Chris Burket presents Magnetic Shield Influence on Low Power Wireless Power Designs as part of the International Wireless Power Summit 2013 in Austin, Texas.

1. Magnetic Shield Influence
on
Low Power Wireless Power Designs
Presented
by:
Chris T. Burket
Sr. Marketing Engineer - TDK
chris.burket@us.tdk.com
714-934-7373

2. Shield and Coil Assembly Basics

3. Wireless Power Summit - 2013
Magnetic Induction System Example

4. Wireless Power Summit - 2013
•
•
•
•
•
•
Design Issues of Coil Assemblies
Required inductance value (key to coupling/resonance)
• Permeability of magnetic shield
• # of turns
• # of winding layers
Saturation current of shield material (Bs of material)
Stability of inductance value (both inductive and resonance applications)
• Over temp
• Over current
• Due to proximity to battery
• Due to end application packaging
Coil type selection
• Litz, bifilar, single strand wire, PCB coils, flex printed circuit, etc.
• Gauge, Cu weight
• Cost vs. performance between PCB coils and wire coils
Coil winding diameter (ID – inner diameter)
Coil wire (windings) proximity issues (resonance applications)

5. Wireless Power Summit - 2013
Role of Magnetic Shields
• “u” value helps determine inductance value of coil assembly
• Completes the magnetic loop path between Tx – Rx coil
assemblies (1/2 transformer per side)
• Shapes the magnetic flux field
• Maintains (encapsulates) magnetic flux field in between 2
parallel shields  improving coupling/efficiency
• Keeps magnetic flux field from escaping out top/bottom and
thus getting into the battery area (or other metal objects)
causing Eddy current losses  heat build up  reduction of
inductance  reduction of coupling  loss of efficiency
• u’ and u” (u’/u” or Q) greatly influenced by shield material 
critical for resonance technologies
• Helps keep the Ls value constant in various environments

6. Wireless Power Summit - 2013
Role of Magnetic Shields

7. Wireless Power Summit - 2013
Magnetic Field Analysis

8. Wireless Power Summit - 2013
Inductive System Losses ~ Equal Thirds
Courtesy of TI

9. TDK Shield Material Comparison

10. Wireless Power Summit - 2013
TDK Material Comparison
TDK currently uses 6 main shield materials for Tx, Rx
coils used in WPC, PMA and resonant applications:
• FK2 – Ni-Zn solid ferrite or pre-cracked (used for most inductive
Tx coil assemblies, small size Rx coil assemblies)
• FK5 – Ni-Zn ferrite sheet, pre-cracked, Rx side
• FJ3 – Ni-Zn ferrite sheet, pre-cracked, Rx side (used for NFC
combo coils, inductive and resonant applications)
• FJ7 – Ni-Zn ferrite sheet, pre-cracked, Rx side for
magnetic resonant applications in the 6.78 MHz range
• MS2 – powdered iron sheet, pre-cracked, Rx side (used in high
saturation applications)
• MS6 – polymer material impregnated with powdered iron, Rx
side (used for ultra-thin applications)

11. Wireless Power Summit - 2013
Material Comparison

12. Wireless Power Summit - 2013
Material Comparison – Ferrite Based

13. Wireless Power Summit - 2013
Material Comparison – Powdered Iron Based

14. Magnetic Induction Shield Influence Comparison

15. Wireless Power Summit - 2013
Magnetic Shield Influence on System Performance
1) Efficiency of coil w/ battery for a WPC Tx A1/WR-4832 Rx coil system
2) Efficiency vs shield thickness w/ and w/o battery vs material
3) Efficiency using WPC Tx A1 w/ and w/o magnet (same Tx coil) vs material
4) Efficiency vs. Rx Ls value
5) Efficiency vs. Rx coil size
6) Efficiency vs. distance between Tx-Rx coils
7) Rs (Rac) of multi-strand Litz wire vs. single strand wire vs. frequency
8) Cracked vs. non-cracked sheet efficiency
9) Efficiency of coil w/ Cu sheet for a WPC Tx A1/WR-3832 Rx coil system

