Wirelessly Charge Wrist Wearable Devices with NFC2023-01-18T20:00:29+00:00

Wirelessly Charge Wrist Wearable Devices Devices with NFC

To Qi or Not to Qi: Wrist Wearable Devices Introduction

NuCurrent has been driving the wireless power industry for over a decade, launched more product categories into mass production than any other company in the world, and developed a global reputation for technology innovation with over 150 patents and many contributions to global standards bodies like the WPC, Airfuel Alliance, and NFC. NuCurrent sets customers up for wireless power success by posing five essential questions to determine which wireless power transfer method will be most effective for wrist wearable devices.

While Qi-based (inductive) charging is the best-known method of wireless power transfer, there are several other options for product developers to consider. In the To Qi or Not to Qi: Wrist Wearable Devices Introduction webinar, NuCurrent explores and defines the different wireless power transfer methods available and details key terminology and concepts along the way.

(logo swoosh) Good morning and good afternoon and good evening to everybody from around the world joining us today. My name is Mike Harmon, I'm the Director of Marketing at NuCurrent and today I'm once again joined by my esteemed colleague and Senior Field Applications Engineer, Jason Luzinski for a topic that is clearly a popular one, what we call To Qi, or Not To Qi. I'd like to take a minute to address why I think this topic is so popular and why NuCurrent is a really good resource to address it. Wireless power and wireless charging means different things to different people. This is an emerging and high growth industry with a lot of different technologies and techniques and terminology and so on. So, for someone walking into this or even someone who's spent a little bit of time with it it can be pretty confusing and that's where NuCurrent comes in. We've been driving this industry for over a decade and we've launched more product categories into mass production than anyone in the world. And we've developed a global reputation for technology innovation with over 150 patents, granted (indistinct) many major contributions to global standards bodies like the WPC and AirFuel and NFC. We've developed a center of excellence with over 50 engineers representing RF mechanical, electrical, and software disciplines all serving major challenges in wireless power. And we do so in a way that we aim to be easy to work with and customer centric, which has qualified us to be among other things and an NXP Gold Partner. And ultimately we're driven to be the world's go-to resource for wireless power, which is why we put on programs like the ones that we have today. As we get started into the meat of this, we're really gonna dig into wireless power over the next hour and before we do we ought to review some important concepts excuse me, and terminology. So first on the left-hand side of the screen, we're gonna take a quick peak at transfer methods and the different types of methods of power transfer that are gonna be applicable for the conversation today. The first is low frequency, which is operating at 110 to 220 kilohertz via magnetic induction. And so Qi, which is, you know, the most well-known standard and method of power transfer is regarded as a low-frequency method. So we wanna make sure everybody understands that. Also Apple's new MagSafe protocol is a low frequency at around 360 kilohertz. Moving on to NFC which is a relatively new transfer method for wireless power operates at 13.56 megahertz and employs magnetic resonance. Similarly high-frequency using magnetic resonance is at 6.78 megahertz. We've got a high frequency solution as well that tends to map to what AirFuel has done as well as NuCurrent through our NuEva Development Platform. The next transfer method we refer to as a hybrid which is really sort of application specific but it employs both low frequency and high frequency. So inductive and NFC for kitchen appliances and the cordless kitchen through the Ki standard. And then lastly, the RF method of transfer which operates around 915 megahertz to 2.4 gigahertz using radio frequency power transfer. And so these methods are gonna come into play as we go through the different product examples that we have. But before we jump into those, there are some definitions and some terminology that I would like to have Jason walk us through so that everybody's aware of these terms as we talk about them through the products that we'd go through. So Jason, can you walk us through the concepts of magnetic induction and magnetic resonance for the audience? Yeah, sure. So magnetic induction is what we coined low-frequency technologies as we are operating like an air transformer where we are relying on coupling to deliver majority of the efficiency as we're trying to harness as many of the flux lines or the magnetic field lines going from the transmitter to the receiver. Magnetic resonance is a blend of a magnetic induction but it also has a little bit of more RF components attached to it such as energy storing items such as a tuning network that allow us to utilize and store energy to not have to rely on coupling as much. Magnetic resonance technologies are in the high frequency range of the NFC or 6, 7, 8, 13.56 megahertz. The next definition we are going to define is going to be antenna and coil. So we are gonna be talking about our RF wireless power technologies which rely on electromagnetic fields as opposed to just magnetic fields. So antenna will relate to something like a Bluetooth antenna, something that is able to harness the field similar to the RF technologies of 915 megahertz to 2.4 gigahertz, and the coil will relate to the inductor that we use for wireless power. The third definition we're gonna cover is coupling and mutual inductance. So this is related to the magnetic technologies. Essentially coupling is the ability of a receiver to capture a percentage of the magnetic flux that is created by a transmitter. This is usually on a scale from zero to one where one is you're capturing all of the fields and zero is capturing your none of the fields. Mutual inductance is the interaction between your receiver and transmitter coil. So essentially you have windings on both sides that are creating an inductance, the transmitter creates a field that generates a voltage on the receiver windings and that receiver winding actually, when you generate that voltage creates a current which inherently has an effect back on the transmitter. So, by understanding that value, you can kind of estimate the amount of power you're able to transfer over a certain area. Third item that we are going to define is