Wireless Power

Wireless power ing capacitive coupling between electrodes.[5][8] Applica- tions of this type are electric toothbrush chargers, RFID tags, smartcards, and chargers for implantable medical devices like artificial cardiac pacemakers, and inductive powering or charging of electric vehicles like trains or buses.[9][11] A current focus is to develop wireless sys- tems to charge mobile and handheld computing devices such as cellphones, digital music player and portable com- puters without being tethered to a wall plug. In radiative or far-field techniques, also called power beaming, power is transmitted by beams of electromagnetic radiation, like microwaves or laser beams. These techniques can trans- port energy longer distances but must be aimed at the receiver. Proposed applications for this type are solar power satellites, and wireless powered drone aircraft.[9] An important issue associated with all wireless power sys- tems is limiting the exposure of people and other living things to potentially injurious electromagnetic fields (see Electromagnetic radiation and health).[9] 1 Overview Antennas or Inductive charging pad for LG smartphone, using the Qi (pro- Coupling Devices nounced 'Chi') system, an example of near-field wireless transfer. When the phone is set on the pad, a coil in the pad creates a magnetic field which induces a current in another coil, in the Vs phone, charging its battery. Power Source Transmitter Receiver Load Wireless power transfer (WPT)[1] or wireless energy transmission is the transmission of electrical power from Generic block diagram of a wireless power system a power source to a consuming device without using solid wires or conductors.[2][3][4][5] It is a generic term “Wireless power transmission” is a collective term that that refers to a number of different power transmis- refers to a number of different technologies for trans- sion technologies that use time-varying electromagnetic mitting power by means of time-varying electromagnetic fields.[1][5][6][7] Wireless transmission is useful to power fields.[1][5][8] The technologies, listed in the table below, electrical devices in cases where interconnecting wires differ in the distance over which they can transmit power are inconvenient, hazardous, or are not possible. In wire- efficiently, whether the transmitter must be aimed (di- less power transfer, a transmitter device connected to rected) at the receiver, and in the type of electromagnetic a power source, such as the mains power line, trans- energy they use: time varying electric fields, magnetic mits power by electromagnetic fields across an interven- fields, radio waves, microwaves, or infrared or visible ing space to one or more receiver devices, where it is con- light waves.[8] [1] verted back to electric power and utilized. In general a wireless power system consists of a “transmit- Wireless power techniques fall into two categories, non- ter” device connected to a source of power such as mains radiative and radiative.[1][6][8][9][10] In near-field or non- power lines, which converts the power to a time-varying radiative techniques, power is transferred over short dis- electromagnetic field, and one or more “receiver” devices tances by magnetic fields using inductive coupling be- which receive the power and convert it back to DC or tween coils of wire or in a few devices by electric fields us- AC electric power which is consumed by an electrical 1 2 2 FIELD REGIONS load.[1][8] In the transmitter the input power is converted technologies are used for transmitting power: to an oscillating electromagnetic field by some type of "antenna" device. The word “antenna” is used loosely • Near-field or nonradiative region - This means here; it may be a coil of wire which generates a magnetic the area within about 1 wavelength (λ) of the field, a metal plate which generates an electric field, an antenna.[1][4][10] In this region the oscillating electric antenna which radiates radio waves, or a laser which gen- and magnetic fields are separate[6] and power can erates light. A similar antenna or coupling device in the be transferred via electric fields by capacitive cou- receiver converts the oscillating fields to an electric cur- pling (electrostatic induction) between metal elec- rent. An important parameter which determines the type trodes, or via magnetic fields by inductive cou- of waves is the frequency f in hertz of the oscillations. pling (electromagnetic induction) between coils of The frequency determines the wavelength λ = c/f of the wire.[5][6][8][9] These fields are not radiative,[10] waves which carry the energy across the gap, where c is meaning the energy stays within a short distance the velocity of light. of the transmitter.