wireless charging of mobile phones

CHAPTER-1

INTRODUCTION

Cellular telephone technology became commercially available in the 1980’s. Since

then, it has been like a snowball rolling downhill, ever increasing in the number of users and

the speed at which the technology advances. When the cellular phone was first implemented,

it was enormous in size by today’s standards. This reason is two-fold; the battery had to be

large, and the circuits themselves were large. The circuits of that time used in electronic

devices were made from off the shelf integrated circuits (IC), meaning that usually every part

of the circuit had its own package. These packages were also very large. These large circuit

boards required large amounts of power, which meant bigger batteries. This reliance on

power was a major contributor to the reason these phones were so big.

Through the years, technology has allowed the cellular phone to shrink not only the

size of the ICs, but also the batteries. New combinations of materials have made possible the

ability to produce batteries that not only are smaller and last longer, but also can be recharged

easily. However, as technology has advanced and made our phones smaller and easier to use,

we still have one of the original problems: we must plug the phone into the wall in order to

recharge the battery. Most people accept this as something that will never change, so they

might as well accept it and carry around either extra batteries with them or a charger. Either

way, it’s just something extra to weigh a person down. There has been research done in the

area of shrinking the charger in order to make it easier to carry with the phone. One study in

particular went on to find the lower limit of charger size. But as small as the charger

becomes, it still needs to be plugged in to a wall outlet.

Most people don’t realize that there is an abundance of energy all around us at all

times. We are being bombarded with energy waves every second of the day. Radio and

television towers, satellites orbiting earth, and even the cellular phone antennas are constantly

transmitting energy. If it could be possible to gather the energy and store it, we could

potentially use it to power other circuits. In the case of the cellular phone, this power could be

used to recharge a battery that is constantly being depleted.

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The potential exists for cellular phones, and even more complicated devices - i.e.

pocket organizers, person digital assistants (PDAs), and even notebook computers - to

become completely wireless. Of course, right now this is all theoretical. There are many

complications to be dealt with. The first major obstacle is that it is not a trivial problem to

capture energy from the air. We will use a concept called energy harvesting. Energy

harvesting is the idea of gathering transmitted energy and either using it to power a circuit or

storing it for later use. The concept needs an efficient antenna along with a circuit capable of

converting alternating-current (AC) voltage to direct-current (DC) voltage. The efficiency of

an antenna, as being discussed here, is related to the shape and impedance of the antenna and

the impedance of the circuit. If the two impedances aren’t matched then there is reflection of

the power back into the antenna meaning that the circuit was unable to receive all the

available power. Matching of the impedances means that the impedance of the antenna is the

complex conjugate of the impedance of the circuit.

Another thing to think about is what would happen when you get away from major

metropolitan areas. Since the energy we are trying to harness is being added to the

atmosphere from devices that are present mostly in cities and are not as abundant in rural

areas, there might not be enough energy for this technology to work. However, for the time

being, we will focus on the problem of actually getting a circuit to work.

This seminar report is considered to be one of the first steps towards what could

become a standard circuit included in every cellular phone, and quite possibly every

electronic device made. A way to charge the battery of an electric circuit without plugging it

into the wall would change the way people use wireless systems. However, this technology

needs to be proven first. It was decided to begin the project with a cellular phone because of

the relative simplicity of the battery system. Also, after we prove that the technology will

work in the manner suggested; cellular phones would most likely be the first devices to have

such circuitry implemented on a wide scale. This advancement coupled with a better overall

wireless service can be expected to lead to the mainstream use of cell phones as people’s only

phones. This thesis is an empirical study of whether or not this idea is feasible. This first step

is to get an external wireless circuit to work with an existing phone by transmitting energy to

the phone (battery) through the air[1].

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CHAPTER-2

NEW TECHNOLOGIES TO IMPROVING MOBILE PHONE BATTERY POWER

One of the biggest challenges facing handset makers today is how to extend the lifetime

of batteries on mobile phones. Advanced mobile phones today require a lot of power to stay

charged since people are using them for all-day activities (like instant messaging) as well as

for activities that quickly drain the battery (like watching TV). Handset makers need to find

ways to make mobile phones that will have a long-lasting battery which isn’t so big that it’s

cumbersome to carry around the phone. Companies are exploring a number of different

methods of making this happen including developing new display technology, creating new

chip technology, exploring the advanced use of multi-core processing for mobile phones,

reviewing options for improving battery chargers and looking into the harvesting of wireless

power for continuous charging of mobile phones. Therefore some of the new technologies to

improve the mobile phone battery power are

New Display Methods for Improved Mobile Battery Power

Mobile Phone Chips as Power Supplies

Use of Multi-Core Processing for Mobile Phones

Harvesting Wireless Power

2.1 New Display Methods for Improved Mobile Battery Power

One of the options that are being explored for extending battery power life in mobile

phones is the option of improving display methods on phones with large screens. The display

module of most smart phones being created today takes up about half of the phone’s available

battery power. If it is possible to improve display methods so that they use less power, then

the phones being created today (and those made in the future) will have longer-lasting

batteries due to the reduced drainage from the display.

The most recent advance in this area of technology that utilizes a technology called

Pixcale to reduce the power drainage of LCD screens. The company has actually been around

for almost a decade and has been highly successful in reduce screen power consumption for

televisions. They haven’t had much success breaking into the mobile phone market in the

past due to the fact that yesterday’s mobile phone screens were too small for their Pixcale

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technology to work effectively. Today’s Smartphone screens are larger and can make use of

the technology. The company says use of its technology can reduce a phone display’s power

consumption by approximately forty percent which would significantly extend the life of

today’s Smartphone batteries.

