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An approach to “Design for Reliability” in solid state lighting systems at high temperatures
Microelectronics Reliability 52 (2012) 783–793
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Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
An approach to ‘‘Design for Reliability’’ in solid state lighting systems at high temperatures S. Tarashioon a,b,⇑, A. Baiano b, H. van Zeijl b, C. Guo a, S.W. Koh b, W.D. van Driel b, G.Q. Zhang b a b
Materials innovation institute (M2i), Mekelweg 2, 2628 CD Delft, The Netherlands Dimes Center for SSL Technologies, Micro/Nanoelectronics System Integration and Reliability, Delft University of Technology, Feldmannweg 17, 2628 CT Delft, The Netherlands
a r t i c l e
i n f o
Article history: Received 29 December 2010 Received in revised form 6 June 2011 Accepted 26 June 2011 Available online 30 July 2011
a b s t r a c t Providing correct reliability information is critical for all manufacturers for both their customers and also their in-house departments intended for product improvements. Solid state lighting (SSL) technology as the novel lighting technology is not an exception. Due to the fact that SSL is a relatively new technology, research on its reliability issues and also systematic methods for design improvement is lacking. In this paper, we introduce an approach for ‘‘Design for Reliability’’ in SSL. Our approach includes three major steps. The first step is the design phase where the physics of failure of the device and virtual assessment of the reliability of the device is investigated. Based on these results, their failure’s causes; and their relationship with reliability and lifetime of the SSL device is defined. Sensors are then assigned to the critical failure causes at the proper positions of the system. The second step of our approach is processing the data from sensors while device is functional. In this phase, gathered data from the sensors, are processed in order to calculate the lifetime of the device. The system should have a processing capability for accomplishing this task and also report the status to the maintenance system. The third step of our reliability approach is to have built-in self-maintenance capabilities. Therefore, the system can predict its failure and be functional for longer time. At the second part of this paper, we introduced a SSL device which has been designed in our group which can fulfill the criteria for our ‘‘Design for Reliability approach’’. This SSL device can provide around 200 lumen and has the capability of sensing and monitoring the high temperature in system. High junction temperature is one of the major failure causes in SSL devices. Due to the additional flexibility that smart controller of this device gives us, self-maintenance solution can be implemented in this device. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction This work focuses on reliability study and introduces an approach for ‘‘Design for Reliability’’ for SSL systems. During the initial stage of any reliability study, it is essential to know why and how this technology is used as reliability, or improvement in reliability is dependent on these parameters. Therefore, a very short introduction about the solid state lighting (SSL) technology will be presented in the following section. The system architecture will be discussed in Section 2. As each parts of a system play an important role in terms of functionality and reliability of the system, a good knowledge on different components of the system is required. The main subject of Section 3 is about the reliability definition, the necessity of this study for SSL and finally how much research is already done in this field. In Section 4 our approach for ‘‘Design for Reliability’’ for SSL systems will be introduced. A flowchart of SSL starting from design phase till the ⇑ Corresponding author at: Materials innovation institute (M2i), Mekelweg 2, 2628 CD Delft, The Netherlands. E-mail addresses: [email protected], [email protected] (S. Tarashioon). 0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2011.06.029
end of the device useful life is the core of this section. In this flowchart, new steps in additional to the standard reliability test for SSL products are introduced for our ‘‘Design for Reliability’’ procedure. Our approach toward having ‘‘Design for Reliability’’ procedure in SSL life cycle will then be discussed. A complete smart SSL device has been designed within the framework of this study which can fulfill the criteria of our reliability approach. In Section 5, this system will be used as a example for our reliability approach. 1.1. Introduction on solid state lighting technology Solid state lighting, commonly called SSL, is the new lighting technology based on light emitting diodes. Although LEDs has been used for a very long time for different applications (mostly as indicators), the idea of using LED for lighting applications has only been introduced in recent decade. The architecture of the first few lighting systems based on LEDs were using a large number (over 10 or 20) of 20 mA 3–5 V conventional LEDs to compensate for the required light output [1]. One of the main reason that that made the idea of using LED as the light source very attractive was its low power consumption
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with respect to the conventional lighting systems (Fig. 1). The electrical energy consumption for lighting in big cities is about 20–25% of the total consumed electrical energy. Therefore energy saving lighting technology is very appealing in our societies. Thus, lots of researches had been focusing on producing high power LEDs with high efficacy and better light quality, especially in the area of white LEDs for general purpose applications. Latest high power LEDs, which also known as high brightness LEDs (HB LEDs), has a power rating of 1 W and even higher. Recent SSL devices have remarkably high efficacy of up to 150 lm/W, as comparison to 15 lm/W for conventional 60–100 W incandescent light bulb; and 60 lm/W for compact florescent light [3]. Another advantage of SSL over conventional lighting systems is its long lifetime. A common incandescent lamp has an average lifetime of around 1000 h whereas the fluorescent lighting technology has an average lifetime of about 10 times longer than incandescent light which is around 10,000 h [1,2]. However, fluorescent lighting technology is a mature technology now and the gain in longer lifetime has reached its limit and it will be very difficult for fluorescent lamps to have dramatically longer lifetime than 10,000 h in the near future. The present LED lighting system technologies have an average of lifetime around 20,000 h (this value varies from manufacturer to manufacturer) with the potential for its lifetime to reach up to 50,000 h and even more up to 100,000 h [4]. In conclusion, even though a single light bulb price of SSL still is much more expensive compared to a fluorescent light bulb price, the total lifetime cost of SSL systems will be much lower than the current lighting technologies considering the saving from the energy consumptions and longer lifetime of each bulb. The third advantage of SSL lighting is that it is an environmental friendly technology. Fluorescent lighting systems which are one of the most efficient lighting systems before SSL systems, contains mercury which is not disposable. In contrary with fluorescent lighting devices, LEDs are semiconductor devices which are free of toxic materials. The dream of having a completely environmental friendly lighting system can be achieved by replacing the electronic components with lead-free devices. SSL technology can be interesting for designers and consumers in lots of other ways. One example is the design flexibility that enables SSL technology to be quickly adopted into different lighting systems market. LEDs are semiconductor devices that their light intensity can be easily controlled. They can be in theory dimmed to 100% but their drivers’ technologies are still not completely compatible with current dimmers in the market. The SSL devices turn-on time is almost zero in contrary to the Fluorescent lighting system turn-on time which is more in the order of seconds. Furthermore, the ease for enabling the color changing capability of SSL system makes SSL appealing to special applications such as dis-
Fig. 1. Historical and predicted of different light sources. It can be seen that the efficacy in all light sources except LED does not have almost any improvement in recent years (Source: Lumileds) [7].
co lighting or shopping center lighting. Due to narrow emission bandwidth of LEDs, there is no infra-red or ultra-violet light emission from SSL devices which is very important for a very good and sharp spot-lights [1,3]. As mentioned in above discussion, there can be a wide variety of applications that SSL devices are very good replacements for traditional lighting systems. In-door lighting, spot lighting, street lighting and lighting of automotive applications are just some of the many examples. There are also lots of new applications which have been introduced to the market like decorative outdoor/indoor lighting. The examples of these new age lighting systems can be seen in lots of historical building, bridges and conference centers all around the world [5,6]. 2. Solid state lighting system architecture Systems in SSL devices include three major parts; optical part, LED electrical driver and interconnections between the latter two parts (Fig. 2). In each SSL system, all these three parts are important for a functional system. In the following paragraphs, the function of each part of the system will be explained [8]. IES standardization in ‘‘ANSI/IESNA RP-16-05 Addendum A’’ [12] has tried to put standard names for different parts of an SSL system, although it does not cover lots of its details. The optical part of the system is the closest part of the system to the users. This part includes the light sources which are the LED (LEDs) and also the reflectors and lenses. Application usually defines the required light intensity and color/s of the light. The designer, based on the application requirements and other factors including the design of LED electrical driver and manufacturer’s concerns, chooses the type and number of LEDs. LEDs can be connected to each other either in series, in parallel or a combination of the two depending on the driver design concerns and LEDs reliability. Reliability information describes the probability of LED failure modes. The first common failure mode of LED is failure induced short circuit and in this case the proper interconnection between LEDs is series connection. The reason is the whole optical part will not fail while just one LED fails. The system will keep functioning but with lower light intensity. The second common LED failure mode is the open circuit, then it is best is to connect LEDs in parallel. The electrical driver of SSL system prepares the regulated power for driving optical part and it will be known as ‘‘SSL driver’’ in the rest of this paper for convenience purposes. As discussed in above paragraphs the types of LEDs, number of LEDs and the way they are connecting together will be defined by the design requirements. In each application the main power supply for whole SSL system can be very different. In general purpose lighting systems, the main power supply using the city power plugs is 110 Vac 60 Hz) 220 Vac 50 Hz. However, for automotive applications, the power
Fig. 2. Different parts of a general SSL system. Optical part is the light source of the system and includes LED (LEDs). LED electrical driver (SSL driver) is the interface of the SSL optical part and the input power of the system. SSL driver also can be more than just a power converter and includes the controller and memory. These two parts of the system are interconnected to each other [28].
