So Jim, we're on to our third method of wireless power transfer, which is inductive resonant wireless power transfer. And this is a category that's really important to us at NuCurrent. We're still in the realm of transferring power through magnetic fields, but can you walk us through this transfer method and help us understand how it's different from inductive? Sure Mike. So we can think of inductive wireless power transfer as an air core transformer that is trying to maximize the coupling between the two coils to maximize the magnetic field interaction. Inductive resonant charging can make do with a much lower level of coupling between the two coils, but still maintain a high level of efficiency. The benefits of this approach is it provides a much improved spatial freedom performance and coil placement over straight inductive coupling. Although the concept of inductive resonance has been around since Nikola Tesla, it really started to gain steam again with work done at MIT in the early 2000s and was picked up by MIT spin out WiTricity and then promoted by what has now become the AirFuel Alliance standards organization. If inductive wireless power transfer can be thought of as a simple, straightforward method of transferring energy over close distances, resonant inductive power transfer is a technologically sexier more elegant solution that provides greater flexibility for the user. The engineering side of me is naturally drawn to the inductive resonant power concept. So let's start by doing an introduction to magnetic resonance and describe the differences versus straight inductive wireless power transfer. So let's take a look at the diagram shown. It is actually very similar to the topology of the inductive wireless power diagram we showed earlier, except that we've pulled the coils further apart from each other. You'll notice that by pulling the coils further apart, less magnetic flux lines generated by the transmit coil actually intersect the receive coil. That instinctively feels like a problem. Shouldn't that mean that there is proportionately less power transferred? Surprisingly, the answer is no. Take a look at all those magnetic flux lines that don't couple. You would think that is a road to nowhere, kind of like a lot of wasted energy, but remember a magnetic field is actually just the repository of stored energy. So even though it's not coupling to the other coil, it's not heating or dissipating energy either. Another way to look at it is from an electrical circuit standpoint. Think of the coupled coil as being split into two parts, the coupled portion and the uncoupled portion. We understand the coupled portion is the transformer, the same as our inductive charging case we talked about. With just a smaller number of flux lines intersecting, the coupled part or the uncoupled part of the coil can now be simply thought of as an inductor, which is a totally separate circuit component. Inductors store energy for part of the alternating cycle and release it at a later time in the cycle. If we can properly tune the inductive reactance with a corresponding capacitive reactance or a matching network, we have effectively balanced or canceled the reactive mismatch and created a resonator out of the stored energy, and that energy can align the voltage and current to maximize transfer to the receiver side. There is a cost for this. It doesn't necessarily come for free. Any significant lossy components in the tuning network, including the coils themselves, can significantly increase the amount of losses in the system and decrease the efficiency. So those losses must be watched very carefully in the design process, but if high quality components are used, then those losses are manageable, and the efficiency of power transfer can actually be very high for resonant inductive systems, even with low coupling. The huge positive, since you don't need the high coupling between the coils, you can get significantly larger freedom of placement than Qi type inductive systems. Very interesting Jim, thank you. Now what types of magnetic resonant systems are out there and how are these being advanced? As I mentioned earlier, the main standard body for coordinating inductive resonant charging is the AirFuel Alliance. The AirFuel Alliance standard operates using the ISM frequency bands at 6.78 megahertz for operation. And just so people on the call know, the ISM band stands for industrial, scientific and medical, and the ISM bands are actually several frequency bands that the FCC and worldwide regulatory bodies have set aside for non-communication purposes. A microwave oven operating at 2.4 gigahertz is a good example of an ISM band application. Used as such, those bands tend to have relaxed EMI regulatory requirements. Now it is interesting to note that 6.78 megahertz is over 50 times higher in frequency than the Qi standard at around 120 kilohertz. By utilizing the higher 6.78 megahertz frequency, the inductive reactance of a given coil is going to be multiplied by the frequency of operation. So this is one way that inductive resonance can achieve higher Q resonators to keep losses low and maintain high efficiency. On the flip side, Qi can often counteract the lower Q of its coils by using Litz wire, usually on the transmit side, which is a more costly wire to use. Now just recently within the past two months, the NFC Forum, the standard body that handles near-field communication devices like security cards, payment cards, and NFC tags, has adopted their first wireless charging specification into NFC that makes it possible to wirelessly charge small battery-operated, consumer devices and IOT devices using NFC with resonant inductive charging up to one watt of power. This opens up a new possibility of wireless charging capabilities, giving access to over 2 billion users of NFC-enabled devices and smartphones that have the ability to charge, to use charging to enhance their devices. NFC is an interesting technology since it allows both