A Novel Passive Off-line Light-Emitting Diode (LED)
Driver with Long Lifetime
S.Y.R. Hui, Fellow IEEE, S.N. Li, X.H. Tao, W. Chen, Member IEEE and W.M. Ng, Member IEEE
Center for Power Electronics
City University of Hong Kong
Hong Kong
eeronhui@cityu.edu.hk
Abstract— This paper describes a patent-pending passive offline LED driver that has no semiconductor switches, electrolytic
capacitors, auxiliary power supply and control board. It can
provide a fairly smooth current from the ac mains to drive LED
strings. The new circuit has the advantages of high input power
factor, high energy efficiency and luminous efficacy, long
lifetime, stable luminous output and high robustness against
extreme weather conditions. In addition, over 90% of the driver
material is recyclable, leading to reduction of electronic waste. It
is particularly suitable public LED lighting systems such as road
lighting systems. Experimental results based on a 50W system
are included in the paper to confirm the validity of the proposal.
Due to the circuit simplicity, an energy efficiency exceeding
93.6% has been achieved.
I.
INTRODUCTION (HEADING 1)
LED technology has emerged as a promising lighting
technology to replace the energy-inefficient incandescent
lamps and mercury-based fluorescent lamps [1]. While LED
devices enjoy relatively long lifetime of typically 80,000 h [2],
the relatively short lifetime of LED drivers, which is limited
by the use of electrolytic capacitors [3], remains a limiting
factor to the lifetime of the overall LED systems. Although
electronic LED drivers without using electrolytic capacitors
have been proposed [4-6], the use of active power electronic
switches requires extra control electronics and auxiliary power
supplies that will increase circuit complexity and reduce
system reliability. In addition, these extra circuit boards may
need electrolytic capacitors, although electrolytic capacitor is
not used in the power circuits. Particularly for outdoor
applications such as road lighting systems, the ballasts (or
drivers for LED lighting systems) must be highly reliable.
Take Hong Kong as an example. The number of lightning
could be 10,000 times or higher in a stormy day in the summer
[7]. With about 130,000 street lamps in Hong Kong, 1% of
the system failure means problems in 1,300 street lamps. So
reliability is a paramount issue in road lighting systems.
Existing street lamps primarily use high-intensitydischarge (HID) lamps and magnetic ballasts. Magnetic
ballasts are highly reliable with lifetime of 20 years,
recyclable and hence highly environmentally friendly [8].
Such environmental friendliness cannot be matched by
978-1-4244-4783-1/10/$25.00 ©2010 IEEE
electronic ballasts due to their short lifetime (typically < 5
years) and their use of toxic and/or non-biodegradable
components. In the International Forum on Novel Light &
Energy Sources held in Shanghai, China, in April 2009,
several road lighting management institutions have expressed
their needs to have LED drivers with lifetime higher than 10
years. This request arises from the experience learnt from
previous trials of LED street lighting products in the last 3
years in China. Sustainable lighting technology should meet at
least three criteria (i) high efficiency or energy saving, (ii)
long product lifetime and (iii) recyclability.
In this paper, a novel and patent-pending [9] passive LED
driver for off-line applications that meet these 3 criteria is
proposed. This passive LED driver consists of passive
components and diodes only, without using any power
electronic switches, auxiliary power supply and control
boards. The proposal features circuit simplicity, reliability and
long product lifetime. A circuit analysis and practical
confirmation of this passive LED driver for a 50W LED
system are included in this paper.
II.
A.
PRINCIPLES OF THE PASSIVE LED DRIVER
Existing concept
ac
mains
AC-DC
power
conversion
stage
Input
power
DC-DC
currentsource
converter
Buffered
power
time
LED
load
LED
power
time
time
Fig.1 Schematic & power profiles of a traditional offline LED system
Fig.1 shows the schematic of a traditional AC-DC power
conversion system for offline LED applications, and the
typical power profiles in the input, intermediate and output
stages. Both single- and two- power stage approaches have
been addressed in [10,11]. For a two-stage approach, a front
594
power stage converts the ac mains voltage into a stable dc
voltage with the help of a large electrolytic capacitor as a
energy storage and buffer. A second power stage then converts
the dc voltage source into controllable dc current source for
driving the LED load. Since the second stage provides a
constant current source, the output power is constant, meaning
that the capacitor in the intermediate stage has to be large
enough to absorb the energy buffer. Therefore, electrolytic
capacitor with large capacitance is usually used as the energy
buffer. The single-stage approach [11-15] essentially
combines part of the two stage circuits together to form a
single-stage circuit. Large storage capacitor is needed as in the
two-stage approach.
