LEDs as Energy Efficient Lighting Systems: A Detail Review

Sohel Uddin, Hussain Shareef, Azah Mohamed, M A Hannan and Khodijah Mohamed Department of Electrical, Electronics and System Engineering

Universiti Kebangsaan Malaysia, Malaysia.

Abstract— Lighting accounts for roughly one-fifth of all global electricity consumption, yet the major share of this energy is emitted not as light but as radiated heat from inefficient bulbs. Compact fluorescent Lights (CFLs) were the initial substitute for incandescent bulbs, but certain characteristics of CFLs, light emitting diodes (LEDs) will be used dramatically in the next few years because of its low energy consumption and long life. This paper presents a survey of literature on the light (LEDs) as energy efficient light bulbs due to their high efficiency. The literature shows an increasing interest in this subject for the last decade, where the enhancement of LED lighting systems using various controllers has been widely investigated. Several technical issues related to thermal properties and LED array configurations have been highlighted. Moreover, some of the developments in LED driver technology have been summarized.

Keywords- Light Emitting Diodes (LEDs); Compact Fluorescent Lamps (CFLs); power quality.

I. INTRODUCTION Recently the idea of using efficient and long life light

emitting diode (LED) in those applications that were traditionally the province of inefficacious and short aged incandescent bulbs and compact fluorescent lamps (CFLs) has resulted in the development of light emitting diode (LEDs) lamps. LED lamps are primarily intended for residential and commercial customers. Lasting much longer and consuming much less energy than incandescent lamps and CFLs with comparable luminous output as shown in Fig. 1, they represented promising new lamp types [1]. Furthermore, the LED lamps are environmental friendly, unlike the CFLs due to the use of mercury inside the CFL [2]. The toxicity of mercury is well known and creates a serious long-term problem. Technically, there are a number of fundamental studies that are needed before they can be widely accepted. For example, long life and energy efficient ballast is needed for using LED as a lighting system. Moreover, suitable methods are required to maintain the temperature and forward voltage because the performance of LED lighting decreases with increasing temperature. Another problem may arise due to mass application of LED lamps in the network due to the accumulation of harmonics from the distorted ballast currents.

Figure 1. LED luminous flux as function of rated power [3].

Fig. 2 depicts a block diagram of typical LED ballast. The

first block contains the full diode bridge rectifier to convert the ac line into a dc stage. It is followed by a smoothing capacitor. It provides the dc link voltage of the current source for the LED load. To minimize the effects of voltage variation, a constant current source is used instead of voltage converter in LED ballast as shown in Fig. 2.

Figure. 2. Schematic diagram of LED system.

The aim of this paper is to survey various expect of LED

lamps by conduction literature survey through IEEE, IET and Elsevier databases over the last 11 years from 2000 to 2011 (July). A large number of papers were published dealing with the behavior of LEDs under various exploitation conditions such as thermal property, LED array design, current balancing converter for LEDs, ballast circuit design etc. Fig. 3 statistically illustrates the number of published research papers on the subject of the LEDs as a lighting source during the last 11years. It shows the intensification of research on thermal property, array design, power factor correction and current control techniques in LED lamps. The following sections elaborate main areas of LED lighting research.

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2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Year

Num

ber o

f Pap

ers

Cons tant Current Contro l

P ower Factor Correction & Life Time

Thermal P roperty

Array & Illumination

Figure 3. Number of papers published in various areas of LED lamp

research.

II. THERMAL PROPERTY The emitted light output of LEDs strongly depends on the

junction temperature. LEDs vary in wavelength and lumen output which is also depend on temperature and time. It is true that the quantum efficiency and junction thermal resistance of LED are the two limiting factors in LED technology [4]. The LED lamp has luminous efficacy of 150 lm/W and junction temperature Tj of 25 ◦C. The luminous efficacy of various LEDs typically decreases by approximately 0.2–1% per degree Celsius rise in temperature. Recent research reports [5]-[6] have highlighted the relationship of efficacy and the junction temperature of the LEDs. Biber reported about the relationship between thermal behavior and luminous output [6]. Since, the junction to thermal resistance is an important factor, [7]-[9] have discussed about junction thermal resistance and configuration of heat sinks. Baureis also proposed a junction thermal resistance for other LED device but not for LED lighting systems [10]. A theory that relates the photometric, electrical, and thermal aspects of an LED system is suggested in [4]. This theory presents about thermal model, heat sink and optimal operating point for touch maximum luminous output when it is operating at normal condition.

