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

Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, 2013
[Technical Paper]
A 36 W Wireless Power Transfer System with 82% Efficiency for
LED Lighting Applications
Wei-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 ClassE 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 circuits 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
1. Introduction
also a switching type amplifier. It has fewer components
Recently, interest in wireless power transfer (WPT)
with high reliability and a low-cost advantage, with a theo-
technology has been growing substantially. This technol-
retical efficiency of 100%.[1] However, compared to Class-
ogy can be applied to charge or power electronic devices
D power amplifiers, Class-E power amplifiers are more
wirelessly. WPT further contributes to technical fields
sensitive to the output impedance. Understanding the
such as medical science and automobile industry. A WPT
methodology on how to design an accurate output imped-
system consists of a transmitter, coupling coils, a receiver,
ance of the Class-E power amplifier is the crucial aspect in
and matching circuits. The matching circuits between
the system implementation. In this study, the matching
each system block are introduced to transform the imped-
circuit between the Class-E power amplifier and transmit-
ance in order to confirm the system blocks operate at cor-
ting coil is combined with the capacitor of the resonating
rect input and output impedance condition. However, the
circuit in the Class-E power amplifier, simplifying the sys-
matching design depends on the load resistance in the
tem structure while keeping the same performance.
receiver. When the resistance changes, the efficiency of
Three main methodologies are introduced to achieve
the receiver drops due to the mismatch, resulting in a drop
wireless power transmission, including electromagnetic
in coupling coefficient which consequently causes a drop
coupling, magnetic resonance, and microwave power
in efficiencies of the transmitter and overall system. In this
transmission.[2, 3] Among these technologies, electro-
study, the turn-on resistance of the LED module is 30 Ω,
magnetic coupling is the most suitable solution for LED
thus, the system design is based on this condition.
lighting applications. It can achieve efficiency levels of at
Another design challenge is the transmitter circuit. It
least 70% for distances of several millimeters to several
determines the performance of WPT systems. Generally,
centimeters between the transmitting and receiving coil.
Class-D power amplifiers are chosen to realize the trans-
For designing the coils, the quality factor Q is the key
mitter, but it requires at least two transistors to achieve
parameter. In order to obtain the high quality factor, this
zero voltage switching (ZVS), increasing the cost of prod-
study uses Litz wire to fabricate the coils to improve the
ucts and introducing process yield problems. This study
efficiency.
selects Class-E power amplifier circuit structure which is
32
In this paper, section 2 describes the design and meaCopyright © The Japan Institute of Electronics Packaging
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
generated by Agilent 33521A waveform generator. Figure
93% efficiency. The content also includes the experiment of
2 shows the implemented Class-E power amplifier. Figure
varying the amplitude of driving clock signal to examine
3–6 show the performance of the circuit, and the wave-
the performance of the Class-E. The implementation of the
forms and output power are measured by using a current
WPT system is demonstrated in section 3. The discussion
probe (Tektronix P6021), voltage probes (Tektronix
of load resistance variation is also presented. Section 4
P2220) and an oscilloscope (Tektronix TDS2004B).
shows the LED lighting module powered by the designed
Figure 3 displays the drain voltage waveform and the
WPT circuit. Finally, the conclusion of this work is pre-
clock signal of 50% duty cycle at the gate of the transistor
sented in section 5.
M1 when VDD of 25 V was applied. As shown, the drain
voltage drops to zero right before the transistor is turned
2. High Performance Class-E Power Amplifier
on, preventing the overlap between these two waveforms.
Figure 1 shows the schematic diagram of the Class-E
This proves that the Class-E power amplifier achieves
power amplifier which is used to convert DC power to AC
proper zero voltage switching condition. Figure 4 shows
power. The output power capability of a WPT system is
the output voltage and current waveforms. Since they are
related to the maximum output power of the DC-AC
both in phase, the Class-E can yield the maximum possible
inverter. In order to obtain a higher output power, the
output power. Figure 5 is the output power and drain effi-
value of load resistor R of the Class-E power amplifier
ciency versus drain supply voltage. When the supply volt-
should be smaller. That is because the Class-E power
age VDD is 32 V, the output power is 38.1 W with a drain
amplifier will be driven by a higher current, and the output
efficiency of 93%. The correlation among the amplitude of
power will dramatically increase when the biasing voltage
the clock signal, the output power and the drain efficiency
is at the same condition. However, according to the equa-
is summarized in Fig. 6. The amplitude of the clock signal
tions listed in [3], the Class-E power amplifier needs a
is varied from 5 V to 10 V while the supply voltage VDD
smaller C1 to operate normally when a smaller R is cho-
remains at 25 V. The result shows how the circuit per-
sen. Therefore, a transistor with small parasitic capacitance 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. Considering the maximum output power is over 38 W, these
Fig. 2 Photograph of the Class-E power amplifier.
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 components. The 240 kHz clock operating the transistor M1 is
Fig. 1 Schematic diagram of the Class-E power amplifier.
