10.

3-3
Wirelessly Charging Mobile Devices
1 from Ambient RF Sources
Hamid Jabbar Student Member, IEEE, Youngseok Song Member, IEEE, Taikyeong Ted. Jeong*, Member, IEEE
{hamid, ttjeong}@mju.ac.kr, youngseoks@dankook.ac.kr

Abstract—A universal wireless battery charger for portable

and mobile devices can use ambient Radio Frequency (RF) or
II. OVERVIEW OF WIRELESS CHARGING
electromagnetic signals as a renewable energy resource, emitted Wireless charging technique already exists in active RFID
by different sources for communications. In this paper, we reader where power is transmitted to Transponder Wirelessly.
designed a RF circuit that enables to charging mobile devices. Commercial products are available in market which transfers
This will result in continuous charging of device especially in electrical power wirelessly to a short distance and works in RF
urban areas where the density of RF sources is high.
and Microwave range. These devices have RF transmission
source which generates the RF energy.
I. INTRODUCTION
The Electromagnetic sources are already existing around us
Electromagnetic (EM) radiation is a self-propagating wave in the form of Mobile phone, base station, wireless internet
in space with electric and magnetic components. One connections, DMB transmitters etc. Using these sources as
important type of electromagnetic radiation on basis of available generating source, there is no need of RF transmitter
frequency is “Radio Waves” with frequency range of 30Hz to thus saving power, space and energy.
300GHz [1]. The frequency ranges of interest reside in Ultra-High-
A circuit can be used with a cellular phone or a mobile Frequency (UHF) band from 300MHz to 3GHz. Broadcast
device to capture the unwanted electromagnetic signals in the television, mobile telephones, wireless networking etc work at
atmosphere and use the power of these signals to power some UHF frequency. These UHF range wirelessly communicating
essential functions of the device or recharge the battery. RFID products already exists in urban environment especially
devices use the principle to transmit power (1-100µW) to commercial areas and their number is increasing day by day.
RFID transponders [5]. In general the power density in a city environment ranges in
The advantage of the designed circuit will be to use the between 0.1 and 0.6mW/m2, where most of this is generated
electromagnetic signals present in atmosphere as a renewable by continuous emitting base stations. The effective base
energy source and transform the electromagnetic waves station power is approx. 400W and for a mobile phone this is
present in atmosphere in to the electric power, making a 0.25W normally and 2W when base station and phone are far
universal charger. away.

Communication Antennas Battery III. WIRELESS CHARGER DEVELOPMENT PROCEDURE

Base stations
Wirless Ineternet
This circuit will charge the battery by utilizing the ambient
Matching Charging
Satellite Communication
Antenna
Circuit
Rectifier
Controller RF signal. Circuit will convert the RF signal to DC signal, and
Radio using the DC signal to charge the battery.
Wireless
TV Device
communication
DMB Terminal
Functions
User Generated

RF Sources

Figure 1: Wireless Charging System Architecture

Fig. 1 show the block diagram of the system, where RF

signals generated by multiple RF sources are captured by
antenna. Using matching circuit for antenna along with
rectifier, generated DC power is used by charging controller to
run the mobile device terminal functions or recharge its Figure 2: Circuit for converting ambient RF waves to DC
battery.
A. Antenna and Matching Circuit
Extracting the electromagnetic energy from environment
and converting it to useful electrical energy is called energy The RF signals can be captured using Multiband antenna as
harvesting. Studies have shown that <1µW/cm2 is available shown in Fig. 2. Antennas such as quad band are easily
for RF sources unless it is near the RF source [3]. Due to this available in market and usually work at
low power generation most work from energy harvesting from 900MHz/1800MHz/1900MHz/2.4GHz. These are of usually
RF is in microwave range. whip type, but small size such as printed, patch, spiral
antennas are also testable.
For a 50Ω antenna, matching circuit is used as shown in
This work is supported by Korea Science and Eng. Foundation (KOSEF) grant funded Fig. 2, to capture the maximum power of frequency range at
by the Korea gov. (MEST) (No.R01-2007-000-20599-0), (No.20090069991), and
(No.M20809005636-08130900-63610). The circuit was designed at IC Design Center. 900MHz. The tuning Capacitor (Ctune) can be verified using

