Wireless Power

Welcome to the Planet Analog blog on wireless power. This is the first in a series of articles that will address the quickly-evolving art and science of transmitting useful amounts of power without wires. Yes, we are all aware and still in awe of Nikola Tesla’s experiments ca. 1890. But this dream is finally becoming a reality, and the topic is more relevant than ever.

Tesla, aged 40, 1896

Tesla, aged 40, 1896

Tesla clearly knew back then that he was on to something — and we are now figuring it out in ways that are safe, useful, and cost-effective. There are now more than 50 million mobile phones that use inductive charging. Electric buses in Turin and Genoa, Italy, induction stovetops, and a plethora of other applications exist today as examples of how the technique is proliferating. So what’s changed to enable this relatively sudden growth in the application of wireless power?

The factors driving this change are what we will discuss in this blog. It’s a fascinating mix of materials development, IC design and process capability, consumer demand, corporate visions of world-dominance, and individual innovation. Over the course of this series, we will explore these topics, dive into the nuts and bolts of various techniques, delve into safety concerns, and explore the ever-present political interests that always accompany the creation of new standards.

Let’s start with the demand-side of the equation. The most powerful forces driving wireless power innovation and adoption are associated with the limitations of mobile-phone battery life and present-day charging techniques. The mobile phone industry even has a name for it: Battery Anxiety .

According to a recent Veloxity study, 84% of mobile phone users in NYC experience the symptoms of battery anxiety. Battery anxiety can be described as that queasy feeling one gets as the phone fuel gauge drops. It results in modified cellphone usage patterns, including decreased web surfing, picture taking, or other data-intensive activities. It also results in decreased revenue for service providers and decreased phone utility.

No wonder battery life is a key selling feature and batteries keep getting bigger. Philips produced a comedic depiction of the disorder here. Typical remedies include anxious searches for a recharge throughout the day, hunts for AC outlets, portable battery packs, and cases with integrated batteries.

Now imagine a world where cellphone charging spots are embedded into your environment wherever you need them to be. A typical day might include moving your phone from your bedside charger to your car charger to your office charger to a train or plane charger to a hotel or restaurant charger… well, you get the idea. In this scenario, phones are not going from full to empty to full again as fast as possible; they are always being topped up. More like energy sipping than gulping. Add to that image the ability of the phone to adapt to the location in which it is charging: A phone placed in a car charging spot will, via NFC, respond to the user’s program and start streaming music, GPS, contact data, and whatever else the user wants available in the car.

Access to these applications and data would be made through safe user interface systems like the in-dash touchscreen, steering wheel controls or voice-command. The same phone placed on a hotel guest room charge spot will immediately stream favorite photos to the bedside digital picture frame, stream video and music to the room’s TV and music system, link to HVAC or room service… Restaurants will offer charging and new services like at-table ordering and payment, loyalty programs, etc.



This vision of ubiquitous wireless charging is driving the investment in numerous startup companies around the globe and is already being realized in cars, hotels, and restaurants. Toyota, Chrysler, Audi, and other automakers now sell models with wireless phone charging. Restaurants and coffee shops around the world are making it available, and we will see other hospitality locations offering wireless charging soon. Major airports around the world (Atlanta, Beijing, Detroit, and Tokyo, for example) now have wireless charging stations, too.

Slowing the adoption of wireless charging are issues common to new technology. There are three competing standards that are not interoperable. Some of the earliest products were, let’s say, best suited to early adopters (they were sometimes quirky or hard to use). Some implementations had technical problems. All the above causes confusion and hesitation with those that would determine large-scale deployments. Because of consumer and commercial demand for wireless charging and its associated benefits, these barriers are being knocked down, and, as I mentioned above, we will discuss these issues in more detail in later posts.

To get us all on even footing, let’s have a look at the basic functionality of a wireless charging system. The following is a generic system, and the functional blocks are common to all modes of wireless charging systems — single or multi-coil, inductive or resonant.

Most of the functional blocks in this diagram are common to the present wireless charging systems. A basic system will have a power stage, a communication stage, and controller engine. Ancillary functions are required to monitor and respond to operational parameters (voltage, current, temperature) and support the communications and control systems (RTC, PLL, etc.).

