Advances in Robotics, Mechatronics and Circuits
Magnetically Dimmable Half Bridge Current
Fed LED Driver
Selim Borekci and Nihal Cetin Acar
and luminous flux adjustment in a space, LED brightness
dimming becomes important. In addition to new generation IC
solutions for dimming, voltage control and pulse width
modulation (PWM) techniques are used to adjust led luminous
flux [3]-[4].
Abstract—In this paper, a new LED (Light Emitting Diode)
driver and dimming techniques are introduced. Half bridge current
fed resonant LED driver is designed and implemented. Rather than
using tradities12onal dimming techniques, dimming is accomplished
magnetically. The proposed driver is implemented on 33 LEDs.
LEDs are dimmed from 33 W to 20 W.
Keywords—dimming, LED driver, resonant converter, variable
inductor
I. INTRODUCTION
AC
T5 and T8 florescent lamps have been used widely for in
indoor and outdoor lightening. Recently LEDs are also
becoming very popular, because of their high efficacy (light
output per watt), long life, no start up and striations problems
and very low maintenance requirements [1]-[2].
AC/DC
Converter
LED
Driver
Stage I
Stage II
Fig. 1 Two Stage LED Driver.
Some of the grid connected AC applications result in
harmonics because of conduction angle. To overcome this
problem and provide isolation between the grid system and
LEDs, high frequency resonant converters with transformers
are implemented.
Varying the LED voltage results in changes in the LED
current . The relation between them is not linear. Because of
that, controlling the LED current requires complex circuit
structure [5]-[7].
Voltage fed high frequency converters such as LLC type
resonant topologies are commonly utilized. For low current
ripple and better starting transients, current fed circuits give
better performance.
In PWM techniques, LEDs are turned on and off in a period
and control scheme adjusts the duty ratio of LEDs [8]-[9]. In
high frequency applications, that reduces LED life span [10][11]. PWM technique may also cause electromagnetic
interference (EMI) noise problems.
Most of the high frequency LED drivers have two stages
shown in (Fig. 1). In the first stage, 50/60 Hz AC power
converted to DC with an IC which provides low THD and
better power factor. In the second stage, DC power is
converted to high frequency AC. In this stage, LEDs are
connected to the circuit with rectifier having an electrolytic
capacitor.
In this paper, a new dimmable LED driver is introduced. A
half bridge current fed topology is used to drive LEDs and also
galvanic isolation is provided.
A new LED dimming
technique is accomplished by using variable inductor. The
proposed technique is designed and implemented on 1 Watt 33
power LEDs.
II. LED AND LUMINANCE
In LEDs data sheets, it can be seen that led luminance is
proportional to its forward led current. LED current is
associated with LED illumination linearly. For energy saving
V-I characteristic of power LEDs is defined by an
exponential function. Nonlinearity characteristics of LEDS
make their driver circuits complicated. Current and voltage
relationship is given by equation (1) and (Fig. 2) shows a
typical V-I cure of a LED.
S. Borekci was working in lighting industry for several years in MS, USA.
He is now with the Department of Electrical and Electronics Engineering at
Akdeniz University, Antalya, TURKEY (e-mail: sborekci@akdeniz.edu.tr).
Nihal C. Acar is working as an research and teaching assistant at Electrical
and Electronics Engineering Dept at Akdeniz University, Antalya, Turkey (email: nihalcetinacar@gmail.com).
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Advances in Robotics, Mechatronics and Circuits
(1)
where I
Is
VD
n
VT
k
T
q
There are several topologies for LED drivers as mentioned.
Commonly used ones have two stages as shown in (Fig.1). In
the first stage, AC grid is converted to DC. ICs are utilized for
better power factor correction, THD and DC voltage
regulation. The function of the first stage is a DC power
supply.
In the second stage, a LED driver is designed such that LED
current results in desired led brightness. Dimming feature can
also be added in this stage.
diode forward current
reverse bias saturation current
diode forward voltage
diode ideality factor
thermal voltage
Boltzman’s constant
temperature
charge on an electron
III. PROPOSED SOLUTION
The second stage is the focus of this research. A current fed
half bridge converter shown in (Fig. 5) is implemented. In the
circuit, two electrolytic capacitors Cdc are used to divide the
DC voltage and two choke inductors Ldc takes place to
regulate the current. A snubber capacitor C is used across
power switches. There are two MOSFETs which are driven by
IR2153. Magnetization impedance of the transformer Lp and
resonant capacitor Cr determines the resonant frequency of the
converter. The secondary voltage of the transformer is
rectified by diodes. For the better output regulation, an
electrolytic capacitor Co is placed in parallel with the LEDs.
I( V)
V
Fig. 2 V & I Characteristic of a LED.
As seen in (Fig. 3), there are two operating states; on and
off. Figure (a) and (b) show the equivalent circuit of LED for
on and off states respectively. Capacitive and resistive effects
of LEDs during on and off are illustrated in (Fig. 3) as Con,
Coff, Ron and Roff respectively. In the figure, Von represents the
forward bias voltage of a LED.
