a 36 w wireless power transfer system with 82% efficiency

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Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, 2013

1. IntroductionRecently, interest in wireless power transfer (WPT)

technology has been growing substantially. This technol-

ogy can be applied to charge or power electronic devices

wirelessly. WPT further contributes to technical fields

such as medical science and automobile industry. A WPT

system consists of a transmitter, coupling coils, a receiver,

and matching circuits. The matching circuits between

each system block are introduced to transform the imped-

ance in order to confirm the system blocks operate at cor-

rect input and output impedance condition. However, the

matching design depends on the load resistance in the

receiver. When the resistance changes, the efficiency of

the receiver drops due to the mismatch, resulting in a drop

in coupling coefficient which consequently causes a drop

in efficiencies of the transmitter and overall system. In this

study, the turn-on resistance of the LED module is 30 Ω,

thus, the system design is based on this condition.

Another design challenge is the transmitter circuit. It

determines the performance of WPT systems. Generally,

Class-D power amplifiers are chosen to realize the trans-

mitter, but it requires at least two transistors to achieve

zero voltage switching (ZVS), increasing the cost of prod-

ucts and introducing process yield problems. This study

selects Class-E power amplifier circuit structure which is

also a switching type amplifier. It has fewer components

with high reliability and a low-cost advantage, with a theo-

retical efficiency of 100%.[1] However, compared to Class-

D power amplifiers, Class-E power amplifiers are more

sensitive to the output impedance. Understanding the

methodology on how to design an accurate output imped-

ance of the Class-E power amplifier is the crucial aspect in

the system implementation. In this study, the matching

circuit between the Class-E power amplifier and transmit-

ting coil is combined with the capacitor of the resonating

circuit in the Class-E power amplifier, simplifying the sys-

tem structure while keeping the same performance.

Three main methodologies are introduced to achieve

wireless power transmission, including electromagnetic

coupling, magnetic resonance, and microwave power

transmission.[2, 3] Among these technologies, electro-

magnetic coupling is the most suitable solution for LED

lighting applications. It can achieve efficiency levels of at

least 70% for distances of several millimeters to several

centimeters between the transmitting and receiving coil.

For designing the coils, the quality factor Q is the key

parameter. In order to obtain the high quality factor, this

study uses Litz wire to fabricate the coils to improve the

efficiency.

In this paper, section 2 describes the design and mea-

[Technical Paper]

A 36 W Wireless Power Transfer System with 82% Efficiency for

LED Lighting ApplicationsWei-Ting Chen*, Raul A. Chinga**, Shuhei Yoshida***, Jenshan Lin**, and Chao-Kai Hsu*

*Electronics and Optoelectronics Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan, ROC

**Department of Electrical and Computer Engineering, University of Florida, Gainesville, Florida 32611, USA

***Green Innovation Research Laboratories, NEC Corporation, Tsukuba, Ibaraki 305-841, Japan

(Received July 26, 2013; accepted October 25, 2013)

Abstract

This paper presents the design and implementation of an LED lighting module powered by a wireless power transfer

system. The overall system achieves an efficiency of 82% with an output power of 36 W when the load resistance in the

receiver is 30 Ω, which is the turn-on resistance of the LED lighting module. The transmitter of the system adopts Class-

E power amplifier structure instead of Class-D, to decrease the number of transistors and its cost. The coils of the system

are designed by electromagnetic coupling methodology and realized by Litz wire to reach high efficiency. Matching cir-

cuits between the system blocks are also discussed in this paper in order to obtain the excellent system performance.

Keywords: Wireless power transfer, Class-E power amplifier, Electromagnetic coupling, Litz wire, LED

Copyright © The Japan Institute of Electronics Packaging

33

Chen et al.: A 36 W Wireless Power Transfer System with 82% Efficiency (2/6)

surement results of a 38 W Class-E power amplifier with

93% efficiency. The content also includes the experiment of

varying the amplitude of driving clock signal to examine

the performance of the Class-E. The implementation of the

WPT system is demonstrated in section 3. The discussion

of load resistance variation is also presented. Section 4

shows the LED lighting module powered by the designed

WPT circuit. Finally, the conclusion of this work is pre-

sented in section 5.

