The High-efficiency LED Driver for Visible Light Communication Applications

Abstract

This paper presents a LED driver for VLC. The main purpose is to solve the low data rate problem used to be in switching type LED driver. The GaN power device is proposed to replace the traditional silicon power device of switching LED driver for the purpose of increasing switching frequency of converter, thereby increasing the bandwidth of data transmission. To achieve high efficiency, the diode-connected GaN power transistor is utilized to replace the traditional ultrafast recovery diode used to be in switching type LED driver. This work has been experimentally evaluated on 350-mA output current. The results demonstrate that it supports the data of PWM dimming level encoded in the PPM scheme for VLC application. The experimental results also show that system’s efficiency of 80.8% can be achieved at 1-Mb/s data rate.

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Scientific RepoRts | 6:30991 | DOI: 10.1038/srep30991
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The High-eciency LED Driver
for Visible Light Communication
Applications
Cihun-Siyong Alex Gong1,2,3, Yu-Chen Lee4, Jyun-Liang Lai4, Chueh-Hao Yu4, Li Ren Huang4 &
Chia-Yen Yang5
This paper presents a LED driver for VLC. The main purpose is to solve the low data rate problem used
to be in switching type LED driver. The GaN power device is proposed to replace the traditional silicon
power device of switching LED driver for the purpose of increasing switching frequency of converter,
thereby increasing the bandwidth of data transmission. To achieve high eciency, the diode-connected
GaN power transistor is utilized to replace the traditional ultrafast recovery diode used to be in
switching type LED driver. This work has been experimentally evaluated on 350-mA output current. The
results demonstrate that it supports the data of PWM dimming level encoded in the PPM scheme for
VLC application. The experimental results also show that system’s eciency of 80.8% can be achieved
at 1-Mb/s data rate.
e light-emitting diode (LED) has been the environmentally friendly mainstream for lighting applications since
its high eciency and long lifetime were established. e incandescent lamp and uorescent lamp are going to
be replaced by LED gradually. Recently, it has been discovered that LED is more suitable for the high speed com-
munications than the traditional light sources due mainly to its wider modulation bandwidth. is has taken a
known technology called the visible light communication (VLC) to its next level1.
Research works have been focusing on realizing high data rate transmission for VLC. e Bias-T circuit tech-
nique is usually utilized to combine DC driving current for lighting with the data for transmission2,3. e use of
Bias-T and single (multiple) power amplier(s) is needed on the data path4,5. However, this causes high cost and
large size on the system as well as severe decay in the eciency. To build VLC technology at existing facility while
at the same time making better tradeos among cost, speed, and eciency, this paper presents a LED driver with
maximum data rate of 1 Mb/s. Also, 80.8-% system eciency can be demonstrated at this data rate.
Proposed VLC LED Architecture
For the VLC technology, switching type LED driver is a general structure. It not only oers advantage of high
eciency but also great potential in lighting market. To regulate the current of LED with simple and economic
method, the conventional structure shown in Fig.1 possesses oating buck converter with peak current mode
control6. This structure also has pulse width modulation (PWM) circuit to process illumination dimming.
However, the switching type LED driver has limitation of the transmission speed for VLC application. e main
reason of such a limitation is its switching frequency limited by switching speed of traditional silicon power
device and its signicant switching loss. e use of traditional silicon power device results in low switching fre-
quency of the driver, and the frequency is usually slower than several hundreds of kHz. Furthermore, for the
purpose of minimizing the current and voltage ripple of LED with such a low switching frequency, large passive
device is unavoidable for the energy storage. Unfortunately, the large passive device brings about the lower reso-
nance frequency. On the other hand, the loop bandwidth is usually lower than one tenth of switching frequency
for the purpose of stability, and consequently the maximum data rate of switching type LED driver is oen limited
by several tens of kb/s.
1Department of Electrical Engineering, College of Engineering, Chang Gung University, Taoyuan 33302, Taiwan.
2Portable Energy System Group, Green Technology Research Center, College of Engineering, Chang Gung University,
Taoyuan County 33302, Taiwan. 3Chang Gung Memorial Hospital at Linkou, Taoyuan 33304, Taiwan. 4Department
of Green Electronics Design and Application, Industrial Technology Research Institute, Hsinchu 31040, Taiwan.
5Department of Biomedical Engineering, Ming-Chuan University, 5 De Ming Rd., Guishan Township, Taoyuan County
33348, Taiwan. Correspondence and requests for materials should be addressed to C.-S.A.G. (email: alex.mlead@
gmail.com or alexgong@mail.cgu.edu.tw) or C.-Y.Y. (email: cyyang@mail.mcu.edu.tw)
Received: 19 May 2016
Accepted: 08 July 2016
Published: 08 August 2016
OPEN