Another company called Unipixel is also working on extending mobile phone battery

life by improving display screens on mobile phones. Since display screens take up so much of

a phone’s battery life today, this is the area of technological development that is of most

interest to many of today’s handset makers[2].

2.2 Mobile Phone Chips as Power Supplies

Display screens are not the only part of mobile phones that are getting attention by

scientists who want to improve the battery life of cell phones. Another thing that is being

studied and improved upon is chip technology for mobile phones. One example of this came

at the end of last year when a company called Tyndall created new silicon chip technology

dubbed Power Supply on Chip (PwrSoC). This uses micromagnetics to improve mobile

phone power supply by up to ninety percent. Chip technology has been successful in

improving battery life of some mobile computers so it’s certainly an area of interest for

people who want to extend battery life on mobile phones[3].

2.3 Use of Multi-Core Processing for Mobile Phones

Another technological advance that scientists are exploring is the development of

multi-core processing options for mobile phones. This option is based upon a similar

development in the world of computing. Back when computers were first developed, they

used single-core processors. This placed a lot of demand on the computers. Once dual-core

and multi-core processing developed personal computers became capable of doing a lot more

(and becoming the machines that we know them to be today). Scientists believe that the same

thing could be done with mobile phones. If so, phones could use one processor for phone

tasks (like voice calls) and the other processor for computing tasks (like surfing the mobile

web). The second processor would be powered down when the first one was in use. This

would allow battery life to be extended even as functionality of cell phones improved.

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2.4 Harvesting Wireless Power

At the other end of the spectrum is technology that would allow for phones to never

need re-charging. Or rather, the phones would continually recharge using existing wireless

power so that the consumer wouldn’t have to do anything to keep the battery charged. This is

a technology that is being looked into by Nokia. The company says that ambient

electromagnetic radio waves can be harvested from Wi-Fi resources and used to slowly

recharge cell phones. This is currently one of the more far-reaching options for improving

cell phone battery life but Nokia says that it’s a distinct possibility for the future of cell

phones. Two methods of wireless charging are

Through microwaves

Using power mat

Among the above four technologies first three are depends on the manufacturer. So,

harvesting wireless power is efficient one[4].

2.5 Different methods of charging

1. Inductive charging

2. Radio charging

3. Resonance charging

2.5.1 Inductive charging

Inductive charging is used for charging mid-sized items such as cell phones, MP3

players and PDAs. Inductive charging uses the electromagnetic field to transfer energy

between two objects. A charging station sends energy through inductive coupling to an

electrical device, which stores the energy in the batteries. Because there is a small gap

between the two coils, inductive charging is one kind of short-distance wireless energy

transfer.

In inductive charging, an adapter equipped with contact points is attached to the

device's back plate. When the device requires a charge, it is placed on a conductive charging

pad, which is plugged into a socket.

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Advantages

Inductive charging carries a far lower risk of electrical shock, when compared with

conductive charging, because there are no exposed conductors. The ability to fully enclose

the charging connection also makes the approach attractive where water impermeability is

required; for instance, inductive charging is used for implanted medical devices that require

periodic or even constant external power, and for electric hygiene devices, such as

toothbrushes and shavers, that are frequently used near or even in water. Inductive charging

makes charging mobile devices more convenient; rather than having to connect a power

cable, the device can be placed on a charge plate.

Disadvantages

One disadvantage of inductive charging is its lower efficiency and increased ohmicness

(resistive) heating in comparison to direct contact. Implementations using lower frequencies

or older drive technologies charge more slowly and generate heat for most portable

electronics, the technology is nonetheless commonly used in some electric toothbrushes and

wet/dry electric shavers, partly for the advantage that the battery contacts can be completely

sealed to prevent exposure to water. Inductive charging also requires drive electronics and

coils that increase manufacturing complexity and cost.

2.5.2 Resonance charging

Resonance charging is used for items that require large amounts of power, such as an

electric car, robot, vacuum cleaner or laptop computer. In resonance charging, a copper coil

attached to a power source is the sending unit. Another coil, attached to the device to be

charged, is the receiver. Both coils are tuned to the same electromagnetic frequency, which

makes it possible for energy to be transferred from one to the other. The method works over

short distances (3-5 meters).

2.5.3 Radio charging

Radio charging is used for charging items with small batteries and low power

requirements, such as watches, hearing aids, medical implants, cell phones, MP3 players and

wireless keyboard and mice. Radio waves are already in use to transmit and receive cellular

telephone, television, radio and Wi-Fi signals. Wireless radio charging works similarly[5].

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CHAPTER-3

WIRELESS CHARGING OF MOBILE PHONES USING MICROWAVES

The basic addition to the mobile phone is going to be the rectenna. A rectenna is a

rectifying antenna, a special type of antenna that is used to directly convert microwave

energy into DC electricity. Its elements are usually arranged in a mesh pattern, giving it a

distinct appearance from most antennae. A simple rectenna can be constructed from a

Schottky diode placed between antenna dipoles. The diode rectifies the current induced in the

antenna by the microwaves.

Rectenna are highly efficient at converting microwave energy to electricity. In

laboratory environments, efficiencies above 90% have been observed with regularity. Some

experimentation has been done with inverse rectenna, converting electricity into microwave

energy, but efficiencies are much lower--only in the area of 1%. With the advent of

nanotechnology and MEMS the size of these devices can be brought down to molecular level.

