Temperature Dependency in Performance of Solid State Lighting Drivers S. Tarashioon1,2, S.W. Koh, W. D. van Driel, G.Q Zhang 1 Delft Institute of Microsystems and Nanoelectronics (DIMES), Delft University of Technology, Feldmannweg 17, 2628 CT Delft, The Netherlands, Phone: +31 (0) 15 27 87063, Fax: +31 (0) 15 26 22163 , 2 Material and Innovation Institute (M2i), Mekelweg 2, 2628 CD Delft, The Netherlands Email address:
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Abstract— Solid state lighting (SSL) is a new lighting technology based on high brightness light emitting diodes (LED). This technology because of being much more energy efficient, having longer lifetime and design flexibility has attracted the attention of both manufacturers and consumers. This technology looks very promising because of its advantages with respect to conventional lighting systems. Especially for some applications like outdoor lighting and automotive applications, using SSL can be very beneficial. Nevertheless there is still requirement to study about the reliability and performance of these lighting systems in different environments [1]. Each SSL lighting system is constructed by two major parts; optical part and electrical driver. The effect of temperature on performance and reliability of SSL system electrical driver is the focus of this research. In the beginning we will discuss about the fundamental SSL electronic driver building blocks; control/information, switching and electromagnetic energy storage. It will be explained that the last two blocks, switching and electromagnetic energy storage, are the heart of power conversion and thus more sensitive to the temperature. In this paper we focus on electromagnetic energy storage part. Ceramic capacitors are the most commonly used devices as electromagnetic energy storage component in SSL drivers. Thermal studies generally focus on steady state temperature dependency of the devices and they ignore the transient part of the temperature variation. Although in applications like automotive SSL lighting and also outdoor lighting the transient part of the temperature change should be also taken into account. In this paper we show that the transient part of the temperature change can also play an important role in performance of the most common type of the electromagnetic storage part of an SSL driver.
part, thus it is necessary that weakest part to be distinguished and then be analyzed. A SSL module includes two major parts, optical part and electrical driver. SSL electrical driver is the interface of optical part and the input power. Based on the literature one of the weakest parts of an SSL module is its electrical driver [3]. Therefore, understanding the performance and reliability of SSL driver is an essential requirement in order to to understand the performance and reliability of whole module. One of the important failure causes of an SSL driver is high temperature. To the best of author’s knowledge, there is very little high temperature study on the reliability of SSL drivers [4]. One of the challenges in this research is the variety of the SSL drivers. It depends on variety of LED specifications, SSL module applications and design requirements of SSL module manufacturers. Due to this large variety of SSL drivers, it will be more efficient to first start with breaking down the fundamental common structures of SSL drivers and afterward distinguishing the sensitive parts of SSL driver to the high temperature. In this study we have performed temperature measurements on one of the building blocks of SSL driver which is electromagnetic storage device. The structure of this paper is as follows; Section II will discuss about the major building blocks of SSL driver and distinguishing the sensitive blocks to the temperature. Then Section III is about thermal measurement test setup and experimental results. The last sections will be conclusion and future work.
Index Terms— Electronic thermal tests, Solid State Lighting (SSL), LED driver, Reliability
I. INTRODUCTION
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olid State Lighting (SSL) technology has promised to replace conventional light sources with impressive economic and environmental saving because of its high efficacy and long lifetime. Each SSL module includes two major parts; optical part and electrical driver. Every part plays role in performance and reliability of whole module. The reliability of the module [2] is often defined by its weakest
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II. SSL DRIVERS BUILDING BLOCKS As we have mentioned in the previous section, SSL electrical driver, or shortly called SSL driver, is the electrical circuit which is the interface between optical part of SSL module and input power. With respect to the application field the input power can be different; for example 220V ac in in-door lighting applications and 12V dc in automotive lighting applications. In any case, SSL driver is a power converter for the power range between less than 1Watt up to few Watts. Fundamental functional elements of this power converter are switching function,
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2011 12th. Int. Conf. on Thermal, Mechanical and Multiphysics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2011
electromagnetic energy storage function and control and information function [5],[6]. Switching function controls the flow of electromagnetic energy through the power converter. Electromagnetic energy storage function provides the continuity of energy when interrupted by the switching function. And finally the Control/information function enables the required relationship among the previous functions.
instruments to online/ offline monitoring the electrical properties of the components and circuit. Figure 1 shows the oven temperature on some SMD ceramic capacitors. The temperature is rising up linearly up to three different temperatures; 70°C, 90°C and 110°C. The maximum temperature that we can apply on our test components is 120°C.
The control/information function element due to the application can vary from a simple switching controller chip [8] up to a complex microcontroller circuit with capability of wireless communication [9],[10]. This part usually consumes very low power. Thus reliability wise, this part usually is not a critical part of SSL driver with respect to the rest of the parts. Although this statement is true in the condition that control/information function elements are placed far enough from the high power part of the circuit. Two parts of the circuit which tolerate the high power and high frequency are the electromagnetic storage functional element and switching functional element. In this paper we focus on the electromagnetic storage functional elements. In the future works of this author there will be the study of the switching functional elements as well. Electromagnetic functional element can be a capacitor or an inductor. Inductors are usually more expensive and also occupy more space in the circuit. Thus in most of the cases ceramic capacitors which are tolerant to the higher temperature are more desirable choices for the designers. In the following section we will show the result of thermal testing on some ceramic capacitors. We will show the effect of the transient part of temperature variation on these elements.
