1. 1University of Colorado Boulder
30 W Capacitive Wireless Power Transfer System
with 5.8 pF Coupling Capacitance
Chieh-Kai Chang, Guilherme Goularte da Silva,
Ashish Kumar, Saad Pervaiz,
Khurram K. Afridi
University of Colorado Boulder
WPTC
May 13, 2015

2. 2University of Colorado Boulder
Outline
• Motivation
• Proposed Capacitive Wireless Power Transfer Architecture
• Prototype Design and Experimental Results
• Comparison with Existing Systems
• Summary and Conclusions

3. 3University of Colorado Boulder
Why Capacitive Wireless Power Transfer?
Inductive WPT system
• Bulky and expensive shielding ferrites required
• Cannot transfer power across metallic barriers
Capacitive WPT system
• Eliminates ferrite weight and cost
• Capable of transferring power across metallic barriers

4. 4University of Colorado Boulder
State of the Art
• Our target air-gap: 5 mm
• Our target plate area: 25 cm2
• Resultant coupling capacitance: < 6 pF
Inductive WPT Capacitive WPT
Maximum
Power
20 kW [1]
𝜂 = 87%
Air-gap = 250 mm
1.2 kW [3]
𝜂 = 83%
Air-gap = 0.25 mm
Maximum
Efficiency
96% [2]
Pout = 100 W
Air-gap = 200 mm
84% [4]
Pout = 3.72 W
Air-gap = 0.13 mm
[1] B. Song et al., “Design of a high power transfer pickup for on-line electric vehicle,” IEVC 2012.
[2] T. Imura et al., “Basic experimental study on helical antennas of wireless power transfer for Electric Vehicles by
using magnetic resonant couplings,” VPPC 2009.
[3] J. Dai et al., “Wireless Electric Vehicle Charging via Capacitive Power Transfer Through a Conformal Bumper,”
APEC 2015.
[4] M. Kline et al., “Capacitive Power Transfer for Contactless Charging,” APEC 2011.

5. 5University of Colorado Boulder
Typical Capacitive WPT System
Limitations
• The maximum power transfer capability is given by:
Pmax = π 2 KrecVOUT C VC,maxfs
• C determined by plate area and air-gap
• VIN, and fs limited by available devices
• VOUT limited by applications
• VC,max is desired to be as high as possible
LcompC
C
High
Frequency
Inverter
High
Frequency
Rectifier
Load
VIN VOUT
-
Lcomp
+

6. 6University of Colorado Boulder
Proposed Capacitive WPT System Architecture
Composite Matching Network
• Provides voltage or current gain
• Reduces compensating inductance requirements by two orders of magnitude
Load
Composite
Matching
Network
High
Frequency
Inverter
VIN
Composite
Matching
Network
High
Frequency
Rectifier
VOUT
+
-
C
C

7. 7University of Colorado Boulder
Proposed Capacitive WPT Topology
Our Design
• Composite matching network comprises
─ Transformer (voltage gain, parasitic capacitance Cp)
─ Parallel L-C matching network (voltage gain, compensating inductance)
• Compensating inductance (Lcomp) reduced by 99.5% (from 2.3mH to 12uH)
VOUT
VIN
Q1
Q2
Q3
Q4
CIN COUT
D1
D2
D3
D4
Lcomp
C
C
1:NTX
Cp

8. 8University of Colorado Boulder
Prototype Design
Parameter Label Value
Air-gap lg 5 mm
Plate Area A 25 cm2
Effective Coupling Capacitance Ceff 5.76 pF
Compensating Inductance Lcomp 12 μH
Operating Frequency fs 1.4 MHz
Transformer Turns Ratio NTX 5.8
Matching Network Voltage Gain NMN 2.4
Rated Output Power: 100 W

9. 9University of Colorado Boulder
Prototype

10. 10University of Colorado Boulder
Experimental Results
VIN = 28.8 V,
VOUT = 251 Vrms,
POUT = 31.8 W
η = 84.13 %

