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.