It has been theorized that similar devices, scaled down to the proportions used in

nanotechnology, could be used to convert light into electricity at much greater efficiencies

than what is currently possible with solar cells. This type of device is called an optical

rectenna. Theoretically, high efficiencies can be maintained as the device shrinks, but

experiments funded by the United States National Renewable energy Laboratory have so

far only obtained roughly 1% efficiency while using infrared light. Another important part of

our receiver circuitry is a simple sensor[6].

3.1 Electromagnetic spectrum

To start with, to know what a spectrum is: when white light is shone through a prism it is

separated out into all the colors of the rainbow; this is the visible spectrum. So white light is a

mixture of all colors. Black is NOT a color; it is what you get when all the light is taken away.

Some physicists pretend that light consists of tiny particles which they call photons. They travel

at the speed of light (what a surprise). The speed of light is about 300,000,000 meters per

second. When they hit something they might bounce off, go right through or get absorbed. What

happens depends a bit on how much energy they have. If they bounce off something and then go

into your eye you will "see" the thing they have bounced off.

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Some things like glass and Perspex will let them go through; these materials are

transparent. Black objects absorb the photons so you should not be able to see black things: you

will have to think about this one. These poor old physicists get a little bit confused when they try

to explain why some photons go through a leaf, some are reflected, and some are absorbed.

They say that it is because they have different amounts of energy. Other physicists pretend that

light is made of waves. These physicists measure the length of the waves and this helps them to

explain what happens when light hits leaves. The light with the longest wavelength (red) is

absorbed by the green stuff (chlorophyll) in the leaves. So is the light with the shortest

wavelength (blue). In between these two colors there is green light, this is allowed to pass right

through or is reflected. (Indigo and violet have shorter wavelengths than blue light.)

Figure 3.1: Electromagnetic spectrum

The visible spectrum is just one small part of the electromagnetic spectrum. These

electromagnetic waves are made up of to two parts. The first part is an electric field. The second

part is a magnetic field. So that is why they are called electromagnetic waves. The two fields are

at right angles to each other[7].

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3.2 Microwave region

Microwave wavelengths range from approximately one millimeter (the thickness of a

pencil lead) to thirty centimeters (about twelve inches). In a microwave oven, the radio waves

generated are tuned to frequencies that can be absorbed by the food. The food absorbs the

energy and gets warmer. The dish holding the food doesn't absorb a significant amount of

energy and stays much cooler. Microwaves are emitted from the Earth, from objects such as cars

and planes, and from the atmosphere. These microwaves can be detected to give information,

such as the temperature of the object that emitted the microwaves.

Microwaves have wavelengths that can be measured in centimeters! The longer

microwaves, those closer to a foot in length, are the waves which heat our food in a microwave

oven. Microwaves are good for transmitting information from one place to another because

microwave energy can penetrate haze, light rain and snow, clouds, and smoke. Shorter

microwaves are used in remote sensing. These microwaves are used for clouds and smoke, these

waves are good for viewing the Earth from space Microwave waves are used in the

communication industry and in the kitchen as a way to cook foods. Microwave radiation is still

associated with energy levels that are usually considered harmless except for people with pace

makers.

Figure 3.2: Microwave Region

9

Here we are going to use the S band of the Microwave Spectrum.

Table 3.1: Bands in Microwave spectrum

The frequency selection is another important aspect in transmission. Here we have selected the

license free 2.45 GHz ISM band for our purpose. The Industrial, Scientific and Medical (ISM)

radio bands were originally reserved internationally for non-commercial use of RF

electromagnetic fields for industrial, scientific and medical purposes.

The ISM bands are defined by the ITU-T in S5.138 and S5.150 of the Radio Due to

variations in national radio regulations[8]. In recent years they have also been used for license-

free error-tolerant communications applications such as wireless LANs and Bluetooth:

900 MHz band (33.3 cm) (also GSM communication in India)

2.45 GHz band (12.2 cm)

IEEE 802.11b wireless Ethernet also operates on the 2.45 GHz band.

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Designation Frequency range

L Band 1 to 2 GHz

S Band 2 to 4 GHz

C Band 4 to 8 GHz

X Band 8 to 12 GHz

Ku Band 12 to 18 GHz

K Band 18 to 26 GHz

Ka Band 26 to 40 GHz

Q Band 30 to 50 GHz

U Band 40 to 60 GHz

Mobile phone

R F

Circulator Waveguide

Rectenna

Sensor

SLOTTED

W.G.

ANTENNA

3.3 Block diagram

Figure 3.3: Block Diagram

3.4 Working of block diagram

3.4.1 Transmitter

The most basic transmitter setup consists of a piece of equipment that generates a signal

whose output is then fed into an amplifier that is finally output through a radiating antenna –

the air interface. A condition must be met where the antenna operates optimally at the desired

frequency output from the signal generator. In the current case, an antenna was connected

through an amplifier to a radio-frequency (RF) source. The RF source is a circuit that outputs

a signal at a user- specified frequency and voltage. The range of frequencies of the signal

generator resides in the radio frequency band, 3 mega-hertz (MHz) to 3 giga-hertz (GHz).

The output power of this device is limited.