foreign object detection. This is kind of an industry standard term where we're talking about as you're transferring power from a transmitter to a receiver and someone places let's say a coin or a key into the field, that is considered a foreign object, something that shouldn't be there. And we wanna be cognizant of these items because they cause safety issues and degrade performance of the system. The next term is charging infrastructure. This is related to the global standards that we have out currently, that is related to AirFuel to Qi to Ki and basically in terms of what is currently available the global leader in charging infrastructure is Qi. You know, all the pads that you see laying around the tables to charge your iPhone, or your Samsung phone, you know, those are related to the Qi charging infrastructure. And there's lots of great companies that are integrating these things into public spaces, so that as you walk around, you don't need to bring your charger anymore. you can just place it down and be able to charge, And the last term that we are going to define is proprietary solution or let's say closed ecosystem. All of these technologies can be developed into a proprietary solution for a specific customer. Now, if we talk about a larger ecosystem, such as Qi that is more charging infrastructure. So we want to create that separation between the two to ensure that you understand in certain situations where, you know, you really don't care about utilizing, for example, the Qi standards, but you want to be able to create your own ecosystem within a family of products, that's how we delineate the two. Jason thanks for running through that. The next slide here is just one of the things that really helps us focus. The session that we have today with some really useful questions and coming from outside the industry, this was one of the things that really helped develop my own understanding of how to map solutions to products. And there are two real breakthroughs I think in having these questions. The first is that we've got these five really smart questions that help quickly eliminate certain options and shine a light on what might be the best solution candidates. And then the next part about this is that based on the nature of the product, whether it's industrial design or its use case or the power levels, some questions are better asked first in this process than others. So we actually give ourselves the flexibility to do that. And as we go through the products that we have today we are gonna see that we approach all of these questions but we sometimes do them in different orders based on what we know about that product. So Jason, let's go through these questions and talk about the rationale for why they matter. So why would someone care about this first question? What is the size of the product and size of the antenna required when it comes to wireless power? So, this is one of the questions that I like to ask is because the size of the product will automatically dictate the maximum size of the receiver core. This will right away eliminate certain wireless power technologies due to the need for specific resistance and coupling characteristics to hit necessary thermal and user experience specifications. Great, so moving on to question two, what is the coil-to-coil distance and how many receivers need to be charged? So, wireless power requires appropriate mutual inductance which is a function of the inductance of the transmitter receiver and coupler. If the charge distance is greater than four to five millimeters, or requires a larger XY offset, it may be necessary to move to a high frequency technology as these do not rely on coupling as heavily as low frequency technologies. With this in mind, it is possible to eliminate certain tech paths based on the unique attributes of the product. Additionally, if there's a desire to charge multiple devices on a singular pad, high-frequency and NFC is usually the desired path due to the ability to operate under low coupling conditions. Great, so the first two are really about size and antenna related, the third one says, does the product need data transfer capabilities greater than 10 kbps? What's up with that? So, wireless power technologies have certain data transfer capabilities. Low frequency technology such as Qi was originally designed to be solely for power and the low frequency of operation limits the total data transfer through but about two kilobits a second from the receiver to the transmitter and on the order of bits from the transmitter to the receiver. NFC comes from a data background and with its higher frequency of operation at about 13.56 megahertz, allows up to 848 kilobits a second throughput. High frequency also supports in band communication up to about 10 kilobits a second. So, if that is a key requirement of a system, this question can be used to eliminate certain technologies. All of these technologies can potentially use auto band communication such as BLE, or ZigBee or a similar scheme, but this usually increases the cost and complexity of the system.
Great, so the next question,
do you need this product to be interoperable with existing infrastructure such as Qi?
This is an easy elimination question.
As of right now, there are only three viable charging infrastructure standards, Qi, NFC, and Ki. So if your product requires the use of an existing Qi charging pads, or wishes to use NFC data infrastructure, you can easily remove certain technology.
Perfect and then the last one,
what is the power level required to power or charge the device?
So power level is usually
one of the first questions you can ask to eliminate certain technologies. If the powers required is greater than three watts, you can immediately remove NFC and RF. However, if the power requirements is less than three watts, you know, you may need to start with a different question and to kind of touch on these five questions, the order that I asked them in is based off my previous experience being in the industry for six, seven years now and applying these questions to the different use cases of products that we see coming in. Wireless power is an interesting blend of mechanical, electrical, RF engineering, in addition to helping shape the industrial design and user experience of a product. So depending on the product that you're looking at you may have a different approach of how you ask these questions for a project that you're looking at. However, as we go through the presentation, I'm gonna give you an insight kind of how I approach this, but I urge everyone to look at the questions and understand what is really key and important for your product and the experience that you're trying to bring your end customers. (logo swoosh)

