Wireless Power Cutting the cord in today’s mobile world

Contents

Introduction……………………………………………………………………………………....

A Brief History of Wireless Energy Transfer………………………………………………….

Wireless Energy Transfer Techniques………………………………………………………..

Transformers and Induction……………………………………………………………....

Radio Coupling…………………………………………………………………………….

Electro-dynamic Induction………………………………………………………………..

Regulatory Issues……………………………………………………………………………….

Communication…………………………………………………………………………….

Industrial, Scientific, and Medical (ISM) Bands, and ISM Equipment………………..

Vehicle Standards Development…………………………………………………………

Modular Approval………………………………………………………………………….

Testing Methods…………………………………………………………………………..........

Radiated Power……………………………………………………………………………

Radiated Spurious Emissions and Harmonics…………………………………………

AC Line-Conducted Emissions…………………………………………………………..

Frequency Stability………………………………………………………………………..

Human RF Exposure………………………………………………………………………

FCC Inquiry…………………………………………………………………………………

Methods of Demonstrating RF Exposure Compliance……………………………….

Measurement……………………………………………………………………………..

SAR Testing………………………………………………………………………………

Calculation………………………………………………………………………………..

Mathematical Modeling………………………………………………………………….

Conclusion……………………………………………………………………………………..

About Intertek…………………………………………………………………………………..

Introduction

With the proliferation of battery-operated portable electronic devices in today’s society, consumer demand for wireless power solutions is growing at a rapid pace. Consumers are looking for products which offer convenience and ease of use. Eliminating the power cord can reduce the hassle of cable tangles, wire routing or wear issues in systems with moving subsystems, and can offer improved marketability and flexibility in form factor by removing the need for an input cable or charging terminal on the device exterior. Wireless charging can also offer improved user safety by eliminating the possibility of electric shock from devices which are used near water. In applications where a power cable is undesirable or impractical, wireless power transfer techniques can provide innovative solutions for extending the battery lifetime of devices which would otherwise be inaccessible.

A Brief History of Wireless Energy Transfer

The concept of wireless energy transfer is not a new one and is as old as the field of electromagnetism itself. Wireless energy transfer was actually demonstrated before Marconi proved that radio communication was possible in 1895. As early as 1820, Andre-Marie Ampere demonstrated that a current in a wire produces a magnetic field, and in 1831 Michael Faraday

showed that a time-varying magnetic flux induces a current on a wire. The transformer, developed in 1836 by Nicholas Callan initially used these concepts to step up the voltage from batteries, but was an early form of wireless energy transfer. Perhaps the most famous historical example of wireless energy transfer is Nikola Tesla’s demonstration in 1893 at the Chicago World’s Fair, where light bulbs were lit wirelessly with high frequency ambient electric fields. Medical Implants have been using wireless charging systems since the 1960s. Another common example of wireless charging systems is the electric toothbrush, which has been charged wirelessly since the 1990s. More recently, wireless charging mats and other wireless charging accessories for cell phones and related portable devices have appeared, and this technology will soon be a standard feature in many cell phone, laptop, and tablet computer models.

Wireless Energy Transfer Techniques

Transformers and Induction Perhaps the most familiar example of a wireless energy transfer system is a transformer. In a transformer, a coil of wire with alternating current generates a time-varying magnetic flux, which couples into an adjacent coil of wire and generates a corresponding current on the secondary coil through magnetic induction. The coils are often wound around a ferromagnetic core or may only be separated by an air gap. Use of a core concentrates the bulk of the magnetic field in the core and provides an efficient “bridge” between the two coils, improving transformer efficiency. However, ferrite cores add significant weight to devices, and a solid core between the two coils makes it difficult to remove one when desired. In a wireless inductive charging system, the primary coil resides in the charging device, and the secondary coil is located in the portable device. Therefore it is necessary to use either an air gap transformer, or a split-core transformer with an air gap between the two cores to improve the efficiency. When the secondary coil is brought in close proximity to the primary coil, the transformer is formed and energy transfer occurs. This method of wireless energy transfer can be fairly efficient as the majority of the magnetic flux resides in the core of the transformer, and therefore losses due to leakage fields are low except at the air gap between the two cores. Air gap transformers are not as efficient as magnetic core transformers due to interaction of the magnetic field with nearby objects, dissipating additional power and thus reducing efficiency.