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from the car battery is 12 Vdc–24 Vdc. Furthermore, SSL systems for replacement of halogen lamps have a power rating of 12 Vac– 100 kHz [1,9]. The primary and fundamental task of the SSL driver is to provide electrical power to the optical component of the system. However, there are lots of other functionalities can be defined and implemented in SSL driver. Dimming and color changing capabilities are two examples of SSL system’s additional value added functionalities which can already be found in a lot of commercial products. Other value added function that are still in the development phase are the built-in sensors, wireless communication, processing capacity for having a smart lighting network [10,11] and, etc. Interconnections between optical part and SSL driver is the last integral part of the system. Their responsibility is to provide the electrical connection between the driver and the optical components. Their design can be varies from the simplest case, where it can be just two wires or tracks on the printed circuit board, to the more complex systems with shielded cables for power and controller for signals with special frequency or power requirements. Each of the mentioned three parts of the system can have nonelectrical parts for different purposes like optical lenses, heat spreaders and enclosures. The shape and their material are chosen based on the application electrical, optical and mechanical requirements. 3. Reliability in SSL system A general definition of reliability is; the ability of a product or a system to perform as intended for a specific time in its life cycle environment [20]. The system is an SSL system in this discussion. A system can be classified whether it is functional from the measurable outputs of the system. In this case, the measurable output of a basic SSL system is its output light intensity and color of the light. The SSL system life cycle includes all stages of a life of a product from manufacturing, storing, handling and operation conditions. In some studies, the life of a product after its complete failure and component recycling also is counted as a part of life cycle of the product. But in this discussion the main focus is still on their main function. As with any other types of products, a reliability study informs both the customers and producers about life and the performance of the product. Customer uses reliability information deduced mainly from the product warranty in order to compare between different products such as the frequency they need to replace their product. For the manufacturer, this information is much more insightful, in addition to warranty information; reliability information can be used for giving feedback to their design department. This information will not only help them to design a more reliable product but also assist them to formulate their maintenance and logistic plan. Therefore, reliability study for any product is essential. There are some factors that make SSL reliability distinguished from the other lighting system. First of all SSL system is a new technology and very little field information exists. The second issue is the long lifetime of LED, over 20,000 h, which makes testing till end of its lifetime before releasing the product into the market almost impossible. The other factor is more complex failure modes in SSL, as compared to the incandescent lighting systems naming the catastrophic and light depreciation failure mode. In light depreciation failure mode, the device is counted as failed device when the light intensity goes below certain percentage (70% is commonly used based on ‘‘ASSIST test recommendation’’) of initial light intensity [14]. Since there are very limited researches on reliability of SSL devices and the SSL system manufacturers are still in the learning curve, most of the claimed lifetime for SSL devices is from high
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brightness LED (HB-LED) itself and not the whole system. Since the lifetime is also depend on the lifetime of all the components in the system, more research attention needed for other component such as the driver. Even the reliability data of HB-LEDs is insufficient. The methods for testing LEDs are mostly a direct translation from the methods which are applied for common semiconductor devices [15] which is not sufficient. In 2008, IESNA standard organization published two standard methods for testing HB-LEDs which is namely LM-80-08 and LM-79-08 [16–18] but LM-08 need at least 6000 h to conduct. In conclusion, there is an essential need to develop a methodology to conduct reliability study for the whole SSL system. In the coming section we will discuss more about the different methods of reliability study in SSL systems. 4. Our approach to SSL ‘‘Design for Reliability’’ Since it has been mentioned in the preceding sections of this paper that SSL is a new technology and there is not enough reliability researches from the literature, there is an essential need for a reliability study of SSL system. As with any other new technology, there is not much information from literature to be used for design improvement or prediction the functionality during its lifetime. Reliability prediction methods [19] can be based on either four references; test data, field data, stress and damage model and lifetime based on reliability handbooks. The most consistent method for reliability prediction is using the stress and damage model which is based on physics of the device [19,20]. In the following paragraphs we will discuss about the pros and cons of the standard adopted methods for SSL. Field data reliability prediction method is based on the data gathered from the products during their operation time in their real application environment. The information required for using this method are namely; product initial operation time, life cycle and operating life history and also failure time (or current time if the device has not failed). In SSL system, there is very little valuable field information because of SSL devices’ long lifetime and being a new technology. Test data reliability methods use data from either accelerated or non-accelerated tests. Data from non-accelerated tests will be similar to the field data which have very low availability for SSL devices. Data from standard testing methods for SSL nonaccelerated tests performed by manufacturer have been published [13,14,17,18]. In LM80, SSL device in operation are monitored for 6000 h. Then based on the monitored light intensity of device during these 6000 h, the lifetime of the device will be predicted. Accelerated tests or overstress tests for each kind of product should be designed based on the physics of failure of the device. In the early stage of a new product design, there is not very useful due to very limited data. Handbook reliability prediction methods for electronic equipments [21,22] can be traced back to MIL-HDBK 217, published in 1960s. These methods based on curve fitting a mathematical model to historical field failure data to determine the constant failure rate of parts. The major parts of these handbooks are lookup tables for different electronic components and their failure rates based on gathered information from different manufacturers. However, one of the drawbacks of this method is that they do not have correct information for HB-LEDs due to the lack of filed data. Furthermore, the failure rate for integrated circuits depends on the number of transistors which is a very rough estimation because a lot of failures of integrated circuits are in interconnections and packaging. The other important limitation is limited used for devices exposed to harsh environment such as for out-door lighting and automotive application; these handbook do not give any reliability information temperature higher than a threshold. As an example in Telcordia
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SR-332 [22] the reliability information is given only up to 65 °C. The last but not the least, failure data in reliability handbooks are gathered decades ago which may not be applicable for the manufactured devices in the recent years. The last method which is the most consistent method for reliability prediction is based on stress and damage model. We need to know the Physics of Failure of different parts of SSL device in order to have the reliable and valuable reliability information for SSL devices. The requirement of this method is to know each part of the SSL system material construction, geometry, its operational requirement and environmental conditions. The failure models are classified into two categories; overstress failures which occurs based on single failure exposure to a defined stress condition and wear-out which is an exposure time required to induce failure based on a defined stress conditions. 4.1. Reliability method based on stress and damage model From the above discussion, the most consistent method for SSL reliability study is the method based on stress and damage model. In this method we will discuss about failure modes, mechanisms and causes. There is four major categories from failure causes namely design process induced failure causes, manufacturing process induced failure causes, environment induced failure causes and operator and maintenance causes. The discussion about the manufacturing induced failure causes are out of the scope of our discussion. We will focus on the rest of the three. In Figs. 3a–d, the flowchart for SSL system design phase and life cycle has been illustrated. In this flowchart, there are additional steps with respect to the common procedure in SSL systems which are required for the reliability method based on stress and damage model and ‘‘Design for Reliability’’. These extra steps are marked with dashed circles around them in Figs. 3a–d. The proper application of these extra steps will have a big improvement on the reliability of SSL device from the design process induced failure causes, environment induced failure causes and operator and maintenance causes. For fulfilling the reliability improvement for the three mentioned failure causes we apply ‘‘Design phase reliability assessment’’, ‘‘System reliability diagnostics’’ and ‘‘System selfmaintenance’’. The design phase of new SSL products has been started by either a new idea or a requirement in the market (Figs. 3a–d). After defining the application requirements and design rule, the design phase is started. In SSL technology, these design rules usually come from general semiconductor knowledge since it is a relatively new technology. In the best case scenarios, design simulations can be employed to check for the prototype’s functionality and conditions for satisfactory results. The new methodology introduced in this paper is to add two more steps to study the reliability of the designed product, which is based on damage and stress reliability method, namely to study the failure mode, mechanism and effect analysis (FMMEA) and risk assessment. In the virtual assessment, reliability assessment which helps to modify the design rules will be conducted in addition to functionality checking. This will be the ‘‘Design phase reliability assessment’’. The output of FMMEA and risk assessment can be used to modify the tests which are performed on the devices in the mass production phase. It can make the tests more exact and efficient. After the manufacturing phase of the device, each product will undergo periods of storage, handling and installing. These three phases are also part of lifetime of the device where failures may occur. However, the failures occur during these periods are usually ignored for simplification purposes because of difficulty in monitoring the devices status. From the practical point of view, the operational phase is the most important phase of a device lifetime. Generally, the device
is put in operation and when it fails, it is maintained or replaced by a new one. Devices failures can be found from random checking like for in-door applications or by regular maintenance visual inspection like in out-door applications. In the flowchart Figs. 3a–d, there is a diagnostics monitoring part which regularly monitors the device status. These monitoring part can be any kind of sensor like light sensor, temperature sensor or humidity sensors, etc. The type of sensor and their locations are defined in FMMEA and risk assessment part of the device. The data collected from the sensors are then processed to define the status of the system and consequently the lifetime of the device. In this part ‘‘System reliability diagnostics’’ is implemented. An example of this approach is in the work of Jianfei Dong et al. [27]. After the failure or degradation of the system is detected, the system should be able to compensate for the failure through self maintenance or reporting the failure to the maintenance operator. Self-maintenance methods can be any kind of smart system to keep the device functional for a longer lifetime or till the maintenance can be done. In critical applications such as automotive lighting and street lighting, these methods can decrease the number of accidents due to light’s failure. One of the simplest methods for self-maintenance is to include a backup for critical parts or components of the system. Although in some of the reliability engineering studies the end of lifetime of the device is when it fails for the first time [23]. In Figs. 3a–d flowchart, reporting to the maintenance operator is a part of the procedure in operational period of device. In this case and also for ‘‘System reliability diagnostics’’, the system should be a smart system with processing capacity and communication capacity.