Wirelessly Charge Wrist Wearable Devices

Stylish wrist wearable devices such as fitness trackers and smartwatches require a safe and waterproof design that is comfortable 24-hours a day. Additionally, they must support wireless power transfer and data transfer from transmitters to receivers while maintaining a sleek and compact form factor. In To Qi or Not To Qi: Wrist Wearable Devices, NuCurrent explores the best wireless power transfer solution for wrist wearable devices.

(Intro Sound) - [Jason] So, to start off with, you know, a Fitbit style wearable, you know, I'm thinking something that is relatively narrow, needs to be comfortable to wear 24 hours a day, usually has a very sleek form factor to make sure that it's, you know, as least noticeable as possible. As a result, the size of the product instantly makes me think, well, if I want to use low frequency, I will probably need to have a coil that's relatively thick, has a lot of windings, in order to get the necessary electrical characteristics to get efficient power transfer. There are better options. RF, NFC, and high-frequency both utilize, in the case of RF, you know, a Bluetooth antenna, something similar of that nature where it could be printed on a flex substrate and NFC and high-frequency which, you know, the coils are on the range of nanohenries, which means one or two turns on a coil as opposed to qi, which requires microhenries which means, you know, tens to twenties of turns. So, from my thought process, RF, NFC, and high-frequency both allow you to integrate wireless power into this specific industrial design while maintaining the electrical performance that you're looking for. - [Speaker] Great. And so, when we get to question two, Jason, the coil to coil distance and how many receivers need to charge, we see that NFC and high-frequency are still tracking okay, but what's going on with RF to make that a bit of a concern? - [Jason] So with- with RF, the current FCC certified RF transmitters have a max output power of about 1.5 Watts. With RF, according to Friis equation, every time you double the distance from the transmitter, your power transfer capability will drop by half. So, if you look at these products that, you know, usually require on the order of, you know, hundreds of milliwatts to about 1.5 watts. With RF, you start to get really limited with the ability to charge your device on the go. If there was an RF infrastructure, you know, having RF chargers placed everywhere, you might be able to get enough trickle charge throughout the day to support your product. However, you know, when you're thinking about fitness wearables, such as a Fitbit, you're out and about, you're outside, you are running, you're biking. So, the ability to charge your device reliably becomes very difficult and in order to get that quick charge ability, you would essentially need to place your wearable right next to the RF transmitter, which kind of, you know, is impractical and kind of eliminates the purpose of RF charging. So, from that case, you know, we have the ability to transfer power to the wearable in the ranges that we're looking for, but from a use case, NFC and high frequency make a lot more sense. You have a higher power capability, you know, up to three watts with NFC and more with high- with high frequency, which gives you that same user experience. And now, you know, people are pretty accustomed to the- the apple watch experience, right? Where you have the little dongle that attaches to the back or you have a cradle where you place your device into. You want to get that topped off, You want to get it charged, and you want to be on your way, and you don't want to have to worry about it. That's why I give NFC and high-frequency the plus in this category because we can charge a device quickly and it also allows the same user experience people are currently used to. - [Speaker] Perfect. So, when we move to question three, we start to see even a little bit more separ- separation of- of a preferred method here. Does the product need data transfer capabilities greater than 10 kilobits per second? First, why would a wrist wearable need something like that, and since it does, you know, why- why are we seeing some clear- clear winner in the pack here? - [Jason] So, you know, the apple watch has wireless charging in it. However, if you remove one of the straps, there is a hidden debug port off to the side. So, if you truly want to make a wireless device, you need to be able to provide a data channel that would give, you know, developers, debuggers the ability to enter low level, let's say, programming, or, you know, communication with the device without having to rely on the battery or the other radios inside the device. You know, if you're picturing a situation where your watch is dead and you're trying to debug something, you're still gonna need to charge the device up. So, in this case, starting with RF, you know RF has the ability to transfer data, you know? It's based off of similar frequencies like wifi and Bluetooth. We have very high data capabilities. However, in a situation where you are in a dead battery situation, you just don't have enough power to be able to boot up the system and provide any meaningful data back and forth. High frequency, you can supply the power, but like I mentioned earlier, with the inbound communication, you