Figure 1: The device under tests are tested in an oven while temperature is rising linearly up to 70°C, 90°C and 110°C. The oven temperature is monitored by thermocouples situated in the middle of the oven. Thermocouples have been calibrated by measuring the crushed ice temperature which in any place should be 0°C.
III. THERMAL MEASUREMENT TEST RESULTS The goal of these series of studies is to be able to model the behavior of LED driver in different temperatures. In this paper specifically the focus is on the performance in transient part of temperature variation. The experiments are on ceramic capacitors which they are electromagnetic energy storage functional element of the SSL driver. There is lack of studies on the effect of transient part of temperature on the devices performance. In datasheets of most of the electrical components, there are tables of thermal characteristics which show the value changing in different steady state temperatures. In applications like automotive applications having knowledge about the device performance in transient part of temperature variation is very valuable. The performance variation can lead to some voltage/current over shoot as well which can affect the reliability of the whole SSL driver and even the optical part.
Figure 2: Monitoring the capacitance value of 4.7uF SMD ceramic capacitors while the temperature is rising linearly up to 70°C, 90°C and 110°C. (Refer to Figure 1). The maximum temperature that capacitors based on datasheets can tolerate is 120°C.
In Figure 2, it is shown that the capacitance value changes while temperature is changed. From this curve, it is obvious that the capacitance value is not linear in transient part of temperature curve. There is a requirement for studying further on high temperature performance of the whole module, because these The thermal test setup includes oven, instruments in order to kinds of non-linear behavior of the components may lead to a online monitoring the temperature and also electrical weak performance of the device which can make the whole device not acceptable for some applications. —2/3— 2011 12th. Int. Conf. on Thermal, Mechanical and Multiphysics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2011
[9] Cheng Guo, Henk van Zeijl, Guo Qi Zhang, R. Venkatesha Prasad, An integrated large SSL system with wireless communications, in Proceedings of 2010 International Conference on Electronic Packaging Technology & High Density Packaging (ICEPT-HDP)
IV. CONCLUSION One of the challenges for studying the performance of LED drivers in different temperatures is variety of designs and topologies with respect to different lighting applications. Our methodology is to break down the module as a system to its fundamental building blocks and then distinguishing the parts which are more sensitive to the temperature. Out of three major Fundamental functional elements of SSL electrical driver, two of them because of working in high power part and high frequency are more sensitive to temperature. These two parts are switching function and electromagnetic energy storage function. The third part, control and information function, is a very low power electrical/electronic circuit thus less critical in reliability study. This paper focus was on electromagnetic energy storage function. This functional element is either an inductor or capacitors. In most of the cases ceramic capacitors are the choice of designer due to lower price, being smaller.
[10] A. Biano, S. Tarashioon, H.W. van zeijl, G. Cheng, , P. Sarro, G.Q. Zhang, Compact and cost effective system integration for solid state lighting module, Proceedings of 7th China international forum on solid state lighting, October 2010, Shenzhen, China
In this paper we showed that the performance of the common electromagnetic energy storage functional element of SSL driver has non-linear dependency to the transient part of temperature variation. This non-linear dependency in some applications like automotive lighting can play important role in working out of specification range and therefore not being acceptable. In the other applications it can lead to some voltage/current overshoot while temperature is changing. Then it can affect the reliability of the whole module. This information is also very useful for the thermal management solution design by considering the transient part of temperature as well. REFERENCES [1] Clemens J.M. Lasance, “Challenges in LED thermal characterization”, 10th Int. Conf. on Thermal, Mechanical and Multiphysics simulation and Experiments in Micro-Electronics and Micro Systems, EuroSimE 2009 [2] U. S. DOE, "LED Measurement Series: LED Luminaire Reliability." vol. 2009. p. 742209-742209-8 [3] G. Archenhold, "Driving Responsibly," in Mondo arc United Kingdom: Mondiale Publishing Ltd, 2009, pp. 93-94. [4] L. Han and N. Narendran, "Developing an accelerated life test method for LED drivers," 2009, p. 742209-742209-8 [5] P. T. Krein, “Elements of Power Electronics”, Oxford University Press Inc. 1998. [6] J. D. Van Wyk, “Power electronics technology at the dawn of a new century-past achievements and future expectations” in Proceedings of The Third International Power Electronics and Motion Control Conference, 15-18 Aug. 2000, Volume: 1, Page(s): 9 – 20. [7] The EDFAS desk reference committee, “Microelectronics failure analysis”, Desk Reference, Fifth edition, 2004 [8] Lighting drivers and controller ICs, NXP Semiconductor products, www.nxp.com
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