5. Design a SSL device based on ‘‘Design for Reliability’’ System diagnostics can be implemented in a system by first knowing its major failure cause and its relationship with the life-
Fig. 3a. A SSL system four major phases from design phase till the end of lifetime of the system. It starts from Phase (1) which is the design phase. The following are the Phase (2) manufacturing phase, and Phase (3) the after production Passive phase. This phase is the all activities after manufacturing till the time the system becomes operational for the first time. Phase (4) is the last phase. In this phase the system can be operational, standby or under maintenance. In each four phases, the blocks which thicker line are added and modified in the new approach introduced in this paper.
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Fig. 3b. The blocks which thicker line are added and modified in the new approach introduced in this paper. Phase (1) is the design phase. In this phase the failure mode, mechanism and effect analysis (FMMEA) and risk assessment helps to be able to design the monitoring part to monitor the critical part of the system. If the system design is in the design revision state then we can also use the collected data from monitoring part to modify the design rules.
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Fig. 3c. The blocks which thicker line are added and modified in the new approach introduced in this paper. Phase (2) is the manufacturing phase which the test procedure can be designed more mature by means of getting data from the assessments in design phase. If the system design is in the design revision state then we can also use the collected data from monitoring part to modify the test procedure. Phase (3) is the passive phase after production of the device; it includes the three steps of storage, handling and installation. Our new approach in this paper does not affect this phase.
time of the device. During the explaining the general approach for Design for Reliability in SSL system (Figs. 3a–d) the input can be strain, stress, temperature and, etc. In this case study the focus is
on the temperature. In HB-LEDs, the relationship between lifetime and junction temperature is known by their manufacturers [24]. Fig. 4 shows an example of the relationship between junction
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Fig. 3d. The blocks which thicker line are added and modified in the new approach introduced in this paper. Phase (4) is the active phase of the device after production. In this phase, the device is put into operation. This paper new approach has a great effect on this phase. The monitoring part of the device is continuously gathering the data from the device status. The data are being processed and the lifetime is estimated. The system has the possibility of self-maintenance and reporting the status to the server.
temperature and lifetime of an HB-LED manufactured by Philips Lumileds Lighting Company. We can observe from Fig. 4 that increasing the junction temperature decreases the lifetime of the device operating in a constant forward current. The relationship between lifetime and LED forward current is also illustrated in this figure. The indication for end of life of the LED is not standardized but one of the most common indications is defined by ‘‘ASSIST recommendation’’ [14] which is B50/L70. It means 50% of the products have at least 70% lumen maintenance for the projected number of operating hours. Based on the above discussion, the lifetime of the device can be estimated from the regular monitoring the junction temperature of the LED in the SSL system. In this work, a low cost but high precision methodology for measuring the junction temperature has
been implemented. In this system also build in light sensor in LEDs has been implemented and thus there will be no need for an external light sensor. This LED will only work as the light sensor when its light is off so that it monitor only the system ambient light but not its own emitted light. This system has a Microcontroller board (lC board) which is the processing center of the device. In the following sections, the details of this system will be explained. 5.1. Device architecture Fig. 5 shows the block diagram of our implemented system [25,26]. This device fulfills the three major points of ‘‘Design for Reliability’’ which is ‘‘Design phase reliability assessment’’, ‘‘System reliability diagnostics’’ and ‘‘System self-maintenance’’. The system
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Fig. 4. The correlation of lifetime of InGaN Luxeon K2 high brightness LED to its Junction temperature and forward current. The failure time is shown for B50/L70 which means 50% of the products have at least 70% lumen maintenance for the projected number of operating hours [14].
architecture will be first explained, and then it will be follow by explanation of the system functionality and finally how it can fulfill the three major reliability issues. As discussed in Section 2 of this paper about system architecture, there are three major part in each SSL system; optical part, SSL driver and interconnections. Four high power LEDs are electrically connected in series to construct the optical part of this system (Fig. 5). Each LED is a power rating of 0.76 W and it can illuminate around 50 lumen. The whole optical part can illuminate maximum 200 lumen. The SSL driver in this system is more than just a power converter for optical part. It includes power
converter to power up the LEDs, the light intensity control (dimmer), built-in light sensing part, built-in LED junction temperature sensing part and the microcontroller board with wireless communication part. 5.1.1. LED as a junction-temperature sensor Fig. 6 shows that the I–V characteristics of the two devices ((1) and (2)) under tests match each other for a bias voltage that is higher than the forward voltage (VF around 40 V). Consequently, the results are reproducible in working region (above forward voltage). A wide deviation of the I–V characteristics is shown for
Fig. 5. (a) The block diagram of a smart SSL system. The different modules can communicate with each other and also with a server. (b) The block diagram of one system. The four LEDs construct the optical part of the system and the rest of the blocks in the module are the SSL driver part. Interconnections in this system are some connectors which makes the electrical connections between optical part and SSL driver.
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voltages lower than the forward voltage. However, this deviation does not influence the objective of this work. As known, there is a temperature dependency in LED I–V curve. This dependency is observable in the I–V diode characteristics reported in the Eq. (1).
eV I ¼ IO eKT 1
ð1Þ
Based on this dependency, one of the LEDs in LED chain can be used as the temperature sensor. This LED is first subjected to a forward biased using three or four different currents values where the output voltage and the current it-self are monitored by a voltage– current monitor chip set (high-voltage analog–digital converter). The more LED voltage and current points are monitored, the higher is the precision of junction-temperature calculation. The voltage– current monitoring chip set is a high voltage monitoring device and connected to the intelligent part (l-controller) of the system via a serial data line. The intensity of light changes by changing the voltage–current of LEDs during the junction-temperature sensing mode, but this transient time is not very critical. The reason is that the other LED remains in ‘‘ON’’ state and the human being eyes is not capable of detecting such a short duration of change in light intensity.
5.1.2. LED as an ambient light sensor In reverse bias, LEDs can be used as ambient light sensor as shown in Fig. 6. It can be observed from Fig. 6 that a change in output current can be induced by the ambient light when a reverse bias is applied. Thus, in this region LED behaves as a photo-diode and SSL module can self-detect ambient light without the need for an extra light sensor. This functionality will provide the fundamental information for detecting the actual state of the LED module, detecting performance degradation and prevent possible failure by regulating the driven power. In this work, detection of the ambient light can be achieved through the measurement of the current, while reverse biasing one of the LED and maintaining the rest of the LEDs in OFF state. This measured current is then converted to voltage to be readout through the microcontroller’s analog–digital converter. Different from the junction-temperature sensing mode, the transient response of ambient-light sensing mode is more critical. Indeed, in this mode, all LEDs are switched to ‘‘OFF’’ state. Therefore, it is important to keep this transient response as short as possible to prevent flickering in the light.