[18] If there is no receiving de- Wireless power uses much of the same fields and waves vice or absorbing material within their limited range [18] as wireless communication devices like radio,[6][12] an- to “couple” to, no power leaves the transmitter. other familiar technology which involves power trans- The range of these fields is short, and depends on mitted without wires by electromagnetic fields, used in the size and shape of the “antenna” devices, which cellphones, radio and television broadcasting, and WiFi. are usually coils of wire. The fields, and thus In radio communication the goal is the transmission of the power transmitted, decrease exponentially with [4][17][19] information, so the amount of power reaching the re- distance, so if the distance between the two ceiver is unimportant as long as it is enough that the signal “antennas” Dᵣₐₑ is much larger than the diameter to noise ratio is high enough that the information can of the “antennas” Dₐ very little power will be re- be received intelligibly.[5][6][12] In wireless communica- ceived. Therefore these techniques cannot be used tion technologies generally only tiny amounts of power for long distance power transmission. reach the receiver. By contrast, in wireless power, the amount of power received is the important thing, so the Resonance, such as resonant inductive efficiency (fraction of transmitted power that is received) coupling, can increase the coupling be- is the more significant parameter.[5] For this reason wire- tween the antennas greatly, allowing ef- less power technologies are more limited by distance than ficient transmission at somewhat greater wireless communication technologies. distances,[1][4][6][9][20][21] although the fields still decrease exponentially. Therefore the These are the different wireless power range of near-field devices is conventionally technologies:[1][8][9][13][14] devided into two categories: • Short range - up to about one antenna diameter: Dᵣₐₑ ≤ Dₐ.[18][20][22] This is 2 Field regions the range over which ordinary nonreso- nant capacitive or inductive coupling can Electric and magnetic fields are created by charged parti- transfer practical amounts of power. cles in matter such as electrons. A stationary charge cre- • Mid-range - up to 10 times the antenna ates an electrostatic field in the space around it. A steady diameter: Dᵣₐₑ ≤ 10 Dₐ.[20][21][22][23] current of charges (direct current, DC) creates a static This is the range over which resonant ca- magnetic field around it. These fields contain energy. pacitive or inductive coupling can trans- The above fields cannot carry power because they are fer practical amounts of power. static, but time-varying fields can.[16] Accelerating electric charges, such as are found in an alternating current • Far-field or radiative region - Beyond about 1 (AC) of electrons in a wire, create time-varying electric wavelength (λ) of the antenna, the electric and mag- and magnetic fields in the space around them. These fields netic fields are perpendicular to each other and prop- can exert oscillating forces on the electrons in a receiving agate as an electromagnetic wave; examples are “antenna”, causing them to move back and forth. These radio waves, microwaves, or light waves.[1][4][9] This represent alternating current which can be used to power part of the energy is radiative,[10] meaning it leaves a load. the antenna whether or not there is a receiver to ab- The oscillating electric and magnetic fields surrounding sorb it. The portion of energy which does not strike moving electric charges in an antenna device can be di- the receiving antenna is dissipated and lost to the vided into two regions, depending on distance Dᵣₐₑ from system. The amount of power emitted as electro- the antenna.[1][4][6][8][9][10][17] The boundary between the magnetic waves by an antenna depends on the ratio regions is somewhat vaguely defined.[8] The fields have of the antenna’s size Dₐ to the wavelength of the different characteristics in these regions, and different waves λ,[24] which is determined by the frequency: 3.2 Capacitive coupling 3 λ = c/f. At low frequencies f where the antenna is much smaller than the size of the waves, Dₐ << λ, very little power is radiated. Therefore the B near-field devices above, which use lower frequencies, radiate almost none of their energy as electro- Vs magnetic radiation. Antennas about the same size Power Oscillator L1 L2 Rectifier Load as the wavelength Dₐ ≈ λ such as monopole or Source dipole antennas, radiate power efficiently, but the electromagnetic waves are radiated in all directions Generic block diagram of an inductive wireless power system. (omnidirectionally), so if the receiving antenna is far away, only a small amount of the radiation will hit it.[10][20] Therefore these can be used for short range, by an electric current to induce a current in a second con- inefficient power transmission but not for long range ductor. This effect occurs in the electromagnetic near transmission.[25] field, with the secondary in close proximity to the pri- mary.

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