Wireless Charging Without Qi: Testing ROHM's NFC Power ICs on a Homemade Smart Band
イチケン / ICHIKEN
Qi wireless charging is familiar from smartphones, but its coils are too large for very small wearables. In this sponsored video, the electronics channel Ichiken tests a different approach: using NFC, the short-range technology behind transit IC cards, to deliver real power rather than only data. The presenter uses ROHM's NFC power-transfer evaluation boards to measure how much power actually gets across and whether it can run a small smart band they built. They also test NFC data transfer.
From Data Exchange to Power Delivery
The presenter starts with the basics. NFC (Near Field Communication) is a short-range wireless technology that operates at 13.56 MHz. Its best-known everyday use is transit cards such as Suica in Japan. Strictly speaking, Suica uses FeliCa, but the presenter describes FeliCa as a type of NFC.
NFC is mainly a way to exchange data. However, a tiny amount of power also passes between the devices during communication, which is how a transit card works without a battery. The theme of the video is to scale up that side effect: use NFC for "full-scale" power delivery, not just data. The presenter wants to find out how much power this actually delivers.
The Principle: Electromagnetic Induction
Before the ROHM hardware, the presenter demonstrates the physics with simple props.
- Magnet and coil. Moving a magnet toward or away from a coil changes the magnetic flux through the coil, and a voltage appears across its ends. The presenter stresses that the change in flux is what matters. Moving the magnet too slowly produces no visible voltage, while moving it fast enough does.
- Two coils. The magnet is replaced by a larger transmitting coil driven with alternating current, which creates a continuously changing flux. Bringing it near the smaller receiving coil induces a voltage, just as the moving magnet did.
- Orientation and distance. The induced voltage depends on how the coils are oriented and how close they are. The presenter compares this to a phone that won't charge, or a Suica card that won't register, when held at an odd angle or too far away.
- Lighting an LED. With an LED attached to the receiving coil, moving the transmitting coil closer makes the LED light up, with no wire connecting the two.
According to the presenter, NFC power transfer relies on this same principle.
Qi Versus NFC Power Transfer
The presenter then compares Qi and NFC power transfer on several points.
Frequency. Qi uses 110–205 kHz. NFC power transfer uses 13.56 MHz.
Power. This is where the gap is largest. The presenter cites Qi at 5–25 W, which is enough for sizable loads. NFC power transfer delivers a few watts at most, and the module used in this video is rated at 250 mW. On power alone, the presenter says, Qi looks better.
Size. NFC's main advantage is that the coils and the whole power system can be much smaller. The presenter shows Qi coils that are often around 50 mm in diameter and sometimes paired with ferrite or other magnetic materials to improve efficiency, which makes the system larger. NFC's higher frequency lets both the transmitting and receiving coils shrink.
Data communication. Qi charging includes control communication to monitor the battery's charge state, but it offers no channel for exchanging arbitrary user data. NFC power transfer is built on NFC technology, so it can also carry user data. The presenter demonstrates this later.
These differences, the presenter argues, lead to different uses. Qi suits devices like smartphones that need high power and have room for large coils. NFC power transfer suits small devices such as smart rings, smart bands, wireless earphones, and hearing aids.
ROHM's Evaluation Boards
The hardware is a ROHM evaluation kit for its NFC power-transfer ICs:
- Transmitter: the larger board, the ML7671.
- Receiver: the small board, the ML7670.
Because the chips are meant for small products, they use small IC packages. The receiver in particular targets ultra-miniaturization and uses a wafer-level chip-size package.
The coils are quite small because of the high frequency and low power. Unlike Qi's wound copper wire, they are printed as patterns on the circuit board.
The presenter points out that antenna design matters a lot in NFC power transfer, because matching adjustment at 13.56 MHz is demanding. They note that ROHM works with antenna manufacturers to provide verified antennas, matching tools, and reference designs, and they think this should lower the barrier to antenna design considerably.
Existing Products and Other Variants
The presenter mentions that a product using ROHM's NFC power IC is already on the market: the SOXAI RING 2 smart ring, which charges its internal battery wirelessly. According to the presenter, its power-receiving section uses the same ROHM IC as in this video.