charging and data transmission over the same antenna and has a fairly high data rate built into the technology so it can enable power plus data in small power devices like fitness trackers, wireless earbuds, digital pens, and other small consumer devices. Unlike the AirFuel Alliance, the NFC forum and its wireless charging specification uses 13.56 megahertz, which is also an ISM frequency band, and not coincidentally, two times the 6.78 megahertz frequency. The NFC standard uses 13.56 megahertz for both communication and wireless power transfer control. That all said, not all companies interested in designing wireless power into their products require a standard. Many customers actually prefer a closed system for their product lines, and they may want some level of customization in the design that they can manage and optimize themselves. NuCurrent has been actively engaged in a number of customers for the development of such products. As a result, NuCurrent has created its own proprietary version of an inductive resonant charging system that uses 6.78 megahertz that is used for closed systems. It is a simplified implementation of inductive resonance that uses in-band communication. So Jim, what kind of power levels and distances are we looking at here for inductive resonant? The AirFuel Alliance allows for a wide range of potential receive charging powers. They have designed a flexible system of six classes of transmitters and seven categories of receivers with a maximum power draw of its category 7 receivers as high as 50 watts. On the low end, they can handle small devices that draw only 1 1/2 watts of power. They advertise distances up to 50 millimeters, which is about two inches. The NFC charging specification allows charging between two NFC-enabled devices in either static mode or negotiated modes. Static mode is where standard RF field strengths and provide a consistent power level. Negotiated mode uses a higher RF field supporting power transfer classes from 250 milliwatts up to 1 watt.
So what are some of the advantages and challenges that come with this higher frequency method of power transfer? The thing that really puts inductive resonance on the map is the greater positional freedom over conventional inductive charging like Qi, and it can also handle a wide range of power levels. In addition, the higher positional freedom allows it to be designed to go through tables or walls or other obstacles, allowing for many expanded charging applications. In addition, as mentioned previously, the AirFuel standard can accommodate multiple receivers from one transmitter. AirFuel handles communications differently than Qi in that it communicates out-of-band via Bluetooth at 2.4 gigahertz. Qi uses N band communication where the receiver can toggle a capacitor in parallel with the receiver coil to vary the amplitude of the transmit carrier. The transmitter can interpret this as amplitude modulation. This load modulation tends to be the simplest form of communication. The AirFuel auto band communication using Bluetooth adds some additional complexity and costs, but also adds benefits like communicating and charging multiple devices. Compared to a Qi system, the AirFuel Alliance 6.78 megahertz system has arguably higher cost of implementation. As mentioned, the out-of-band communication has a separate transmitter and receiver. In addition, 6.78 megahertz is a fixed frequency of operation, and power control is varied by moving the transmitter rail voltage up and down to vary power. This tends to be somewhat more costly than the lowest cost versions of Qi transmitters, which vary power by varying the frequency of the transmit signal away from the Qi resonance point. That essentially varies the matching of the transmit signal and is a form of power control. Also to maximize efficiency, AirFuel's higher frequency would naturally gravitate to GaN type power amplifier devices, which are very fast and high performing, but are currently more costly than silicon parts. In general, EMI also tends to be somewhat more challenging for 6.78 megahertz systems and higher. Though the AirFuel standard has the benefit of using the ISM band, it can use FCC Part 18 and get some harmonic allowance on carrier and other harmonics that can help in regulatory issues. NFC operates at higher frequencies and its charging is mostly built around charging smaller devices. So you can have very small antennas pushing a good amount of power into pen type stylists and other wearables, but similar to AirFuel, regulatory issues with EMI are going to be more of a concern. Thanks, Jim. So what are some of your favorite applications of resonant charging that you've been involved in? Well as a former basketball gym rat, one fun custom solution we designed using inductive residence was to place a wireless charging system inside of a basketball. The basketball contained various electronic sensors to aid in various athletic diagnostic parameters. However, despite our requirement to put wireless charging into a basketball, the basketball is still needed to meet all of the mechanical spec constraints of a basketball, meaning it would need to bounce true and uniform when bounced in any orientation, it could have no dead spots, and it couldn't rotate out of balance or off center when the ball was shot. And it had to meet its weight constraints. We designed a very custom 3D transmit coil system for the transmitter and a very small embedded receive coil at 6.78 megahertz that allowed for a very loose placement accuracy on the charging stand, but still allowed uniform charging. That was a very different and a very fun project. Yeah, that was an interesting one. And we've got some other really exciting ones coming up with the high-frequency solutions at 6.78 and the NFC charging at 13.56. That was a ton of content and insight, Jim. Thank you so much. I think you've earned a break to take a sip of water. That concludes our third method of power transfer, inductive resonant.