III. ARRAY DESIGN LED is a small source of light. A single LED is not

sufficient to emit light like Fluorescent Lamps. Thus it is necessary to use several LEDs operating simultaneously. A compact lamp based on LEDs was suggested in [1], [11] for uniform illumination distribution. The purpose is to get parameters which state the maximum LED density and the minimum LED to detector distance for different configurations of LED arrays which produce satisfactory uniformity. Unfortunately, this method may not provide agronomic design of an LED array. To improve this problem, [12] discussed about design method by systemic concepts and focuses on the study of optical properties. With the proposed procedure, it is possible to design efficiently a LED lighting module and achieve a satisfactory uniformity by the design of

a uniform LED illumination system. In this method, it not only reaches the maximally flat illumination distribution but also the emitting angle of the system can be designed. Several researchers concerned in light distribution and visual performance of LED lamps [13]-[15]. The drawbacks of currently available white LEDs include the point source nature and colorimetric of their light output. Currently available LED array design includes circular, linear and square arrays.

IV. BALLAST CIRCUIT DESIGN A. LED Driver for Constant Current Control

LED arrays have three possible connections such as series, parallel and series-parallel connections. All arrays types contain advantages and disadvantages. Among them series-parallel connection is considered better according to some authors but in this configuration, the current will not be same in all branches. As a result, a difference in luminous intensity in each LED can occur. So many researchers proposed current balanced control technique for LED lamps [16]-[19].

To control the current, a ballast resistor is added in series

with each LED string to minimize current differences in each LED strings [20]. In fact, it suffers from poor operating efficiency due to the excessive power losses dissipated on the added current balancing resistors. In [18] a current balancing converter was introduced for driving parallel LED strings, so as to make sure that the required current is shared among all LED string. The efficiency of linear regulator LED drivers with a pre-regulator can be improved by sensing and regulating the minimum voltage of the linear regulators [21]. An approach that integrates the controls for parallel LED string current regulation was presented in [17]. Moreover, depending on strong temperature, the forward voltage of the sensing diodes which actually sense the minimum voltage drop across the linear regulators varies with the operating temperature causing a significant variation of the power loss. To improve this drawback, a novel linear regulator LED driver for paralleled LED strings was employed in [22], whose a voltage pre-regulator with an adaptive output voltage offers the maximum efficiency. In the work of Hsieh et al., a reference tracking technique was presented to reduce the power dissipation of the LED driver when the LED strings are controlled by the digital PWM dimming signal [19]. The proposed LED driver consists of multiple parallel current sink regulators and a dc/dc boost converter. An effective two secondary output windings structure was proposed in [23] for multiple LED strings with independent brightness control but it has some limitations such as its only suitable for constant input voltage and a separate controller is needed for each LED string. To develop multiple LED strings, a dimmable LED driver based on one master power converter with cascaded magnetic amplifier based converter was investigated in [24], which control the part of the voltage range that governs the voltage variations of the parallel LED strings. Modeling of LED lamp with ballast circuit depends on the power required of LEDs and power delivered by the ballast. Normally, the power requirement of LEDs is less than power rating of ballast

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circuit. A driving technique which matched a difference between the LEDs and rated power of ballast circuit is presented in [25].

B. Power Factor Correction (PFC) and Life Time

LED is a semiconductor device which operates in DC power. For this reason, as a replacement of AC light bulbs, a ballast is needed for LED lamps for lighting purpose. High power factor, long lifetime, accurate current control and high efficiency are the key factors to design of LED ballast circuits. Many researchers highlight about the design of ballast circuit and how to improve the ballast circuit performance of LEDs lamps [26]-[29]. There are many types of converters such as buck, boost, cuk and sepic converters for power factor correction in LED driver.