Fig. 3 Drain and gate waveform.
33
Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, 2013
VDD
L1
Clock
L2
M1
M
C2
L3
C3
L4
C4
Rload
Fig. 7 Schematic diagram of the WPT system, consisting of
Class-E, coupling coils, rectifier, and matching networks.
Fig. 4 Voltage and current waveforms at R.
Fig. 8 Photograph of the WPT system, including transmitter
and receiver.
coils, a rectifier circuit and matching networks.[3] Wireless power transfer is accomplished by the coupling coils.
These coils, designed by using electromagnetic field analysis method, were made by using Litz wire to form an 8-turn
rectangular shape on a paperboard. Due to its multipleFig. 5 Output power (solid squares) and drain efficiency
strand structure, this type of wire can reduce the power
(hollow circles) versus drain supply voltage.
loss caused by skin effect and proximity effect at high frequency, 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 implement low loss electromagnetic coupling coils. The fullbridge rectifier, composed of four SK310A diodes and a
capacitor C4 of 68 uF, was selected to convert the AC
Fig. 6 Output power (solid squares) and drain efficiency
power to DC power. The series-series matching network
(hollow circles) versus amplitude of clock signal.
structure[5, 6] is chosen to transform the impedance in
order to ensure the system blocks operating with correct
forms more efficiently when it is driven by clock signals
input and output impedances when the load resistance R is
with the amplitude over 5.5 V.
30 Ω. C2 of 38 nF, combining the capacitor of the resonator
in the Class-E power amplifier with the capacitance from
3. Design and Verification of WPT System
series matching network, is designed to obtain the maxi-
Figure 7 illustrates the schematic diagram of the WPT
mum power transmission. C3 of 10 nF performs the same
system, consisting of the Class-E power amplifier, coupling
role. The photograph of the WPT system is shown in Fig. 8.
34
Chen et al.: A 36 W Wireless Power Transfer System with 82% Efficiency (4/6)
M
C1
Zin
C2
L1
L2
Rload
Fig. 11 Simplified circuit model of Class-E output impedance.
Fig. 9 Power delivered to a 30-Ω load (solid squares) and
system efficiency (hollow circles) versus drain supply voltage.
Fig. 12 Photograph of the LED Module.
Fig. 10 Power delivered to Rload (solid squares) and system
efficiency (hollow circles) versus load resistance.
Figure 9 shows the power delivered to the load and the
Fig. 13 Photograph of the measuring setup.
system efficiency versus drain supply voltage when the
load resistance is 30 Ω. The system delivers 35.9 W to the
system achieves the highest efficiency and output power
load resistance R with a system efficiency of 81.8%. The
at an optimal Rload.
system efficiency versus load resistance when a supply
voltage VDD of 34 V is shown in Fig. 10, which verifies
Zin =
ω2M 2 Rload
(1)
that the system achieves the best performance when the
load resistance is 30 Ω. Figure 11 illustrated the simplified
4. LED Lighting Module With WPT System
circuit model of the Class-E output impedance. Equation
Figure 12 displays the photograph of the LED module
(1) further explains why the performance of the system
consisting of 12 LED chips bonded on a flexible PCB that
degrades fleetly when the load resistance changes in Fig.
can be attached to a curved surface. Measurement setup
10. This equation represents the impedance looking into
for testing the LED lighting module powered by the WPT
the transmitting coil at the resonance which is also the
system is illustrated in Fig. 13. The receiving coil is on top
output impedance of the Class-E power amplifier. When
of the transmitting coil, and the distance between the two
the load resistance Rload of the system changes, the out-
coils is 5 mm. The system load resistance of 30 Ω is
put impedance of the Class-E power amplifier also
replaced by the LED module and it is connected to the
changes, resulting in a non-optimal operating condition of
rectifier directly. When the LED module turns on, the
the Class-E power amplifier. This is the reason that the
power delivered to the module is 2.5 W while the Class-E
35
Transactions of The Japan Institute of Electronics Packaging Vol. 6, No. 1, 2013
power amplifier consumes 3.1 W from a supply voltage of 9
while the system load is changing is a challenging problem
V. The system efficiency is 80.6%. It is capable of powering
in practical applications.
more LED modules.
References
5. Conclusion
[1] N. O. Sokal and A. D. Sokal, “Class E - A new class of
In this study, a 36 W wireless power transfer system
high-efficiency tuned single-ended switching power
with 82% efficiency had been developed to power an LED
amplifiers,” Solid-State Circuits, IEEE Journal of, Vol.
lighting module. Due to the sensitivity to load variation in
10, pp. 168–176, 1975.
the WPT system, the load is fixed to 30 Ω to meet the turn-
[2] H. Shoki, “Issues and initiatives for practical use of
on resistance of the lighting module. In this load condition,
wireless power transmission technologies in Japan,”
it is confirmed that the system can achieve the best output
in Microwave Workshop Series on Innovative Wire-
power and efficiency when it is connected to the module.
less Power Transmission: Technologies, Systems, and
The measured result proves that the wireless power trans-
Applications (IMWS), 2011 IEEE MTT-S Interna-
fer technology is a suitable solution for powering this type
tional, pp. 87–90, 2011.
of appliances.