978-1-4244-4316-1/10/$25.00 ©2010 IEEE

following formula to resonate with antenna inductance V. CIRCUIT AND CONSUMER ELECTRONICS
(Lantenna), where f is the frequency of operation. The proposed circuit can be used at any place, such as
mobile handset where RF signal are present. The circuit will
be effective in urban areas where high density multiple RF
sources exists. A user required source can be used to provide
B. Voltage Doubler Rectifier wireless charging capability.
The Radio Frequency is AC signal, to get a DC signal out The design is simplest with diode and capacitor as the main
of the AC signal, a rectifier circuit is employed. components as a prototype of wireless charging circuit. Using
The charge pump circuits, such as voltage doubler are used the SMD components, the circuit will have a small size which
to rectify the input voltage (Vin), by employing multiply stages will enable it to be fitted or integrated in cellular or mobile
the required output voltage (Vo) can be obtained. The voltage devices. Fig. 3 shows antenna with developed circuit and
output is twice the input peak voltage, minus twice the diode waveform generation with antenna and matching circuit.
threshold voltage. Under load, output voltage drops due to One other way to charge battery from the circuit is to place
capacitor charge drain. Charge drained (Δq) by the load the circuit near other cellular phone, where present device
current (Iload) per period, where f is input signal frequency, is containing circuit will act as a parasite, draining other mobile
energy.
Circuit can be extended to n stages, producing theoretical
2nVin output voltage. Under ideal condition the output of n-
stage charge pump circuit, with stage capacitance (C) will be

Since the power received by the receiver will be relatively

low and the signal frequency is high, the diodes are required to
have a very low turn on voltage and high operating frequency.
We tested a Schottky diode for rectifier as a safe operation.
Cclamp and D1 form the negative clamp, while Crectifier and D2
form peak rectifier. Crectifier also smooth the output. Figure 3: (a) Antenna with developed circuit (b) Waveform generation
with antenna and matching circuit
C. Charging Controller
The charging controller acts as a switch. Using the battery Table 1: Key Features of Antenna and voltage doubler circuit
power level as a reference it charges the device battery or run Parameters  Features 
Antenna size  50mm x 14mm,  2r = 2mm
the device terminal functions if battery is fully charged or only
Main clock  8 MHz 
running the terminal function is required as in battery less
Power supply 1.2V 
devices. The switch can easily be designed using a low power Clock speed  8 MHz 
MOSFETs as comparator. Power Gain  15dB after test (initial 7dB) 

IV. POWER GENERATED VI. CONCLUSION

The power generated depends on source power and distance Due to its small size it can be fitted in devices such as
R of circuit from the RF power source. The RF power receive mobile handsets, PDA’s etc. We designed a RF circuit that
(PR) is given by enable to charging mobile handsets, result in continuous
charging of device especially in metro areas where the density
of RF sources is high.
Other then these existing RF sources, a user generated RF
Where GT is transmitter antenna gain, GR is receiver antenna source can be used to charge the battery of mobile device.
gain, λ is the wavelength of the radiation, and PT is the power Using existing antennas and system for mobile communication
transmitted. this generation is low cost and feasible.
Also the power losses in the designed circuits are kept at
minimum using the minimum components able to work at low VII. REFERENCE
voltages, 1.2V and frequency of 8MHz.
The important component in presented scheme is the
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antenna, which should be capable of working at multiple Electronics,” IEEE Pervasive Computing, 4, pp.18–27, 2005
frequencies and capture the maximum power of signal at [2] R. Krishnan, “Method and apparatus for Wireless Powering and Recharging”, US
Patent 6127799, Oct 3, 2000
maximum possible frequencies. [3] Lin, P.M., “Topological Generation and Analysis of Voltage Multiplier Circuits”,
Using our circuit with antennas working at single and IEEE Transactions on Circuits and Systems, Vol. CAS-24, no. 10, pp. 528, October 1977.
multiple frequency bands we present the power generated by [4] M. Sperling, “RF to DC Converter in SiGe Process,” Thesis, Carnegie Mellon
University, USA, August 2003.
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using RFID technique,” IEEE Transactions on Consumer Electronics, vol. 45, no 1,
February, 2008