For this reason, an amplifier is required on the output. The transmitting antenna is

called a patch antenna and is fabricated from copper plating that is soldered to a feed wire

and has a ground plane. The frequency of 915MHz was chosen for this project because it is

one at which our team has experience, and it falls in one of the Industrial-Scientific-Medical

(ISM) RF bands made available by the Federal Communications Commission for low power,

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Magnetron

short distance experimentation. This frequency was chosen mostly for simplicity in using the

available equipment. It is not used for mass communication or anything else on a major scale,

and therefore is not going to be interfered with, or interfere with other devices at low power

levels.

Antenna

915MHz Amplifier

Oscillator

Figure 3.4: Transmitter

This also means that transmitters for short distances are readily available. In fact,

915MHz is a very common frequency used in RF research. This makes a transmitter system

easy to construct and manage. The source is nothing more than a signal generator, capable of

outputting a low-noise AC signal at 915MHz. This setup results in the antenna beaming

approximately 6mW of power per square meter. This was the limit of the gain of the

amplifier[9].

3.4.2 Receiver

The receiver’s main purpose is to charge a battery. A simple battery charging theory is

to run current through the battery, and apply a voltage difference between the terminals of the

battery to reverse the chemical process. By doing so, it recharges the battery. There are other

efficient and faster ways to charge the battery, but it requires a large amount of energy which

the wireless battery charger cannot obtain, yet. Therefore, in our design, we use a straight

forward method to charge the battery.

Microwave signal is an AC signal with a frequency range of 1 GHz – 1000 GHz. 915

MHz is in between the RF/ Microwave range. No matter how high the frequency is, AC

signal is still AC signal.

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Antenna

LED

Rectifier Circuit Load(Battery)

Figure 3.5: Receiver

Therefore, the signal can also be treated as a low frequency AC signal. In order to get a

DC signal out of the AC signal, a rectifier circuit is needed. At the output of the rectifier, the

signal is not a fully DC signal yet. Thus, by adding a capacitor and a resistor can smooth out

the output to become DC signal.

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CHAPTER-4

TRANSMITTER DESIGN

The source used to implementing the process is magnetron which is a self contained

microwave oscillator operates differently from the linear beam tubes such as TWT and klystron.

4.1 Magnetron

Magnetron is a high power microwave oscillator and it is used in microwave oven and

radar transmitter.

It is itself a special kind of vacuum tube that has permanent magnet in its constructions.

This magnet is setup to affect the path of travel of electrons that are in transit from

cathode to the plate.

Magnetron is capable to deliver more power than reflex klystron or Gunn diode.

It is a high power oscillator and has high efficiency of 50% to 80%.

Magnetron is a device which produces microwave radiation of radar application and

microwaves.

Magnetron functions as self-excited microwave oscillator.

Crossed electron and magnetic fields are used to produce magnetron to produce the high

power output required in radar equipment.

These multi cavity devices are used in transmitters as pulsed or cw oscillators to produce

microwave radiation.

Disadvantage of magnetron is that it works only on fixed frequency

Figure 4.2 is a simplified drawing of the magnetron. CROSSED-ELECTRON and

MAGNETIC fields are used in the magnetron to produce the high-power output required in

radar and communications equipment[10]. The magnetron is classed as a diode because it has no

grid. A magnetic field located in the space between the plate (anode) and the cathode serves as a

grid. The plate of a magnetron does not have the same physical appearance as the plate of an

ordinary electron tube. Since conventional inductive-capacitive (LC) networks become

impractical at microwave frequencies, the plate is fabricated into a cylindrical copper block

containing resonant cavities that serve as tuned circuits.

14

Figure 4.1: magnetron Figure 4.2: Simplified magnetron

The magnetron base differs considerably from the conventional tube base. The magnetron

base is short in length and has large diameter leads that are carefully sealed into the tube and

shielded. The cathode and filament are at the center of the tube and are supported by the filament

leads. The filament leads are large and rigid enough to keep the cathode and filament structure

fixed in position. The output lead is usually a probe or loops extending into one of the tuned

cavities and coupled into a waveguide or coaxial line. The plate structure is a solid block of

copper.

The cylindrical holes around its circumference are resonant cavities. A narrow slot runs

from each cavity into the central portion of the tube dividing the inner structure into as many

segments as there are cavities. Alternate segments are strapped together to put the cavities in

parallel with regard to the output. The cavities control the output frequency. The straps are

circular, metal bands that are placed across the top of the block at the entrance slots to the

cavities. Since the cathode must operate at high power, it must be fairly large and must also be

able to withstand high operating temperatures. It must also have good emission characteristics,

particularly under return bombardment by the electrons. This is because most of the output

power is provided by the large number of electrons that are emitted when high-velocity electrons

return to strike the cathode. The cathode is indirectly heated and is constructed of a high-

emission material. The open space between the plate and the cathode is called the

INTERACTION SPACE. In this space the electric and magnetic fields interact to exert force

upon the electrons.

15

4.2 Receiver design

The basic addition to the mobile phone is going to be the rectenna. A rectenna is a

rectifying antenna, a special type of antenna that is used to directly convert microwave energy

into DC electricity. Its elements are usually arranged in a mesh pattern, giving it a distinct

appearance from most antennae. A simple rectenna can be constructed from a Schottky diode

placed between antenna dipoles. The diode rectifies the current induced in the antenna by the

microwaves. Rectennae are highly efficient at converting microwave energy to electricity. In

laboratory environments, efficiencies above 90% have been observed with regularity.