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5.1.3. Driver solution In this work, SSL module for 220 Vac 50 Hz main power applications is presented. The driver used in this project is a direct current source for a series of dimmable LEDs. The dimming control signal comes from the smart part of the system which is the microcontroller board. This driver uses not only the intrinsic sensing functionalities of LEDs for sensing ambient light and junctiontemperature (as discussed in the earlier section) but it will also provides the readout circuitry for these two sensing parameters. Fig. 7 shows the main blocks of this work. The LED driving part including the dimmer circuitry composed of AC–DC converter (rectifier and capacitor) and a high efficiency linear regulator Q1 (Fig. 7) as current sink to drive the four LEDs. The efficiency of such driver is measured to be 98–99%. The smart subsystem (l-controller) in this setup controls the gate voltage of Q1 which in turn changes the intensity of the light through a current modulation. Hence, the dimming function has smooth transition from 0% to 100% of the total light intensity. The concept for using LED as sensor is based on changing the LED state from emitting to the sensing mode. Signals to interchange the system to either ambient light or junction temperature sensing mode are regulated using l-controller. The outputs from the sensing mode are also monitored by l-controller. This information is logged in the system memory and it is used based on the defined application for the system. For the temperature sensing-mode shown in Fig. 7, the regulator Q1 regulates three or more different forward bias currents through the LEDs. Consequently, a high-voltage analog–digital converter (ADC) connected serially to one LED, measures the current and voltage and sends this information to the l-controller. The slope of the IV characteristic can be extracted from the above mentioned collected data. Since the bias current can be set around the typical emitting-mode operation values, flicker will not occur. Hence, the operation time needed for the temperature sensingmode is not an issue. For the ambient light sensing-mode, the regulator Q1 stops the sink current from the LEDs. Then one of the LEDs (LED4 in this case) is set to reverse bias by switching on the transistors Q3 and Q4. This is achieved by using the circuit as shown in Fig. 7. Transistor Q2 is fundamental to the interdicting current flowing through the LEDs when transistor Q4 is switched on. A MX resistor, which works as a current to voltage converter, senses the variation of photo-current and converts it into voltage. This voltage is applied to the internal analog to digital converter (ADC) of the l-controller. The dynamic range of using LED as light sensor can be as large as 200 mV, which leads to a high sensitivity factor when using in conjunction with a 12-bit ADC. The reason of using only one LED as photo-detector is due to the advantage of using the low output voltage of the microcontroller for reverse biasing the LED. In contrast with the temperature-sensing mode, the light-sensing mode must be executed as fast as possible in order to avoid any visible flicker. This is because in this mode all LEDs have to be completely switched-off. 5.2. Fulfilling ‘‘Design for Reliability’’ issues in our SSL device
Fig. 6. I–V characteristics of two LEDs from the Seoul Semiconductor (A4 series) in reverse and forward bias. The relationship between reverse current and ambient light has been distinguished.
As explained in details in the previous sections, this design has the capability of addressing the three reliability issues due to the thermal induced failure in LED. In the following section, this will be explained using the system flowchart in Figs. 3a–d. Furthermore, the methodology for accomplishing ‘‘Design for Reliability’’ will also be discussed. The first part of these three issues is ‘‘ Design phase reliability assessment’’. The correlation of the HB-LEDs lifetime versus their junction temperature is always provided by the HB-LED manufacturer. Therefore, the dependence of this failure cause on LEDs
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+
3
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AC plug 1
Q5
2
CAP
R
4
-
1 2
Q2
R
LED1
LED2 Q3
Light Sense Mode Signal
Temp Sense Readout Signal1 Temp Sense Readout Signal2 LED3
Temp Sense Readout Signal3 Light Sense Readout Signal LED4
Light Sense Mode Signal
R (Mohm)
Q4
Diode
Dimming Signal
Q1
Light Sense Mode Signal
R
Fig. 7. SSL driver topology while integrating ambient light sensor and junction-temperature sensor by means of LED intrinsic sensing functionalities.
lifetime is known. The built-in LED junction temperature sensor and built-in light sensor can then be implemented in the design. The second issue is ‘‘System reliability diagnostics’’. The system can monitor its own status regularly by monitoring the LED junction temperature via the built-in junction temperature sensor. The data is gathered using the microcontroller board which is the processing part of the system. The data are processed and the lifetime of the device is calculated. There is wireless communication function in this system which can communicate with a server and sends the system status regarding if it needs any maintenance. In this system, a built-in light sensing part is also implemented. Besides gathering information about the ambient light for smart energy saving light control, it can also play a role in reliability study. The LED itself functions as the light sensor so it is not possible to read its own emitting light intensity. But if more than one of these systems are installed in an area; each system can read the light output of the rest of devices and estimate their lifetimes due to their light degradation. In this system, the intensity of the output light can be controlled by microcontroller. One of the ‘‘System self-maintenance’’ is to run the device on 80–90% of the maximum capacity. In the case of light depreciation due to failure of one of the LED, the other LEDs can compensate by operating at higher capacity. In this way, the light intensity can be maintained and this will keep the system running at a longer lifetime. This is possible due to the microcontroller implemented in this system. Furthermore, this system is controlled by software written in the frame work of this project. Thus, it will give a lot of flexibility to the system.
In the case where the junction temperature of LED is too high, the expected lifetime will become very short. In this case, the device can be regulated to run in lower forward current. In this case, the output light is lower, but the system can last longer. One example can be in automotive application which losing complete light can be very dangerous. The light can stay functional at a lower light intensity before a mechanic can be located. The second option can be in the case of reaching critical temperature system can detect it and shuts down the current through LEDs; this option leads to have no light output.
6. Conclusions Providing reliability information and designing a more reliable product is an essential requirement in solid state lighting (SSL) products. However, due to the fact that SSL is a relatively new lighting technology which is expanding the domain of its applications every day, the need for more precise reliability data becomes more critical. The range of SSL applications varies from simple light bulbs for in-door applications to street lighting, automotive lighting and also a wide range of other fields. In this work, we introduced an approach toward ‘‘ Design for Reliability’’ in SSL systems. This approach adds three major steps to the regular lifecycle of a SSL product; ‘‘Design phase reliability assessment’’, ‘‘System reliability diagnostics’’ and ‘‘System selfmaintenance’’. These three steps are applied in design phase, operation phase and maintenance phase of the lifecycle of a product. In
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the design phase, based on physics of device, the lifetime of the device is predicted. We also find the critical points of the system and their major failure causes. A monitoring system is designed in order to monitor the critical points based on related failure causes, e.g. a temperature sensor if the failure cause is the high temperature. In operation phase, the output data from sensors are read and processed in order to predict the lifetime of the device. In the case of prediction of a very short lifetime, maintenance is required. Afterward system checks if it is capable of doing self-maintenance unless it will request maintenance from operators (human being maintenance instead of system self-maintenance). In this paper, a designed SSL device is presented which covers requirements of the above mentioned three major steps of ‘‘Design for Reliability’’. In this system the critical point of the system is junction high temperature of the high brightness LED. The LED itself is designed to function also as its own junction temperature sensor. This system has microcontroller board which can read the data from junction temperature sensor. Some self-maintenance methods like driving LEDs in lower forward current in order to elongate the system lifetime can be implemented in the software of the microcontroller board. This design can be a good example of a SSL system which accomplishes the goal of ‘‘Design for Reliability’’ for high temperature applications. Acknowledgements This research was carried out within the Project ‘‘Nanoelectronics for Safe, Fuel Efficient and Environment Friendly Automotive Solutions’’ (SE2A), co-funded by the European Commission and Agentschap NL in the framework of the Public-Private Partnership ENIAC JU. The hardware implementation of this work is a part of ENSURE project which is the collaboration between Philips Lighting B.V. and TUDelft University of Technology. We also thank people from Philips lighting and Philips research in Eindhoven who helped us improving this project. References [1] Gilbert Held. Introduction to LED technology and applications. Auerbach Publication; 2008. [2] US Department of Energy. Solid-state lighting research and development: manufacturing roadmap; September 2009. [3] Zukauskas Arturas, Shur Michael S, Caska Remis. Introduction to solid state lighting. A Wiley-Interscience Publication; 2002.