into a current source for driving the LED load. As an
alternative, this output inductor can be replaced by a current
ripple cancellation circuit, which consists of a coupled
inductor and a capacitor [19-21]. This study focuses on the
LED driver performance. Current balancing techniques for
parallel LED strings [22] are not the scope of this study.
LED power
PLED
Fig.3a
Time
B. New Concept
The new concept proposed in this study [9] is illustrated in
Fig.2. A single-stage passive circuit with power factor
correction is proposed to replace the two stage power circuit.
Instead of using a large capacitor to ensure that the output
current is constant, it is proposed that a small current ripple
may be allowed in the output current. In this way, the
requirement for the energy buffer can be reduced and
consequently non-electrolytic capacitors can be used to enable
long lifetime of the overall system. This small current ripple
may cause power variation in the LED load. However, such
power variation will not cause noticeable luminous variation
to human eyes.
With the help of the general photo-electro-thermal theory
for LED systems [16], thermal designs can be made so that the
luminous flux and power of a LED system can follow the
profiles in Fig.3b or in Fig.3c. It can be seen that the slope of
this curve is small at and around the peak flux value in Fig.3b
and Fig.3c. This means that a relatively large power
fluctuation will only lead to a small flux change, i.e. the
sensitivity of the luminous flux with the changes in LED
power is small.
Passive
AC-DC
power
conversion
stage
ac
mains
Pmax
Pmin
Luminous
flux φv
∆φv
∆PLED
Pmin
Fig.3b
Luminous
flux φv
Pmax
LED power
Pmax
LED power
∆φv
∆PLED
Pmin
Fig.3c
Fig.3 Variation of LED power and luminous flux in this proposal
inductor
ac
mains
LED
load
Diode
rectifier
Valleyfill
circuit
LED
load
Fig.4a Schematic of a passive LED driver
Coupled inductor
Input
power
LED
power
Buffered
power
time
time
ac
mains
Diode
rectifier
time
LED
load
Fig.4b Schematic of a passive LED driver with a current ripple
cancellation circuit
Fig.2 Schematic & power profiles of a traditional offline LED
system
Fig.4a shows the schematic of a passive LED driver for
offline applications. It consists of an input inductor, a diode
rectifier, a valley-fill circuit, an output inductor and a LED
load. The input inductor is used to limit the power output of
the load and also to reduce the load power sensitivity against
fluctuation of the ac mains voltage. The diode rectifier turns
the ac voltage into a dc one. Unlike the previous use of the
valley-fill circuit for improving the input power factor [17,18],
a major function of the valley-fill circuit is to reduce the
output voltage ripple [9] so as to reduce the size of the output
filter inductor. This output inductor turns the dc voltage source
Valleyfill
circuit
III.
PASSIVE LED DRIVER AND ANALYSIS
A. Passive LED Driver & Circuit Operation
Fig.5 shows the circuit diagram of one patent-pending
passive LED driver for off-line applications. It consists of only
11 components, namely 7 power diodes, two inductors and
two capacitors. The input inductor Ls is used to (i) to filter the
input current in order to reduce the input current harmonics
and (ii) to control the power sensitivity of the LED load. The
diode-bridge is to rectify the input ac voltage into dc one and
595
the valley-fill circuit is used to reduce the voltage ripple in the
output dc voltage V3. The output inductor L is used to convert
the dc voltage source V3. into a smooth dc current source Io to
drive the LED load. The LED load can be a LED string or a
parallel of LED strings. In order to increase the lifetime of this
circuit, non-electrolytic capacitors can be used for the valleyfill circuit. Variants of this basic circuit can be used to further
reduce the ac ripple in the output current. For example, an
extra capacitor (the 12th component) can be connected across
V3 so as to further reduce the output voltage ripple in V3.