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Scientific RepoRts | 6:30991 | DOI: 10.1038/srep30991
e proposed VLC LED driver is shown in Fig.2. To solve the problem of low data rate, GaN power device has
been utilized to replace the traditional silicon power device. anks to its high switching speed and low switching
loss, the proposed driver achieves 10-MHz switching frequency, increasing the bandwidth of data transmission
accordingly. e maximum data rate can be up to 1 Mb/s. However, the ultrafast recovery diode, shown in Fig.1,
with large reverse recovery charge (Qrr) results in a long switching interval and high switching loss of low side
power MOSFET7. e proposed structure includes diode-connected GaN power transistor as a rectier, shown in
Fig.2. Instead of the traditional ultrafast recovery diode, the diode-connected GaN power transistor M2 featuring
zero Qrr has been used to increase the data rate and eciency of the LED driver, leading to a system’s eciency
of 80.8% at 1-Mb/s data rate. e M2 is OFF when the M1 is ON. Conversely, the M2 is ON while the M1 is OFF.
As shown in Fig.2, the constant o-time controlled loop is used to regulate the LED current stably for wide
input voltage. In addition, there is a data/dimming modulation circuit to combine the PPM data with PWM
diming signal for VLC and dimming applications8. As shown in Fig.3, the predetermined pulse width is used to
decide the dimming level in a modulation period, while at the same time the pulse position is used to determine
if bit 0 or bit 1 happens. As the case shown in Fig.3, the data of bit 0 is represented by the pulse at leading edge
of a modulation period. On the other hand, bit 1 is represented by the pulse at trailing edge. e gate driver of
control IC with independent push and pull paths is used to prevent overvoltage or overcurrent damage on GaN
power transistor. e overvoltage or overcurrent damage is due to high di/dt and dv/dt coming from the parasitic
elements of transistor during switching interval9.
Reverse Recovery Eect on System’s Eciency
In this section, we discusses the reverse recovery eect on system’s eciency. e comparison between the use of
the traditional ultrafast recovery diode and that of the diode-connected GaN power transistor in the proposed
LED driver will be shown. For the purpose of high eciency, the lower MOSFET of LED driver should act like a
switch. For example, when the power MOSFET of the LED driver is switching on, it requires operation from the
cuto region to saturation region, followed by the triode region. e MOSFET of the LED driver can only stay
in the saturation region for a while during switching. at is to say the higher rectier of the LED driver plays an
important role during the process of switching on. As shown in Fig.4, aer the gate-source voltage (VGS) on the
Figure 1. Conventional oating buck LED Driver with pulse width modulation (PWM) dimming.
Figure 2. Proposed VLC LED driver.

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Figure 3. Variable pulse position modulation (PPM) scheme with dierent PWM dimming levels.
Figure 4. Waveforms at the moment that the lower switch is ON. e LED driver serves as (a) a traditional
ultrafast recovery diode or (b) a diode-connected GaN power transistor as a rectier.