Figure 4.3: Set up to charge the mobile phone

Some experimentation has been done with inverse rectennae, converting electricity into

microwave energy, but efficiencies are much lower--only in the area of 1%. With the advent of

nanotechnology and MEMS the size of these devices can be brought down to molecular level. It

has been theorized that similar devices, scaled down to the proportions used in nanotechnology,

could be used to convert light into electricity at much greater efficiencies than what is currently

possible with solar cells. This type of device is called an optical rectenna.

16

Theoretically, high efficiencies can be maintained as the device shrinks, but experiments

funded by the United States National Renewable energy Laboratory have so far only obtained

roughly 1% efficiency while using infrared light. Another important part of our receiver

circuitry is a simple sensor. This is simply used to identify when the mobile phone user is

talking. As our main objective is to charge the mobile phone with the transmitted microwave

after rectifying it by the rectenna, the sensor plays an important role. The whole setup looks

something like this[11].

4.3 Process of rectification

A rectifying antenna rectifies received microwaves into DC current .A rectenna

comprises of a mesh of dipoles and diodes for absorbing microwave energy from a

transmitter and converting it into electric power. Its elements are usually arranged in a mesh

pattern, giving it a distinct appearance from most antennae.

Figure 4.4: Rectenna

A simple rectenna can be constructed from a Schottky diode placed between antenna

dipoles as shown in Fig.. The diode rectifies the current induced in the antenna by the

microwaves. Rectenna are highly efficient at converting microwave energy to electricity. In

laboratory environments, efficiencies above 90% have been observed with regularity. In

future rectennas will be used to generate large-scale power from microwave beams delivered

from orbiting SPS satellites.

17

4.4 Schottky barrier diode

A Schottky barrier diode is different from a common P/N silicon diode. The common

diode is formed by connecting a P type semiconductor with an N type semiconductor, this is

connecting between a semiconductor and another semiconductor; however, a Schottky barrier

diode is formed by connecting a metal with a semiconductor. When the metal contacts the

semiconductor, there will be a layer of potential barrier (Schottky barrier) formed on the

contact surface of them, which shows a characteristic of rectification. The material of the

semiconductor usually is a semiconductor of n-type (occasionally p-type), and the material of

metal generally is chosen from different metals such as molybdenum, chromium, platinum

and tungsten. Sputtering technique connects the metal and the semiconductor.

A Schottky barrier diode is a majority carrier device, while a common diode is a

minority carrier device. When a common PN diode is turned from electric connecting to

circuit breakage, the redundant minority carrier on the contact surface should be removed to

result in time delay. The Schottky barrier diode itself has no minority carrier, it can quickly

turn from electric connecting to circuit breakage, its speed is much faster than a common P/N

diode, so its reverse recovery time T rr is very short and shorter than 10 nS. And the forward

voltage bias of the Schottky barrier diode is under 0.6V or so, lower than that (about 1.1V) of

the common PN diode. So, The Schottky barrier diode is a comparatively ideal diode, such as

for a 1 ampere limited current PN interface.

Below is the comparison of power consumption between a common diode and a

Schottky barrier diode:

P=0.6*1=0.6W

P=1.1*1=1.1W

It appears that the standards of efficiency differ widely. Besides, the PIV of the

Schottky barrier diode is generally far smaller than that of the PN diode; on the basis of the

same unit, the PIV of the Schottky barrier diode is probably 50V while the PIV of the PN

diode may be as high as 150V. Another advantage of the Schottky barrier diode is a very low

noise index that is very important for a communication receiver; its working scope may

reach20GHz.

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4.5 Sensor circuitry

The sensor circuitry is a simple circuit, which detects if the mobile phone receives any

message signal. This is required, as the phone has to be charged as long as the user is talking.

Thus a simple F to V converter would serve our purpose. In India the operating frequency of

the mobile phone operators is generally 900MHz or 1800MHz for the GSM system for

mobile communication. Thus the usage of simple F to V converters would act as switches to

trigger the rectenna circuit to on.

A simple yet powerful F to V converter is LM2907. Using LM2907 would greatly

serve our purpose. It acts as a switch for triggering the rectenna circuitry. The general block

diagram for the LM2907 is given below. Thus on the reception of the signal the sensor

circuitry directs the rectenna circuit to ON and the mobile phone begins to charge using the

microwave power.

A sensor is devised to sense the activities such as texting, calling, SMS and MMS,

being carried out in a cell phone within a specified range. It is an easy to use handy mobile

device, sometimes also called as sniffer or pocket-size mobile transmission detector. A

number of phone sensor manufacturing companies have sprouted in the industry, each

offering some or the other exceptional features in their products. You can choose the one as

per your own requirements[12].

Figure 4.7: Sensor circuit

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A cell phone sensor can sense the presence of an activated cell phone within the range

of around one and a half meters. The cell phone sensor circuit has been designed to perfection

so that it may be able to track the appearance of a mobile phone and all its activities,

including SMS, video transmissions, incoming calls as well as outgoing calls. The device is

quiet capable to function properly even if the cell phone under surveillance is on silent mode.

As soon as the sensor senses the RF transmission signals from a phone located somewhere in

its vicinity, it starts raising a beep alarm which continues till the signal transmission is not

ceased.

4.6 Advantages:

Use of separate chargers is eliminated

Electricity is saved

The phone can be charged anywhere anytime

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CHAPTER-5

POWER MAT

A system will be presented using existing antenna and charge pump technology to

charge a cellular phone battery without wires. The wireless charger will convert the RF/

microwave signal at 915 MHz frequency into a DC signal, and then store the power into a

battery. In this first step, a standard phone is used, and incorporates the charging technology

into a commercially available base station. The base station will contain an antenna tuned to

915MHz and a charge pump. We will discuss the advantages and disadvantages of such a

system, and hopefully pave the way for a system incorporated into the phone for charging

without the use of a base station.