2. High Performance Class-E Power AmplifierFigure 1 shows the schematic diagram of the Class-E

power amplifier which is used to convert DC power to AC

power. The output power capability of a WPT system is

related to the maximum output power of the DC-AC

inverter. In order to obtain a higher output power, the

value of load resistor R of the Class-E power amplifier

should be smaller. That is because the Class-E power

amplifier will be driven by a higher current, and the output

power will dramatically increase when the biasing voltage

is at the same condition. However, according to the equa-

tions listed in [3], the Class-E power amplifier needs a

smaller C1 to operate normally when a smaller R is cho-

sen. Therefore, a transistor with small parasitic capaci-

tance Cds is a better choice to design a high output power

Class-E power amplifier.[4] In this study, the transistor

IRF640 with less than 200 pF parasitic capacitor Cds was

selected when the drain to source voltage is over 30 V.

Considering the value of load resistor R should be higher

than parasitic resistance of L2 to reduce the power loss in

the inductor, a 10 Ω load resistor was used to be the initial

condition for the equations listed in [3]. Selecting 240 kHz

as the operating frequency, the other component values

are L1 = 500 µH, C1 = 14.4 nF, C2 = 54 nF, L2 = 18 µH. Con-

sidering the maximum output power is over 38 W, these

components have the characteristic of high voltage and

current rating. The quality factor of the components was

also considered to reduce the power loss in the compo-

nents. The 240 kHz clock operating the transistor M1 is

generated by Agilent 33521A waveform generator. Figure

2 shows the implemented Class-E power amplifier. Figure

3–6 show the performance of the circuit, and the wave-

forms and output power are measured by using a current

probe (Tektronix P6021), voltage probes (Tektronix

P2220) and an oscilloscope (Tektronix TDS2004B).

Figure 3 displays the drain voltage waveform and the

clock signal of 50% duty cycle at the gate of the transistor

M1 when VDD of 25 V was applied. As shown, the drain

voltage drops to zero right before the transistor is turned

on, preventing the overlap between these two waveforms.

This proves that the Class-E power amplifier achieves

proper zero voltage switching condition. Figure 4 shows

the output voltage and current waveforms. Since they are

both in phase, the Class-E can yield the maximum possible

output power. Figure 5 is the output power and drain effi-

ciency versus drain supply voltage. When the supply volt-

age VDD is 32 V, the output power is 38.1 W with a drain

efficiency of 93%. The correlation among the amplitude of

the clock signal, the output power and the drain efficiency

is summarized in Fig. 6. The amplitude of the clock signal

is varied from 5 V to 10 V while the supply voltage VDD

remains at 25 V. The result shows how the circuit per-

Fig. 1 Schematic diagram of the Class-E power amplifier.

Fig. 2 Photograph of the Class-E power amplifier.

Fig. 3 Drain and gate waveform.

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forms more efficiently when it is driven by clock signals

with the amplitude over 5.5 V.

3. Design and Verification of WPT SystemFigure 7 illustrates the schematic diagram of the WPT

system, consisting of the Class-E power amplifier, coupling

coils, a rectifier circuit and matching networks.[3] Wire-

less power transfer is accomplished by the coupling coils.

These coils, designed by using electromagnetic field analy-

sis method, were made by using Litz wire to form an 8-turn

rectangular shape on a paperboard. Due to its multiple-

strand structure, this type of wire can reduce the power

loss caused by skin effect and proximity effect at high fre-

quency, thus the quality factor is increased. The measured

mutual inductance between the two coils separated by 5

mm is 21.59 µH while the inductance of the transmitting

coil L3 is 17 µH with a parasitic resistance of 0.17 Ω, and

the inductance of the receiving coil L4 is 17.36 µH with a

parasitic resistance of 0.17 Ω. Those values were measured

by using an Agilent impedance analyzer 4192A at 240 kHz.

The quality factor of the designed coils is higher than 150.

It proves that Litz wire has excellent performance to imple-

ment low loss electromagnetic coupling coils. The full-

bridge rectifier, composed of four SK310A diodes and a

capacitor C4 of 68 uF, was selected to convert the AC

power to DC power. The series-series matching network

structure[5, 6] is chosen to transform the impedance in

order to ensure the system blocks operating with correct

input and output impedances when the load resistance R is

30 Ω. C2 of 38 nF, combining the capacitor of the resonator

in the Class-E power amplifier with the capacitance from

series matching network, is designed to obtain the maxi-

mum power transmission. C3 of 10 nF performs the same

role. The photograph of the WPT system is shown in Fig. 8.