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lower MOSFET reaches the threshold voltage, the lower MOSFET forms current in the saturation mode, causing
increase in the MOSFET current (Imos). en, common-source inductor of MOSFET resists the change of VGS
and slows down the increasing speed of VGS and Imos. e drain-source voltage (VDS) of MOSFET will start to
decrease until the reverse recovery charge (Qrr) of diode is depleted completely while the voltage on diode (VD)
is turning into zero.
Next, the Imos will decrease to the designed value, followed by the process of switching on becomes complete.
As shown in Fig.4(a), it costs 3.4 ns from the timing of VG turns on to that of VD turns back to 0 V. In Fig.4(b),
it costs 2 ns for it. e results show that the traditional ultrafast recovery diode, which has more Qrr than that of
the diode-connected GaN power transistor, takes more time to deplete charges. As a result, the MOSFET will stay
long in the saturation mode and the peak current (Ipk) of it will be higher than that of the driver with the diode
having lower Qrr. As shown in Fig.4(a,b), the Ipk at the moment that the lower switch is ON will be 2.5A and
1.78A for the LED driver serving, respectively, as a traditional ultrafast recovery diode and a diode-connected
GaN power transistor as a rectier. To sum up, the LED driver serves as a large Qrr diode as a rectier will extend
the interval of switching and increase the current overshoot on MOSFET. is phenomenon not only results in
another switching loss of the LED driver but also brings about possible damage in transistor.
e simulation results of the power loss distribution in the proposed LED driver are shown in Fig.5 where a
comparison between the use of traditional ultrafast recovery diode and that of the diode-connected GaN power
transistor is shown. e simulation was carried out under the conditions 1) the converter is operated at 10 MHz,
2) the input voltage is 53 V, 3) the output voltage is 28.8 V, 4) the LED current is 350 mA with 100-mA ripple, and
5) the VLC input data is 50% PWM dimming level encoded in a 1-MHz PPM. From Fig.5(a,b), aer the use of
diode-connected GaN power transistor in the LED driver, the system’s eciency is increased from 84.47% to
85.16%. e diode and MOSFET are the two items that have the most impact on the power distribution. Although
the power distribution of diode is increased by 2.04% from Fig.5(a,b), the power distribution of MOSFET is
decreased by 2.76%. e reason is that the forward voltage of diode-connected GaN power transistor is 2.4 V
with 350 mA, and the traditional ultrafast recovery diode’s is 0.67 V. erefore, although the LED driver serves the
diode-connected GaN power transistor as a rectier will increase the power loss of diode, the decrease in switch-
ing loss and current overshoot on MOSFET results in higher system eciency.
Experimental Results
e proposed VLC LED driver is designed with 350 mA and transmission data rate of 1 Mb/s. e converter is
operated at 10 MHz with the input voltage of 50 V and output voltage of 25 V. e data of VLC used in the meas-
urement is a 50-% PWM dimming level encoded in 1-MHz PPM. e control IC is shown in Fig.6(a) where it
was fabricated in a standard 0.5-μ m CMOS technology. It measures 1930 μ m × 1250 μ m. A standard SOP-32 pin
package has been used for measurement. e LED Driver module is shown in Fig.6(b) where its discrete compo-
nents involve diode-connected GaN power transistor (EPC, EPC8010), GaN power transistor (EPC, EPC8010),
and 18-μ H inductor. e verication platform is shown in Fig.7. e power supply is used to light up the LED,
and the signal of 50-% PWM dimming level encoded in 1 MHz PPM is input through the VLC input pin of the
LED driver module. e current of LED and receiver was measured by current probe and oscilloscope.
As shown in Fig.8, the waveforms from top to bottom are the gate voltage of the lower GaN power transistor
in the proposed LED driver, the LED current, and the receiver current, respectively. e measurement results
show that the average current of LEDs is 172 mA, and consequently the 50% PWM dimming is achieved. In addi-
tion, the waveforms of the LED and receiver current show that the data rate of 1 Mb/s has been demonstrated,
while at the same time having measured power consumption of 5.68 W and 4.59 w for the system and the LED,
respectively. e eciency of the whole system is 80.8%. As a result, a LED driver with high data rate and high
Figure 5. Power distribution of the LED Driver. (a) With traditional ultrafast recovery diode. (b) With diode-
connected GaN power transistor. “PLED” denotes “power dissipation of LED string”. “PIC” denotes “power
dissipation of integrated circuit”. “PMOS” denotes “power dissipation of MOS”. “PD” denotes “Power dissipation
of diode”.

