 |
BMe Research Grant |
|
Hegedűs János
BMe Research Grant - 2019
IIIrd Prize
Doctoral School of Electrical Engineering
BME VIK, Department of Electron Devices
Supervisor: Dr. Poppe András
Power LED reliability and lifetime testing and modelling
Introducing the research area
My research work is based on the strongly temperature dependent operation of power LEDs. Both efficiency and reliability of LED based products can be further enhanced by appropriate characterisation and development techniques. Investigating and modelling of reliability issues are also organic parts of my work. The final goal is an LED model which enables determining the condition of a luminaire any time, already in the design stage.
Brief introduction of the research place
My research work is located in the thermal laboratory of the Department of Electron Devices. The most important equipment of the lab includes two LED characterisation stations with the T3Ster thermal transient tester and CAS 140CT spectroradiometer, a Weiss WK3-340/70 climate chamber, a standard LM-80-08 aging chamber and various thermostating solutions. LEDs are measured in integrating spheres.
History and context of the research
The rapid expansion of solid state lighting devices (Figure 1) is the result of their continuously increasing efficiency and efficacy. LEDs however, also constitute a new technical challenge to designers and manufacturers. Proper cooling of the luminaires has become a key issue as the efficiency and the expected product lifetime decreases significantly as the temperature of the LEDs increases. In the case of an inadequate cooling, not only the LEDs operate under more disadvantageous conditions, but also the device aging is faster, which further decreases light efficiency, resulting in higher probability of a sudden failure. LED based lamp and luminaire designers still rely on rules of the thumb in most cases, and although these rules are constantly reviewed and adjusted by many manufacturers, knowledge-based designing still offers additional benefits.
Figure 1: Influence of “LED-fication” on the streetscape (left) compared to the yellowish light of high pressure sodium lamps (right) [Source]
The research goals, open questions
Thermal issues of power LEDs are often discussed as the function of the ambient temperature, however, the forward voltage and the luminous flux values depend on the pn-junction which is significantly warmer than its environment. In many cases this inconsistency is present in manufacturer datasheets as well, but even providing the parameters at pn-junction temperatures will not in itself allow getting a comprehensive and detailed view of the temperature dependences. The main reason can be that a so-called isothermal forward current - forward voltage - radiant flux characterization that conforms to the relevant JEDEC standards is rather complicated and time consuming. However, an appropriate and detailed LED model is required to perform system level simulations for the determination of the operating luminous flux, forward voltage, consumed electrical power and operating temperature conditions of LED based luminaires.
The main goal of my research work is to base an “Industry 4.0” compatible design solution for a control scheme that compensates not only the effects of the ambient temperature changes but also the effects of LED aging, so that the luminous flux output of the luminaire is maintained at a constant level throughout its whole lifetime. This method decreases the electric consumption while increases the expected product lifetime. Last but not least, this luminaire can enhance visual comfort as its luminous flux does not have to be overdesigned by 10-30% in order to compensate for the continuous luminous flux decrease of the LEDs throughout their lifetime.
Methods
In case of constant current driving, luminous flux of an LED based luminaire decreases with the increase of the ambient temperature.
Providing the minimum required luminous flux at any environmental condition means that the constant forward current of the LEDs should be set according to the highest expected ambient temperature. This will, however, lead to higher luminous flux output than required throughout most of the year, which unnecessarily increases the electrical power consumption and accelerates the device aging. A similar situation occurs due to the continuously decreasing luminous flux output (known as total luminous flux maintenance) as the LEDs age during the operation time. To provide the required illumination at any time, the lamp should be designed so that its expected luminous flux remains above the specified value until the intended replacement time. This results in significant extra energy consumption and faster degradation of the device during the whole lifetime.
The new methodology is based on the so called multi-domain circuit simulation model of the LEDs [1] that can appropriately describe the electrical, thermal and optical operation as well as their interactions. This model also contains the compact thermal model of the LED package which can be further extended by the compact thermal model of the cooling assembly. Input parameters of the Spice-like simulations are forward current and ambient temperature by which the operating parameters of the LEDs mounted onto the luminaire, such as the pn-junction temperatures, forward voltages, efficiency values and finally the total radiant and luminous flux values can be calculated.
To generate this LED model the above discussed isothermal characteristics are needed. Exact values of the model parameters can be determined by an appropriate fitting procedure.
The most commonly used LED aging test is described by the IES LM-80 approved method, for which the TM-21-11 technical memorandum – and the Arrhenius-equation – provides a methodology to further extrapolate the test results. Having these complemented with the zero-hour multi-domain LED model makes it possible to perform lifetime simulations with some neglect; the final and complete solution would be provided by the elapsed lifetime dependent LED multi-domain model.
Results
System-level simulation of a luminaire
I have performed the system-level simulation of a streetlighting luminaire with 48 LEDs mounted onto it, with the help of the compact thermal model of the luminaire and the LED multi-domain models that I had generated previously (Figure 2). That way I determined the amount of “hot lumens” and consumed electrical power of the warmed up luminaire as a function of ambient temperature and opening current. The importance of the system-level simulation is obvious: extrapolation of the operating parameters of the individual LEDs is not sufficient due to the thermal interactions in the whole thermal network [2].