The ICs tested here are consumer parts with a maximum output of 250 mW. The presenter says ROHM also has an industrial product line that supports up to 1 W.
Testing in an Anechoic Chamber
The presenter can't simply switch the boards on at the Ichiken office. They explain that doing so would emit a 13.56 MHz high-frequency magnetic field from the coil, which would be a problem under Japan's Radio Act. So the experiments were done in a radio-wave anechoic chamber rented from ROHM, part of the EMC measurement site at ROHM's Yokohama Technology Center.
With 5 V supplied to the transmitter, the board lights up. When the receiving module is brought close, the two sides negotiate power transfer and power starts flowing. An LED connected to the receiver lights once the coils are near each other.
With an oscilloscope probe held near the transmitting coil, the 13.56 MHz magnetic field is clearly visible. The presenter explains that the receiving coil picks up this field and rectifies it to produce usable power.
How Much Power Actually Gets Through
Next, the presenter tests the rated 250 mW maximum. A 3 mm spacer sits between the boards, and a resistive load on the receiver is gradually made heavier:
- At 1 mA and 2 mA, the output is steady.
- At 10 mA, the presenter reports about 50 mW being transferred.
- As the load increases, the current occasionally drops out. The voltage briefly falls to 0 V. The presenter attributes this to the system adjusting its transmitted power.
- At 20 mA, about 100 mW; then about 150 mW; at 40 mA, about 200 mW.
- The presenter overshoots to around 70 mA, backs off, and reaches about 50 mA, or roughly 250 mW.
The presenter notes that operation near the limit is sensitive to the coil gap and supply conditions, and that drawing too much makes the output drop off. Under these test conditions, they conclude that stable transfer tops out at a load current of about 30–40 mA, or roughly 150–200 mW.
Building a Smart Band Powered Over NFC
To show a practical application, the presenter built a smart band on a breadboard. The system has two parts:
- Transmitter: the same ML7671 evaluation board.
- Receiver: the wrist-worn band. It uses the ML7670, and after the charging stage a battery module converts the output to 3.3 V. That 3.3 V runs a microcontroller, an accelerometer, and a display.
Running on wireless power alone. The presenter first tested the band with no battery attached. Coil positioning was fairly touchy, so they adjusted it with spacers. Once powered, the display showed live three-axis acceleration readings, and the values changed when the accelerometer was tilted. This showed that the microcontroller, display, and sensor were all running on wireless power alone.
The presenter adds a caveat. The parts are low-power devices, but they still draw more than the components a real product would use. Even so, power delivery stayed stable.
Why a battery is still needed. Running on wireless power alone has an obvious limit: move the band away from the transmitting antenna and it stops, and bring it back and it starts again. The presenter concludes that, in practice, NFC power transfer needs to be paired with a battery.
With a battery. The presenter then added a battery. Placed on the transmitting coil, the band charges wirelessly. Taken off, it keeps running on the battery. The presenter notes this is how ordinary wearables work.
Sending Data Over the Same Link
Until now the video focused on power. The presenter returns to the point that NFC was originally a data technology, one where transit cards exchanged tiny amounts of power during communication. The ROHM parts raise that power to a few hundred milliwatts. The next test is whether the same ICs can also carry data.
In this setup, the smart band sits on top and a receiving device with its own display sits below. Accelerometer data captured on the band is sent to the transmitting side over the power IC's NFC communication and shown on the lower display. When the band is held over the receiver, NFC communication starts and the lower display mirrors the band's accelerometer values as the band moves. When NFC communication is turned off, the lower display keeps showing the last values, because it is implemented as a ring buffer. Holding the band over it again resumes the updates. The presenter reports that data transfer worked without problems.
A Key Limitation: Power and Data Can't Run at the Same Time
The presenter highlights one important constraint. The ICs used in this video cannot do data communication and power delivery simultaneously. Power delivery pauses while data is being sent, so the band needs a battery whenever NFC is used for data.
The presenter mentions another ROHM pair, the ML7661 and ML7660, which have a protocol that automatically alternates between power delivery and communication. With those parts, data can be exchanged without a battery on the receiving side. The presenter suggests they would suit cases where you don't want a battery in the receiver and only need to read data while the devices are in contact.