Ldc
S1
Vdc/2
Ideal diode
Con
Ron
Vin
Coff
Cdc
C
Cr
+
_
Roff
Lp
D3
D5
D4
D6
Ls
C0
LED
S2
Vdc/2
Von
(a)
D1
Vgs2
Vgs1
Cdc
D2
(b)
Fig. 3 Electrical Model of a LED.
Ldc
It is important that LED forward current determines the
brightness of a LED. Figure 4 shows a typical relation between
the forward current and relative luminous of a LED. LED
luminous flux increases linearly with its current, while the
voltage is almost constant. Therefore, the efficacy of LEDs
can be assumed to be constant.
Fig. 5 Proposed LED Driver.
DC voltage, Vin, is applied to the half bridge current fed
resonant converter. Primary and secondary voltages of the
transformer are in sinusoidal shape. The peak value of the
primer voltage for the first harmonic is calculated as
(2)
Number of LEDs m and forward voltage of selected LEDs
Vled are determined at the beginning of the design. Therefore,
the peak value of the secondary voltage can be obtained from
equation (3).
(3)
When the primary and secondary voltages are known, the turn
ratio of the transformer can be calculated as
Fig. 4 Lamp Current versus LED Relative Luminous Flux.
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(4)
For high frequency applications, ferrite cores can be used.
The saturation of the ferrite core is around 0.3 T. Better
material can have higher saturation level. Selected frequency
of the circuit, effective cross section area of the core, and the
secondary voltage of the transformer determine the number of
secondary turns as in equation (5). Multiplying the turn ratio
with the number of secondary turns gives the number of
primary turns.
Lvrb
Idc
Fig. 6 Current Controlled Inductor.
To dim LEDs, the inductor is integrated with the circuit
given in (Fig. 5). Figure 7 shows the magnetically dimmable
half bridge current fed LED driver. The current controlled
inductor acts as an additional impedance in the secondary of
the transformer and functions as ballast.
(5)
Selected magnetic core has its own effective permeability
µeff, cross section area Aeff and effective magnetic length
Leff. From the number of primary turns and core information,
primer reactance can be calculated by equation (6).
IV. EXPERIMENTAL AND SIMULATION RESULTS
(6)
The proposed circuit shown in (Fig. 7) is implemented on
33 LEDs. Resonant frequency is tune to 22 800 Hz and
Mosfets are driven by 23 500Hz by IR2153. The circuit has
400 Vdc input and 100 Vdc output.
Table I shows the
components used in the implementation.
The relationship between primary and secondary currents of
the transformer is given in equation (7). Rp and Rs represent
cupper losses for the primary and secondary respectively. kps
is the coupling coefficient between primary and secondary
windings. Vp and Vs are primary voltage and secondary
voltages and Ip and Is are primary and secondary currents
respectively. ω is the radian frequency of the circuit and RLed
is the equivalent resistance of LEDs.
Ldc
S1
Vgs2
Vdc/2
Vin
(7)
Cdc
D1
LVrb
C
Cr
+
_
Lp
D3
D5
D4
D6
Ls
C0
LED
S2
where,
Vdc/2
Vgs1
Cdc
D2
,
Ldc
Fig. 7 Proposed LED Driver with dimming function.
Table 1The List of components.
Item
Font
5mH
730uH
65uH
68nF
33 x1W Power Led
350mA, 100
lumen
C
2nF
47uF
100uF
Mosfets
SK2605
Lvrb
1470uH-175uH
To tune the resonant frequency, resonant capacitor Cr is
selected from equation (6).
(8)
For dimming purpose, a current controlled inductor shown
in (Fig. 6) is used. When dc control current is increased, the
value of the inductor decreases. Detailed analyses of
magnetically control inductor design can be found in [12].
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D3,D4,D5,D6
It can be seen that in the proposed approach, the switching
frequency is kept constant and LEDs are not subjected to be on
and off in a dimming period.
MUR160
400V
To meet the desired LED current, the value of inductance of
the variable inductor is tuned by the control current. Figure 8
shows the relations between the control current and inductance
value. When the inductor value is high, LEDs’ current
becomes low and vice versa.
Led Power & Idc
40,0
35,0
30,0
Watt
25,0
Lvrb mH
Lvrb & Idc
20,0
15,0
1600
10,0
1400
5,0
1200
0,0
0,00
1000
0,20
0,40
0,60
0,80
1,00
1,20
1,40
1,60
Idc
800
600
Fig. 11 LEDs Power vesus Control Current.
400
200
0
0,00
0,20
0,40
0,60
0,80
1,00
1,20
1,40
Zero voltage switching is also accomplished with the
proposed technique. While the body diode is in forward bias,
MOSFET becomes on. As an example, 350 mA led current,
drain current and drain source voltage of upper MOSFET are
shown in (Fig. 12).
1,60
Idc
Fig. 8 Inductance Variation with Control Current.
