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; 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. 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 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 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 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 wheth er or not this idea i dea 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] [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 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 i mproving display methods on phones with large scr eens. 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 s o 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 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 3
technology to work effectively. Today’s Smartphone screens are larger and can make use of the technology. 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 technol ogical development that is of most interest to many of today’s handset makers[2]. 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). (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 s o 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 batt ery 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 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]. 6
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 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 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. 7
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 magnetic field. field. So that is why why they they are are called electromagn electromagnetic etic waves. waves. The 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 centimeters (about twelve inches). inches). In a microwave 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 good for transmitting information from one place place to another because microwave energy can penetrate haze, haze, light rain and snow, clouds, and 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
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Here we are going to use the S band of the Microwave Mi crowave Spectrum. Designation Designati on
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
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 licensefree 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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3.3 Block diagram
Magnetron
Circulator
S L O T T E D
Rectenna
Waveguide W.G.
Sensor
A N T E N N A
Mobile phone
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, 11
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] [9].
3.4.2 Receiver
The receiver’s main purpose is to charge a battery. A simple battery charging theor y is to run current through the battery, and apply a voltage difference between the terminals of the battery to reverse r everse the chemical process. By doing so, it recharges the battery. There are a re 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 – 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 t he 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 output required required in radar radar equip equipment. ment.
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.
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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 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 parallel with regard regard to the output. output. The cavities cavities control control the output output frequency 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 particularly under under return bombardmen bombardmentt by the electrons. electrons. This is because because most of the output output power is provided provided by the large large number number of electrons electrons that are are emitted emitted when when high-v high-velocity elocity electrons electrons return to strike the cathode. The cathode is indirectly heated and is constructed of a highemission 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 current induced induced in the antenna 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 possible with with solar solar cells. cells. This This type type of device is called called an optical optical rectenna. rectenna.
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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 circuitr circuitry y 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, rectenna, the sensor plays plays an important important role. The whole setup looks looks something 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.
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4.4 Schottky barrier diode
A Schottky barrier diode is different from a common P/N silicon diode. The 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
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 ca rried out in a cell phone within a specified range. It is an easy to t o 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 i n 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 perfe ction 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 phone battery without without wires. The wireless charger will will convert the RF/ RF/ microwave signal at 915 MHz frequency into a DC signal, and then store the power into a battery. In this first fi rst step, ste p, a standard phone is used, us ed, 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.
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, 21
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. 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 wirelessl y.
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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. 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 23
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 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. 24
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.
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, 25
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.
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, 26
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 half -wave rectifier, rect ifier, but the circuit itself is much more complicated with the introduction of a transformer. This ess entially 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.
Figure 5.7: Voltage Double Schematic
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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, 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 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 t he 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 t he 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. 28
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 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 suc h 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 t his project they would be a hindrance.
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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 r eceiver that gets its power wirelessly, but has a wired mini-USB charger on the end and comes with 7 other adapters that adapt from miniUSB to other form factors for charging.
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].
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5.7 Advantages
1. No tangled wires – wires –
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 – 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 – Energy –
Power mat draws less power in standby mode than the vast majority of chargers for handheld electronic devices. 31
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 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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