PowerbyProxi has been in the wireless power business for just over 9 years (that’s if you don’t count many of our individual backgrounds with the University of Auckland). 9 years is a substantial amount of time as a company, because the industry itself is still very young. Consider that the Qi specification, which continues to make giant strides to becoming the default standard for wireless charging, was only established in 2009. Other standards groups including the Power Matters Alliance (PMA), The WiPower Alliance, which became the Alliance for Wireless Power (A4WP) and then AirFuel) were not formed until 2012.
The point I am trying to make is that we have seen a lot of developments in wireless power and understand the vast challenges in bringing the technology to commercial reality. We have remained successful during this time, partly due to a mix of some very bright engineers, and a focus on working hard to develop real, working solutions for our customers with a deep understanding on fundamentals. No wireless trickery and showy demos that cannot be commercialized here!
Over the past few years we (the collective public) has witnessed the development of a number of technologies which propose the transfer of power over vast distances, via various matters i.e. ultrasound, and RF. Such technologies promise features such as extensive transmission distance and safe power transfer well beyond the extent of traditional inductive or resonant technologies. For those people considering any sort of wireless power solution, I want to talk directly to you. I want to provide you with the means and tools required to be able to help you come to your own conclusions about the suitability of the various approaches.
This toolset takes the form of a series of questions you should be asking yourself and your proposed wireless power technology supplier. These questions are based on our experiences and the challenges we have faced during our 9 years developing a range of industrial and consumer electronics solutions for customers.
Let’s begin with the basics:
Essentially the ICNIRP determine specific guidelines on the health hazards associated with radiation exposure for certain technologies.
Therefore, based on which type of wireless power transfer technology you are considering;
It may not always be easy to track down the performance of some of these technologies vs. these limits, therefore ask the company to provide a technical analysis of their wireless power solution in relation to the ICNRIP guidelines.
If they are unable to provide a technical analysis, then ask why not?
Now for your own consideration….
We find that for most people, the top priorities will include a selection of the following: Power Level (charging speed), Distance, Spatial freedom and Interoperability with existing products. Does that sound about right?
From our experience, Safety, EMC and increasingly Efficiency are non-optional top priorities that block the ability to ship product. Therefore the next question to ask is:
If you have been reassured that safety, efficiency and EMC are not a concern, then that’s fantastic. But just to be sure, its best to double check on this, because it is a notoriously tough balance between power output/transmission distance/EMC performance/safety/efficiency.
..and one more (necessary) check…
If it all checks out, then it shouldn’t be too much hassle for them to let you do some tests yourself, right?
Efficiency is crucial in determining the overall performance of the system. It directly impacts overall charging speed and transmission distance by examining power loss from the power source to the receiving device. Not to mention, it provides a good gauge about associated emissions.
Now be careful here – an age-old trick is to provide efficiency figures for the ‘sweet spot’ of the system, which isn’t characteristic of the whole system. So make sure you ask…
And if they don’t know what the best and worst case scenario is, then find out, why not?!
There is no reason why any of this information should be made unavailable to you, the customer. Furthermore, you should never be denied the opportunity to test the system independently.
By being able to clarify each of these questions, you will be able to understand exactly what the proposed system is capable of and how ‘real’ or commercially viable the technology actually is.
So the next time you are thinking about a wireless power alternative, make sure you don’t get caught up in the hype – do your due diligence and make sure the respective company can back up their assertions with real data.
Blog first published and contributed to Planet Analog
Blog first published and contributed to Planet Analog
Recent innovations have enabled the creation of robust and reliable wireless power systems that can be tailored for use in a wide variety of industrial and consumer settings. There are two key design considerations when building such a system. One is frequency of operation, which we’ve explored before and will again more in-depth in our next piece. The other is coil geometry.
Coil geometry refers to the design of the transmitter coil (or transmitter coils) that create the electromagnetic field for the transfer of power to the receiver. There are two proven and reliable geometries that are available on the market today — coil array and perimeter coil. Each has features that make it well-suited to certain situations.
A coil array is as it sounds — a collection of small coils placed next to each other in a grid fashion across a flat surface. (Often some sort of charging pad.) The key benefit of a coil array system is that each coil can be individually turned on or off by a special detector circuit. When nothing is near a transmitter, all the coils are off. Placing a phone or other device on or near the transmitter creates a slight magnetic disturbance. That then triggers the coils to scan, determine that a valid device is present (not simply a foreign object like keys or coins), and activate the coil beneath the device.
A perimeter coil is made up of a single large transmitter coil that fills a large area with magnetic current or flux when turned on. Perimeter coils fill the air with flux, enabling 3D charging at very low power levels. However, it is less efficient than a coil array. It’s also not as effective for charging multiple devices simultaneously, or charging at high power levels (>2W).