16. Wireless Power Summit - 2013
1 - 3) Efficiency vs. Shield Material vs. Shield Thickness
•
Tested with WPC Tx/A1 coil, output current at 0.7A

17. Wireless Power Summit - 2013
WPC A1 Magnet Impact on Performance

18. Wireless Power Summit - 2013
1 - 3) Efficiency vs. Various Shield Materials
Test with Tx/A1 coil, output current is 0.7A
•
80.0
75.0
Efficiency(%)
FK5(48x32xt0.3)
FK5(48x32xt0.5)
70.0
FJ3(48x32xt0.25)
FJ3(48x32xt0.5)
65.0
MS2(48x32xt0.3)
MS2(48x32xt0.5)
MS6(48x32xt0.25)
60.0
MS6(48x32xt0.5)
55.0
50.0
Only Coil
with Magnet
w/o Battery
w/o Magnet
with Battery
with Magnet
with Battery

19. Wireless Power Summit - 2013
4) Efficiency vs. Ls Value
Using 3 different shield materials for Rx coil, varying windings  inductance
Tested with Tx/A1 coil, output current at 0.7A
FJ3(44x40xt0.3)
MS6(44x40xt0.25)
MS2(48x32xt0.5)
Ls(uH)
Efficiency(%)
Ls(uH)
Efficiency(%)
Ls(uH)
Efficiency(%)
23.1
65.6
19.7
63.4
12.6
70.8
21.8
66.3
18.8
62.9
11.8
70.8
20.4
66.9
17.7
63.7
10.8
70.6
19.1
66.2
16.9
63.7
9.5
69.1
16.1
66.4
14.5
62.7
8.0
67.6
11.6
63.8
12.1
60.9
10.0
61.6
9.6
55.1
80.0
75.0
Efficiency(%)
•
•
70.0
FJ3(44x40xt0.3)
MS6(44x40xt0.25)
MS2(48x32xt0.5)
65.0
60.0
55.0
50.0
5.0
7.0
9.0
11.0
13.0
15.0
Ls(uH)
17.0
19.0
21.0
23.0
25.0

20. Wireless Power Summit - 2013
5) Efficiency vs. Rx Coil Size
All data is with battery. (Battery size is 54 x 83 x t2.9 mm)
All sheet thickness at t=0.5mm
Tested with Tx/A1 coil, output current at 0.7A
MS2(with
MS2(w/o
FK5(with
FK5(w/o
Magnet)
Magnet)
Magnet)
Magnet)
4832size
68.5
71.1
68.9
71.7
3832size
66.3
70.3
58.0
71.6
3030size
61.3
65.2
57.5
65.9
80.0
75.0
Efficiency(%)
•
•
•
70.0
MS2(with Magnet)
MS2(w/o Magnet)
65.0
FK5(with Magnet)
FK5(w/o Magnet)
60.0
55.0
50.0
4832size
3832size
3030size

21. Wireless Power Summit - 2013
6) Efficiency vs. Distance Between Tx-Rx Coils
Tested with Tx/A1 coil, output voltage at 5V
•
Distance(mm)
2.4
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
MS2(38x32xt0.5)
Efficiency(%)
71.7
71.7
71.1
70.2
69.5
69.2
69.1
68.7
68.0
66.6
0.0
MS2(48x32xt0.5)
MS2(38x32xt0.5)
with Battery
MS2(48x32xt0.5)
with Battery
Efficiency(%)
72.8
72.8
72.6
72.4
72.1
71.2
70.8
70.6
69.1
68.3
0.0
Efficiency(%)
68.9
68.9
67.6
67.6
65.8
63.5
63.5
62.6
60.3
56.4
0.0
Efficiency(%)
72.1
71.7
70.7
69.8
68.3
67.4
65.6
63.9
61.1
58.5
0.0
80.0
75.0
Efficiency(%)
MS2(38x32xt0.5)
70.0
MS2(48x32xt0.5)
MS2(38x32xt0.5)
with Battery
65.0
MS2(48x32xt0.5)
with Battery
60.0
55.0
50.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
Distance between Tx-Rx(mm)
8.0
9.0
10.0
11.0
12.0