With the proposed technique, lamp current is changed from
215 mA to 350 mA. The waveforms of 215mA and 350mA
LED currents and their voltages are illustrated in (Figs 9 and
10) respectively.
Fig. 12 LED current, Drain Current and Drain Source Voltage
of MOSFET.
V. CONCLUSION
Fig. 9 LED Current and Voltage for Low Power.
A unique dimmable current fed half bridge led driver is
proposed in this study.
Dimming is accomplished
magnetically to meet desired led luminous flux. The proposed
circuit is implemented on 33x1W LEDs. LEDs are driven
within 20 W to 33 W. That range may be increased by
integrating pulse density modulation technique with the current
controlled inductor.
Fig. 10 LED Current and Voltage for Nominal Power.
The control current of the inductor is increased from 0 to
1.6 A. As a result of this, LEDs luminous flux is increased
too. Led power versus dc control current is plotted in (Fig. 11).
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Advances in Robotics, Mechatronics and Circuits
REFERENCES
[1]
J. Zhang, L. Xu, X. Wu, and Z. Qian, “A precise passive current
balancing method for multi-output LED drivers,” IEEE Trans. Power
Electron., vol. 26, no. 8, pp. 2149–2159, Aug. 2011.
[2] D. Rand, B. Lehman, and A. Shteynberg, “Issues, models and solutions
for TRIAC modulated phase dimming of LED lamps,” in Proc. IEEE
Power Electron. Spec. Conf., 2007, pp. 1398–1404.
[3] O. Tetevnoks, I. Galkin and A. Avotins, “Illumination Detection for
LED Dimming Process Efficiency Evaluation”, Elektronika ir
Elektrontechnika (Electronics and Electrical Engineering), vol. 19, no.
1, pp 44-47,2013.
[4] I. Galkin, O. Teteryonok and I. Milashevski, “Weight and Size
Estimation of Energy Efficient LED Ballast”, Elektronika ir
Elektrontechnika (Electronics and Electrical Engineering), vol. 120,
no. 4, pp. 55-60, 2012.
[5] Qingcong Hu; Zane, R., "LED Driver Circuit with Series­InputConnected Converter Cells Operating in Continuous Conduction
Mode," IEEE Transactions on Power Electronics,vo1.25, no.3, pp.574582, March, 2010.
[6] Ray-Lee Lin, Yi-Chun Chang, and Chia-Chun Lee, "Optimal Design of
LED Array for Single­Loop CCM Buck-Boost LED Driver," IEEE
Transactions on Power Electronics, vo1.49, no.2, pp761-768, March,
2013.
[7] Y. Hu and M. M. Jovanovi'c," LED Driver With Self­Adaptive Drive
Voltage," IEEE Transactions on Power Electronics, vol.23, no.6, pp.
3116-31241, Nov., 2008.
[8] Y. Chen, Y. Nan, and Q. Kong ,"A Loss-Adaptive Self­Oscillating
Buck Converter for LED Driving," IEEE Trans.Power Electron., vol.
27, no. 10, pp. 4321-4328, Oct. 2012
[9] C.S.Moo, Y.J. Chen, and W.C. Yang, "An Efficient Driver for
Dimmable LED Lighting, "IEEE Trans. Power Electron.,vol. 27, no.
11, pp. 46l3-4618, Nov. 2012.
[10] M. S. Lin, and c.L. Chen, "An LED Driver With Pulse Current Driving
Technique," IEEE Transactions on Power Electronics, vol.27, no.Il,
pp.4594-460l, Nov., 2012.
[11] W. K. Lun, K. H. Loo, S. C. Tan, Y. M. Lai, and C. K. Tse, "Bilevel
current driving technique for LEDs," IEEE Trans. Power Electron., vol.
24, no. 12, pp. 2920-2932, Dec. 2009.
[12] Medini G, Michael G, Sam B.Y, “Inductor-Controlled Current
Sourcing Resonant Inverter and İts Application As A High Pressure
Discharge Lamp Driver.” Applied Power Electronics Conference and
Exposition, Vol.1, pp:434 440, Feb. 1994
Selim Borekci was received the M.Sc and Ph.D. degrees in electrical
engineering from New Mexico State University, Las Cruces, New Mexico,
USA, in 1997 and 2003, respectively.
He was worked as a designed engineer at Howard Ind. Laurel, MS, USA
from 2001 to 2004. He worked with Electrical and Electronic Engineering
Department of Pamukkale for 6 years. Since 2010, He has been working in
Electrical and Electronics Eng. Dept. at Akdeniz Universities, Turkey. Now,
he is an Associate Professor at Akdeniz University. His research interest
includes the applications of power electronics such as power filters, reactive
power control techniques, power supplies, multi-level inverters, electronic
ballasts, resonant circuits, and induction heating systems.
Nihal C. Acar was received the M.Sc and Ph.D. degrees in electrical
engineering from Akdeniz University, Antalya, Turkey. Her research interests
are application of power electronics such as LED driver design and induction
heating systems.
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