Note that communications can be in-band (by modulating the load current and demodulating the reflected signal) or out-of-band (through, say, Bluetooth). This data stream is used to communicate authentication, foreign object detection, charge status, and temperature. In this diagram, the receiver is controlling the charge by communicating its needs back to the transmitter.

Some transmitters are designed to connect to a local or wide-area network (Ethernet, automotive bus, etc.) and may have additional connectivity blocks like a network interface function. The transmission frequency can be between 100 MHz and 6.78 MHz, depending on the control function and the standard. Each of these blocks and architectural choices will be the stuff of upcoming articles. Our next article will dive into the specific design challenges and functionality of an inductive wireless charging system.

31 comments on “Wireless Power

  1. Davidled
    June 12, 2014

    Some clients might wonder how much energy will be wasted through air charging. Wire charging process could be more reliable than wireless charging. Wireless charging efficiency could be variable as system grows older, compared with wire charging process. Also, Efficiency would be depending on mechanism of controller (state machine).

  2. timmluca14
    June 13, 2014

    Great learning process for me. It can help me writing just like that.

    ferrari yellow and black race replica jacket

  3. etnapowers
    June 13, 2014

    Agreed, moreover the wireless charging could collect energy from the environment , increasing the overall system efficiency, the wire charging is more reliable but it cannot gather electromagnetic energy in the air.

  4. Davidled
    June 13, 2014

    ->Agreed, moreover the wireless charging could collect energy from the environment , increasing the overall system efficiency, the wire charging is more reliable but it cannot gather electromagnetic energy in the air.

    Energy source charged from the environment could be feed into charging station. In other words, hybrid charging station which is the combination of both wire and wireless could be used, as long as cost is not expensive. Wire charging would be only supplemented to compensate about 20 % energy loss due to the wireless charging station.

  5. Netcrawl
    June 13, 2014

    @Daej, its growth is currently hindered by the lack of common standard ( I believe we dont have) that guarantees interoperability between devices and charging devices from different vendors, the solution available in the market force us into a closed system, this closed system limits the ability of end users to charge their devices only with compatible charging pads. The lack of standard will definitely limit the adoption rate by consumers.     

  6. Netcrawl
    June 13, 2014

    @etnapowers efficiency and low standby power are important aspects for wireless charging, lower efficiency translates into longer chargng time, waste of energy and increased heat dissipation.

    The wireless charging should also contain “some smart” or intelligence or, able to detect or sense presence of a client, if no client it should be switched off.  

  7. Davidled
    June 14, 2014

    Wireless charging station is still on development stage. But, the Society of Automotive Engineer (SAE) J1772 illustrated AC level 1&2 and DC level 1&2 with faster charging time and SOC value.  I think that the requirement of wireless charging pad could be included in the similar category.

  8. JDPerzow
    June 14, 2014

    Yes–that is one of the advantages of modern wireless charging systems. The client lets the transmitter know of it's power needs and the transmitter responds. No client, no power. As power requirements are decreased, the transmiter decreases power. The Qi system uses transmission frequency to decrease power: higher frequency = lower transmitted power. If only a trickle is needed, the system goes into pulse-width modulation. Here is a paper discussing how wireless charging can be as efficient as wired–if slightly different partitioning is used:

  9. JDPerzow
    June 14, 2014

    Interesting–especially since the CE4A, after a 3 year study, made recommendations on in-cab wireless charging. You are probably aware that Toyota, Chrysler, Honda, Mercedes and other auto companies now offer wireless charging. It is a great way to bring smart-phone features like GPS, favorite locations, contacts, etc. into the car in ways that are safe, and without killing your phone battery.

  10. JDPerzow
    June 14, 2014

    The lack of a single standard is really slowing the adoption of wireless charging. The vision is that this is much more than a convenient replacement for the wall wart. There are three groups with three very different motivations for developing a standard, and this is making it difficult for the three groups to cooperate.

  11. etnapowers
    June 16, 2014

    The hybrid charging station is a good solution , I guess that both the costs and the effectiveness of this solution might be considered and, if the evaluation is positive, depending on the particular environment in which the station works, this could be a reliable system.

