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

2University of Colorado Boulder

Outline

• Motivation

• Proposed Capacitive Wireless Power Transfer Architecture

• Prototype Design and Experimental Results

• Comparison with Existing Systems

• Summary and Conclusions

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

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

MaximumPower

20 kW [1] = 87%

Air-gap = 250 mm

1.2 kW [3] = 83%

Air-gap = 0.25 mm

Maximum Efficiency

[2]Pout = 100 W

Air-gap = 200 mm

[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.

5University of Colorado Boulder

Typical Capacitive WPT System

Limitations

• The maximum power transfer capability is given by:

• determined by plate area and air-gap

• , and limited by available devices

• limited by applications

• is desired to be as high as possible

LcompC

C

High

Frequency

Inverter

High

Frequency

RectifierLoadVIN VOUT

-

Lcomp

+

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

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)

VOUTVIN

Q1

Q2

Q3

Q4

CIN COUT

D1

D2

D3

D4

Lcomp

C

C

1:NTX

Cp

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

9University of Colorado Boulder

Prototype

10University of Colorado Boulder

Experimental Results

VIN = 28.8 V,

VOUT = 251 Vrms,

POUT = 31.8 W

= 84.13 %

11University of Colorado Boulder

Efficiency v.s. Power Transferred

Efficiency stays above 80% from 10 to 105 W

0 20 40 60 80 100 12050%

60%

70%

80%

90%

Efficiency

Power Transferred [W]

12University of Colorado Boulder

Comparison with Existing Systems

Transfers higher power per unit area while having 5 times bigger air-gap

0 1 2 3 4 5 60

1

2

3

4

5

6

7

[6]

Air-gap [mm]

Po

we

r T

ran

sfe

r p

er

Un

it A

rea

[kW

/m2

]

[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.

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.

[3] [4] [5] [6] [7] [8] This Paper

0

2

4

6

0.1 0.06 0.12 0.03 0.27 0.04

5.52

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

15University of Colorado Boulder

Thanks!

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.