Fig. 4 Voltage and current waveforms at R.

Fig. 5 Output power (solid squares) and drain efficiency (hollow circles) versus drain supply voltage.

Fig. 6 Output power (solid squares) and drain efficiency (hollow circles) versus amplitude of clock signal.

Fig. 7 Schematic diagram of the WPT system, consisting of Class-E, coupling coils, rectifier, and matching networks.

M1

VDD

L2L1

ClockL3 L4

MC3

C4Rload

C2

Fig. 8 Photograph of the WPT system, including transmitter and receiver.

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Figure 9 shows the power delivered to the load and the

system efficiency versus drain supply voltage when the

load resistance is 30 Ω. The system delivers 35.9 W to the

load resistance R with a system efficiency of 81.8%. The

system efficiency versus load resistance when a supply

voltage VDD of 34 V is shown in Fig. 10, which verifies

that the system achieves the best performance when the

load resistance is 30 Ω. Figure 11 illustrated the simplified

circuit model of the Class-E output impedance. Equation

(1) further explains why the performance of the system

degrades fleetly when the load resistance changes in Fig.

10. This equation represents the impedance looking into

the transmitting coil at the resonance which is also the

output impedance of the Class-E power amplifier. When

the load resistance Rload of the system changes, the out-

put impedance of the Class-E power amplifier also

changes, resulting in a non-optimal operating condition of

the Class-E power amplifier. This is the reason that the

system achieves the highest efficiency and output power

at an optimal Rload.

ZM

Rloadin

2 2

= ω (1)

4. LED Lighting Module With WPT SystemFigure 12 displays the photograph of the LED module

consisting of 12 LED chips bonded on a flexible PCB that

can be attached to a curved surface. Measurement setup

for testing the LED lighting module powered by the WPT

system is illustrated in Fig. 13. The receiving coil is on top

of the transmitting coil, and the distance between the two

coils is 5 mm. The system load resistance of 30 Ω is

replaced by the LED module and it is connected to the

rectifier directly. When the LED module turns on, the

power delivered to the module is 2.5 W while the Class-E

Fig. 9 Power delivered to a 30-Ω load (solid squares) and system efficiency (hollow circles) versus drain supply voltage.

Fig. 10 Power delivered to Rload (solid squares) and system efficiency (hollow circles) versus load resistance.

Fig. 11 Simplified circuit model of Class-E output imped-ance.

L1 L2

C1

MC2

RloadZin

Fig. 12 Photograph of the LED Module.

Fig. 13 Photograph of the measuring setup.

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power amplifier consumes 3.1 W from a supply voltage of 9

V. The system efficiency is 80.6%. It is capable of powering

more LED modules.

5. ConclusionIn this study, a 36 W wireless power transfer system

with 82% efficiency had been developed to power an LED

lighting module. Due to the sensitivity to load variation in

the WPT system, the load is fixed to 30 Ω to meet the turn-

on resistance of the lighting module. In this load condition,

it is confirmed that the system can achieve the best output

power and efficiency when it is connected to the module.

The measured result proves that the wireless power trans-

fer technology is a suitable solution for powering this type

of appliances.

The Class-E power amplifier was chosen as the DC-AC

inverter of the WPT system, achieving 38.1 W output

power with 93% efficiency. In the design, the small load

resistance and the transistor with low parasitic capacitance

were selected to generate higher current, yielding more

output power. Choosing high quality passive components

also reduced the power loss, thus improving the circuit

efficiency. The wireless power transfer was realized by

using electromagnetic coupling technology and Litz wire,

which achieved a good power transfer performance at a

distance of 5 mm between coils. In addition, the matching

networks were used to enable each system block to oper-

ate at the correct input and output impedance condition.

For the future work, the sensitivity to load variation

which causes the system performance to degrade must be

solved. This situation appears when the WPT technology

is used to charge batteries. The impedance looking into a

battery is changing during the charging period, which

means that the WPT system cannot operate at the optimal

load condition. How to maintain a good system efficiency

while the system load is changing is a challenging problem

in practical applications.