  12. etnapowers
    June 16, 2014

    “The wireless charging should also contain “some smart” or intelligence or, able to detect or sense presence of a client, if no client it should be switched off.  “


    Good point. The smart sensors require some power as well, a good feature might be a DC emergency storage power source, able to feed the sensors while searching for “clients”.

  13. etnapowers
    June 16, 2014

    DaeJ: you're right, this solution is still under development, I guess that the requirements and the standards will be fully developed once the solution will be at the production stage on the market.

  14. etnapowers
    June 16, 2014

    The wireless charging  technology in car holds promises of great diffusion , provided that it works for all types of devices, some car makers support only a particular brand , I guess that when the technology will be more diffused this issue will be solved.

  15. samicksha
    June 18, 2014

    Initially i was curious to find how exactly this mechanism works but then i realized its simple, an alternating current in the transmitter coil generates a magnetic field, which induces a voltage in the receiver coil. This voltage is then used to charge up the device.

  16. JDPerzow
    June 20, 2014

    Hi Samicksha, Yes, in principle it is simple. But transferring power efficiently, safely and with low EMI is pretty tricky. There is a secure communication path to enable authentication, significant coil and magnetics design, foreign object detection, etc. We'll talk about some of the more recent innovations that have gotten us to this point.

    June 21, 2014


    Nice to have a series on the subject. It would be helpful to cover some application areas, especially medical devices, where people frown upon anything that is wireless due to valid reasons. You may discuss the relevant applicable standards, if possible.

  18. samicksha
    June 23, 2014

    Thank's JP for puting up these points, with these points would also like ask as i understand that inductive charging is low in efficiency so does that impact the device.

  19. JDPerzow
    June 23, 2014

    Hello Samicksha,

    It's true that there are inefficient wireless power systems out there, but some systems have shown to be almost as efficient as a wired system.  We will get into the efficiency factors, losses and measurement techniques. For example, inductive systems with good Q achieve over 80% (power into the load / power into the transmitter). I would be interested to know how you have come to understand that inductive charging is low in power transfer efficiency. What does “low” mean (<50%? for example?).

    As we prepare future articles on this subject, we would like you; all readers of this blog, to weigh in with your questions and areas of interest.

  20. samicksha
    June 24, 2014

    What i understand from my past experiance is that wireless takes more time in charging device than a wired charger may be because of transfer losses through the use of thik coils.

  21. geek
    June 25, 2014

    @John: Great article. I think this is one of the most interesting blogs at PA and a topic that's going to get a lot of attention in the future. As far as wireless power is concerned, are there any health or safety issues surrounding it that people must be made aware of?

  22. geek
    June 25, 2014

    @JDPerzow: What about the normal range that is covered by these wireless devices? How big of an area can each device covere and what's the restriction when it comes to expanding the area?

  23. RedDerek
    June 26, 2014

    Back from a three week vacation, I am catching up on all the discussions. Problems I see with wireless charging that I would like to see are:

    1. Efficiencies

    2. Health considerations, especially for the high-power charge applications. How well is the magnetic field generation kept within the charging device?

    As for standards… I can understand the challenges of putting a standard together. When I worked with the 802.3at development, there seemed to be one or two companies pushing their method. Primarily because of patents in place to sell rights to. I suspect this is the same thing for the wireless standards.

  24. yalanand
    June 30, 2014

    The concept of wireless charging will be really ideal especially when it comes to car charging. The only thing that they should ensure is that it works in all devices. Cars operate differently depending with the brand of the vehicle. The wireless charging process should ensure that it takes this into serious consideration. This will enable drivers to comfortably charge their phones when they are driving and this will be a good boost to technology. All that needs to be done is just energy diffusion.

  25. etnapowers
    July 1, 2014

    @samicksha, that's correct, the voltage of the receiving coil has to be converted by mean of a AC/DC converter to charge finally the device.

  26. JDPerzow
    July 1, 2014


    These are valid and central points. We will get into some of these topics in detail, but I'll address them at a high level one-by-one. But first, a note about releasing a responsible product into the market: All the companies I am aware of working on wireless power products or standards are top-notch and of high integrity. So it goes without saying that, despite design trade-offs that could impact efficiency or safety, any product or standard released will certainly meet regulatory and industry requirements for efficiency and safety.