Figure 2: CAD model of the PearlLight 48 luminaire showing the LED positions
(left);
simulated pn-junction temperatures (right) of the CREE XPE LEDs assembled to
the luminaire (the grid points indicate elements of the 6x8 LED matrix)
Constant light output driving mode and its board model
Due to the temperature dependent forward current – forward voltage characteristics of LEDs the operating “hot lumens” of a luminaire decrease with increasing temperature. The above discussed system-level simulations enabled determining a driving scheme that provides a constant luminous flux output of the luminaire, independently of the ambient temperature changes (Figure 3) [3-7]. A sample luminaire provided by the manufacturer was modified to match the new driving scheme and the methodology was verified both by field and laboratory measurements (Figure 4). To estimate the annual attainable energy savings, I conducted a case study using archive meteorological data (Figure 5). For this luminaire type the energy savings – as a function of the expected maximum temperature during its operation – was calculated to be 3 to 5%.
Figure 3: Results of the system-level simulations of the luminaire as the function of forward current and ambient temperature: the total “hot lumens” (left) and the consumed electrical power (right) – the black curves indicate the constant light output operation
Figure 4: Relative radiant flux of the luminaire measured in the field (left) and in the climate chamber of the laboratory (right), normalized to the 24 °C values, at constant current and constant light output operation
Figure 5: Simulation results of the temperature dependent luminous flux (left) and consumed electrical power (right) of the luminaire, according to the archive daily mean temperature values at Szombathely. In case of constant current driving the luminous flux is significantly higher than necessary almost at all times.
Lifetime-long constant luminous flux output
During the standard LM-80-08 life testing measurements of Luxeon Z LED samples, the discussed isothermal characterisation was also performed. Using these results, the multi-domain models of the aged LEDs were generated, which were then theoretically connected to the compact thermal model of the luminaire. In this way, I could compare the operating characteristics of a new and a virtually aged luminaire.
The luminous flux of the theoretical luminaire was calculated to be 8200 lumens at the nominal operating current of 700 mA. The operating conditions resulting in this 8200 lumens are indicated in Figure 6 (right), see the curves with green and red markers for the new and aged luminaires, respectively. The 6000-hours aged luminaire needs ~780 mA forward current to provide the necessary light output; in case of constant current driving this should be the initially set value. [7-10]
Figure 6: Radiant flux curves of the new and aged Luxeon Z LEDs (left) and the simulated power consumption of the virtually assembled luminaire (right); the 8200 lumen conditions are also indicated
With the help of the constant luminous flux controlling scheme the operating temperature of the LEDs can be reduced, which significantly increases the expected lifetime of the LEDs.
In my publications, I proposed a new end-of-life condition for LEDs: in the case of aging-compensated driving methods, I proposed the possibility of using luminous efficacy maintenance (such as ηv90) instead of the luminous flux maintenance (like L90).
To investigate the increase of the expected lifetime thanks to reduced operating pn-junction temperature, I developed a new theory [11] that can describe the current status (i.e. the aging process) of the LEDs as the function of changing temperature and changing forward current. With the help of the LM-80-08 test results and the initial multi-domain model of the LEDs, I generated a theoretical model for constant luminous flux control for the complete device lifetime (Figure 7), furthermore I also examined its beneficial effects on both energy savings and increased lifetime (Figure 8).
Figure 7: Modelling the aging process (left) and luminous flux (right) of a power LED in case of constant current and constant light output operation, using archive meteorological data
Figure 8: Estimated energy consumption and lifetime of the tested LED type for the constant current and the constant light output driving
Elapsed lifetime dependent multi-domain model
With the help of the isothermal characteristics of the new and aged Luxeon Z LED samples, I created the elapsed lifetime dependent multi-domain models and examined the change in the obtained model parameter values [12]. Figure 9 indicates that the parameters of the five individual LED samples show very similar trend; these trends, however, are a result of the combination of different aging mechanisms (Figure 10).
An important finding of this investigation is the fact that for Luxeon Z LED the 6000 hours aging time required by the standard is not sufficient to explore and precisely extrapolate any degradation process.
Based on this finding, I launched a new and broader aging test which still continues. Further research on this subject is the main field of my future work.
Figure 9: Elapsed-time dependent change of some parameters of the multi-domain model
Figure 10: Averaged forward voltage (left) and radiant efficiency (right) of the tested Luxeon Z LED samples, as a function of elapsed operating time
Expected impact and further research
My past and planned research work is strongly based on scientific-technical cooperation (e.g. Delphi4LED H2020 ECSEL project) with Hungarian and foreign lighting and industrial LED photometry experts (HungaroLux Light Ltd., LightingLab Kalibráló Kft., Mentor - Siemens business, Signify, Lumileds, PI-Lighting, etc.). The scientific background of the work is provided by the solid state lighting research group and by the thermal and reliability laboratory of BUTE Dept. of Electron Devices. The results of my further research should be a useful contribution to the currently running K 128315 NKFIH (OTKA) project, and also to the proposed Digital4LED (New Digital Flow For LED-based Product Development) EIT Digital 2020 project, with the consortium of PISEO, Signify and BUTE. This way my partial results could be immediately applied, gaining feedback from several leading European lighting companies (e.g. from Signify).