There is generally a trade-off between coil size, performance, and cost. Coil size and cost are inversely related — smaller coils give a better performance, but at a higher cost. This means that a perimeter coil geometry costs less than a coil array for a given charging area.
Beyond cost, the selection of which geometry to use is based on a variety of considerations, including safety, Z-height, efficiency, and power level.
Safety & EMC interference
The key consideration from a safety perspective is the amount of current or flux that’s released or lost during power transfer. Radio frequency (RF) emissions can be dangerous to human health in excessive quantities, while electromagnetic compatibility (EMC) emissions can interfere with other devices. A coil array design allows individual coils to be turned on, which ensures that the receiver coil — the coil on the device being wirelessly charged — almost always covers most of the transmitter coil. The result is that minimal flux is leaked out. Thus it’s easier to achieve safety compliance as power is scaled up.
It is more difficult to control the direction of the magnetic flux with a perimeter coil. Since it is one continuous coil, flux is not contained to the receiver. Safety testing for EMC compliance in particular becomes difficult with restrictions for the amount of power output that is allowable.
Z-height refers to the distance of power transfer between the transmitter and receiver coils, measured in vertical height. It is particularly important for the integration of wireless power systems into infrastructure where Z-height enables charging through surfaces — think table tops, office desks, or car dashboards.
Both designs enable Z-height charging, but to slightly different degrees. A coil array presents some difficulty — making coils larger to enable greater distance of power transfer can introduce dead spots. (Think of coils like pixels of a TV screen: The more you have, the greater the resolution, and the finer the level of control.) A perimeter coil system achieves Z-height more easily with increased flux — but again, the safety considerations can restrict the amount of output power.
Efficiency equates to the percentage of power lost from the power source to the battery of the device. Higher efficiency means faster charging, responsible use of resources, and overall lower cost (lower power transmitter for a given receiver power requirement).
By energizing specific coils, a coil array provides a better coupling coefficient and thus efficiency. Selective energization of localized coils does not vary significantly based on the position of the receiver on the transmitter. Selective energization also prevents foreign objects — even metal in the receiver device — from getting hot.
A perimeter coil has a lower coupling coefficient that equates to higher power loss — more power is needed to ensure enough is sent to the receiver. Imagine a widespread magnetic field sending out power over the extent of the transmitter charging area. Without selective energization, foreign metal objects may get hot, further reducing efficiency.
Large variations in the induced field depend on the receiver position on the transmitter, which may also result in uneven efficiency across the transmitter, although newer perimeter-coil designs have improved flux-density somewhat. Overall system efficiency is improved if several receivers are placed in the charging area and share the charging energy. In this case, sophisticated communication and control techniques are required to allow the transmitter to recognize the unique receiver requirements and eliminate crosstalk.
One of the key considerations from a design perspective is ensuring that the charging speed of a wireless power system is better than, or at least equivalent to, that of a wired charger. This means different things for array and perimeter set-ups.
Perimeter coil systems are challenged to stay within ICNIRP (International Commission on Non-Ionizing Radiation Protection) limits as power is increased beyond 40W (although this shouldn’t be a factor when powering one or two cell phones). In terms of scalability, a larger pad area will act to lower the efficiency further, thus potentially reducing the power capacity in the center of the transmitter.
Coil-array type systems, because of their higher coupling coefficients, are not as power-constrained. It is easier to scale the power up (beyond 2k watts) without approaching ICNIRP EMI or RF safety limits. This also enables scalability of the transmitter to cover multiple devices of different power requirements and scalability of charging area without sacrificing power transfer efficiency (although at the cost of more coils).
Coil array is optimal is any environment where you want:
Perimeter coil is optimal in situations where:
It was interesting to see these comments from Qualcomm in this recent Computerworld article by Lucas Mearian: Samsung uses Qi charging for Galaxy S4, but sees A4WP as the future.
“The frequencies at which tightly coupled solutions operate are not that far from the frequencies that are used for conductive cooking,” he said. “The tightly coupled solutions today have a problem where they can heat the metal surfaces in the smartphone & or metal objects. The result is that a lot of times [with] the tightly coupled solutions, the foreign object detection either dials back the power or simply turns the power off”
Let’s get a few things clear first of all:
You can have tightly coupled systems operating at high frequencies and loosely coupled systems operating at low frequencies.
And the good news here at PowerbyProxi is that in none of these cases do we design wireless charging systems like you would design induction cookers! With an induction cooker inefficiency is the target, the less efficient the better – it’s how you create heat. The opposite is true for any respectable wireless power supply. High efficiency is the target.