This revolutionary new wireless charging technology allows users to wirelessly charge

multiple devices simultaneously and eliminate the tangle of wires that accumulate in the

home and behind work stations. It is a two part system pairing a sleek, ultra-thin mat with

receivers that attach to your device, enabling you to harge by simply placing those devices on

the mat.

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Figure 5.1: Pictures of power mat

Power mat uses induction to charge all your electronics, including Blackberries,

iPhones, iPods, cameras, portable gaming devices, and other small electronics. Basically,

when you drop a Power mat-enabled device on the mat, it identifies the device, determines

how much power it needs, then starts transferring energy to it. Once a device is fully charged,

Power mat will stop the electricity from flowing. It charges gadgets pretty quickly. It takes

only 90 minutes to completely charge an iPod, an iPhone, and an HTC Touch Smart phone.

Figure 5.2: Power mat with multiple devices

Smartphone customers use their devices so much, they often need to charge it more

than once a day. Power mat gives them an easy charging solution to maximize their time and

how they use their Smartphone. Not only does Power mat provide the freedom to charge a

Blackberry or any other Smartphone without the constant plugging and unplugging, but they

can also charge the rest of their handheld gear without the hassle and tangle of cords.

22

A unique sound tells you that a solid connection has been made and your device is

charging. A second, similar sound is heard when the device is removed. The level of these

sounds can be changed between high and low or they can be turned off. The Power mat also

features indication lights. Each of the three wireless charging access point has a

corresponding light indicator that informs you that your device is charging wirelessly.

The intensity of these lights can be changed between high and low or they can be

turned off. The Power mat can charge a fourth device via the USB port on the rear of the Mat

as well[13].

5.1 Transmitter

The most basic transmitter setup consists of a piece of equipment that generates a signal

whose output is then fed into an amplifier that is finally output through a radiating antenna

the air interface. A condition must be met where the antenna operates optimally at the desired

frequency output from the signal generator. In the current case, an antenna was connected

through an amplifier to a radio-frequency (RF) source. The RF source is a circuit that outputs

a signal at a user-specified frequency and voltage. The range of frequencies of the signal

generator resides in the radio frequency band, 3 mega-hertz (MHz) to 3 giga-hertz (GHz).

The output power of this device is limited. For this reason, an amplifier is required on the

output. The transmitting antenna is called a patch antenna and is fabricated from copper

plating that is soldered to a feed wire and has a ground plane.

The frequency of 915MHz was chosen for this project because it is one at which our

team has experience, and it falls in one of the Industrial-Scientific-Medical (ISM) RF bands

made available by the Federal Communications Commission for low power, short distance

experimentation. This frequency was chosen mostly for simplicity in using the available

equipment. It is not used for mass communication or anything else on a major scale, and

therefore is not going to be interfered with, or interfere with other devices at low power

levels. This also means that transmitters for short distances are readily available. In fact,

915MHz is a very common frequency used in RF research. This makes a transmitter system

easy to construct and manage. The source is nothing more than a signal generator, capable of

outputting a low-noise AC signal at 915MHz. This setup results in the antenna beaming

approximately 6mW of power per square meter. This was the limit of the gain of the

amplifier.

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5.2 Design aspects

The design aspect of this is focused on the receiving side. For this stage of research, of

which the goal is to prove that the wireless battery charger idea is feasible, it was decided to

incorporate the energy harvesting circuitry and antenna in some sort of base station or

charging stand. It is necessary to hide the components for demonstration purposes. This being

the case, two phones were chosen that have accessories currently available to use as our

charging stands. The Nokia 3570 was the first phone that was received for the research. This

phone comes standard with a battery and an AC/DC travel charger. The battery included with

the phone has a voltage range from 3.2V - when the phone shuts off - to 3.9V when fully

charged. This battery only takes about 2 hours to charge when plugged into the wall through

the travel charger supplied with the phone.

This charger has an unloaded, unregulated direct current (DC) output voltage of 9.2V.

When connected to the phone, the charging voltage goes to the battery voltage,

approximately 3.6V, and then slowly increases until it saturates at 3.9V. This charger

regulates the current to around 350mA. The other phone that was chosen is the Motorola

V60i. This phone has many of the same features as the Nokia above, and it also comes

standard with its own battery and travel charger. The battery for this phone is a 3.6V battery

like the Nokia battery. The travel charger shown is quite different from its Nokia counterpart.

First of all, there are 3 pins going to the phone, not just the 2 needed for power and ground.

Two of these pins are at a ground potential, and the other one is 6.09V higher than the other

two. This is very close to the regulated voltage of 5.9V seen by the phone during charging. It

runs at 400mA, a little higher than the Nokia charger.

5.3 Basic concept

This is based on a very simple concept, capture RF energy using an antenna, input it

into a charge-pump and use this energy to power some other circuit. As a precursor to this

thesis, there have been many projects involving charge pumps. These projects range from

tuning the charge pump to using results from existing charge pumps to drive other circuits.

For the tuning projects, usually the testing is done using a light emitting diode (LED). RF

energy is transmitted to the circuit and the charge pump stores the energy in a large capacitor.