Our results in this field were published and presented at the 2017 THERMINIC Conference in Amsterdam, which was recognized with the "Harvey Rosten Award for Excellence" prize on the 22nd of March, 2018 in San Jose, California.
Publications, references, links
List of related own publications
[1] Poppe, A; Hegedüs, J; Szalai, A
Multi-domain modeling of power LEDs based on measured isothermal I-V-L characteristics
In: anon (eds.) Proceedings of the CIE Lighting Quality & Energy Efficiency Conference; Wien, Austria: International Commission on Illumination (CIE), (2016) pp. 318-327. Paper: OP56, 10 p.
[2] János, Hegedüs; András, Poppe
Simulation of luminaires based on chip level multi-domain modeling of power LEDs
In: Dariusz, Sawicki; Piotr, Pracki (eds.) Proceedings of the VI. IEEE Lighting Conference of the Visegrad Countries LUMEN V4; New York, United States of America: IEEE, (2016) pp. 59-64., 6 p.
[3] János, Hegedüs; Gusztáv, Hantos; András, Poppe
Embedded Multi-domain LED Model for Adaptive Dimming of Streetlighting Luminaires
In: András, Poppe (eds.) Proceedings of the 22nd International Workshop on THERMal INvestigation of ICs and Systems (THERMINIC'16); Budapest, Hungary: BTU Dept. of Electron Devices, (2016) pp. 208-212., 5 p.
[4] János, Hegedüs; Péter, Horváth; Gusztáv, Hantos; Tamás, Szabó; András, Szalai; András, Poppe
A New Dimming Control Scheme of LED Based Streetlighting Luminaires Using an Embedded LED Model Implemented on an IoT Platform to Achieve Constant Luminous Flux at Different Ambient Temperatures
In: Matej, B Kobav (eds.) Proceedings of Lux Europa 2017; Ljubljana, Slovenia: Lighting Engineering Society of Slovenia, (2017) pp. 87-92., 6 p.
[5] Hegedüs, János; Poppe, András
Közvilágítási lámpatestek karakterizálása multi-domain LED modellekkel – a LED karakterisztikáktól a lámpatest üzemi fényáramáig
ELEKTROTECHNIKA 110: 3-4 pp. 13–20. (2017)
[6] Hegedüs, J; Horváth, P; Szabó, T; Szalai, A; Poppe, A
A New Dimming Control Scheme of LED Streetlighting Luminaires Based on Multi-Domain Simulation models of LEDs in order to Achieve Constant Luminous Flux at Different Ambient Temperatures
In: P, Zwick (eds.) PROCEEDINGS of the Conference on "Smarter Lighting for Better Life" at the CIE Midterm Meeting 2017; Vienna, Austria: Commission Internationale de l'Eclairage, (2017); pp. 267–276., 10 p.
[7] Hegedüs, János; Hantos, Gusztáv; Poppe, András
Light output stabilisation of LED based streetlighting luminaires by adaptive current control
MICROELECTRONICS RELIABILITY 79 pp. 448-456. , 9 p. (2017)
[8] Hegedüs, János
LED-es lámpatestek többlet energia megtakarítási lehetőségei termikus és élettartam szempontokat figyelembe vevő, modell alapú tervezéssel
ELEKTROTECHNIKA 111: 6-7-8 pp. 21–26. , 6 p. (2018)
[9] Poppe, András; Farkas, Gábor; Gaál, Lajos; Hantos, Gusztáv; Hegedüs, János; Rencz, Márta
Multi-Domain Modelling of LEDs for Supporting Virtual Prototyping of Luminaires
ENERGIES 12: 10 Paper: 1909, 30 p. (2019)
[10] Hegedüs, János; Hantos, Gusztáv; Poppe, András
LED-es lámpatestek modell alapú tervezése
In: Némethné dr. Vidovszky, Ágnes; Poppe, András (eds.) Világítástechnikai Évkönyv 2018-2019; Budapest, Hungary: MEE Világítástechnikai Társaság, (2019) pp. 83–89. , 7 p.
[11] Hegedüs, János; Hantos, Gusztáv; Poppe, András
Lifetime Iso-flux Control of LED based Light Sources
In: W, Luiten; J, Janssen; G, Martin (eds.) Proceedings of the 23rd International Workshop on THERMal INvestigation of ICs and Systems (THERMINIC'17); New York, United States of America: IEEE, (2017); Paper_181, 5 p.
[12] J., Hegedüs; G., Hantos; A., Poppe
A step forward in lifetime multi-domain modelling of power LEDs
In: Proceedings of the 29th Session of the CIE; Vienna, Austria: International Commission on Illumination (CIE), (2019) pp. 1154–1161. , 8 p.
MTMT link of the whole list of own publications
Table of links
TeraLED LED characterisation station
T3Ster thermal transient tester
Total luminous flux maintenance