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[4] US Department of Energy (DOE). Solid state lighting manufacturing roadmap, Vancouver, Washington; June 2009. [5] Richard Karney. Review of the ENERGY STARÒ solid state lighting luminaire criteria. Solid-state lighting market introduction workshop materials. US Department of Energy (DOE); July 2008. [6] US Department of Energy. Using LED to its best advantage, LED application series; 2008. [7] Navigant Consulting, Inc. Multi-year program plan FY’09-FY’15, prepared for US department of energy, Radcliffe Advisors, Inc., and SSLS, Inc.; March 2009. [8] Mottier Patrick. LED for lighting application. John Wiley & Sons; 2009. [9] Winder Steve. Power supplies for LED lighting. Elsevier Inc.; 2008. [10] Cheng Guo, Guoqi Zhang, Henk van Zeijl, Ignas Niemegeers. Controlling large lighting systems with networked wireless devices. In: Proceedings of green lighting Shanghai forum 2010, Shanghai, China; April 2010. [11] 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). [12] ANSI/IESNA RP-16-05 Addendum A. Nomenclature and definitions for illuminating engineering. IES Standard; 2005. [13] US Department of Energy (DOE). CALiPER program; 2009. [14] ASSIST Recommendation. LED life for general lighting: definition of life, vol. 1, issue no. 1; February 2005. [15] Clemens JM, 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. [16] US Department of Energy (DOE). SSL standards; 2009. [17] IESNA LM-80-08 Standard. IES approved method for lumen maintenance of LED light sources; 2008. [18] IESNA LM-79-08 Standard. IES approved method for the electrical and photometric measurements of solid state lighting products; 2008. [19] IEEE Std 1413.1-2002. IEEE guide for selecting and using reliability prediction based on IEEE 1413. IEEE standard coordinating committee 37. [20] Pecht Michael G. Prognostics and health management of electronics. John Wiley & Sons, Inc.; 2008. [21] MIL HDBK217-E, Notice 1. Reliability prediction of electronic equipment. Military Handbook; 1990. [22] Telcordia Technologies Special Report. Reliability prediction procedure for electronic equipment, SR332; 2001. [23] Klaassen Klaas B, van Peppen Jack CL. System reliability concepts and applications. VSSD; 2006. [24] Philips Technology White Paper. Understanding power LED life time analysis. [25] Baiano A, van zeijl HW, Cheng G, Tarashioon S, Sarro P, Zhang GQ. System integration options for LED module. Proceedings of LS12-WLED3 conference, Eindhoven, The Netherlands; July 2010. [26] Baiano A, Tarashioon S, van zeijl HW, Cheng G, Sarro P, Zhang GQ. Compact and cost effective system integration for solid state lighting module. Proceedings of 7th China international forum on solid state lighting, Shenzhen, China: October 2010. [27] Jianfei Dong, Willem van Driel, Guoqi Zhang. Automatic diagnosis and control of distributed solid state lighting systems. OPTICS EXPRESS 5784, vol. 19, issue no. 7; March 2011. [28] http://www.usa.lighting.philips.com/connect/LED_modules/fortimo_DLM.wpd.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
An approach to ‘‘Design for Reliability’’ in solid state lighting systems at high temperatures S. Tarashioon a,b,⇑, A. Baiano b, H. van Zeijl b, C. Guo a, S.W. Koh b, W.D. van Driel b, G.Q. Zhang b a b
Materials innovation institute (M2i), Mekelweg 2, 2628 CD Delft, The Netherlands Dimes Center for SSL Technologies, Micro/Nanoelectronics System Integration and Reliability, Delft University of Technology, Feldmannweg 17, 2628 CT Delft, The Netherlands
a r t i c l e
i n f o
Article history: Received 29 December 2010 Received in revised form 6 June 2011 Accepted 26 June 2011 Available online 30 July 2011
a b s t r a c t Providing correct reliability information is critical for all manufacturers for both their customers and also their in-house departments intended for product improvements. Solid state lighting (SSL) technology as the novel lighting technology is not an exception. Due to the fact that SSL is a relatively new technology, research on its reliability issues and also systematic methods for design improvement is lacking. In this paper, we introduce an approach for ‘‘Design for Reliability’’ in SSL. Our approach includes three major steps. The first step is the design phase where the physics of failure of the device and virtual assessment of the reliability of the device is investigated. Based on these results, their failure’s causes; and their relationship with reliability and lifetime of the SSL device is defined. Sensors are then assigned to the critical failure causes at the proper positions of the system. The second step of our approach is processing the data from sensors while device is functional. In this phase, gathered data from the sensors, are processed in order to calculate the lifetime of the device. The system should have a processing capability for accomplishing this task and also report the status to the maintenance system. The third step of our reliability approach is to have built-in self-maintenance capabilities. Therefore, the system can predict its failure and be functional for longer time. At the second part of this paper, we introduced a SSL device which has been designed in our group which can fulfill the criteria for our ‘‘Design for Reliability approach’’. This SSL device can provide around 200 lumen and has the capability of sensing and monitoring the high temperature in system. High junction temperature is one of the major failure causes in SSL devices. Due to the additional flexibility that smart controller of this device gives us, self-maintenance solution can be implemented in this device. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction This work focuses on reliability study and introduces an approach for ‘‘Design for Reliability’’ for SSL systems. During the initial stage of any reliability study, it is essential to know why and how this technology is used as reliability, or improvement in reliability is dependent on these parameters. Therefore, a very short introduction about the solid state lighting (SSL) technology will be presented in the following section. The system architecture will be discussed in Section 2. As each parts of a system play an important role in terms of functionality and reliability of the system, a good knowledge on different components of the system is required. The main subject of Section 3 is about the reliability definition, the necessity of this study for SSL and finally how much research is already done in this field. In Section 4 our approach for ‘‘Design for Reliability’’ for SSL systems will be introduced. A flowchart of SSL starting from design phase till the ⇑ Corresponding author at: Materials innovation institute (M2i), Mekelweg 2, 2628 CD Delft, The Netherlands. E-mail addresses: [email protected], [email protected] (S. Tarashioon). 0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2011.06.029
end of the device useful life is the core of this section. In this flowchart, new steps in additional to the standard reliability test for SSL products are introduced for our ‘‘Design for Reliability’’ procedure. Our approach toward having ‘‘Design for Reliability’’ procedure in SSL life cycle will then be discussed. A complete smart SSL device has been designed within the framework of this study which can fulfill the criteria of our reliability approach. In Section 5, this system will be used as a example for our reliability approach. 1.1. Introduction on solid state lighting technology Solid state lighting, commonly called SSL, is the new lighting technology based on light emitting diodes. Although LEDs has been used for a very long time for different applications (mostly as indicators), the idea of using LED for lighting applications has only been introduced in recent decade. The architecture of the first few lighting systems based on LEDs were using a large number (over 10 or 20) of 20 mA 3–5 V conventional LEDs to compensate for the required light output [1]. One of the main reason that that made the idea of using LED as the light source very attractive was its low power consumption
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with respect to the conventional lighting systems (Fig. 1). The electrical energy consumption for lighting in big cities is about 20–25% of the total consumed electrical energy. Therefore energy saving lighting technology is very appealing in our societies. Thus, lots of researches had been focusing on producing high power LEDs with high efficacy and better light quality, especially in the area of white LEDs for general purpose applications. Latest high power LEDs, which also known as high brightness LEDs (HB LEDs), has a power rating of 1 W and even higher. Recent SSL devices have remarkably high efficacy of up to 150 lm/W, as comparison to 15 lm/W for conventional 60–100 W incandescent light bulb; and 60 lm/W for compact florescent light [3]. Another advantage of SSL over conventional lighting systems is its long lifetime. A common incandescent lamp has an average lifetime of around 1000 h whereas the fluorescent lighting technology has an average lifetime of about 10 times longer than incandescent light which is around 10,000 h [1,2]. However, fluorescent lighting technology is a mature technology now and the gain in longer lifetime has reached its limit and it will be very difficult for fluorescent lamps to have dramatically longer lifetime than 10,000 h in the near future. The present LED lighting system technologies have an average of lifetime around 20,000 h (this value varies from manufacturer to manufacturer) with the potential for its lifetime to reach up to 50,000 h and even more up to 100,000 h [4]. In conclusion, even though a single light bulb price of SSL still is much more expensive compared to a fluorescent light bulb price, the total lifetime cost of SSL systems will be much lower than the current lighting technologies considering the saving from the energy consumptions and longer lifetime of each bulb. The third advantage of SSL lighting is that it is an environmental friendly technology. Fluorescent lighting systems which are one of the most efficient lighting systems before SSL systems, contains mercury which is not disposable. In contrary with fluorescent lighting devices, LEDs are semiconductor devices which are free of toxic materials. The dream of having a completely environmental friendly lighting system can be achieved by replacing the electronic components with lead-free devices. SSL technology can be interesting for designers and consumers in lots of other ways. One example is the design flexibility that enables SSL technology to be quickly adopted into different lighting systems market. LEDs are semiconductor devices that their light intensity can be easily controlled. They can be in theory dimmed to 100% but their drivers’ technologies are still not completely compatible with current dimmers in the market. The SSL devices turn-on time is almost zero in contrary to the Fluorescent lighting system turn-on time which is more in the order of seconds. Furthermore, the ease for enabling the color changing capability of SSL system makes SSL appealing to special applications such as dis-
Fig. 1. Historical and predicted of different light sources. It can be seen that the efficacy in all light sources except LED does not have almost any improvement in recent years (Source: Lumileds) [7].