This interaction is a source of additional heat in the nearby boards as the induced currents in the circuitry dissipate resistively, and can impact thermal design considerations. Due to the need for a small air gap even in split core transformers, the efficiency is lower than more traditional transformer designs, and as the air gap increases the effectiveness of the coupling is decreased significantly. For these reasons this method is only useful for energy transfer over short distances. Air gap transformers which do not use magnetic cores are generally more effective over larger distances, but performance will suffer as the coils are separated, and losses due to interaction with nearby objects are larger since the field is not concentrated in a core. Therefore the potential for heating nearby boards increases correspondingly. Use of coils with small diameters concentrates the power but requires more precise coupling between them. In contrast, larger coils will transfer some energy even if they are not optimally aligned, but suffer from the aforementioned increased interaction with the host device and other nearby objects.

Radio Coupling Radio communication utilizes energy transfer to convey information, by producing a time-varying electromagnetic field at a specific frequency which is modulated in some fashion, and which radiates outward from the source and induces a corresponding current in nearby receiving antennas. In theory, this induced current can be used to trickle charge a battery or power a product directly if the device power requirements are not large. Depending upon the antenna type used, the radiation pattern can either be directional or isotropic. Radio coupling methods suffer from much more inefficiency than transformers as the electromagnetic field strength drops rapidly as distance increases from the source. Unless extremely directional antennas are used, much of the energy is radiated into free space rather than at the intended recipient. Directional antennas can improve this, but achieving optimum coupling at large distances with two directional antennas can be difficult. The radio coupling method has the benefit of allowing multiple devices to be powered from the same source simultaneously - if the device power requirements are low enough – and also allows devices to be charged at longer distances than transformers, including applications such as charging “hot spots” or room-wide charging fields. Isotropic

receiving antennas can be used to allow coupling at multiple angles which is an important consideration for portable devices.

Electro-dynamic Induction Electro-dynamic Induction is a variation on transformer design which uses tuned resonant coils to transfer energy from the primary to the secondary coil at high efficiency over short distances (usually less than a wavelength of the frequency being used). A capacitor is used to form an LRC oscillator that rings at the frequency used. When the primary coil is driven, a large amount

of energy is stored in the capacitor and coil which dissipates slowly. Since both coils resonate at the same frequency the coupling efficiency between the primary and secondary coils is high, and while losses through distance do occur, a large amount of energy can still be transferred. Electro-dynamic induction is especially suitable for high power applications, where large charging currents are needed. In these situations the high power stored in the coil can be significantly dissipated by dielectric and resistive losses in the coil windings and cores, so air-core designs may be beneficial.

Regulatory Issues

Wireless charging systems have a unique set of regulatory considerations. As devices which intentionally generate and radiate energy, they are similar to transmitters, but since they do not always communicate, they often fall under different rules. Due to the fact that wireless charging systems need to be of a much higher power than most transmitters, issues such as Human RF exposure become even more important. Systems which radiate high power also run a higher risk of interfering with adjacent devices. Testing which quantifies the radiated output power and harmonic emissions, as well as any other unwanted emissions, is generally required for most inductive charging systems.

Communication One important decision when implementing a wireless energy transfer system is whether the system will include some form of communication, and if so, how it will be implemented. It can be desirable to have functionality within the charging system that allows the system to behave intelligently; i.e. reporting battery levels back to the charger, managing charging rates and trickle charging or deep cycle battery recharging modes in real time to optimize charge rates and battery performance. Methods of communication include typical radio transmitter frequency, amplitude, and phase modulation methods, as well as on-off keying, and even near-field load modulation. Intentional radiators which convey information are considered to be radio communications devices, regardless of the method of communication used. This includes even simple communications methods such as keying the transmitter in a certain pattern or variations in resistance of load circuits. The FCC and other regulatory agencies require radio communications devices to be certified under the radio transmitter standards, so implementing communication in a wireless charging system can sometimes lead to extremely low regulatory limits on radiated power as the ISM bands with unlimited power cannot be used. For this reason it is often better to include a second radio which performs the communications function, while the charging system simply uses a high power carrier without conveying any information. Another alternative is to implement a switched mode system which operates without modulation at high power for charging, but then switches regularly into a communications mode at a lower power that meets the applicable radio transmitter requirements. This second alternative method suffers from charging being interrupted regularly to perform communication, and leads to a more complex design which may not be optimal for either function.