Io
Ls
V3
0.5Vdc
Io
t
Fig.8 Idealized waveforms of output voltage V3 and current Io of the
valley-fill circuit (with V3 a rectified version of V2)
L
Is
V3
Vdc
LED
load
Vo
Po
V2
Vs
Vo
Fig.5 Basic circuit of the passive LED driver.
In this study, we use the basic circuit in Fig.4a for analysis.
The idealized input voltage Vs and input current Is waveforms
are shown in Fig.6. Due to the use of the input inductor, the
input current is expected to lag behind the input voltage. For
further improvement of the input power factor, a standard
solution is to add an input capacitor across the ac mains.
However, this method is not included in the following circuit
operation description.
The idealized waveforms of the input voltage to the diode
rectifier with the valley-fill circuit and the input current are
shown in Fig.7. Since the equivalent LED load reflected to the
input side of the diode bridge is resistive, V2 and Is are in
phase. The output voltage of the valley-fill circuit should be a
rectified version of V2. Thus, the idealized waveforms of V3
and Io are shown in Fig.8. Assuming that the voltage across
the LED load Vo does not change significantly, the idealized
output voltage, current and power waveforms are included in
Fig.9.
Io
t
Fig.9 Idealized waveforms of voltage across LED load (Vo), output
load current (Io) and the output load power (Po)
B. Circuit Analysis
This circuit analysis starts from the load side. If the voltage
output V3 is considered as an equivalent voltage source, a
simplified circuit of Fig.5 is shown in Fig.10, where R is the
winding resistance in the output inductor.
L
R
Io
Vs
LED
load
Vo
V3
Is
Fig.10 Simplified equivalent circuit of Fig.5 (output side)
_
t
φ
The average output current
I o can be expressed as:
_
_
V 3 − Vo
Io =
R
Fig.6 Idealized waveforms of input ac mains voltage and current
(with a phase shift (φ) between Vs and Is)
Vdc
0.5Vdc
_
where
V2
(1)
V3 is the average voltage of V3 . From the waveform of
V3 in Fig.8,
_
3
V3 = Vdc
4
Is
t
(2)
Rearranging (2) gives:
Fig.7 Idealized waveforms of input voltage V2 and current Is of the
diode rectifier (with V2 and Is in phase)
4 _ 4
Vdc = V3 = (Vo + I o R )
3
3
(3)
Note that the total voltage drop of the LED load is
approximated as a constant Vo. Therefore, Vdc does not change
596
_
_
Io =
I o does not change significantly. In general, Vo
significantly if
is much bigger than
I o R . Thus Vdc is close to 1.33Vo and Vdc
VS − V21
0.98 ⋅ ωLS
(10)
can be considered as a function of the Vo which is determined Differentiating (10) will lead to
in the LED load.
_
∆V
∆ Io =
The next issue is to find out a way to reduce the change
of Io due to fluctuation in the input mains voltage (i.e. to
reduce the sensitivity of the load power with the fluctuation of
the ac mains voltage. By the law of conservation of energy,
input power is equal to the power entering the diode bridge,
assuming that the input inductor Ls has negligible resistance.
Form the waveforms in Fig.6 and note that V21 and IS are in
phase as shown in Fig.7.
(4)
VS I S cos φ = V21I S
_
_
_
average output load current
∆ I o for a given change in the
input ac mains voltage ∆VS . That is, the power sensitivity of
the LED load, which is a function of the output current Io, can
be controlled by the inductance of the input inductor Ls.
Ls
→
VS
φ
→
VS
_
3
VS I S cosφ = V3 I o = Vdc I o = I o2 R + I o Vo
4
(11)
0.98 ⋅ ω LS
Equation (11) is the important equation which shows that the
input inductance Ls can be used to reduce the change of
where V21 is the fundamental component of V2.
Similarly, the input power is also equal to the output
power of the valley-fill circuit, assuming that the power loss in
the diode rectifier and valley-fill circuit is negligible.
