a poweramplifier (PA), a matching circuit and an antenna. Theoscillator produces a periodic signal with the chosen carrierfrequency. Because of combination with standard RFIDprotocols for data transfer, the 13.56 MHz frequency wasused in the present system. This range between 13.553 and13.567 MHz is employed by a wide variety of radio serviceslike press agencies and telecommunications. In addition toRFID systems, remote control systems, remote controlledmodels, demonstration radio equipment and pagers can befound here.The gain of the power amplifier determines the inductoror antenna current and thus the transmission distance and thereceive power at the receiver (see II). This power is limitedaccording to the frequency range, defined from the nationalregulatory authority. In the 13.56 MHz band, the power mustnot exceed 42 dBμA/m at 10 m distance (EN 300 220, EN300330, EN 300 440) [4].The matching circuit implements the matching betweenthe PA output and the antenna to guarantee maximum powertransfer to the antenna. Depending on the frequency used andthe required distance, the size of the antenna has to bechosen. The optimum distance for energy transmission withinductive coupling is half the diameter of the antenna.Figure 1: Block diagram of the transmitterThe receiver of the WET is directly connected to theapplication device. It uses the electro-magnetic flux of thetransmitter to produce a constant voltage for the supply ofthis application device. The receiver comprises an antennatogether with a capacitor to build a resonance circuit at itsinput. Due to this resonance, the maximum possible energyis used for the supply of the device. According to the desiredfrequency the inductance L of the antenna and thecapacitance C must chosen with the following equation:LC1The induced voltage is strongly depended on the distancebetween transmitter and receiver. Varying distances result influctuating voltages. Integrated circuits (IC) work only in alimited voltage range, so special protection or limitingcircuits are used. For commercial ICs this range is e. g. 20V,for ASIC-designs this voltage range is smaller. To avoidovervoltage and a possible destruction of the chip, alimitation circuit necessary. In Figure 2 the voltage isregulated by an NMOS transistor, controlled with anamplifier. This transistor regulates the maximum voltageafter the rectifier indirectly by shortening the current. Afeedback loop divides the voltage and compares it with afixed reference. According to the results of this comparisonthe NMOS transistor is controlled to reduce the voltage.
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