Industrial, Scientific, and Medical (ISM) Bands, and ISM Equipment Most regulatory authorities consider equipment which is designed to intentionally emit RF energy to perform a task other than telecommunications or information technology to be consumer ISM equipment. Since charging systems intentionally emit RF energy to provide power to another device, rather than to communicate, these devices can be approved using the rules contained in the following standards:

United States (FCC): US Code of Federal Regulations (CFR) 47 Part 18

Canada (Industry Canada): Interference Causing Equipment Standard ICES-001

European Union (Various Regulators): CENELEC EN 55011, Group 2 limits

IEEE CB Scheme (International): Cispr 11, Group 2 limits In order to facilitate certain activities that require high output power, spectrum regulators have allocated a set of special high-power or unlimited-power frequency bands set aside primarily for use by industrial, scientific, and medical (ISM), but also for domestic equipment which emits and uses RF energy to perform a task or affect a material. It is not necessary to use these bands, but non-ISM bands are subject to much lower output power limits to prevent interference with primary services such as licensed radio and government frequency allocations. Practically speaking, the limiting factor on output power for devices using ISM bands with unlimited power will be the RF exposure considerations inherent to the application. The ISM bands are structured such that harmonics of lower frequency ISM bands will fall into high frequency ISM bands, where possible. Some lists of the allowed ISM bands in various standards are shown below.

ISM Band FCC Part 18 Power Limits

EN 55011, ICES-001, and Cispr 11 Group 2 Power Limits

6.78 MHz ±15.0 kHz Unlimited Under Consideration

13.56 MHz ±7.0 kHz Unlimited Unlimited

27.12 MHz ±163.0 kHz Unlimited Unlimited

40.68 MHz ±20.0 kHz Unlimited Unlimited

433.92 MHz ±870.0 kHz Not allocated Under Consideration

915 MHz ±13.0 MHz Unlimited Unlimited

2450.0 MHz ±50.0 MHz Unlimited Unlimited

5,800.0 MHz ±75.0 MHz Unlimited Unlimited

24,125.0 MHz ±125.0 MHz Unlimited Unlimited

61.25 GHz ±250.0 MHz Unlimited Under Consideration

122.5 GHz ±500.0 MHz Unlimited Under Consideration

245.00 GHz ±1.0 GHz Unlimited Under Consideration

Vehicle Standards Development Currently IEC subcommittee TC69 is working on developing a new standard for Electric Vehicle wireless power transfer systems, which is intended to be published as IEC 61980. This document is still under development and is not yet available for use. The position of the TC69 committee is similar to that described above, in that wireless energy transfer systems that contain communication elements would be subject to the applicable radio communications standards, while other systems would simply need to meet the applicable non-radio EMC requirements.

Modular Approval Currently, there are no procedures for modular approval of a wireless charging system as there are for radio communications transmitters. However, as wireless charging systems become more and more common, and as the technologies become more well-known, demand for modular solutions and application volume will likely drive the need for a regulatory process to approve modular wireless charging systems which can be placed in multiple devices without extensive additional testing. Due to the nature of the rules, wireless power systems which utilize communication in all modes – and are therefore considered to be communications devices – could potentially be authorized as modules under the modular approval rules that already exist for various regulatory domains.

Radiated Power Radiated Power Testing is the evaluation of the output power at the desired operating frequency of the charging system. Radiated power must be measured and compared to any applicable limits. This often takes the form of a field strength measurement at a specified limit distance. Often limits are extrapolated to a 10m distance due to test site size considerations. The roll-off of the intended signal can become an important factor in this measurement, and it can be useful to quantify the power roll-off using measurements at several distances, rather than using the theoretical adjustment factors which may not always accurately represent the behavior of a particular antenna or coil configuration.

Radiated Spurious Emissions and Harmonics Similar to the radiated power testing, the field strength of harmonics and other emissions from the system in the wireless charging mode are measured on a test site and compared to the

applicable limits. The wireless charging mode and any associated circuitry that would be active in that mode are tested, while other portions of the digital circuitry (including other radios and system functions not related to charging) must be electrically connected but do not need to be active. For instance, a radio board must be present and electrically connected, but need not actually be transmitting. Likewise, a motor or other function can be present but off during this test.

AC Line-Conducted Emissions This test is typically applicable to the charging side of the wireless charging system, where the connection to the AC mains is located. It quantifies the unintended emissions generated at the AC power input of the charger.

Frequency Stability Some applications, such as coupling which includes communication, will be required to demonstrate frequency stability. It is also important to ensure that any device using the ISM band will stay within the assigned tolerances for the unlimited power bands. This test demonstrates that the operating frequency of the wireless charging system will not drift significantly over reasonable variations of voltage and temperature.

Human RF Exposure Often the most important regulatory consideration related to a wireless charging system becomes the question of Human RF Exposure. The high power nature of these technologies – factoring in the use in portable device configurations, the proximity of transmitting coils to the human body, and the potential for room-wide fields in some situations – requires closer scrutiny. Extensive studies of RF exposure since the 1950s have established that there are no reproducible low-level (non-thermal) effects due to non-ionizing RF radiation such as radio emissions, and the consensus is that there are no theoretical mechanisms for such effects. The primary threat from RF exposure is related to tissue heating due to RF energy absorption. Microwave ovens are a common example of a device which uses RF thermal effects for a beneficial purpose.

