Resonant inductive
Wireless energy transfer
Flemming Nyboe (fne@teknologisk.dk)
What is going on now?
„
Recently increased interest in wireless energy transfer is
driven by a growing number of target applications, rather
than new technology
„
Semiconductor industry partners have formed the Wireless
Power Consortium, defining a standardized interface
between wireless chargers and devices
„
As of Oct. 2010, no chipsets for are out, but products
utilizing wireless energy transfer are introduced at an
increasing rate (Dell,
(Dell Nintendo,
Nintendo and many others)
„
Claims that wireless charging conserves energy are dubious
(assume that wireless chargers have lower stand by power
loss)
„
http://en.wikipedia.org/wiki/Wireless_energy_transfer
„
http://www.wirelesspowerconsortium.com/
2
Weak coupling and resonance
„
Coupling coefficient k << 1 is the fraction of shared field
„
Only a small fraction of the oscillating energy is absorbed by
the receiver each cycle
„
Transfer efficiency depends on the ratio of energy absorbed
by the receiver to the energy lost in transmitter (not to the
oscillating energy)
„
This is the essence of resonant energy transfer
3
Link equivalent circuit
„
The inductors form a transformer with coupling coefficient
k << 1
„
Traditional non-ideal
non ideal transformer circuit theory applies
„
The primary side is driven by an oscillator
„
The receiver LC circuit is loaded (except for heater
applications)
4
Coupling coefficient k
„
From http://www.wirelesspowerconsortium.com/
5
Link efficiency
„
The coupling coefficient k is determined only by coil
diameters and positions
„
The fractions of energy lost in the transmitter and receiver
each cycle are quantified by 1/QT and 1/QR respectively
„
The transfer efficiency depends solely on k, QT and QR
k 2 ⋅ QT ⋅ QR
η=
1 + k 2 ⋅ QT ⋅ QR
where QR is loaded receiver Q
„
Independent of frequency, inductances, turns etc.
„
The practical
Th
ti l d
design
i
challenge:
h ll
F
For a given
i
coil
il geometry
t
(given k), design LC circuits with the largest possible Q
„
Free design parameters:
• Frequency
• Coil # of turns / wire diameter (equal cross section)
6
Coil design for maximum Q
1/2
41mm dia
1400
„
Radiation
losses
d
dominate
i t att
high
frequencies
„
Adding turns
Addi
helps, because
L increases
more than Rs
„
Capacitance
values become
impractically
small
1200
Coil Q
1000
800
N=50, 0.25mm wire
N 40 0
N=40,
0.28mm
28
wire
i
N=30, 0.32mm wire
N=20, 0.40mm wire
N=10, 0.56mm wire
600
400
200
0
0
50
100
150
200
250
freqency (MHz)
300
350
400
7
Coil design for maximum Q
2/2
41mm dia, fixed 10pF Capacitance
320
„
Fixed 10 pF
capacitance
(f robustness)
(for
b t
)
„
Weak increase
with frequency
i only
is
l caused
d
by proximity
effect and nonideal magnetic
coupling
between turns
310
300
Coil Q
290
N=10, 0.56mm wire
N=20, 0.40mm wire
N 30 0
N=30,
0.32mm
32
wire
i
N=40, 0.28mm wire
N=50, 0.25mm wire
280
270
260
250
240
4
6
8
10
12
14
freqency (MHz)
16
18
20
8
Transmitter and receiver circuitry
„
Transmitter circuits are oscillators.
„
There are 4 fundamental ways to drive an LC tank:
Parallel or series drive,
drive voltage or current source
„
An efficient choice is Class C or E drivers
(parallel voltage drivers)
„
Receiver circuits may be as simple as a half-wave rectifier,
but additional control is needed for powering electronics, or
charging applications
Class E driver for transcutaneous power and data link for
implanted electronic devices
M di l and
Medical
d Biological
Bi l i l Engineering
E i
i
and
d Computing,
C
ti
Vol.
V l 30
issue 1 (1992)
¾ http://www.iis.fraunhofer.de/EN/bf/ec/pbm/index.jsp
(papers at bottom)
¾
9
Demo
„
Transmitter
• Class E driver, 330 VRMS @ 13.56 MHz, 1W
• 41 mm diameter, 8 turns ø1mm Cu, spaced 1mm
• L=2.7 µH, Rs=0.52 Ω, (C=52 pF), Q > 400
„
Receiver
• 8mm outer diameter, 16 turns ø0.25mm Cu
• L=1.8 µH, Rs=1.50 Ω, (C=78 pF), Q = 100
• 2 / 16 output
t t tap
t
„
Measured link efficiency (excl. driver, incl. receiver rectifier)
• 29 % @ 4 cm (~2 coil diameters, k=0.0083)
• 4 % @ 6 cm (~3 coil diameters, k=0.0030)
10
Compliance – EMC & Radio
„
EMC
• Any radiated emission limit below 30 MHz in Europe ?
„
Danish radio frequency interface 00 032, unlicensed
• 6,765-6,795 MHz 42 dB(µA/m) @10m
BW=0.44%
• 13,553-13,567
,
,
MHz 42 dB(µA/m)
(µ / ) @10m
@
BW=0.10%
• 26,957-27,283 MHz 10 mW e.r.p.
BW=1.20%
• 40,660-40,700 MHz 10 mW e.r.p.
BW=0.10%
• 10mW
10 W e.r.p. in
i a loop
l
is
i 45.2dB(µA/m)
45 2dB( A/ ) @ 10m
10
• Demo transmission coil has Rs=1.24Ω and Rr=228μΩ @
27 MHz. Burning 1W in Rs results in 0.18mW radiated
power. This
Thi iis 25
25.9dB(µA/m)
9dB( A/ ) @ 10m
10
11
Compliance – Exposure limits
„
Figure from ICNIRP guidelines 1998
„
Regulated by
EN 62233
„
Measurement
distance is 30
cm for most
applications
li
i
„
Limit @ 13.56
MHz is 92 nTRMS
„
6.25 µTRMS
allowed below
150 kHz
12
LC tuning and manufacturability
„
Alignment of 3 frequencies
• Transmitter LC resonance (mostly for drivability)
• Receiver LC resonance
• Transmission frequency
„
1.2% ((best case)) allowed transmission frequency
q
y window
can not be hit by an out-of-the-box LC, so transmission
frequency must be determined by other means than
transmitter LC resonance
„
LC circuit Q > 200 implies that in practice, center frequency
variance will be much larger than usable bandwidth
„
It would be desirable to auto
auto-tune
tune resonances during
operation. Some methods for this have been described in
literature.
¾
Inductive Powering Van Schuylenbergh, Puers, p. 173
13
Conclusions and outlook
„
Practical transfer distance is limited by roughly 3 times coil
diameter
„
EMC, radio and exposure limits are not necessarily
EMC
prohibitive
„
Special attention must be paid to manufacturability of tuned
high Q LC circuits
high-Q
„
Auto-detection of load/transmitter presence is often
desirable
14