1. A study on Optimization of the wireless power transfer
using the half-bridge flyback converter
Jin-Ju Jang, Won-Yong Chae, Ho-Sung Kim, Dong-Gil Lee, Hee-Je Kim*
Department of Electrical Engineering Pusan National University
Jangjeon-dong, Geumjeong-gu, Busan 609-735, Republic of Korea
Jinjuj83@hanmail.net
Abstract—we considered about a wireless power transfer for
breaking inconvenience of wiring power sources. The best way
how to supply electric power through wireless system is using
the electromagnetic coupled resonance phenomena. It can be
supplied the electric power to the load. It is analyzed the
transfer characteristics through the basic experiment with
variable LED load. And we could apply protection circuit to
find the accurate resonance point that can get the high
efficiency result of wireless power transfer
Keywords- Wireless Power Transmission(WPT), resonance
frequency, protection circuit
I. INTRODUCTION
The Wireless Power Transmission (WPT) technology has
been treated to a wide extent in recent years. And it has been
a dream of human beings. Many scientists have researched it
uninterruptedly but little progress has been made [1]. Until
now, there are two kinds of wireless power transfer types
that one is non-contact electromagnetic induction and the
other is radio waves transceiver [2]. Efficiency of non-
contact electromagnetic induction could reach up to 80% but
in a very short distance such as within 1m. Radio waves can
transfer energy in 10m with very low efficiency and only
1~100MW power could be transferred. Due to the conflict
between transfer efficiency and the distance, they are not
used in a very wide range. And more, low efficiency results
in serious heat problems [3]. Comparatively speaking,
wireless power transfer of resonant coupling and transfer
energy in 5m and its efficiency up to 40%, which will be a
new technology with wider ranges [4].
However, resonance coupling wireless power transfer is
still in its infancy, whose theoretical and experimental
analysis are in lacks, especially for efficiency analysis[5-6].
During the wireless power transfer process, resonant
frequency maybe change because of changing resonant
inductance with obstacles, parasitical parameters, impacts of
receiving loop, temperature rising in circuit and so on. Once
detuning happens, the efficiency will drop rapidly.
Technologies of frequency tracking focused on CD4046 and
all digital PLL have been more matured [7-8], but they can
not meet fast tracking in higher ranges more than 1MHz.
We propose the new protection circuit and half bridge to
overcome these problems and get the higher transmission
efficiency in this system. The operating frequency of this
protection circuit is maintained close to and on the right side
of the resonant frequency and always works in the ZVS
condition. The technique of ZVS is usually preferred in
modern power converters. If we want to minimize the
switching loss and EMI, it is preferable to operate each
MOSFET and diode with ZVS [9].
II. THE SYSTEM OF WIRELESS POWER TRANSMISSION
Fig. 1 shows a block diagram for a generalized wireless
power system. This technology, unlike traditional remote
electromagnetic radiation is used frequency/wavelength
shorter than the distance passed on to their traditional usage
and effectiveness with narrow electrical field, further
guidance on the transmitter/receiver for matching the
resonant frequency and turn to a very high efficiency. And
the exceeding losses are far superior and harmless to humans.
Figure 1. Diagram of Wireless Power Transmission.
Wireless power transmission for the module is largely
comprised of sources and the receiver. Power Transmission
division circuit is configured as a transmitting coil and a
half-bridge, the receiving division is divided into a receiving
coil, a rectifying circuit and a LED.
Fig. 2 shows relationship between various frequencies
and operation mode of converters. The half bridge converter
using MOSFETS must be operated under ZVS condition for
improvement in efficiency. Proposed protection circuit
prevents the transition from the right side of the resonant
frequency to the left side under variable frequency operation
[9].
Second International Conference on Computer Research and Development
978-0-7695-4043-6/10 $26.00 © 2010 IEEE
DOI 10.1109/ICCRD.2010.164
717

2. Figure 2. Relationship between varying frequency and operation mode of
converter.
The design must be optimized for each part and the entire
system must consider the individual characteristics of the
device. Rectifier circuit of the WPT system in a portion of
the RF-DC conversion must be used a diode with a big
reverse voltage and fast switching speeds.
Resonance frequency is changed depending on values of
L and C, so the proper values of L and C is very important.
Fig. 3 shows the variable resonance frequency with various L
and C. We use the highest value of 360 kHz in this
experiment.
Figure 3. Resonance frequencys depening on value of L, C
WPT should be affected by the distance. Efficiency of
long distance transferring power is lower than that of short
distance. Fig. 4 shows those results in case of 5cm.
Figure 4. Efficiency curves in different distance
III. EXPERIMENTS
Make the receiver coil with a LED and a transmitter coil
with a half-bridge to test for WPT (Wireless Power
Transmission). Transmitting coil is consisted of a 16cm x
18cm with spiral 13turns which is designed by the technique
described in [10], and a 4cm x 5cm with spiral 6turns as the
receiving coil. Both coils were made by American Wire
Gauge Lutz wires to minimize coil parasitic. The switching
transistor was adopted a MOSFET, which was chosen to
minimize its output capacitance. And a MOSFET was driven
with a gate voltage of 5V and a supply voltage of 12V,
corresponding to a turn-on resistance of 0.125Ω. The LED
rating is 3V, 20mA. And the resonance frequency was 360
kHz.
First, the wireless transmission system is verified through
the experiment which is implemented about theoretical
frequency and then the bright light is emitted on that
frequency. In this research, we propose the protection circuit
for fixing the resonant frequency because the resonant
frequency is changed according to the value of LED. Fig. 5
shows the protection circuit. And table1 shows the designed
parameters for this experiment.
Figure 5. Full diagram of the WPT with protection circuit
TABLE I. DESIGN PARAMETERS
Parameter Value Parasitic Resistance ( )
L1(measured) 36.5 μH 0.42
L2(measured) 4.01 μH 0.08
M(measured) 8 μH
Q 2
IV. RESULTS
Fig. 6, 7 shows the result of various values of LED. And
this was not including protection circuit. Fig. 6 shows three
LEDs in parallel connection and Fig. 7 shows five LEDs in
the same condition. The yellow line is the input and the blue
line is output of LED, so we could know that the changing
value of LED impacts on the input. The input was changed
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3. by the value of LED. From those results, we could
understand that the exact frequency will be required to
uphold.
Figure 6. Switching frequency = 360 Khz, three LEDs
Figure 7. Frequency = 360 khz, five LEDs
Fig. 8 shows the output voltage waveform to the
protection circuit. From those experimental results, this
frequency protection circuit is better than that of no
protection circuit.
(a) Protection circuit (b) No protection circuit
Figure 8. Output voltage wave forms of LED
V. CONCLUSION
This paper focuses on the theoretical analysis of resonant
coupling wireless power transfer. To get a higher efficiency,
the accurate resonant frequency will be required. But if the
LED is variable, resonant frequency is not stable. Therefore,
we propose the new protection circuit. This optimized
method was tested in a system composed of a 16cm x 18cm
primary coil and a 4cm x 5cm secondary coil with a variable
LED. This system shows power delivery of over 3.4W and
peak efficiency of over 61%. In addition, the desirable trend
is to be seen with the decreasing power and efficiency as the
increasing load resistance.
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