_
S
V21
→
(5)
φ I
S
V21
→
r
VL = jωLS I S
Using Fourier analysis on the waveform of V2, the Fig.11 Simplified equivalent circuit & vector diagram of Fig.5 (input
side).
fundamental component V21 of V2 can be determined as:
V21 =
(2 + 2 )V
dc
π
_
sin (ωt − φ ) = 1.087 ⋅ Vdc sin (ωt − φ ) (6a)
In order to relate
I o with Vs, using (6), (7) and (8) gives:
2
_



4  
VS2 = (0.77 ) (Vo ) + ωLS  0.98 I o 
3
   



The root-mean-square value of V21 is therefore
V21 _ rms
1.087
=
⋅ Vdc = 0.77 ⋅ Vdc
2
(6b)
2
_
Io =
Io .
_
Vs2 − (1.024 ⋅ Vo )
_
4
(Vo + I o R )× I S = I o2 R + I o Vo
3
2
_
⇒ I S = 0.98 I o
Po = Vo ⋅
(7)
Now consider the equivalent circuit and the vectorial
relationship between Vs and V21 as shown in Fig.11.
2
V = V + (ωLS I s )
2
S
2
21
(8)
and
r r
r Vs − V21
Is =
jωLs
(13)
0.98 ⋅ ωLs
Note that Vo can be determined from the number of LED
devices in the LED strings. If Ls is chosen, then (13) provides
the relationship between the average output current and the
input ac mains voltage. The LED load power is therefore:
V21 I s = I o2 R + I o Vo
0.77Vdc I S = 0.77 ×
(12)
Solving (12) gives:
Based on (4), (5) and (6) and assuming that winding resistance
is negligible, one can relate Is and
2
(9)
From (6), it can be seen that V21 depends on Vdc, which is
approximately close to Vo (approximated as a constant value).
With the help of (7) and (9),
Vs2 − (1.024 ⋅ Vo )
0.98 ⋅ ωLs
IV.
(14)
EXPERIMENTAL VERIFICATION
A passive LED driver based on the circuit in Fig.5 has
been designed and built for a LED load. The load consists of a
LED string using 16 Sharp LED (model number:
GW5BWC15L02) in series. Its peak current rating is 400mA.
The total voltage across this LED load is Vo=158V. The input
inductor Ls is 1.47 H and it has a winding resistance RLs=2.7Ω.
Two polypropylene capacitors of 20µF each are used in the
valley-fill circuit. The output inductor L is 2.3H and it has a
winding resistance RL= 3Ω. Based on these reactive
parameters (with all resistance ignored), the theoretical output
current should be about 0.36A and the LED load power 57W.
597
A 50W LED system has been developed and tested with
the proposed passive offline LED driver. Fig.11 shows the
measured input voltage Vs and current Is of the entire system.
It can be seen that the input current is highly sinusoidal. The
measured Vs and Is agree with predictions in Fig.6. Fig.12
shows the measured V2 and Is. Vs is found to be a stepped ac
voltage which is in line with the idealized stepped voltage in
Fig.7, and is in phase with Is as predicted. The measured V3
and Io are captured in Fig.13. V3 is found to be a rectified
version of V2 as expected in Fig.8. The output current is fairly
smooth with a small ripple only. The measured output current
Io, LED string voltage Vo and output power of one LED string
Po are shown in Fig.14. These results are highly consistent
with the theoretical predictions.
The energy efficiency of the LED driver is also measured.
Power measurements are made with the use of Voltech
PM6000 power analyzer. [Note: readings from the Power
Analyzer are slightly different from and more accurate than
those displayed in a digital storage oscilloscope.] With an ac
mains voltage of 230V, the total input power is 49.12W and
the output power consumed by the LED load is 46W. The
system loss is only 3.12W. Thus, a high efficiency of 93.6%
has been achieved.
Fig. 13 Measured V3 and output current Io (L=2.3H)
Fig. 14 Measured output current Io, LED string voltage Vo
& output power Po (L=2.3H)
Fig. 11 Measured input voltage Vs & current Is
The output current ripple can be reduced further by either
using a larger output inductor or a current ripple cancellation
circuit. To illustrate this point, a larger output inductor L of
about 5H is used. At an ac mains voltage of 230V, the total
input power is 52.29W and the output power consumed by the
LED load is 48.64W. A high efficiency of 93.0% can still be
achieved.
The corresponding measurements are recorded in Fig.15Fig.18. The waveforms of Vs, Is, V2 and V3 in Fig.15 and
Fig.16 are of the same forms as those in the previous case.