Bonus: The Receiver Also Works as an NFC Tag
As a brief aside, the presenter holds a smartphone near the receiver board, and it behaves as an NFC tag. Contents stored in the ROHM IC's ROM, in this case a URL for the channel's YouTube page, open on the phone. The phone's NFC antenna communicates with the board through its high-frequency magnetic field, and the board appears to run on that field alone.
Conclusions
The presenter concludes that NFC power transfer delivers much less power than Qi, but lets the antenna and overall system be much smaller. That makes it, in their view, an effective technique for small devices that need contactless power, such as smart rings and earphones. Their own smart band, with a fairly large breadboard circuit, ran without problems on a 250 mW supply.
The main caveat is that the parts tested here cannot deliver power and data at the same time. The presenter points viewers who need both toward ROHM's other model that handles it.
This video is brought to you courtesy of ROHM. Hello. Today we will try wireless power transfer using NFC.
You've all heard the word NFC at least once. I'm sure you've heard of it. It's an abbreviation for Near Field Communication, a short-range wireless communication technology using the 13.56 MHz frequency. Familiar examples include Suica and other transportation system IC cards. NFC technology is used in them. Suica, for example, actually uses FeliCa technology, but this is a type of NFC.
NFC basically uses wireless technology to exchange data. In fact, when communicating, a tiny amount of power is also exchanged. This is why a transportation IC card works even though it does not have a battery in it.
This time, the idea is to increase NFC power so it can be used not only for data communication but also for full-scale power supply. That's today's theme. The key point is that electricity can also be sent. How much power can be delivered specifically? I've also made a device for demonstration. I'll check on that and other things.
First, how does wireless power transfer work? We'll take a quick look at how it's working. The principle is electromagnetic induction. Here are the coil and magnet. When this magnet is moved closer to or further away from the coil, the magnetic flux through the coil changes, and voltage is generated at both ends of the coil. The voltage comes from the change in magnetic flux. This is electromagnetic induction.
This change in flux is critical. If it's too slow, the voltage won't show up. If it's fast enough, you get voltage like this. This is electromagnetic induction.
Now, I'll replace the magnet with this coil. This larger coil is the coil on the power transmission side, and the smaller coil I was using earlier is the coil on the receiving side. An alternating current is passed through this power transmission coil. The alternating current produces a changing magnetic flux. As this transmission coil is moved closer to the receiving coil, voltage comes from the receiving coil, just like when we did this with the magnet.
The magnitude of the voltage coming out also varies depending on the orientation of the coils relative to each other and how close they are. It's the same as when charging your phone or using Suica: if the orientation is strange or they're too far apart, it won't work.
In addition, LEDs have been installed on the coil on the receiving side. As we move the power transmission coil closer, can you see it? You can see the LEDs glowing. Even though they are not connected by a wire, we are able to transmit power. This is the basic mechanism of wireless power transfer using electromagnetic induction. NFC power transfer is also based on this principle.
So far, we have looked at the principle of wireless power transfer. There are various types of wireless power supply. Here is a comparison between Qi power transfer and NFC power transfer. First, you're probably familiar with Qi because it's commonly used for wireless charging of phones.
The major differences between Qi and NFC are, first of all, the frequency bands used. Qi is 110 kHz to 205 kHz. NFC power transfer is 13.56 MHz. Secondly, there is a big difference in the power that can be supplied. Qi is 5 W to 25 W and can deliver a fairly large amount of power. On the other hand, NFC power supply can only provide a few watts at most. The NFC module used in this project is 250 mW. Compared to Qi, NFC power transfer can only send a small amount of power.
If you just look at power, Qi looks better. There are some good points to NFC power transfer: the coil and power supply system can be made smaller. As you can see here, Qi power transmitting and receiving coils are often about 50 mm in diameter. To further increase transmission efficiency, ferrite and other magnetic materials are sometimes used together with them. Therefore, it has the disadvantage of making the system itself larger.
On the other hand, what about NFC? Thanks to higher frequencies, both transmitting and receiving coils can be smaller.