know, we're kind of limited to about 10 kilobits a second. That might be enough for some basic charging information going back and forth, but if you're trying to, you know, do UART or send some kind of higher level data, the speeds aren't really there. With NFC, with the ability to transfer up to 848 kilobits a second, you know, we have been able to design systems that essentially are over their UART. So, we are giving developers and let's say product designers the ability to keep those necessary low level functions enabled while completely making the device sealed. So, that's why I give NFC the benefit here, because it has a higher data transfer capability that we can use to address some of the issues that, let's say, product designers have had because they need to keep a debug port attached to their product. - [Speaker] Great. And so on question four, we see a little bit of a question mark, with regards to power level needed to- to- to- power and charge the device on the RF side. Why is that Jason? - [Jason] So like I mentioned a little bit earlier with RF technologies, your power from the transmitter to the receiver drops by two every time you double the distance. So, in the case of RF, you know, the- the idea is to be able to walk into a room and deliver power to your device and keep it charged up. Unfortunately, that's on the order of milliwatts if you're walking through a room. Because of that, I give it- I give it a minus. The infrastructure's not there yet, the locations where these wearables are being used probably will not have an RF power transmitter. So, you can only expect a couple milliwatts unless you place this device directly in front of the transmitter. So, I give it a minus from a use case. and from a cost perspective. These transmitters tend to be a lot more expensive because they have to be able to supply power to an entire room. NFC and high-frequency both, you know, are closely coupled and have the ability to, you know, deliver the necessary power level up to three watts. That is more than enough for a lot of these products. So, that's why I give NFC and high-frequency the thumbs up in this specific question. - [Speaker] Sure. And for question five, you've mentioned a couple times that RF doesn't have the infrastructure there and we had the same thing come up with the laptops for high-frequency, but you've given a real positive mark here for question five for NFC. Why is that? - [Jason] So, in 2019, the NFC form ratified the WLC wireless charging standard for NFC. There are multiple people currently developing, the Universal Stylist Initiative is using NFC as its main tr- main standard for wireless power transfer. In addition to that, you know, there are rumors that mobile phones are also going to start having NFC charging enabled inside of them. So, from that aspect, from a charging infrastructure aspect, NFC seems like the clear path forward here because the infrastructure is being developed and being creative. In addition to that, NFC also has its data infrastructure. So, the same coil that we'd be using for NFC, charging your device can also be used for tap to pair, potentially tap to pay, and other user benefits that come from that data link. Based off that, NFC makes the clear path to me, whether it is going to be something that requires kind of a proprietary ecosystem or utilizing the entire NFC infrastructure, it gives developers the options that they need to give the necessary user experience they're looking for. - [Speaker] Sure. And because of some of the similarities in terms of question one and question two and even question three, another category like smart glasses really comes into play here as a kind of map, similarly to this. We're using our NFC- Nueva NFC development platform on both wrist wearables and smart glasses based on these product constraints and coming up with some really exciting solutions. (suspenseful music)

5 Essential Questions

  1. What is the size of the product and the size of the antenna required?
  2. What is the coil-to-coil distance, and how many receivers need to charge?
  3. Does the product need data transfer capabilities greater than 10kb/s?
  4. Do you need this product to be interoperable with existing infrastructure?
  5. What is the power level required to power/charge the device?

Solution for

Wrist Wearable Devices

NuCurrent’s NuEva NFC platform delivers a premium wireless charging solution faster, with industry-leading performance and less risk.


  • Greater spatial freedom

  • Multi-device charging

  • Thin, efficient coils for small electronics

  • Large component ecosystem
  • Enables fully sealed waterproof design

  • Data transfer capabilities up to 848k

  • Over-the-air UART

Is Qi Charing for Laptops the Way to Go?

Qi-based (inductive) charging is the most popular form of wireless power, and for good reason. But there are many more power transfer methods to consider when deciding which is best for your device.

In this 60-minute session, NuCurrent will look into the different wireless power transfer methods available and walk through the tradeoffs that come with each method, followed by a recorded Q&A session.

The Power to Get to Production





QI: 15W

WHOOP mobile


Want to discuss a
NFC or custom low frequency solution?

Ready to discuss a premium wireless charging experience for your product?

Go to Top