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 timevarying 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
6.78 MHz ±15.0 kHz
13.56 MHz ±7.0 kHz
27.12 MHz ±163.0 kHz
40.68 MHz ±20.0 kHz
433.92 MHz ±870.0 kHz
915 MHz ±13.0 MHz
2450.0 MHz ±50.0 MHz
5,800.0 MHz ±75.0 MHz
24,125.0 MHz ±125.0 MHz
61.25 GHz ±250.0 MHz
122.5 GHz ±500.0 MHz
245.00 GHz ±1.0 GHz
FCC Part 18
Power Limits
EN 55011, ICES-001, and
Cispr 11 Group 2 Power Limits
Unlimited
Unlimited
Unlimited
Unlimited
Not allocated
Unlimited
Unlimited
Unlimited
Unlimited
Unlimited
Unlimited
Unlimited
Under Consideration
Unlimited
Unlimited
Unlimited
Under Consideration
Unlimited
Unlimited
Unlimited
Unlimited
Under Consideration
Under Consideration
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.
ii.
iii.
iv.
v.
vi.
vii.
viii.
ix.
x.
In the "Subject" line, fill the field as follows: Seeking guidance for wireless chargers;
complete product description;
the rule part(s) the device will operate in and the reasoning for rule part(s);
planned equipment authorization procedure;
drawings, illustrations;
frequencies;
radiated power;
operating configurations;
conditions for human exposure [1], and
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 electrodynamic 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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