There are also differences in data communication. Qi charging has control communication to monitor the battery's state of charge, but no communication function that allows users to exchange arbitrary data. NFC power transfer, on the other hand, is originally based on NFC technology. It can be used not only for power supply but also for user data communication. We'll demonstrate this later.
As we have seen so far, there is a big difference between Qi and NFC in their features, so the places where they're used differ. Qi is used for things like charging a smartphone, which requires high power and a device large enough to fit the coils. What about NFC power transfer? It is suitable for small devices such as smart rings, smart bands, wireless earphones, and hearing aids. Next, we'll see how much power can actually be supplied using NFC power transfer.
You've seen a little of this already. This is an evaluation board using NFC power supply ICs from ROHM. The large board on the left is the power transmission side. The small board on the right is the power receiving side. In terms of model numbers, the transmission side is ML7671, and the receiving side is ML7670.
Since these parts will be going into small products, small IC packages are used. The receiving side in particular is designed for ultra-miniaturization, using a wafer-level chip-size package.
Let's look at the coils. Because the frequency is high and the power is small, the coil size is quite small. The coils themselves are not wound copper wire like with Qi. They are made with a pattern on a printed circuit board.
By the way, antenna design is quite important with NFC power supply. Since the frequency is as high as 13.56 MHz, antenna matching adjustment is demanding. But ROHM is working with antenna manufacturers to provide verified antennas, matching tools, and even reference designs. I think this will lower the hurdles to antenna design quite a bit.
Incidentally, products using ROHM's NFC power supply IC are already out in the world. There is this smart ring, the SOXAI RING 2. There's a battery inside this ring, and it is powered wirelessly this way. The IC in that power supply section is the same ROHM NFC power supply IC I've been using this time.
The IC used in this project is for consumer products, with a maximum power supply of up to 250 mW. Another line of products for industrial use is also available. That one supports a maximum power supply of up to 1 W.
I'd like to try NFC wireless power transfer. Now let's actually turn on the power... We can't, can we? Actually, because of the Radio Law, we can't turn this on at Ichiken's office and emit a 13.56 MHz high-frequency magnetic field from the coil. I really want to do the experiment, so this time we rented a radio-frequency anechoic chamber that ROHM has and decided to experiment in it.
Here we are in the anechoic chamber. There is an EMC measurement site at the ROHM Yokohama Technology Center. We are borrowing part of it for the experiment.
Now let's turn on the evaluation board again. I just supplied 5 V to the transmission side. It's lighting up. As we move it closer to the power receiving module, it negotiates power transmission, and wireless power supply to the receiver starts. For example, connect an LED to the power receiving side. If you move the modules' coils closer to each other, the LED lights up. It's working fine.
Next, we'll use an oscilloscope to look at the high-frequency magnetic field coming from the power transmission module's coil. If you move the probe closer like this, you can see the 13.56 MHz high-frequency magnetic field pretty clearly. The power receiving module's coil receives this magnetic field, and through rectification, we are able to extract power.
To look at this in a little more detail, the maximum power supply capacity of this evaluation board is 250 mW. I'll do an experiment to see how much power we can actually get. We'll experiment with a 3 mm spacer sandwiched between them like this.
I'm connecting a resistive load right now. A current of 1 mA is flowing. Let's make the load even heavier. It looks like this. It's 2 mA now. Increasing the current further, 10 mA is flowing. This is now transmitting 50 mW of power.
Then you can make the load even heavier. As you increase the load, you can see that the current is occasionally interrupted. That is, there is a period when the voltage is 0 V. This is because the wireless power transmission power is being adjusted.
As the load is made even heavier, it's 20 mA. The power is transmitted at about 100 mW. Now 150 mW. We're at 40 mA now, which is about 200 mW. Increasing the load even further, I think I let a little too much current flow, about 70 mA. This is running at 50 mA now. It's about 250 mW.
But the power supply is quite sensitive to things like the gap between these coils. If you draw too much with the load, it drops out. In this case, a load current of about 30 mA to 40 mA, or about 150 mW to 200 mW in terms of power, was the upper limit for stable power transmission.