FCC Inquiry Due in part to the sudden popularity and proliferation of wireless power systems, the FCC in particular has become especially concerned about controlling the RF exposure impact of these types of devices. Previously, devices operating under part 18 were categorically excluded from RF exposure evaluations. However this has changed and the FCC recently issued Knowledge Database (KDB) article #680106 which now requires an inquiry to be submitted to the FCC lab in order to confirm categorization of the device under part 18 when no communication is present, and to address how RF exposure compliance will be demonstrated for the particular application. This

KDB article also contains further guidance on the FCC’s approach to wireless charging systems of various types. The FCC inquiry must contain the following information:

i. In the "Subject" line, fill the field as follows: Seeking guidance for wireless chargers; ii. complete product description; iii. the rule part(s) the device will operate in and the reasoning for rule part(s); iv. planned equipment authorization procedure; v. drawings, illustrations; vi. frequencies; vii. radiated power; viii. operating configurations; ix. conditions for human exposure [1], and x. operating configurations for different charging devices.

Methods of Demonstrating RF Exposure Compliance RF exposure levels can be determined in many ways, including various types of testing, calculation, or modeling, some of which are discussed here. They are based on average values of power, with an averaging time of 6 minutes for general population applications, and 30 minutes for occupational exposure where personnel are trained on RF exposure and RF exposure mitigation techniques.

Measurement Measurements of RF exposure can take several forms. Determination of the actual radiated power using standard emissions measurement techniques can be used to show exemption from RF exposure compliance if the power is low enough, or can be used as a starting point for calculations of RF exposure at typical distances from users. The resultant calculated power densities are then compared with the limits for exposure. SAR Testing SAR testing uses a dielectric solution containing salt, sugar and other constituents to fill a human body “phantom” to determine the amount of power radiated by a transmitter in mW/g of tissue. A probe on a robotic arm maps the electric field, which is converted to a power density chart, similar to a topographical map. SAR testing is the preferred RF exposure compliance test technique and is a highly accurate way to characterize actual RF exposure from devices. However, well developed SAR test methods are not currently available below 150 MHz, where many magnetic systems are designed to operate. Therefore another approach is often required for these devices, such as

calculation or mathematical modeling.

Calculation Calculations of the potential RF exposure from a wireless power system can be made using information such as coil diameter, number of coil turns, coil shape, frequency of operation, and the coil current for both coils. The effects of any cores used on the magnetic field intensity due to changes between the magnetic permeability of free space and that of the material used should be taken into consideration. Antenna power and gain are used for radio coupling systems. Conservative estimates should be used which overestimate the potential for exposure. If calculations do not show compliance with the applicable RF exposure limits, further evaluation in the form of SAR testing, if possible, or mathematical modeling are often required.

Mathematical Modeling Mathematical modeling uses 3D software to analyze the fields generated by the wireless power system and its associated circuitry, and to predict power densities around the device. For magnetic devices which are not exempt due to low output power measurements or calculations, and which cannot be tested using SAR techniques due to low frequency, mathematical modeling is often the only remaining option for demonstrating compliance. For medical implant devices used within the human body, mathematical modeling is required as even SAR test methods do not adequately represent the RF exposure potential of the implant with the surrounding tissue.

Conclusion

Wireless energy transfer in the form of inductive charging, radio reception, or resonant electro-dynamic induction is a powerful tool that can be used to improve the convenience, safety, and usefulness of products. While there are some tradeoffs in the form of energy efficiency, product complexity, human RF exposure issues, and additional regulatory approval, for many portable products it is the way of the future. In this paper, Intertek provides an overview of the considerations and regulatory requirements involved in selecting, implementing, and bringing a wireless charging system design to market. Of course, it is not possible to address every detail related to wireless charging systems in this white paper, or to provide information on the requirements of every country where your products will be sold. For more information, contact Intertek’s experts on wireless charging systems for more details on other regulations or test procedures and regulatory inquiries and approvals. Our team can help you to wade through the murky world of compliance for these cutting edge technologies. With hundreds of labs in many countries around the world, including 20 in the U.S. alone, Intertek can provide the answers you need – often within 24 hours.

About Intertek

Intertek is a leading provider of quality and safety solutions serving a wide range of industries around the world. From auditing and inspection, to testing, quality assurance and certification, Intertek people are dedicated to adding value to customers' products and processes, supporting their success in the global marketplace. Intertek has the expertise, resources and global reach to support its customers through its network of more than 1,000 laboratories and offices and over 30,000 people in more than 100 countries around the world. Intertek Group plc (ITRK) is listed on the London Stock Exchange in the FTSE 100 index.

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