PowerbyProxi continues to demonstrate real wireless power solutions that prove loosely coupled systems operating at low frequencies, when designed properly, actually have better thermal performance to tightly coupled systems (when measured on all key areas of the phone like the LCD, back cover and battery as well as the transmitter surface area). This is because loosely coupled systems operating a low frequencies have superior average efficiency. Average efficiency is what the user experiences day to day (peak efficiency is what only test engineers experience). Please see Kunal’s blog on average efficiency if you don’t know what I am talking about.
Let’s remember that the user does not care about how you achieve loosely coupled or what the frequency is. The user wants to place his or her phone and other electronics devices anywhere on the pad without any thought and have it recharge as fast as a wired charger. Furthermore they want to know that is is safe to use, will not cause interference with other devices and is environmentally friendly.
The average efficiency of PowerbyProxi’s Proxi Smartphone pad (loosely coupled and operating at low frequencies) is almost triple that of a loosely coupled system operating at high frequencies. I know which one our customers call the induction cooker.
If you would like more information please contact us directly at firstname.lastname@example.org.
The past year has seen an infiltration of wireless charging technologies in the consumer electronics space. A number of mobile phone operators are rushing to build wireless capabilities for their products as the promise of true, flexible wireless charging becomes a reality. As is often the case with new technology, one of the first questions raised is how safe is it for use by humans – in this situation: how safe is wireless charging? The post that follows presents our case as to the safety of our technology and specifically our own loosely coupled charging pad (Proxi Smartphone Solution) by independently measuring the level of emissions in relation to international safety guidelines for consumer electronic devices.
In order to measure the emissions level and respective safety of the product, we have looked to verify emissions in terms of the electric, magnetic and electromagnetic emissions compliance with the major American and European safety guidelines. In the United States of America, the Federal Communications Commission (FCC) mark is used to show safety compliance. In the European Union, the European Commission CE mark is used.
PowerbyProxi’s inductive power transmitter (IPT) systems are designed to operate far into the near field, using magnetic loops to generate the magnetic flux field. Therefore, only magnetic field emissions need to be considered when determining emissions safety. Electric field emissions are very low due to the near field operation of these magnetic loops. Further, because PowerbyProxi’s wireless power systems are designed to operate so far into the near field, far field emissions – electromagnetic waves – are not relevant from a safety perspective, as this far field will only occur at distances of around 100m away from the transmitter, by which point the emissions will be very weak.
It is also important to note that testing of the Proxi Smartphone Solution was conducted by an independent tester – EMC Technologies Melbourne (an accredited FCC testing and CE testing laboratory). The maximum magnetic field strength observed was 0.28 A/m, or equivalently 0.352µT.
In the United States, electric, magnetic, electric and electromagnetic emission safety is demonstrated through compliance with FCC standards. The relevant FCC parts are covered in detail in this section.
Radiofrequency (RF) radiation exposure limits (FCC Part 1.1310)
The FCC Par 1.1310 RF exposure limits are given in Figure 1 below. The operating frequency of the Proxi Smartphone Solution is 285kHz – at the nearby frequency of 300kHz, the magnetic field exposure limit is 1.63 A/m.
Independent tests conducted on the Proxi Smartphone Solution showed that the system does not emit more than 0.28A/m field strength, which is approximately 83% under the FCC limits. Likewise the electric field strength for the Proxi Smartphone Solution is very low and is estimated to be around 0.066 V/m at 10cm distance, which is 99.99% under the FCC MPE limit.
Radiofrequency radiation exposure evaluation: portable devices (FCC Part 2.1093)
This measures the specific absorption rate (SAR) of radiation allowed into human tissue. In practice, measuring SAR at frequencies as low as 285kHz (the operating frequency of the Proxi Smartphone Solution), is challenging. This is because, for a given field strength, the SAR falls as frequency decreases and so the SAR is very low at 285kHz. The most stringent SAR limits on both the FCC and ICNIRP standards are the same, at 0.08W/kg for general public exposure. The Proxi Smartphone Solution is already compliant with ICNIRP reference levels by a wide margin, so it can be inferred that it will comply with the ICNIRP SAR limit of 0.08W/kg also.
The FCC regulation states:
In the European Union, magnetic, electric and electromagnetic emission safety is demonstrated through compliance with ICNIRP standards. The relevant ICNIRP documents are covered in detail in this section.
ICNIRP 1998 reference levels and basic restrictions
Most European Union members still use the ICNIRP 1998 standard for emissions safety, even though ICNIRP 2010 is the more recent standard.