Next, I'll use this evaluation board to do a little crafting. I'm building a circuit on a breadboard like this. By the way, we're planning a giveaway of the light blue electronics mat you can see now. Please watch the video to the end. I hope you will apply. This electronics mat is quite useful.
This time, as an example application of NFC power transfer, I made a smart band. Here it is. First, let me explain the configuration. The system is divided into two main parts: the transmission side and the smart band side that goes on your wrist. This is the receiving side.
For the transmission side, this is the ML7671 evaluation board we just used. The smart band on the receiving end also uses ML7670. After charging, the power goes through the battery module and is converted to 3.3 V to drive the microcontroller, accelerometer, and display.
First, without connecting the battery, we'll check whether the smart band works just on power from wireless power transfer. The positioning of the coils was a bit demanding, so I adjusted the position with spacers like this. When I turned on the power, something appeared on the display. It displays acceleration for three axes measured by the accelerometer. If you tilt the accelerometer here, the values change properly, so the sensing is working.
In this way, the microcontroller, display, and sensor are all working on wireless power transfer alone. Although the microcontroller and other components used here are low-power devices, their power consumption is still high compared to devices that would be used in a product. Even with such devices, power is being supplied stably.
However, since it is running only on wireless power, if you move it away from the antenna on the transmission side, the power supply is naturally interrupted and operation stops. It works like this: it stops when moved away, and when you bring it close, it works again. Therefore, when using wireless power supply such as NFC power transfer, a battery will be needed in combination with it.
So I built a battery into the smart band. Battery charging has begun. Since the battery is in place, the smart band keeps working away from the power transmission coil. When placed on the power transmission coil, it charges wirelessly. When you take it off and carry it around, it runs on battery power. We've got something similar to common wearable devices.
So far, I have mainly been using the evaluation board for wireless power supply. Originally, NFC was used for data communication. Even during data communication with transportation IC cards, there was a tiny amount of power exchange. This ROHM device increases that to a few hundred milliwatts so it can be used for a full-scale power supply. Next, I'll try using it to send data.
I created this experimental setup. Above is the smart band. Below is the device that receives the smart band's data. What does that mean? Data acquired by the accelerometer on the smart band side is sent to the power transmission side using the power supply IC's NFC communication. It is then displayed on a display on the power transmission side.
If you hold it up like this, NFC communication starts, and the data is displayed on the receiving device. When you move the accelerometer like this, can you see it? The same data on the smart band side is also shown on the transmission side. It looks like this.
Here's what happens when you turn off NFC communication. It goes like this. The lower display uses a ring buffer to keep displaying that value. If you hold it up again, it looks like this. I am able to use NFC for data communication without any problem.
There is one important point here. The wireless power transfer IC used in this project cannot perform data communication and power supply at the same time. Power supply is temporarily stopped during communication. Therefore, when using NFC for data communication, a battery is required on the smart band side.
Incidentally, ROHM's other models, ML7661 and ML7660, are equipped with protocols that automatically alternate power supply and communication. In that case, data can be exchanged without a battery. If you don't want to put a battery on the receiving end and only need to get data when in contact, these seem like a good choice.
To digress for a moment, if you hold your phone closer to the board on the receiving end like this, it works as an NFC tag. The contents are recorded in ROM in the ROHM IC. In this case, I've set it to open a YouTube URL. Data is communicated through the high-frequency magnetic field coming from the phone's NFC antenna. The board appears to operate even with the high-frequency magnetic field used for data communication.
Today, we tried wireless power transfer using NFC from ROHM. NFC can send less power than Qi, but the antenna and system can be made much smaller. Therefore, I thought it was quite an effective technique for small devices that require a contactless power supply, such as smart rings and earphones.
I actually made a smart band this time. With a circuit of this size, it worked without any problems even with a 250 mW power supply. The model we used this time could not perform data communication and power supply at the same time. Depending on your use, there is an industrial model that can do that, so you could use it.
Last but not least, we would like to announce a giveaway! An electronics work mat with the ROHM logo and ESD tweezers, as used in this video, will be given to 10 winners selected by drawing. For more information on how to apply, please check the link in the description.
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