    1. Efficiencies. Wireless power can be transferred with almoast 100% efficiency (perfect coupling, no circuit or switching losses) all the way to 0% (no received power as described in the article). Distance between the transmitter and receiver, switching speed and the amount of the flux field captured by the receiver are the first-order factors that determine efficiency. Resonant techniques can compensate somewhat for distance. GaN and other semiconductor technologies can compensate for switching losses.  Coil design, coil arrays and magnetic shielding can help improve how much magnetic energy is captured by the receiver over the charging area. We are seeing tightly-coupled systems transmit power with over 80% total efficiency.  For purposes of discussion, we will define efficiency as power into the load divided by the power into the transmitter.

    2. Health considerations. The International Commission Non-Ionizing Radiation Protection (ICNIRP) has set clear limits for safety and modeling techniques exist to enable accurate testing. Some topologies have inherently more EMI than others, but all systems released to the market will certainly operate well below ICNIRP safety limits. Operating frequency, coil design, waveform, distance, power levels and shielding all impact the design trade-offs required to operate below ICNIRP safety limits.  These are typical design considerations for any power system, but different wireless power implementations present different design challenges.

    3. Conflicts of interests in standards development. I too was involved with 802.3at and af and witnessed individual companies vie for control of the I.P. Some standards are created by a single company and result in great benefit to consumers and markets. Some do not. When groups of competitors agree to work together to develop a standard on a level playing field (RAND terms), the result is often reliably good, but there are no guarantees, either. We will focus primarily on circuit design and system challenges, but stark differences exist in the composition and motivation between wireless power standards groups. It will be very interesting to see how this plays out.

  27. lcgallo
    July 16, 2014


    Did you really mean ” The transmission frequency can be between 100 MHz and 6.78 MHz,” ?

    There seem to be information on transmitting energy up tp 30 W, what about receiving it?


  28. JDPerzow
    July 16, 2014

    Hi Icgallo,

    Yes, transmission frequency for Qi is 105 ~ 210 kHz (they control power by modulating the frequency). PMA uses 200 to 300 kHz. The A4WP Rezence standard uses 6.78 MHz.  5W to 7.5W are the typical low-power standards out there today and this power level is great for cell phone charging. The standards groups are working on extensions to their spec to allow for higher power, which will charge tablets and notebooks more quickly. Those devices will indicate how much power they want back to the transmitter. So, for example, one may put a phone and a tablet in a charging area, and the phone will communicate a 5W need, the tablet may communicate a 15W need and the transmitter will provide power as appropriate. The WPC has just demonstrated a 2000 W system (transmitter and receiver) for kitchen appliances. The demo powered a food processor through a 30mm kitchen counter. 


  29. lcgallo
    July 16, 2014

    Hi John,

    The 100 MHz I was asking about is then a typo.

    I see published information about transmitters but none of a 30 W receiver, is the receiver where the state of the art is found?

    In a wireless “set up” is the magnetic field generated and radiated by the Tx coil like the near field of an ordinary AM transmitter antenna?

    The 3KW transmission sounds very interesting, in that demonstration what was the distance Tx – Rx and how close could people be?



  30. JDPerzow
    July 16, 2014

    Ah, yes, the 100 MHz is a typo. The cutting edge is efficiency, not power. Receivers will appear after transmitters are available, so 30W receivers after 30W or greater transmitters are standardized.

    For close-coupled systems, near-field techniques are used. For loosely-coupled systems, the transmitter and receiver coils are tuned to the same frequency to increase the power transfer efficiency over distance (Z-axis).

    Any system that is released to the market will be safe for people to be around, and must meet government and safety standards.  Just like induction cook-tops, people can be as close as they want to the transmitter.


  31. lcgallo
    July 17, 2014

    Hi John,

    Thank you for yor answers.

    I am a student of history of technology and an electrical engineer no longer working for a living.

    Over the years, Tesla has been in my line of interests, particularly because of his concept of wireless transmission of power. Very recently I got quizzed on how to best transmit energy at micro-watt level and also reminded that some individuals, particularly the sick ones, are discomforted already at micro-watt levels.

    From here my interest in bringing my knowledge up to the state of the art.

    While I see the convenience and the possibility of charging a phone wirelessly, I wonder is something more could be done by harvesting, for that purpose, the energy that is already around us.


Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.