Generally, the isolated switching mode converter, which is

classified into three types such as flyback, forward and push-pull converter have been developed for both LED driver and PFC stage [30]-[32]. Comparing the three different topologies, it appears that the fly back converter is the best for this application. A Type-II fly back compensator was proposed in [33] to reduce the sensitivity of the LEDs to the voltage variation. It eliminates the need to sense the input voltage. Moreover, it has simple feedback control, high power factor and dimmable LED current. A different converter topology that belongs to the family of D2 converters which employs only one switch and three diodes which helps in achieving an acceptable efficiency unlike conventional converter requiring four diodes was employed in [29]. In the work of Kening et al., a quasi active PFC scheme assisted by a power converter was presented which precedes a driving power stage [34]. It uses a passive circuit to implement power factor correction function. Due to the passive circuit, PFC function increases the reliability and reduces the cost. It also makes the size of the components in the passive circuit small. In [35] a high turn’s ratio transformer is used to provide a low voltage level at the secondary of LED controller but due to transformer, a leakage inductance on the primary side of the transformer causes dramatic increase in power loss, thermal stress and causes the lifetime of converter much shorter than that of LED lamps. To improve this problem an LED ballast with a first stage isolated current fed power factor correction pre-regulator was proposed in [28]. It introduces the short lifetime of high voltage storage capacitor to the secondary, thus extending the overall system lifetime.

However the lifetime of an electrolytic capacitor is quite

limited with only several thousand hours under rated operating conditions which is very much shorter than lifetime of the LEDs. Thus, the electrolytic capacitor is a key issue to the overall long term reliability of the LED and its power supply. Although LED ballast without electrolytic capacitors have been used. Recently, many researchers are used a film capacitors instead of electrolytic capacitors [36]-[38], which can reduce the requirement for a high storage capacitance to achieve a long lifetime of the PFC converter. But this is not an

ultimate solution for achieving a better performance for all applications such as outdoor application. A passive offline LED driver without controlled power electronic switches, electrolytic capacitors and control board was described in [26], [39]. Its only contains passive components and diodes. So majority driver materials are recyclable, leading to reduction of electronic waste and the size of driver. A resonance assisted buck converter was suggested in [27] to achieve a high voltage step down ratio and high converter efficiency, whilst maintaining durability and compatibility with existing incandescent dimmers. LED lamps can also operate directly from AC line. For this case a resistor in needed before LEDs but more heat is generated due to resistance. In this paper [40] a driver control circuit is presented for operating LEDs directly from AC supply source. This circuit improved heat problem. Some literature also discusses flicker generation in LED lamps [41], [42]. In the work of Taekhyun et al. [42], it has demonstrated that LED lamps exhibit flicker response similar to that of CFLs, which are sensitive to high frequency interharmonics located close to the odd order harmonics.

V. CURRENT CHALLENGES Although LED lamps are more efficient than conventional incandescent bulbs and also ahead in terms of environmental friendliness, there are few issues and challenges in this new lighting technology. The real issue is about worthiness of investing on these LED lamps. Compared with incandescent bulbs, they certainly are economical. However, compared with CFLs, LEDs aren’t as economical. One would save maybe 50 kWh and no more than $10 for replacements [43]. Furthermore, in order to label a new lamp technology as green, it is not enough to just consider energy consumption in use. Instead, the complete life cycle covering resources, energy needed during manufacturing, transport and use as well as the end-of-life needs to be analyzed. In contrast to the primary energy consumption of incandescent lamps of around 3,302 kWh, CFL and LED lamps use less than 670 kWh of primary energy during their entire life. Fig. 4 shows the primary energy demand for various types of lamps. Form Fig. 4, it can be seen that LED lamps consume a lot of energy in heat sinks manufacturing [44]. Although LED lamps produce far less heat than incandescent bulbs, dissipating what heat they do create is the primary factor limiting their brightness. According to regulations, LED lamps must remain below 90 °C. Therefore, researchers and engineers are thinking of making specifically designed fixtures to dissipate LED heat thereby possibly obtaining more lumens in LED lamps. Another challenge for LED lamps is the tradeoff between color temperature and lighting efficiency. The semiconductor itself produces a much bluer light than most other types of bulbs. Currently manufacturers lower the color temperature by using phosphors. However, as more long-wavelength phosphors get added, more energy gets lost in the conversion process.

All in all, it is expected that future improvements of LED

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Figure. 4. Primary Energy Demand for manufacturing of all three lamps

lamps will further cut down energy demand by solving the challenges faced in LED lighting technology.

VI. CONCLUSION This paper presents a bibliographical survey of the work

published on the application of LEDs as lighting systems. The current development in LED lighting system to improve their quality such as ballast design, illumination distribution, thermal property etc is discussed. Different technical problem, such as power factor and current control methods are address and discussed. About forty research publications have been classified, discussed, and appended for a quick reference.

ACKNOWLEDGMENT This work was carried out with the financial support from

the MOHE under the research grant UKM-KK-02-FRGS0193-2010.

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