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eciency has been realized. Comparison between the proposed design and state-of-the-art works is tabu-
lated in Table1. The work presented in ref. 4 utilizes multiple-resonant equalization and blue filter to achieve
a transmission rate of 80 Mb/s. The advantage of its design is high transmission rate. However, it requires
integrating extra T Bias-tee and multiple power amplifiers with the LED driver, not only increasing overall
cost but also decreasing energy conversion efficiency. The work presented in ref. 10 utilizes shunt switch
technique to transmit data. It achieves both high transmission rate and high energy conversion efficiency.
The data rate can be as high as Mb/s class without being limited by the switching frequency of LED driver.
However, it needs an extra power transistor as the shunt switch. The addition of power-transistor-associated
gate driver is also unavoidable. As a result, both cost and power are compromised. In our work, we have uti-
lized the GaN power devices as replacements of not only a silicon power device but also an ultrafast recovery
diode for switching. Our design can be an effective way to achieve Mb/s-class transmission. Compared with
Figure 6. (a) Control IC (b) LED driver module.
Figure 7. Verication platform of the VLC system.

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the works in refs 4 and 10, our proposed work integrates both the DC lighting driving signal with the data signal
to drive a single power transistor. By taking the advantage, the visible light communication technique can be built
directly on the existing indoor illumination infrastructure without additional devices and energy loss, thereby
oering higher energy conversion eciency.
Conclusion
e proposed LED driver in this paper solves the low data rate problem of conventional switching type LED
driver. It is designed for 350 mA output current and supports the data of PWM dimming level encoded in
PPM scheme for VLC application. In this paper the GaN power device is used to increase the switching
frequency of the LED driver, and consequently the bandwidth of the LED driver can be extended and the
data rate can be increased. Moreover, it has been demonstrated that the proposed LED driver with the
diode-connected power transistor has higher efficiency than the LED driver with traditional ultrafast recov-
ery diode. The measurement results show that the proposed LED driver achieves 80.8% efficiency under
1-Mb/s data rate. Commercial white (fluorescent powder) LEDs have about 1 MHz~2 MHz modulation
bandwidth. The bandwidth is mainly limited by the inherent light emission mechanism. As a matter of fact,
despite higher bandwidth in the design of transmitter, the modulation bandwidth is still limited by the LED
itself during the electrical-to-optical conversion for visible light communication. To solve the problem, a blue
light filter can be added to the front of the receiver front end to remove the yellow light with longer response
time. The filter can be an effective way to achieve higher bandwidth, thereby achieving higher switching
frequency of the driver in the proposed design. The higher the switching frequency, the higher the data rate.
Figure 8. Experimental waveforms. From top to bottom are (a) Gate voltage of the lower GaN power
transistor, (b) LED current, and (c) Receiver current.
is Work [4] [10]
Phillips
US8150269
Panasonic
US20130015784
Max. Date Rate 1 Mb/s 80 Mb/s 2 Mb/s < 10 kb/s 4 kb/s
Eciency 81.5% (85.8%) — 74.6% — —
Architecture Switched-mode
w/GaN device
Multiple-resonant
equalization w/blue ller Switched-mode Switched-mode Switched-mode
Table 1. Comparison between the proposed design and state-of-the-art works.

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Acknowledgements
is study was funded in part with grants from the MINISTRY OF SCIENCE AND TECHNOLOGY (MOST),
Taiwan, under Grants MOST 104-2815-C-182-052-E, MOST 104-2632-E-130-001, MOST 105-2221-E-182-039-
and MOST 104-2221-E-182-044. is work is also supported in part by the CHANG GUNG UNIVERSITY
(CGU) and CHANG GUNG MEMORIAL HOSPITAL (CGMH) under Contracts UERPD2E0031 and
CMRPD2F0101.
Author Contributions
C.-S.A.G. wrote the manuscript, analyzed the data, and validated the results. Y.-C.L. and J.-L.L. performed
experiments. C.-H.Y., L.R.H. and C.-Y.Y. served as consultants and assisted in the experiments.
Additional Information
Competing nancial interests: e authors declare no competing nancial interests.
How to cite this article: Gong, C.-S. A. et al. e High-eciency LED Driver for Visible Light Communication
Applications. Sci. Rep. 6, 30991; doi: 10.1038/srep30991 (2016).
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