co lighting or shopping center lighting. Due to narrow emission bandwidth of LEDs, there is no infra-red or ultra-violet light emission from SSL devices which is very important for a very good and sharp spot-lights [1,3]. As mentioned in above discussion, there can be a wide variety of applications that SSL devices are very good replacements for traditional lighting systems. In-door lighting, spot lighting, street lighting and lighting of automotive applications are just some of the many examples. There are also lots of new applications which have been introduced to the market like decorative outdoor/indoor lighting. The examples of these new age lighting systems can be seen in lots of historical building, bridges and conference centers all around the world [5,6]. 2. Solid state lighting system architecture Systems in SSL devices include three major parts; optical part, LED electrical driver and interconnections between the latter two parts (Fig. 2). In each SSL system, all these three parts are important for a functional system. In the following paragraphs, the function of each part of the system will be explained [8]. IES standardization in ‘‘ANSI/IESNA RP-16-05 Addendum A’’ [12] has tried to put standard names for different parts of an SSL system, although it does not cover lots of its details. The optical part of the system is the closest part of the system to the users. This part includes the light sources which are the LED (LEDs) and also the reflectors and lenses. Application usually defines the required light intensity and color/s of the light. The designer, based on the application requirements and other factors including the design of LED electrical driver and manufacturer’s concerns, chooses the type and number of LEDs. LEDs can be connected to each other either in series, in parallel or a combination of the two depending on the driver design concerns and LEDs reliability. Reliability information describes the probability of LED failure modes. The first common failure mode of LED is failure induced short circuit and in this case the proper interconnection between LEDs is series connection. The reason is the whole optical part will not fail while just one LED fails. The system will keep functioning but with lower light intensity. The second common LED failure mode is the open circuit, then it is best is to connect LEDs in parallel. The electrical driver of SSL system prepares the regulated power for driving optical part and it will be known as ‘‘SSL driver’’ in the rest of this paper for convenience purposes. As discussed in above paragraphs the types of LEDs, number of LEDs and the way they are connecting together will be defined by the design requirements. In each application the main power supply for whole SSL system can be very different. In general purpose lighting systems, the main power supply using the city power plugs is 110 Vac 60 Hz) 220 Vac 50 Hz. However, for automotive applications, the power
Fig. 2. Different parts of a general SSL system. Optical part is the light source of the system and includes LED (LEDs). LED electrical driver (SSL driver) is the interface of the SSL optical part and the input power of the system. SSL driver also can be more than just a power converter and includes the controller and memory. These two parts of the system are interconnected to each other [28].
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from the car battery is 12 Vdc–24 Vdc. Furthermore, SSL systems for replacement of halogen lamps have a power rating of 12 Vac– 100 kHz [1,9]. The primary and fundamental task of the SSL driver is to provide electrical power to the optical component of the system. However, there are lots of other functionalities can be defined and implemented in SSL driver. Dimming and color changing capabilities are two examples of SSL system’s additional value added functionalities which can already be found in a lot of commercial products. Other value added function that are still in the development phase are the built-in sensors, wireless communication, processing capacity for having a smart lighting network [10,11] and, etc. Interconnections between optical part and SSL driver is the last integral part of the system. Their responsibility is to provide the electrical connection between the driver and the optical components. Their design can be varies from the simplest case, where it can be just two wires or tracks on the printed circuit board, to the more complex systems with shielded cables for power and controller for signals with special frequency or power requirements. Each of the mentioned three parts of the system can have nonelectrical parts for different purposes like optical lenses, heat spreaders and enclosures. The shape and their material are chosen based on the application electrical, optical and mechanical requirements. 3. Reliability in SSL system A general definition of reliability is; the ability of a product or a system to perform as intended for a specific time in its life cycle environment [20]. The system is an SSL system in this discussion. A system can be classified whether it is functional from the measurable outputs of the system. In this case, the measurable output of a basic SSL system is its output light intensity and color of the light. The SSL system life cycle includes all stages of a life of a product from manufacturing, storing, handling and operation conditions. In some studies, the life of a product after its complete failure and component recycling also is counted as a part of life cycle of the product. But in this discussion the main focus is still on their main function. As with any other types of products, a reliability study informs both the customers and producers about life and the performance of the product. Customer uses reliability information deduced mainly from the product warranty in order to compare between different products such as the frequency they need to replace their product. For the manufacturer, this information is much more insightful, in addition to warranty information; reliability information can be used for giving feedback to their design department. This information will not only help them to design a more reliable product but also assist them to formulate their maintenance and logistic plan. Therefore, reliability study for any product is essential. There are some factors that make SSL reliability distinguished from the other lighting system. First of all SSL system is a new technology and very little field information exists. The second issue is the long lifetime of LED, over 20,000 h, which makes testing till end of its lifetime before releasing the product into the market almost impossible. The other factor is more complex failure modes in SSL, as compared to the incandescent lighting systems naming the catastrophic and light depreciation failure mode. In light depreciation failure mode, the device is counted as failed device when the light intensity goes below certain percentage (70% is commonly used based on ‘‘ASSIST test recommendation’’) of initial light intensity [14]. Since there are very limited researches on reliability of SSL devices and the SSL system manufacturers are still in the learning curve, most of the claimed lifetime for SSL devices is from high
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brightness LED (HB-LED) itself and not the whole system. Since the lifetime is also depend on the lifetime of all the components in the system, more research attention needed for other component such as the driver. Even the reliability data of HB-LEDs is insufficient. The methods for testing LEDs are mostly a direct translation from the methods which are applied for common semiconductor devices [15] which is not sufficient. In 2008, IESNA standard organization published two standard methods for testing HB-LEDs which is namely LM-80-08 and LM-79-08 [16–18] but LM-08 need at least 6000 h to conduct. In conclusion, there is an essential need to develop a methodology to conduct reliability study for the whole SSL system. In the coming section we will discuss more about the different methods of reliability study in SSL systems. 4. Our approach to SSL ‘‘Design for Reliability’’ Since it has been mentioned in the preceding sections of this paper that SSL is a new technology and there is not enough reliability researches from the literature, there is an essential need for a reliability study of SSL system. As with any other new technology, there is not much information from literature to be used for design improvement or prediction the functionality during its lifetime. Reliability prediction methods [19] can be based on either four references; test data, field data, stress and damage model and lifetime based on reliability handbooks. The most consistent method for reliability prediction is using the stress and damage model which is based on physics of the device [19,20]. In the following paragraphs we will discuss about the pros and cons of the standard adopted methods for SSL. Field data reliability prediction method is based on the data gathered from the products during their operation time in their real application environment. The information required for using this method are namely; product initial operation time, life cycle and operating life history and also failure time (or current time if the device has not failed). In SSL system, there is very little valuable field information because of SSL devices’ long lifetime and being a new technology. Test data reliability methods use data from either accelerated or non-accelerated tests. Data from non-accelerated tests will be similar to the field data which have very low availability for SSL devices. Data from standard testing methods for SSL nonaccelerated tests performed by manufacturer have been published [13,14,17,18]. In LM80, SSL device in operation are monitored for 6000 h. Then based on the monitored light intensity of device during these 6000 h, the lifetime of the device will be predicted. Accelerated tests or overstress tests for each kind of product should be designed based on the physics of failure of the device. In the early stage of a new product design, there is not very useful due to very limited data. Handbook reliability prediction methods for electronic equipments [21,22] can be traced back to MIL-HDBK 217, published in 1960s. These methods based on curve fitting a mathematical model to historical field failure data to determine the constant failure rate of parts. The major parts of these handbooks are lookup tables for different electronic components and their failure rates based on gathered information from different manufacturers. However, one of the drawbacks of this method is that they do not have correct information for HB-LEDs due to the lack of filed data. Furthermore, the failure rate for integrated circuits depends on the number of transistors which is a very rough estimation because a lot of failures of integrated circuits are in interconnections and packaging. The other important limitation is limited used for devices exposed to harsh environment such as for out-door lighting and automotive application; these handbook do not give any reliability information temperature higher than a threshold. As an example in Telcordia