Within ICNIRP 1998, there are two ways to check emissions safety: the reference levels and the basic restrictions. The reference levels, shown in Figure 3 below, are by far the more stringent of the two, and serve as a “quick check” that a system will be compliant. The reference levels are chosen by ICNIRP such that if a system passes the reference levels, it will definitely pass the basic restrictions. Compliance with the reference levels is relatively straightforward to measure as the reference levels are given in terms of E-field strength, H-field strength and B-field strength, at the frequencies relevant for wireless power systems. Measuring these quantities is relatively straightforward using standard test equipment.
As mentioned previously the electric field strength for the Proxi Smartphone system is very low and is estimated to be around 0.066 V/m at 10cm distance, which is 99.92% under the ICNIRP reference level limit. At 285kHz (the operating frequency of the Proxi Smartphone system), the maximum magnetic field strength allowed under the ICNIRP 1998 reference levels is 2.56A/m, and the maximum allowed flux density is 3.22µT. From the testing data alluded to earlier, the Proxi Smartphone system does not emit more than 0.28A/m field strength and just 0.35µT flux density, both of which are around 89% under the reference level limit.
If a wireless power system fails to pass the reference levels, the basic restrictions, shown in Figure 4 below, can be used to assess compliance. The quantities given in the basic restrictions cannot be easily measured and must be assessed using sophisticated numerical modelling techniques, taking into account a range of possible consumer use cases and human body models. While some of PowerbyProxi’s competitors in the consumer space are forced to fall back on these basic restrictions in order to prove safety compliance, PowerbyProxi’s system is designed with consumer safety in mind and as such passes the more stringent reference levels test by a wide margin.
ICNIRP 2010 reference levels and basic restrictions
At 285kHz, the reference levels for magnetic field strength and magnetic flux density of ICNIRP 2010 are significantly less stringent than those in ICNIRP 1998. In ICNIRP 2010 and tested at 285kHz, the maximum magnetic field strength is 21 A/m, and the maximum magnetic flux density is 27µT. Since the Proxi Smartphone system is compliant with ICNIRP 1998 reference levels, it will also be compliant with ICNIRP 2010 reference levels.
The ICNIRP 2010 reference levels and basic restrictions are given in Figures 5 & 6 below.
Figure 6: Basic restrictions for human exposure to time-varying electric and magnetic fiel
Tests have shown that the Proxi Smartphone Solution system is compliant with electric field, magnetic field and electromagnetic field emissions safety regulations, in both the US and Europe. Given that the FCC and ICNIRP regulations represent the benchmark for emissions safety testing, the Proxi Smartphone Solution is considered safe for use by the general public.
In my previous blog I talked about efficiency and using it to measure “how loose” a loosely coupled system actually is. The next question is how much does an end-user actually care about the efficiency of sub 20W consumer device charging solutions. When was the last time you checked the efficiency of your wall wart for your smartphone or your laptop for that matter? Is this data even easily available to curious end-users?
To get an appreciation for how close to the thermal edge smartphones operate at today, you only need to play music or stream a video over 3G / WiFi on a sunny day and see how long it takes before smartphone goes into self-preservation mode. It is said that computer design is more like refrigerator design these days to see who can design the best heat sinks. For a long time Apple did not put i7 processors in their MacBook Pros due to the inability to get heat outside the slick Aluminium shell.
To ensure that wireless charging for consumer devices is widely adopted (such as smartphones & tablets), the technology should not limit the usability of devices while charging is taking place. In my view efficiency is actually a means to achieving thermal performance which is the “end”, and NOT the “end” itself. Other parameters that matter are; cost, Human RF Exposure, EMC performance, Rx size, and how quickly the device charges.
A wireless power standard is essential to achieving true ubiquity of wireless charging of electronic devices. This is something wired charging has struggled to achieve just ask anyone who has owned a couple or more laptops. Lets not even get started about the iPhone 5 wired charger!
One of the key questions is what is the magic power level or range that will create the most user friendly ecosystems. Ecosystem being defined as compatible transmitters and recievers. Is there an ecosystem for cellphones / smartphones, another for laptops and so on? Or is the real need to have compatibility across the board for all “general” household consumer devices.
The downside of having a one size fits all type of solution is that you will need to trade-off performance and cost against ecosystem expansion from 0-3.5W to 0-100W. A transmitter that can charge 2 smartphones @ 3.5W each only, will look very different to one that can charge 2 smartphones @ 3.5W each as well as a laptop at 90W.
Most of the smaller consumer electronics devices would only require an ecosystem operating in the sub 10W range – this includes smartphones, cellphones, tablets and the like. On the face of it then, a logical demarcation point for ecosystems might be <10W for smaller devices, and 11W to 100W per receiver for larger capacity devices like laptops?