However, it can be seen from the waveforms of the output
current in Fig.17 and Fig.18 that the current ripple has been
reduced. Consequently, the variation of the load power as
shown in Fig.18 is also reduced.
Fig. 12 Measured V2 (stepped) and input current Is (sinusoidal)
It is increasing to note that the change of the output
inductor does not significantly change the input and output
power. The total input power is just slightly higher than the
previous case with L=2.3H. The system loss has increased
from 3.12W to 3.65W due to the increase in the winding
resistance of the inductor. As illustrated in (14), it is the input
inductor Ls that controls the load power for a given input
voltage Vs and load voltage Vo.
598
Fig. 15 Measured input voltage Vs & current Is
Fig. 18 Measured output current Io, LED string voltage Vo
& output power Po (L=5H)
V.
Fig. 16 Measured V2 (stepped) and input current Is (sinusoidal)
CONCLUSION
A novel single-stage passive LED driver for offline
applications has been presented. This driver contains only
passive and robust components without using any power
switches, auxiliary power supply and control boards. Circuit
analysis and experimental verification on a 50W prototype has
been provided to confirm the feasibility of this proposal. As
the driver consists of only a few components and there is no
switching loss, a high efficiency of 93.6% has been achieved.
Since only a few robust components are used in the passive
LED driver, it is envisaged that this circuit provides other
advantageous features such as low cost, low maintenance
requirements and good robustness against extreme weather
conditions such as lightning and wide temperature variation.
Since the metallic materials of the cores and windings of
the two inductors, which contribute to the majority of the
product material, can be recycled, this passive driver offers
high efficiency, recyclability and long lifetime. These three
factors are the essential criteria for sustainable lighting
technology.
ACKNOWLEDGMENT
The authors would like to thank the Hong Kong Research
Grant Council for its support for Project CityU 123508 and
also the Centre for Power Electronics, City University of
Hong Kong for the support provided for this project.
Fig. 17 Measured V3 and output current Io (L=5H)
REFERENCES
[1]
[2]
[3]
599
E.F. Schubert, “Light-emitting diodes”, Cambridge, Second Edition,
2006
“Datasheet of Luxeon Emitter”, DS51, LUEXON POWER LEDS.
http://www.lumileds.com/pdfs/DS51.pdf
H. S.-H Chung; N.-M. Ho; W. Yan; P. W. Tam; S. Y. Hui:
“Comparison of Dimmable Electromagnetic and Electronic Ballast
Systems—An Assessment on Energy Efficiency and Lifetime”, IEEE
Transactions on Industrial Electronics, Volume 54, Issue 6, Dec. 2007
Page(s):3145 – 3154
[4]
Y.X. Qin,; H.S.H. Chung,; D.Y. Lin,; S.Y.R. Hui,; “Current source
ballast for high power lighting emitting diodes without electrolytic
capacitor”, 34th Annual Conference of IEEE Industrial Electronics,
2008. IECON 2008. 10-13 Nov. 2008 Page(s):1968 – 1973
[5] P. T. Krein, R. S. Balog, “Cost-Effective Hundred-Year Life for SinglePhase Inverters and Rectifiers in Solar and LED Lighting Applications
Based on Minimum Capacitance Requirements and a Ripple Power
Port”, in Applied Power Electronics Conference and Exposition, 2009,
APEC’09, Feb. 2009, pp. 620 – 625.
[6] L. Gu, X. Ruan, M. Xu, K. Yao, “Means of Eliminating Electrolytic
Capacitor in AC/DC Power Supplies for LED Lightings”, IEEE Trans.
Power Electronics, vol.24, no.5, May.2009, pp.1399-1408
[7] Hong Kong Observatory official Website http://www.hko.gov.hk/
Chung, H. S.-H.; Ho, N.-M.; Yan, W.; Tam, P. W.; Hui, S. Y.;
“Comparison of Dimmable Electromagnetic and Electronic Ballast
Systems—An Assessment on Energy Efficiency and Lifetime”, IEEE
Transactions on Industrial Electronics, Volume 54, Issue 6, Dec. 2007
Page(s):3145 – 3154
[8] S.Y.R. Hui and W. Yan, “Re-examination on Energy Saving &
Environmental Issues in Lighting Applications”, Proceedings of the
11th International Symposium on Science 7 Technology of Light
Sources, May 2007, Shanghai, China (Invited Landmark Presentation),
pp.373-374
[9] S,Y,R, Hui, “Apparatus and methods of Operation of Passive LED
Lighting Equipment”, US patent application 12/429,792, 24 April 2009
[10] Zhongming Ye; Greenfeld, F.; Zhixiang Liang; “A topology study of
single-phase offline AC/DC converters for high brightness white LED
lighting with power factor pre-regulation and brightness dimmable
feature”, 34th Annual Conference of IEEE Industrial Electronics, 2008.