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SR-332 [22] the reliability information is given only up to 65 °C. The last but not the least, failure data in reliability handbooks are gathered decades ago which may not be applicable for the manufactured devices in the recent years. The last method which is the most consistent method for reliability prediction is based on stress and damage model. We need to know the Physics of Failure of different parts of SSL device in order to have the reliable and valuable reliability information for SSL devices. The requirement of this method is to know each part of the SSL system material construction, geometry, its operational requirement and environmental conditions. The failure models are classified into two categories; overstress failures which occurs based on single failure exposure to a defined stress condition and wear-out which is an exposure time required to induce failure based on a defined stress conditions. 4.1. Reliability method based on stress and damage model From the above discussion, the most consistent method for SSL reliability study is the method based on stress and damage model. In this method we will discuss about failure modes, mechanisms and causes. There is four major categories from failure causes namely design process induced failure causes, manufacturing process induced failure causes, environment induced failure causes and operator and maintenance causes. The discussion about the manufacturing induced failure causes are out of the scope of our discussion. We will focus on the rest of the three. In Figs. 3a–d, the flowchart for SSL system design phase and life cycle has been illustrated. In this flowchart, there are additional steps with respect to the common procedure in SSL systems which are required for the reliability method based on stress and damage model and ‘‘Design for Reliability’’. These extra steps are marked with dashed circles around them in Figs. 3a–d. The proper application of these extra steps will have a big improvement on the reliability of SSL device from the design process induced failure causes, environment induced failure causes and operator and maintenance causes. For fulfilling the reliability improvement for the three mentioned failure causes we apply ‘‘Design phase reliability assessment’’, ‘‘System reliability diagnostics’’ and ‘‘System selfmaintenance’’. The design phase of new SSL products has been started by either a new idea or a requirement in the market (Figs. 3a–d). After defining the application requirements and design rule, the design phase is started. In SSL technology, these design rules usually come from general semiconductor knowledge since it is a relatively new technology. In the best case scenarios, design simulations can be employed to check for the prototype’s functionality and conditions for satisfactory results. The new methodology introduced in this paper is to add two more steps to study the reliability of the designed product, which is based on damage and stress reliability method, namely to study the failure mode, mechanism and effect analysis (FMMEA) and risk assessment. In the virtual assessment, reliability assessment which helps to modify the design rules will be conducted in addition to functionality checking. This will be the ‘‘Design phase reliability assessment’’. The output of FMMEA and risk assessment can be used to modify the tests which are performed on the devices in the mass production phase. It can make the tests more exact and efficient. After the manufacturing phase of the device, each product will undergo periods of storage, handling and installing. These three phases are also part of lifetime of the device where failures may occur. However, the failures occur during these periods are usually ignored for simplification purposes because of difficulty in monitoring the devices status. From the practical point of view, the operational phase is the most important phase of a device lifetime. Generally, the device
is put in operation and when it fails, it is maintained or replaced by a new one. Devices failures can be found from random checking like for in-door applications or by regular maintenance visual inspection like in out-door applications. In the flowchart Figs. 3a–d, there is a diagnostics monitoring part which regularly monitors the device status. These monitoring part can be any kind of sensor like light sensor, temperature sensor or humidity sensors, etc. The type of sensor and their locations are defined in FMMEA and risk assessment part of the device. The data collected from the sensors are then processed to define the status of the system and consequently the lifetime of the device. In this part ‘‘System reliability diagnostics’’ is implemented. An example of this approach is in the work of Jianfei Dong et al. [27]. After the failure or degradation of the system is detected, the system should be able to compensate for the failure through self maintenance or reporting the failure to the maintenance operator. Self-maintenance methods can be any kind of smart system to keep the device functional for a longer lifetime or till the maintenance can be done. In critical applications such as automotive lighting and street lighting, these methods can decrease the number of accidents due to light’s failure. One of the simplest methods for self-maintenance is to include a backup for critical parts or components of the system. Although in some of the reliability engineering studies the end of lifetime of the device is when it fails for the first time [23]. In Figs. 3a–d flowchart, reporting to the maintenance operator is a part of the procedure in operational period of device. In this case and also for ‘‘System reliability diagnostics’’, the system should be a smart system with processing capacity and communication capacity.
5. Design a SSL device based on ‘‘Design for Reliability’’ System diagnostics can be implemented in a system by first knowing its major failure cause and its relationship with the life-
Fig. 3a. A SSL system four major phases from design phase till the end of lifetime of the system. It starts from Phase (1) which is the design phase. The following are the Phase (2) manufacturing phase, and Phase (3) the after production Passive phase. This phase is the all activities after manufacturing till the time the system becomes operational for the first time. Phase (4) is the last phase. In this phase the system can be operational, standby or under maintenance. In each four phases, the blocks which thicker line are added and modified in the new approach introduced in this paper.
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Fig. 3b. The blocks which thicker line are added and modified in the new approach introduced in this paper. Phase (1) is the design phase. In this phase the failure mode, mechanism and effect analysis (FMMEA) and risk assessment helps to be able to design the monitoring part to monitor the critical part of the system. If the system design is in the design revision state then we can also use the collected data from monitoring part to modify the design rules.
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Fig. 3c. The blocks which thicker line are added and modified in the new approach introduced in this paper. Phase (2) is the manufacturing phase which the test procedure can be designed more mature by means of getting data from the assessments in design phase. If the system design is in the design revision state then we can also use the collected data from monitoring part to modify the test procedure. Phase (3) is the passive phase after production of the device; it includes the three steps of storage, handling and installation. Our new approach in this paper does not affect this phase.
time of the device. During the explaining the general approach for Design for Reliability in SSL system (Figs. 3a–d) the input can be strain, stress, temperature and, etc. In this case study the focus is
on the temperature. In HB-LEDs, the relationship between lifetime and junction temperature is known by their manufacturers [24]. Fig. 4 shows an example of the relationship between junction
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Fig. 3d. The blocks which thicker line are added and modified in the new approach introduced in this paper. Phase (4) is the active phase of the device after production. In this phase, the device is put into operation. This paper new approach has a great effect on this phase. The monitoring part of the device is continuously gathering the data from the device status. The data are being processed and the lifetime is estimated. The system has the possibility of self-maintenance and reporting the status to the server.
temperature and lifetime of an HB-LED manufactured by Philips Lumileds Lighting Company. We can observe from Fig. 4 that increasing the junction temperature decreases the lifetime of the device operating in a constant forward current. The relationship between lifetime and LED forward current is also illustrated in this figure. The indication for end of life of the LED is not standardized but one of the most common indications is defined by ‘‘ASSIST recommendation’’ [14] which is B50/L70. It means 50% of the products have at least 70% lumen maintenance for the projected number of operating hours. Based on the above discussion, the lifetime of the device can be estimated from the regular monitoring the junction temperature of the LED in the SSL system. In this work, a low cost but high precision methodology for measuring the junction temperature has
been implemented. In this system also build in light sensor in LEDs has been implemented and thus there will be no need for an external light sensor. This LED will only work as the light sensor when its light is off so that it monitor only the system ambient light but not its own emitted light. This system has a Microcontroller board (lC board) which is the processing center of the device. In the following sections, the details of this system will be explained. 5.1. Device architecture Fig. 5 shows the block diagram of our implemented system [25,26]. This device fulfills the three major points of ‘‘Design for Reliability’’ which is ‘‘Design phase reliability assessment’’, ‘‘System reliability diagnostics’’ and ‘‘System self-maintenance’’. The system
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Fig. 4. The correlation of lifetime of InGaN Luxeon K2 high brightness LED to its Junction temperature and forward current. The failure time is shown for B50/L70 which means 50% of the products have at least 70% lumen maintenance for the projected number of operating hours [14].
architecture will be first explained, and then it will be follow by explanation of the system functionality and finally how it can fulfill the three major reliability issues. As discussed in Section 2 of this paper about system architecture, there are three major part in each SSL system; optical part, SSL driver and interconnections. Four high power LEDs are electrically connected in series to construct the optical part of this system (Fig. 5). Each LED is a power rating of 0.76 W and it can illuminate around 50 lumen. The whole optical part can illuminate maximum 200 lumen. The SSL driver in this system is more than just a power converter for optical part. It includes power
converter to power up the LEDs, the light intensity control (dimmer), built-in light sensing part, built-in LED junction temperature sensing part and the microcontroller board with wireless communication part. 5.1.1. LED as a junction-temperature sensor Fig. 6 shows that the I–V characteristics of the two devices ((1) and (2)) under tests match each other for a bias voltage that is higher than the forward voltage (VF around 40 V). Consequently, the results are reproducible in working region (above forward voltage). A wide deviation of the I–V characteristics is shown for
Fig. 5. (a) The block diagram of a smart SSL system. The different modules can communicate with each other and also with a server. (b) The block diagram of one system. The four LEDs construct the optical part of the system and the rest of the blocks in the module are the SSL driver part. Interconnections in this system are some connectors which makes the electrical connections between optical part and SSL driver.
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voltages lower than the forward voltage. However, this deviation does not influence the objective of this work. As known, there is a temperature dependency in LED I–V curve. This dependency is observable in the I–V diode characteristics reported in the Eq. (1).
eV I ¼ IO eKT 1
ð1Þ
Based on this dependency, one of the LEDs in LED chain can be used as the temperature sensor. This LED is first subjected to a forward biased using three or four different currents values where the output voltage and the current it-self are monitored by a voltage– current monitor chip set (high-voltage analog–digital converter). The more LED voltage and current points are monitored, the higher is the precision of junction-temperature calculation. The voltage– current monitoring chip set is a high voltage monitoring device and connected to the intelligent part (l-controller) of the system via a serial data line. The intensity of light changes by changing the voltage–current of LEDs during the junction-temperature sensing mode, but this transient time is not very critical. The reason is that the other LED remains in ‘‘ON’’ state and the human being eyes is not capable of detecting such a short duration of change in light intensity.