22. Wireless Power Summit - 2013
7) Rs of Multi-strand Litz Wire vs. Single Strand Wire at 100 KHz
Using Tx/A1 coil structure
Inner coil diameter set to φ8.0 mm for both.
•
•
Wire type
(wire dia mm)
Sheet
No of Turn
Outer size of
coil(mm)
Ls
(uH)
Rs
(Ω)
Q
Single
(φ0.45mm)
FK2(1mm)
24
φ31.2
17.17
0.41
25.29
FK2(1mm)
26
φ33.0
21.09
0.47
26.90
Litz wire
(φ0.08mmx30)
FK2(1mm)
24
φ41.0
20.25
0.23
52.80
FK2(1mm)
26
φ44.0
24.76
0.26
56.90
2.0
24
1.8
1.6
Litz wire(φ0.08mmx30)
- 24 Turns
22
20
Litz wire(φ0.08mmx30)
- 26 Turns
18
Single(φ0.45mm)
- 24 Turns
16
Single(φ0.45mm)
- 26 Turns
14
Litz wire(φ0.08x30mm)
- 24 Turns
1.4
Rs(Ohm)
Inductance(μH)
26
1.2
Litz wire(φ0.08x30mm)
- 26 Turns
1.0
Single(φ0.45mm)
- 24 Turns
0.8
0.6
Single(φ0.45mm)
- 26 Turns
0.4
12
0.2
0.0
10
50
100
150
200
Frequency(kHz)
250
300
50
100
150
200
Frequency(kHz)
250
300

23. Wireless Power Summit - 2013
8) Cracked vs. Non-cracked Shield Efficiency
•
•
•
Using FJ3 material, shield size 43.5 x 39.5 x t0.3 mm
Tested with battery
Tested with Tx/A1 coil, output current at 0.7A

24. Wireless Power Summit - 2013
9) Efficiency of Coil w/ Cu Plate
•
•
Using Rx coil/WR-383250 (MS2)
Test with Tx/A1 coil, output current is 0.7A

25. Magnetic Resonance Shield Influence

26. Wireless Power Summit - 2013
Magnetic Resonance Shield Influence
•
•
•
•
•
Key parameters in resonance system are coupling (typically K < 0.5) and
Quality Factor (Q) of the coil assemblies.
K depends on:
• u’ of material, some minimum value, above which, limited impact
• Coil winding dimensions
• Shield dimensions/area
• Distance between coil assemblies
Q depends on:
• Size/shape, configuration of shield
• u’, u” of shield material
• Frequency of application
• Type of coil used, winding proximity, coil pattern for field shaping
• Operating power level
Figure of merit value of
is used to determine coil parameters
Higher Q material/coils, allow for use over broader range of distances

27. Emerging Applications Technical Challenges

28. Wireless Power Summit - 2013
15-30W Application Issues
• Targeting tablets, notebooks, laptops, power tools, appliances
• Higher current applications need higher Bs shield material to
reduce saturation effects
• AC resistance (Rac) is critical for reducing losses  typically
Litz wire, I2 x Rac losses create heat problems
• Adding thin sheet of higher “Bs” value magnetic material for
handling higher current/preventing saturation is an option
• Shield size important to ensure good coverage over battery,
other critical components
• Thickness limitations per end application affect performance