IECON 2008. 10-13 Nov. 2008 Page(s):1961 – 1967
[11] Xiaohui Qu; Wong, S.C.; Tse, C.K.; Xinbo Ruan; “Isolated PFC PreRegulator for LED Lamps”, 34th Annual Conference of IEEE
Industrial Electronics, 2008. IECON 2008. 10-13 Nov. 2008
Page(s):1980 – 1987
[12] F.S.D. Reis,; J.C. Lima,; R., Jr. Tonkoski,; V.M. Canalli,; F.M. Ramos,;
A. Santos,; M. Toss,; U. Sarmanho,; F. Edar,; L. Lorenzoni,;, “Single
stage ballast for high pressure sodium lamps”, IECON 2004. 30th
Annual Conference of IEEE Industrial Electronics Society, 2004.
Volume 3, 2-6 Nov. 2004 Page(s):2888 - 2893 Vol. 3
[13] Jinrong Qian; F.C. Lee,; “A high efficient single stage single switch
high power factor AC/DC converter with universal input”, Twelfth
Annual Applied Power Electronics Conference and Exposition, 1997.
APEC '97 Conference Proceedings 1997., Volume 1, 23-27 Feb. 1997
Page(s):281 - 287
[14] C. Qiao,; K.M. Smedley,; “A topology survey of single-stage power
factor corrector with a boost type input-current-shaper”, IEEE
Transactions on Power Electronics, Volume 16, Issue 3, May 2001
Page(s):360 – 368
[15] C.K. Tse,; M.H.L. Chow,; “Single stage high power factor converter
using the Sheppard-Taylor topology”, 27th Annual IEEE Power
Electronics Specialists Conference, 1996. PESC '96 Record., Volume 2,
23-27 June 1996 Page(s):1191 - 1197 vol.2
[16] S.Y.R Hui. and Y.X. Qin, “General photo-electro-thermal theory for
light-emitting diodes (LED) systems”, IEEE Transactions on Power
Electronics, Volume 24, Issue 8, Aug. 2009 Page(s):1967 – 1976
[17] K. Kit Sum, “Improved Valley-Fill Passive Current Shaper”, Power
System World 1997, p.1-8
[18] Lam, J.; Praveen, K.; “A New Passive Valley Fill Dimming Electronic
Ballast with Extended Line Current Conduction Angle”, INTELEC '06.
28th Annual International Telecommunications Energy Conference,
2006. 10-14 Sept. 2006 Page(s):1 – 7
[19] Hamill, D.C.; Krein, P.T.; “A `zero' ripple technique applicable to any
DC converter”, 30th Annual IEEE Power Electronics Specialists
Conference, 1999. PESC 99. Volume 2, 27 June-1 July 1999
Page(s):1165 - 1171
[20] Schutten, M.J.; Steigerwald, R.L.; Sabate, J.A.; “Ripple current
cancellation circuit” Eighteenth Annual IEEE
Applied Power
Electronics Conference and Exposition, 2003. APEC '03. Volume 1,
9-13 Feb. 2003 Page(s):464 - 470
[21] Cheng, D.K.W.; Liu, X.C.; Lee, Y.S.; “A new improved boost
converter with ripple free input current using coupled inductors”,
Seventh International Conference on Power Electronics and Variable
Speed Drives, 1998. (Conf. Publ. No. 456) 21-23 Sept. 1998
Page(s):592 - 599
[22] Hwu K. and Chou S., “A simple current-balancing converter for LED
lighting”, IEEE Applied Power Electronics Conference, Feb. 2009,
Washington DC, USA, paper: 16.7.
600