When the amount of charge is large enough, the LED uses the stored energy to light for a

moment. This is called a charge-and-fire system. In other research, charge pumps were tested

24

from earlier projects that were used to power other circuits. This type of technology is very

useful in Radio Frequency Identification (RFID) applications.

The way RFID systems work is that when a chip passes through a scanner device,

power is sent to the chip from the scanner. In older systems, the frequency or amplitude of

this signal was modulated by the chip and sent back. This technique is called backscatter.

But, in more recent systems, the chips are getting more complicated and require much more

power to run. The RFID system is unsuitable for batteries mostly because they have to be

small, but also because the batteries will eventually die and require changing. But, with a

good antenna, a charge pump should be able to handle the powering of these circuits and

never will need to be serviced. Because the circuits are small, the power required is minimal.

5.4 Charge pump

At this point, it is necessary to explain what exactly a charge pump is, and how it

works. A charge pump is a circuit that when given an input in AC is able to output a DC

voltage typically larger than a simple rectifier would generate. It can be thought of as a AC to

DC converter that both rectifies the AC signal and elevates the DC level. It is the foundation

of power converters such as the ones that are used for many electronic devices today. These

circuits typically are much more complex than the charge pumps used in this thesis. Power

converter circuits have a lot of protective circuitry along with circuitry to reduce noise. In

fact, it is a safety regulation that any power-conversion circuits use a transformer to isolate

the input from the output. This prevents overload of the circuit and user injury by isolating

the components from any spikes on the input line. For this thesis, however, such a low power

level is being used that a circuit this complex would require more power than is available,

and it would therefore be very inefficient and possibly not function. In that case, it is

necessary to use a simple design.

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Figure 5.3: Peak Detector

The simplest design that can be used is a peak detector or half wave peak rectifier. This

circuit requires only a capacitor and a diode to function. The schematic is shown in Figure

5.3. The explanation of how this circuit works is quite simple. The AC wave has two halves,

one positive and one negative. On the positive half, the diode turns on and current flows,

charging the capacitor. On the negative half of the wave, the diode is off such that no current

is flowing in either direction. Now, the capacitor has voltage built up which is equal to the

peak of the AC signal, hence the name. Without the load on the circuit, the voltage would

hold indefinitely on the capacitor and look like a DC signal, assuming ideal components.

With the load, however, the output voltage decreases during the negative cycle of the AC

input, shown in Figure 5.4.

Figure 5.4: Half-wave Peak Rectifier Output Waveform

This figure shows the voltage decreases exponentially. This is due to the RC time

constant. The voltage decreases in relation to the inverse of the resistance of the load, R,

multiplied by the capacitance C. This circuit produces a lot of ripple, or noise, on the output

DC of the signal. With more circuitry, that ripple can be reduced.

26

Figure 5.5: Full-wave Rectifier

The next topology presented in Figure 5.5. is a full-wave rectifier. Whereas the

previous circuit only captures the positive cycle of the signal, here both halves of the input

are captured in the capacitor. From this figure, we see that in the positive half of the cycle,

D1 is on, D2 is off and charge is stored on the capacitor. But, during the negative half, the

diodes are reversed, D2 is on and D1 is off. The capacitor doesn’t discharge nearly as much

as in the previous circuit, so the output has much less noise, as shown in Figure 3.4. It

produces a cleaner DC signal than the half-wave rectifier, but the circuit itself is much more

complicated with the introduction of a transformer. This essentially rules this topology out for

this research because of the space needed to implement it.

Figure 5.6: Full-wave Rectifier Output Waveform

There are other topologies for charge pumps but they will not be covered here. The

others are more complex and all involve transformers, like the full-wave rectifier, and

therefore take up more room than there is real estate for in this project. Instead, the circuit

that was chosen to be used will now be presented. The charge pump circuit is made of stages

of voltage doublers. This circuit is called a voltage doubler because in theory, the voltage that

is received on the output is twice that at the input.

27

Figure 5.7: Voltage Double Schematic

The schematic in Figure 5.7. represents one stage of the circuit. The RF wave is

rectified by D2 and C2 in the positive half of the cycle, and then by D1 and C1 in the

negative cycle. But, during the positive half-cycle, the voltage stored on C1 from the negative

half-cycle is transferred to C2. Thus, the voltage on C2 is roughly two times the peak voltage

of the RF source minus the turn-on voltage of the diode, hence the name voltage doubler[14].

The most interesting feature of this circuit is that by connecting these stages in series,

we can essentially stack them, like stacking batteries to get more voltage at the output. One

might ask, after the first stage, how can this circuit get more voltage with more stages

because the output of the stage is DC? Well, the answer is that the output is not exactly DC.

5.5 Antenna

The most straightforward option for the receiving antenna is to use an existing antenna

that can be obtained commercially. This idea was explored along with fabricating a new

antenna. As can be seen from Figures 3.1 and 3.2, there is a coaxial connector to connect to

the antenna. For the initial research, a quarter-wave whip antenna was used for all the testing

purposes. This antenna is similar to that used on car radios. It is called a quarter-wave

antenna because it is designed so that its length is approximately one quarter of the

wavelength of the signal.

This means that for a 915MHz signal, with a wavelength equal 32cm, a quarter-wave

antenna would have an 8cm length. The main dilemma in using this type of an antenna is that

it requires a rather large ground plane in order to work properly. This is fine for car radios

that can be grounded to the frame of the car. But, for this project, the ground plane needed to

28

receive enough of a signal to power the charging circuit is larger than the form factors of the

charging stands chosen to house the circuits. A picture of the quarter-wave whip antenna is

shown in Figure 5.8.