References[1] N. O. Sokal and A. D. Sokal, “Class E - A new class of

high-efficiency tuned single-ended switching power

amplifiers,” Solid-State Circuits, IEEE Journal of, Vol.

10, pp. 168–176, 1975.

[2] H. Shoki, “Issues and initiatives for practical use of

wireless power transmission technologies in Japan,”

in Microwave Workshop Series on Innovative Wire-

less Power Transmission: Technologies, Systems, and

Applications (IMWS), 2011 IEEE MTT-S Interna-

tional, pp. 87–90, 2011.

[3] K. A. Grajski, R. Tseng, and C. Wheatley, “Loosely-

coupled wireless power transfer: Physics, circuits,

standards,” Microwave Workshop Series on Innova-

tive Wireless Power Transmission: Technologies, Sys-

tems, and Applications (IMWS), 2012 IEEE MTT-S

International, pp. 9–14, 2012.

[4] W. Chen, R. A. Chinga, S. Yoshida, J. Lin, C. Chen, and

W. Lo, “A 25.6 W 13.56 MHz wireless power transfer

system with a 94% efficiency GaN class-E power

amplifier,” Microwave Symposium Digest (MTT),

2012 IEEE MTT-S International, pp. 1–3, 2012.

[5] Z. N. Low, R. A. Chinga, R. Tseng, and J. Lin, “Design

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pled planar wireless power transfer system,” Indus-

trial Electronics, IEEE Transactions on, Vol. 56, pp.

1801–1812, 2009.

[6] J. J. Casanova, Z. N. Low, and J. Lin, “Design and opti-

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Wei-Ting Chen received his master’s degree in communication engineering from Yuan Ze University, Taiwan, in 2005. In the same year, he joined the electronic and opto-electronics research laboratories at the Industrial Technology Research Institute, Hsinchu, Taiwan, as an engineer. Between

2011 and 2012, he was sent to the University of Florida to study wireless power transfer technology, serving as a visiting researcher. His major research interests include system in pack-age (SiP) technology, wireless communication systems, and wire-less power transmission (WPT) technology.

Raul A. Chinga began attending the Uni-versity of Florida in 2005 where he went on to receive bachelor’s degree in electrical and computer engineering. Upon finishing his undergraduate curriculum, Raul began working towards his Doctor of Philosophy in electrical and computer engineering in 2008

under the guidance of Dr. Jenshan Lin. The same year, Raul received the Bridge to the Doctorate Fellowship from the Univer-sity of Florida Graduate School. In 2010, Raul received an Honor-able Mention for the Graduate Research Fellowship from NSF, the Verosi Scholarship from the University of Florida in 2012 and the SEAGEP Fellowship from University of Florida in 2013. His work concentrates in switching RF power amplifiers for wireless power transmission applications and dielectric barrier discharge plasma.

Shuhei Yoshida received M.A in school of integrated design engineering from Keio University, Japan in 2007. Since 2007, he has been a research member of Smart Energy research labs at NEC Corporation, Japan. His current research interests are design and analysis of wireless power transmission

system including antennas, rectifiers, amplifiers and control cir-cuits.

Jenshan Lin received the Ph.D. degree in Electrical Engineering from the University of California at Los Angeles (UCLA) in 1994. He worked for AT&T Bell Labs (later became Lucent Bell Labs), Murray Hill, New Jersey, from 1994 to 2001, and its spin-off Agere Systems from 2001 to 2003. In July

2003, he joined University of Florida as an Associate Professor, and became a Professor in August 2007. He has authored or co-authored over 230 technical publications in refereed journals and conferences proceedings, and holds 10 U.S. patents. Dr. Lin is a Fellow of IEEE.

Chao-Kai Hsu received his bachelor’s degree in mechanical engineering from National Chin-Yi University of Technology in 1991. Since 2004, he has served in electronic and optoelectronics research laboratories at the Industrial Technology Research Insti-tute, Hsinchu, Taiwan, and is now an engi-

neer in the advanced packaging technology division of EOL/ITRI. His research interests include wafer-level packaging, optical interconnects packaging, flexible electronic packaging, and 3D IC technologies.

a 36 w wireless power transfer system with 82% efficiency

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