[3] K. A. Grajski, R. Tseng, and C. Wheatley, “Loosely-
The Class-E power amplifier was chosen as the DC-AC
coupled wireless power transfer: Physics, circuits,
inverter of the WPT system, achieving 38.1 W output
standards,” Microwave Workshop Series on Innova-
power with 93% efficiency. In the design, the small load
tive Wireless Power Transmission: Technologies, Sys-
resistance and the transistor with low parasitic capacitance
tems, and Applications (IMWS), 2012 IEEE MTT-S
were selected to generate higher current, yielding more
International, pp. 9–14, 2012.
output power. Choosing high quality passive components
[4] W. Chen, R. A. Chinga, S. Yoshida, J. Lin, C. Chen, and
also reduced the power loss, thus improving the circuit
W. Lo, “A 25.6 W 13.56 MHz wireless power transfer
efficiency. The wireless power transfer was realized by
system with a 94% efficiency GaN class-E power
using electromagnetic coupling technology and Litz wire,
amplifier,” Microwave Symposium Digest (MTT),
which achieved a good power transfer performance at a
2012 IEEE MTT-S International, pp. 1–3, 2012.
distance of 5 mm between coils. In addition, the matching
[5] Z. N. Low, R. A. Chinga, R. Tseng, and J. Lin, “Design
networks were used to enable each system block to oper-
and test of a high-power high-efficiency loosely cou-
ate at the correct input and output impedance condition.
pled planar wireless power transfer system,” Indus-
For the future work, the sensitivity to load variation
which causes the system performance to degrade must be
trial Electronics, IEEE Transactions on, Vol. 56, pp.
1801–1812, 2009.
solved. This situation appears when the WPT technology
[6] J. J. Casanova, Z. N. Low, and J. Lin, “Design and opti-
is used to charge batteries. The impedance looking into a
mization of a class-E amplifier for a loosely coupled
battery is changing during the charging period, which
planar wireless power system,” Circuits and Systems
means that the WPT system cannot operate at the optimal
II: Express Briefs, IEEE Transactions on, Vol. 56, pp.
load condition. How to maintain a good system efficiency
830–834, 2009.
36
Chen et al.: A 36 W Wireless Power Transfer System with 82% Efficiency (6/6)
Wei-Ting Chen received his master’s
Shuhei Yoshida received M.A in school of
degree in communication engineering from
integrated design engineering from Keio
Yuan Ze University, Taiwan, in 2005. In the
University, Japan in 2007. Since 2007, he has
same year, he joined the electronic and opto-
been a research member of Smart Energy
electronics research laboratories at the
research labs at NEC Corporation, Japan.
Industrial Technology Research Institute,
His current research interests are design
Hsinchu, Taiwan, as an engineer. Between
and analysis of wireless power transmission
2011 and 2012, he was sent to the University of Florida to study
system including antennas, rectifiers, amplifiers and control cir-
wireless power transfer technology, ser ving as a visiting
cuits.
researcher. His major research interests include system in pack-
Jenshan Lin received the Ph.D. degree in
age (SiP) technology, wireless communication systems, and wire-
Electrical Engineering from the University
less power transmission (WPT) technology.
of California at Los Angeles (UCLA) in 1994.
Raul A. Chinga began attending the Uni-
He worked for AT&T Bell Labs (later
versity of Florida in 2005 where he went on
became Lucent Bell Labs), Murray Hill, New
to receive bachelor’s degree in electrical and
Jersey, from 1994 to 2001, and its spin-off
computer engineering. Upon finishing his
Agere Systems from 2001 to 2003. In July
undergraduate curriculum, Raul began
2003, he joined University of Florida as an Associate Professor,
working towards his Doctor of Philosophy in
and became a Professor in August 2007. He has authored or co-
electrical and computer engineering in 2008
authored over 230 technical publications in refereed journals and
under the guidance of Dr. Jenshan Lin. The same year, Raul
conferences proceedings, and holds 10 U.S. patents. Dr. Lin is a
received the Bridge to the Doctorate Fellowship from the Univer-
Fellow of IEEE.
sity of Florida Graduate School. In 2010, Raul received an Honor-
Chao-Kai Hsu received his bachelor’s
able Mention for the Graduate Research Fellowship from NSF,
degree in mechanical engineering from
the Verosi Scholarship from the University of Florida in 2012 and
National Chin-Yi University of Technology in
the SEAGEP Fellowship from University of Florida in 2013. His
1991. Since 2004, he has served in electronic
work concentrates in switching RF power amplifiers for wireless
and optoelectronics research laboratories at
power transmission applications and dielectric barrier discharge
the Industrial Technology Research Insti-
plasma.
tute, Hsinchu, Taiwan, and is now an engineer 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.
37