The large copper plate is the ground plane. The antenna is attached to

the copper, with an SMA connector on the underside of the ground plane.

This type of connector uses a simple screw mechanism allowing for easy

connectivity with other circuits and test equipment. The cord is connected on

the other side to the BNC connector of the board. As you can see, this ground

plane is rather large, too large to be used inside the stand for a cellular

phone. It covers almost 50% more area than the stands that were selected

for this research.

Figure 5.8: Quarter-wave Whip Antenna

With this in mind, a different type of antenna needs to be researched

and tested. Other types of antennas to consider are patches, micro strips,

dipoles, and monopoles. The patch antenna has two major problems when

being used with a research project like this. The first is that it also needs to

29

be relatively large, on the order of the ground plane for the quarter-wave

whip antenna. The second reason is that it is highly directional, meaning that

it only radiates, and accepts radiation, in one direction, i.e., it does not have

a good coverage area. These reasons rule out this option.

A micro strip antenna can be any type of antenna discussed previously,

but what makes it unique is that it is “painted” on to a surface so that it is in

the same plane as the printed circuit board. This type of antenna is used

mostly on small surfaces such as silicon die to be used by the circuit on the

same die. By “painted” on, what is meant is that on a silicon die it is etched

onto the surface, or on a printed circuit board, it is part of a conductive layer.

This means that it can be patch, a dipole, or a quarter-wave whip, as long as

all the metal is in the same plane. The main problems with this antenna are

its gain and its directionality. These types of antennas are appropriate to be

used in RFID, but for this project they would be a hindrance.

5.6 Power cube

Both the Home & Office and Portable Power mat come with what is called a Power

cube. The Power cube is basically a universal receiver that gets its power wirelessly, but has a

wired mini-USB charger on the end and comes with 7 other adapters that adapt from mini-

USB to other form factors for charging.

30

Figure 5.9: Power Cube

Power mat claims that this adapter can charge 1,000s of devices including Kindles,

eReaders, digital Kodak cameras, Bluetooth headsets, handheld game players, many cell

phone brands and models and more. They go on to say that the Power cube replaces all of

those individual power adapters, but the kicker here is that you can only charge one device at

a time with the Power cube[15].

5.7 Advantages

1. No tangled wires –

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Figure 5.10: Tangled wires

Power mat is a revolutionary way to charge all your favorite devices from one power source

without the tangled mess of wires. It is fast, efficient, and safe solution, using a combination

of radio frequency identification (RFID) and magnetic induction.

2. One charger for multiple devices –

Figure 5.11: multiple devices on one charger

Power mat delivers real-time, wireless charging to multiple electronics including

mobile phones, music players, handheld games, electronic readers, GPS devices, Bluetooth

headsets, notebooks and laptops.

3. Saving Energy –

Power mat draws less power in standby mode than the vast majority of chargers

for handheld electronic devices.

CONCLUSION

This seminar report demonstrates a method of using the power of the microwave to

charge the mobile phones without the use of wired chargers. Thus this method provides great

32

advantage to the mobile phone users to carry their phones anywhere even if the place is devoid

of facilities for charging. Use of the Rectenna and a sensor in a mobile phone could provide a

new dimension in the revelation of mobile phone. In this modern generation where we prefer

the most efficient gadgets to serve our purposes, not even a slightly deviated device is

acceptable. The highly accomplished cell phone sensors created by the top-notch

manufacturers in the industry befit your needs exactly the best way and proves to be highly

effective tools to combat security breach. Depending on the features they offer, these are

available in different price ranges. We can see that this wireless charging technology is the

way of the future. We can envision a future where furniture and kitchen counters have these

'pads' built in and you can just place your phone or toaster on the counter and it just

magically works without any plugs. Without a doubt the Power mat is ahead of its time and

is a conversation starter to say the least.

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33

[2] Nikola Tesla, “The Transmission Of Electrical Energy Without Wires As A Means

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[4] W. C. Brown., "The History of Power Transmission by Radio Waves". Microwave

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[8] L.W. Epp, A.R. Khan, H.K. Smith, and R.P. Smith, “A Compact Dual-Polarized

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[9] T-WYoo and K. Chang, “Theoretical And Experimental Development Of 10 GHz

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[10] P. Koert and J.T. Cha, “35 GHz Rectenna Development,” in Proc. 1st Annu. Wireless

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[11] Brown, W.C, “The History of the Development of the Rectenna” Proc. Of SPS

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[12] J.C. Lin, “Biological Aspects Of Mobile Communication Fields,” Wireless Networks,

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[13] M. Onda,M. Fujita, Y. Fujino, N. Kaya, K. Tomita, andMYamada, “A Stratospheric

Stationary Ltaplatform Concept And Ground-To-Vehicle Microwave Power

Transmission Tests,” in 37th AIAA Aerospace Sciences Meeting and Exhibit, Reno,

NV, 1999, pp. 1-7.

[14] P. Koert and J.T. Cha, “35 Ghz Rectenna Development,” in Proc.1st Annu. Wireless

Power Transmission Conf., San Antonio, TX, 1993, pp. 457-466.

[15] T.-W. Yoo, “Experimental And Theoretical Study On 35 Ghz RF-To-DC Power

Conversion Receiver For Millimeter-Wave Beamed Power Transmission,” Ph.D.

dissertation, Dept. of Electrical Engineering, Texas A&M Univ., Dec. 1993.

35