What do you think?
Read more about wireless power technology.
Coupling is a term widely used in discussing wireless power systems – it refers to a coupling coefficient ‘k’ which defines how well a transmitter and receiver are magnetically “linked” as a percentage. Generally something like transformers have extremely high coupling coefficients approaching 100%.
Technically, tightly coupled systems are interpreted as having high or transformer like coupling coefficients while loose coupling is interpreted as systems with low coupling coefficients. There is some debate around the cut-off for high and low – 50% is one proposed transition point.
From a usability perspective I would define tightly coupled as a system which requires some form of mechanical alignment to fix orientation and transmission distance. This can be done via a magnet or a mechanical alignment feature on the transmitter and receiver. A loosely coupled system would allow complete flexibility of orientation / misalignment in a 2D target zone. A 2D target zone would be a planar area such as a matt (i.e. Proxi-2D), which can wirelessly charge devices imbedded with a receiver that is a few cm above it.
Systems like the Proxi-3D which enables receivers to work in a three dimensional target zone with an omni-directional receiver are what a user may call as beyond loosely coupled!
In my opinion the usability definition is more relevant for anyone buying the system as really the technical definition is really just a means to an end – the end being ensuring customers don’t have to carefully align their devices with a wireless charger.
Over the last few years Wireless Power has made rapid advances towards becoming a mainstream technology and is often the case, marketing departments become the source of many new inventions. Perhaps the biggest marketing “invention” to date is something called “Magnetic Resonance” (related to Resonant Inductive Coupling) when everyone else is just doing stone age “Inductive Power.”
It’s a term many have now adopted, as if it was some space-age technological leap from “Inductive Power.” When in fact Magnetic Resonance and Inductive Power are EXACTLY the same thing.
Lets dig into this a little more …
I’m sure all EEE majors will remember that Inductive Power uses Magnetic Resonance.
Any Inductive Power system has to have resonance, even the WPC which requires complete alignment between the transmitter and receiver coils uses resonance. Yes, tightly coupled systems, like loosely coupled systems, do use resonance! This is accepted science since The University of Auckland started researching modern day wireless power 20 years ago.
Originally, before induction, wireless power could be achieved by effectively taking a transformer and separating the primary and secondary coils (i.e. a split transformer).
To increase the power efficiency it was worked out a long time ago that we need to use resonant coupling. This is just a fancy way of saying that by adding capacitors on both coils, a resonant circuit is created between the inductance of coil and capacitor. At the resonance frequency, the reactance cancels out and you are left with only the parasitic effects of finite winding resistance, AC resistance (proximity effect) and dielectric losses.
If you had a perfect AC source and drive the resonant circuit you have no losses. Losses are solely limited by your parasitics.
“Magnetic Resonance” is just sticking some capacitors in place. It was great to see Marin Soljacic, the inventor of WiTricity confirm this in the IEEE publication, A Critical Look at Wireless Power. “Resonance enables efficient energy transfer…. …. it’s not a new idea: Tesla’s eponymous coils use that very same principle.”
To summarize in non-technical speak, all Inductive Power systems use magnetic resonance and its certainly not the difference between tightly coupled and loosely coupled systems.
Like most new technologies you need to get underneath the marketing spin to understand the features and benefits that each vendor can deliver to those who matter most – our customers!
In my post last week I started to examine issues around frequency selection for wireless power systems. This week I want to take the discussion a bit further and talk about the benefits (as we perceive) of using lower frequencies (in the kHz range) vs. higher frequencies (MHz) as an industry standard.
As the standards debate rages on, different parties continue to put forward their various interpretations on the ideal frequency range. Various standards utilize higher frequency ranges than others. PowerbyProxi, through the CEA working group, continues to argue that standards that use lower frequency ranges (kHz) are more appropriate based on what we believe is in the best interest for you – the consumer.
There are several factors that need to be taken into account.
Complexity. How complex is the device to manufacture. Controllers used for wireless power systems are far more complex at the MHz range ultimately impacting the cost of manufacture and thus the price that the end-consumer (you) pays.
Interference. As we as we are aware there are no wireless devices operating in the MHz range that meet EMC radiated emission compliance. What it means is that at the moment, there is no proof that they wont cause interference on other devices.
Other factors such as charging distance, transmission efficiency, thermal properties and form factor tend to be implementation specific or require further research to be able to draw clear comparisons.
On the balance of the research done so far, lower frequencies at the kHz allow for more user friendly and functional wireless power systems to be developed. Isn’t that ultimately what it should be all about?