29. Wireless Power Summit - 2013
Typical Coil Assembly Stack-up – 20W Rx Example

30. Wireless Power Summit - 2013
20W Rx Examples
without IFL10M 0.1mm
TDK EX1
TDK EX2
with IFL10M 0.1mm
TDK EX3
TDK EX4
Sharp Inquiry
φ35
WLC Coil total Thickness [mm]
WLC Coil wire
WLC Outer Coil size [mm]
WLC Inner Coil size [mm]
WLC Turn
WLC Ls @100kHz [uH]
WLC Rs @100kHz [Ohm]
WLC Q @100kHz
1.0mm
(FK2)
1.62
0.4mm Copper Wire
φ32.8
φ10
WLC Sheet thickness [mm]
φ35
1.0mm
(FK2)
WLC Outer Size [mm]
2.91
0.08mm x 50
φ33.8
φ7.9
15Ts/1Layer
Total 2Layer
24.68
23.25
17.06
19.11
0.16
0.27
0.60
0.76
101.88
55.54
18.27
16.29
26
1. Only Coil
2. WPC A1 Tx
3. WPC A1 Tx + Batt.
4. WPC A1 Tx + Cu.
1. Only Coil
2. WPC A1 Tx
3. WPC A1 Tx + Batt.
4. WPC A1 Tx + Cu.
1. Only Coil
2. WPC A1 Tx
3. WPC A1 Tx + Batt.
4. WPC A1 Tx + Cu.
24.32
19.84
12.93
15.40
0.46
0.49
0.87
1.08
32.09
25.57
9.33
9.21
φ35
1.0mm (FK2)
+
0.1mm (IFL10M)
1.72
0.4mm Copper Wire
φ32.8
φ10
26
24.41
20.52
14.76
16.08
0.48
0.51
0.79
0.90
32.01
25.55
11.39
10.77
φ35
1.0mm (FK2)
+
0.1mm (IFL10M)
3.01
0.08mm x 50
φ33.8
φ7.9
15Ts/1Layer
Total 2Layer
24.86
24.41
19.38
20.95
0.16
0.28
0.51
0.67
103.21
55.26
24.33
19.54

31. Issues With Small Diameter Rx Coils

32. Wireless Power Summit - 2013
Rx Coil << Tx Coil
• Number of turns need to be higher to achieve required inductance
value  DCR is higher
• Typically target higher inductance  higher coupling  higher
efficiency (for inductive)
• Typically, more turns on smaller shield requires smaller diameter wire to
be used  DCR is higher
• Higher “u”, lower Bs material is used in order to boost inductance
• Typically have height restriction  higher wire gauge, thinner shield
• Current levels are usually lower  Rac not as critical
• Q
winding diameter  Q is lower (key to magnetic resonance)

33. Multiple Winding Coils

34. Wireless Power Summit - 2013
Dual Mode Coils – The Latest Trend
•
•
•
•
•
•
WPC + PMA (Dual Mode coils)
WPC and/or PMA + resonance
WPC and/or PMA + resonance + NFC (13.56 MHz)
WPC or PMA + NFC (Combo coils)
WPC + PMA can use same shield material, issue on resonance caps
WPC or PMA + resonance, issue on optimizing shield material at 100300 KHz and 6.78 MHz. One or both may be reduced in efficiency.
Dual Mode Coils
Combo Coils

35. TDK’s Wireless Power Products

36. Wireless Power Summit - 2013
What is TDK Capable of Doing?
• Supplier of ferrite, powdered iron and polymer cores/shields for:
• Inductive applications (WPC, PMA, others)
• Resonant applications (A4WP, others)
• Supplier of coil assemblies (shield + windings)
• Standard and custom for low power (up to 20W currently)
• Wire and PCB based coils
• Supplier of dual mode (WPC+PMA or WPC or PMA + resonant coil) and
combo coils (WPC or PMA + NFC)
• Supplier of materials
• Ferrite “tiles” which are glued/pieced together to make large sheets
(up to the KW range)
• Shields for applications where the windings are in other format (flex
circuit, PCB, etc.)
• Supplier of complete modules

37. Wireless Power Summit - 2013
Custom Coil Assemblies
Custom coils will be considered on a case by case basis,
depending on many business factors. TDK is current
supporting:






WPC applications
PMA applications
A4WP applications
Witricity
PowerByProxi
Others