11. 11University of Colorado Boulder
Efficiency v.s. Power Transferred
Efficiency stays above 80% from 10 to 105 W
50%
60%
70%
80%
90%
0 20 40 60 80 100 120
Power Transferred [W]
Efficiency

12. 12University of Colorado Boulder
Comparison with Existing Systems
Transfers higher power per unit area while having 5 times bigger air-gap
[3]
[4]
[6]
This Paper
0
1
2
3
4
5
6
7
0 1 2 3 4 5 6
PowerTransferperUnitArea
[kW/m2]
Air-gap [mm]
[3] J. Dai et al., “Wireless Electric Vehicle Charging via Capacitive Power Transfer Through a Conformal Bumper,”
APEC 2015.
[4] M. Kline et al., “Capacitive Power Transfer for Contactless Charging,” APEC 2011.
[6] C. Liu, et al., “Modelling and Analysis of a Capacitively Coupled Contactless Power Transfer System”, IET Power
Electronics, 2011.

13. 13University of Colorado Boulder
Comparison with Existing Systems
More than 20 times higher power transfer per unit coupling capacitance
[3] J. Dai et al., “Wireless Electric Vehicle Charging via Capacitive Power Transfer Through a Conformal Bumper,”
APEC 2015.
[4] M. Kline et al., “Capacitive Power Transfer for Contactless Charging,” APEC 2011.
[5] H. Fnatoet al., “Wireless Power Distribution with Capacitive Coupling Excited by Switched Mode Active Negative
Capacitor,” ICEMS 2010.
[6] C. Liu et al., “Modelling and Analysis of a Capacitively Coupled Contactless Power Transfer System”, IET Power
Electronics 2011.
[7] M.P. Theodoridis, “Effective Capacitive Power Transfer,” IEEE Transactions on Power Electronics 2012.
[8] L. Huang et al., “A Resonant Compensation Method for Improving the Performance of Capacitively Coupled Power
Transfer System,” ECCE 2014.
0.1 0.06 0.12 0.03 0.27 0.04
5.52
0
2
4
6
[3] [4] [5] [6] [7] [8] This
Paper

14. 14University of Colorado Boulder
Summary and Conclusions
• Proposed a novel capacitive WPT architecture
• Designed and built a demonstrative prototype
• More than 20 times higher power transfer capability per unit coupling
capacitance than existing systems
• Very high power transfer density

15. 15University of Colorado Boulder
Thanks!

16. 16University of Colorado Boulder
References
[1] B. Song, J. Shin, S. Lee, S. Shin, Y. Kim, S. Jeon, and G. Jung, “Design of a high power transfer
pickup for on-line electric vehicle (OLEV),” Proceedings of the IEEE International Electric Vehicle
Conference (IEVC), , pp. 1-4, Greenville, SC, March 2012.
[2] T. Imura, H. Okabe, and Y. Hori, “Basic experimental study on helical antennas of wireless power
transfer for Electric Vehicles by using magnetic resonant couplings,” Proceedings of the IEEE Vehicle
Power and Propulsion Conference (VPPC), pp. 936-940, Dearborn, MI, September 2009.
[3] J. Dai and D. C. Ludois, “Wireless Electric Vehicle Charging via Capacitive Power Transfer Through a
Conformal Bumper,” Proceedings of the IEEE Applied Power Electronics Conference and Exposition
(APEC), Charlotte, NC, March 2015.
[4] M. Kline, I. Izyumin, B. Boser and S. Sanders, “Capacitive Power Transfer for Contactless Charging,”
Proceedings of the IEEE Applied Power Electronics Conference and Exposition (APEC), pp. 1398-
1404, Fort Worth, TX, March 2011.
[5] C. Liu, A.P. Hu and N.K.C. Nair, “Modelling and Analysis of a Capacitively Coupled Contactless Power
Transfer System”, IET Power Electronics, vol. 4, issue 7, pp. 808-815, August 2011.