5.1.2. LED as an ambient light sensor In reverse bias, LEDs can be used as ambient light sensor as shown in Fig. 6. It can be observed from Fig. 6 that a change in output current can be induced by the ambient light when a reverse bias is applied. Thus, in this region LED behaves as a photo-diode and SSL module can self-detect ambient light without the need for an extra light sensor. This functionality will provide the fundamental information for detecting the actual state of the LED module, detecting performance degradation and prevent possible failure by regulating the driven power. In this work, detection of the ambient light can be achieved through the measurement of the current, while reverse biasing one of the LED and maintaining the rest of the LEDs in OFF state. This measured current is then converted to voltage to be readout through the microcontroller’s analog–digital converter. Different from the junction-temperature sensing mode, the transient response of ambient-light sensing mode is more critical. Indeed, in this mode, all LEDs are switched to ‘‘OFF’’ state. Therefore, it is important to keep this transient response as short as possible to prevent flickering in the light.
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5.1.3. Driver solution In this work, SSL module for 220 Vac 50 Hz main power applications is presented. The driver used in this project is a direct current source for a series of dimmable LEDs. The dimming control signal comes from the smart part of the system which is the microcontroller board. This driver uses not only the intrinsic sensing functionalities of LEDs for sensing ambient light and junctiontemperature (as discussed in the earlier section) but it will also provides the readout circuitry for these two sensing parameters. Fig. 7 shows the main blocks of this work. The LED driving part including the dimmer circuitry composed of AC–DC converter (rectifier and capacitor) and a high efficiency linear regulator Q1 (Fig. 7) as current sink to drive the four LEDs. The efficiency of such driver is measured to be 98–99%. The smart subsystem (l-controller) in this setup controls the gate voltage of Q1 which in turn changes the intensity of the light through a current modulation. Hence, the dimming function has smooth transition from 0% to 100% of the total light intensity. The concept for using LED as sensor is based on changing the LED state from emitting to the sensing mode. Signals to interchange the system to either ambient light or junction temperature sensing mode are regulated using l-controller. The outputs from the sensing mode are also monitored by l-controller. This information is logged in the system memory and it is used based on the defined application for the system. For the temperature sensing-mode shown in Fig. 7, the regulator Q1 regulates three or more different forward bias currents through the LEDs. Consequently, a high-voltage analog–digital converter (ADC) connected serially to one LED, measures the current and voltage and sends this information to the l-controller. The slope of the IV characteristic can be extracted from the above mentioned collected data. Since the bias current can be set around the typical emitting-mode operation values, flicker will not occur. Hence, the operation time needed for the temperature sensingmode is not an issue. For the ambient light sensing-mode, the regulator Q1 stops the sink current from the LEDs. Then one of the LEDs (LED4 in this case) is set to reverse bias by switching on the transistors Q3 and Q4. This is achieved by using the circuit as shown in Fig. 7. Transistor Q2 is fundamental to the interdicting current flowing through the LEDs when transistor Q4 is switched on. A MX resistor, which works as a current to voltage converter, senses the variation of photo-current and converts it into voltage. This voltage is applied to the internal analog to digital converter (ADC) of the l-controller. The dynamic range of using LED as light sensor can be as large as 200 mV, which leads to a high sensitivity factor when using in conjunction with a 12-bit ADC. The reason of using only one LED as photo-detector is due to the advantage of using the low output voltage of the microcontroller for reverse biasing the LED. In contrast with the temperature-sensing mode, the light-sensing mode must be executed as fast as possible in order to avoid any visible flicker. This is because in this mode all LEDs have to be completely switched-off. 5.2. Fulfilling ‘‘Design for Reliability’’ issues in our SSL device
Fig. 6. I–V characteristics of two LEDs from the Seoul Semiconductor (A4 series) in reverse and forward bias. The relationship between reverse current and ambient light has been distinguished.
As explained in details in the previous sections, this design has the capability of addressing the three reliability issues due to the thermal induced failure in LED. In the following section, this will be explained using the system flowchart in Figs. 3a–d. Furthermore, the methodology for accomplishing ‘‘Design for Reliability’’ will also be discussed. The first part of these three issues is ‘‘ Design phase reliability assessment’’. The correlation of the HB-LEDs lifetime versus their junction temperature is always provided by the HB-LED manufacturer. Therefore, the dependence of this failure cause on LEDs
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+
3
792
AC plug 1
Q5
2
CAP
R
4
-
1 2
Q2
R
LED1
LED2 Q3
Light Sense Mode Signal
Temp Sense Readout Signal1 Temp Sense Readout Signal2 LED3
Temp Sense Readout Signal3 Light Sense Readout Signal LED4
Light Sense Mode Signal
R (Mohm)
Q4
Diode
Dimming Signal
Q1
Light Sense Mode Signal
R
Fig. 7. SSL driver topology while integrating ambient light sensor and junction-temperature sensor by means of LED intrinsic sensing functionalities.
lifetime is known. The built-in LED junction temperature sensor and built-in light sensor can then be implemented in the design. The second issue is ‘‘System reliability diagnostics’’. The system can monitor its own status regularly by monitoring the LED junction temperature via the built-in junction temperature sensor. The data is gathered using the microcontroller board which is the processing part of the system. The data are processed and the lifetime of the device is calculated. There is wireless communication function in this system which can communicate with a server and sends the system status regarding if it needs any maintenance. In this system, a built-in light sensing part is also implemented. Besides gathering information about the ambient light for smart energy saving light control, it can also play a role in reliability study. The LED itself functions as the light sensor so it is not possible to read its own emitting light intensity. But if more than one of these systems are installed in an area; each system can read the light output of the rest of devices and estimate their lifetimes due to their light degradation. In this system, the intensity of the output light can be controlled by microcontroller. One of the ‘‘System self-maintenance’’ is to run the device on 80–90% of the maximum capacity. In the case of light depreciation due to failure of one of the LED, the other LEDs can compensate by operating at higher capacity. In this way, the light intensity can be maintained and this will keep the system running at a longer lifetime. This is possible due to the microcontroller implemented in this system. Furthermore, this system is controlled by software written in the frame work of this project. Thus, it will give a lot of flexibility to the system.
In the case where the junction temperature of LED is too high, the expected lifetime will become very short. In this case, the device can be regulated to run in lower forward current. In this case, the output light is lower, but the system can last longer. One example can be in automotive application which losing complete light can be very dangerous. The light can stay functional at a lower light intensity before a mechanic can be located. The second option can be in the case of reaching critical temperature system can detect it and shuts down the current through LEDs; this option leads to have no light output.
6. Conclusions Providing reliability information and designing a more reliable product is an essential requirement in solid state lighting (SSL) products. However, due to the fact that SSL is a relatively new lighting technology which is expanding the domain of its applications every day, the need for more precise reliability data becomes more critical. The range of SSL applications varies from simple light bulbs for in-door applications to street lighting, automotive lighting and also a wide range of other fields. In this work, we introduced an approach toward ‘‘ Design for Reliability’’ in SSL systems. This approach adds three major steps to the regular lifecycle of a SSL product; ‘‘Design phase reliability assessment’’, ‘‘System reliability diagnostics’’ and ‘‘System selfmaintenance’’. These three steps are applied in design phase, operation phase and maintenance phase of the lifecycle of a product. In
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the design phase, based on physics of device, the lifetime of the device is predicted. We also find the critical points of the system and their major failure causes. A monitoring system is designed in order to monitor the critical points based on related failure causes, e.g. a temperature sensor if the failure cause is the high temperature. In operation phase, the output data from sensors are read and processed in order to predict the lifetime of the device. In the case of prediction of a very short lifetime, maintenance is required. Afterward system checks if it is capable of doing self-maintenance unless it will request maintenance from operators (human being maintenance instead of system self-maintenance). In this paper, a designed SSL device is presented which covers requirements of the above mentioned three major steps of ‘‘Design for Reliability’’. In this system the critical point of the system is junction high temperature of the high brightness LED. The LED itself is designed to function also as its own junction temperature sensor. This system has microcontroller board which can read the data from junction temperature sensor. Some self-maintenance methods like driving LEDs in lower forward current in order to elongate the system lifetime can be implemented in the software of the microcontroller board. This design can be a good example of a SSL system which accomplishes the goal of ‘‘Design for Reliability’’ for high temperature applications. Acknowledgements This research was carried out within the Project ‘‘Nanoelectronics for Safe, Fuel Efficient and Environment Friendly Automotive Solutions’’ (SE2A), co-funded by the European Commission and Agentschap NL in the framework of the Public-Private Partnership ENIAC JU. The hardware implementation of this work is a part of ENSURE project which is the collaboration between Philips Lighting B.V. and TUDelft University of Technology. We also thank people from Philips lighting and Philips research in Eindhoven who helped us improving this project. References [1] Gilbert Held. Introduction to LED technology and applications. Auerbach Publication; 2008. [2] US Department of Energy. Solid-state lighting research and development: manufacturing roadmap; September 2009. [3] Zukauskas Arturas, Shur Michael S, Caska Remis. Introduction to solid state lighting. A Wiley-Interscience Publication; 2002.
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