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
[email protected] 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
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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
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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
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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
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_
_
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.
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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.
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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)
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