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How to accelerate the transition to Industry4.E by reducing development time and cost while maintaining product quality? The Delphi4LED European project ECSEL-JU grant #692465 developed a state-of-the-art methodology to establish a standardized approach for generating and integrating multi-domain LED digital twins from measurement data. The paper presents the approach, the importance of standardization and the latest results in terms of accuracy and calculation time gains for a luminaire level simulation.

In this work we determine the factors that influence the accuracy of LEDs thermal transient analysis, in particular, the leakage of heat into the dome material and the parasitic generation of heat on the reflector cup surface due to the light reflection losses are considered. Our analysis indicates a significant impact of these two factors on interpretation of the thermal transient measurements of mid-power LEDs. The paper demonstrates the significance of the physical phenomena behind these factors for creation of LEDs finiteelement thermal models. We quantify the inaccuracies in the thermal structure functions associated with these effects. We determine possible inaccuracies of finite-element models parameters calibration by thermal structure function or transient thermal response alignment if these factors are not accounted for. We show a substantial impact of the parasitic heat losses on the LEDs' reflector cup surface on the results of thermal transient for LEDs with high internal quantum efficiency encapsulated in packages with low light extraction efficiency.

Light emitting diodes (LEDs) are highly efficient light sources with a long lifetime. LEDs are widely used in modern lighting applications. Reliable thermal modelling of LEDs and lighting solutions based on LEDs is a key aspect for achieving high reliability, desired light properties and lifetime of lighting systems. Though the LEDs are significantly more efficient than convenient incandescent bulbs, still they dissipate a great part of electrical power applied as heat. Lacking such efficient way of cooling as energy radiation LEDs have to rely on other means of heat transfer which complicates their thermal design. Thermal transient testing is a common way to evaluate thermal properties of LEDs. Nevertheless, due to the intrinsic limitations of the method and peculiar architecture of LEDs the data extracted with the method may contain error. This paper quantifies the transient characterization error for LEDs with silicone and phosphor/silicone composite domes and experimentally determines the sources of it.

This paper presents a number of novel methods to measure the junction temperature and to estimate the health of gallium nitride light-emitting diodes (LEDs). The methods are based on measurements of the dynamic impedance and optical output. Our experimental analysis reveals temperature sensitive parameters of the electrical and optical responses. Moreover, a correlation between the non-radiative current characterizing the active region defects and the small-signal impedance is shown. The demonstrated methods can be applied to enhance existing temperature-monitoring techniques. The derived dependencies also build a foundation for advanced in-field health monitoring of the LEDs using the infrastructure of visible light communication systems. Such methods and techniques are valuable for predictive maintenance of solid state lighting systems.

Light-emitting diode (LED) technology has been rapidly developing due to high energy efficiency and longer lifetimes of LED luminaires. One of the main challenges in designing LED components is to manage the inter-twined relation between thermal, electrical, and optical performances. These dependencies are required to be well understood in order to operate LEDs efficiently and have accurate performance prediction at different levels of the product value chain. Delphi4LED project aims at developing standardised method to create multi-domain (thermal, electrical, and optical) compact models from the measurement data. To obtain highly reliable and representative data sets via characterisation and calibration, end-user requirements specification for diverse LED samples are needed. In this paper, we report the lists of LED parameters and components selected for measurements, simulation and calibration considering end-user needs. We also show some of the methodologies for compact thermal model development that are recommended in Delphi4LED, followed by definition of simulation benchmark problems.

Thermal transient testing is widely used for LED characterization, derivation of compact models, and calibration of 3D finite element models. The traditional analysis of transient thermal measurements yields a thermal model for a single heat source. However, it appears that secondary heat sources are typically present in LED packages and significantly limit the model’s precision. In this paper, we reveal inaccuracies of thermal transient measurements interpretation associated with the secondary heat sources related to the light trapped in an optical encapsulant and phosphor light conversion losses. We show that both have a significant impact on the transient response for mid-power LED packages. We present a novel methodology of a derivation and calibration of thermal models for LEDs with multiple heat sources. It can be applied not only to monochromatic LEDs but particularly also to LEDs with phosphor light conversion. The methodology enables a separate characterization of the primary pn junction thermal power source and the secondary heat sources in an LED package.

The uniformity of Electroluminescence (EL) of green and blue/green stacking InGaN Light Emitting Diodes (LED) has been studied and analyzed by various techniques. Comparative measurements of Photo-Luminescence (PL), Cathode-Luminescence (CL) and EL allow us to conclude that non-uniform distribution of EL in studied LEDs is caused by non-uniformity of acceptors distribution over the p-GaN area. It was found that there are some defective areas in p-GaN having locally low level of acceptor concentration. These areas correspond to the surface hexagonal shaped pyramids on p-GaN surface. The locally low doping level changes the field distribution, injection properties and the EL emission properties for these defective points. Optimization of p-GaN growth conditions will improve the EL uniformity and increase the efficiency of green and blue/green LEDs. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The tunneling radiative recombination in green InGaN based Light Emitting Diodes (LEDs) has been studied by micro-Electroluminescence measurements. It was found that this recombination was very local, which has lateral size less than 1 μm and spectral peak position around 600 nm. Changes of growth conditions led to a reduction in the concentration of these defects, improving the current-voltage characteristics and slightly enhancing the optical efficiency of green LEDs. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The structure of a single light-emitting diode (LED) chip for operation under high voltage alternating current (ac) conditions was proposed. The chip was designed and fabricated as an integrated circuit with several isolated LED structures grown on a single insulated sapphire substrate. The efficient blue LED chip for operation directly from 220-V ac power source has been designed, fabricated, and analyzed. The white ac operated LED (ACOLED) chip demonstrates a value of averaged luminous flux ~ 320 lm and luminous efficiency 80 lm/W under operation from 220-V ac source with 4 W of power consumption. The ladder type of circuit and the efficient ACOLED chip configuration results in proficient operation under ac conditions.

Uniformity of luminescence properties in blue InGaN light-emitting diodes has been studied and analyzed by local time-resolved photoluminescence and microelectroluminescence measurements at different biasing. For studied structures, some nonuniform distribution of photoluminescence properties has been observed at reverse biasing conditions. This nonuniformity revealed inhomogeneous distribution of electric field over the active region. It is supposed that nonuniform distribution of acceptors concentration in p-GaN is a source of electric field fluctuations. Microelectroluminescence measurements showed that areas with locally lower acceptor concentration in p-GaN layer emit lower electroluminescence intensity. This was caused by limited hole injection efficiency into multiple quantum wells region at high current.

A new multiple heat path dynamic compact model extraction method for LED packages with silicone domes is proposed. The method enables separate characterization of the LEDs dome and the main heat path. It is based on thermal transient analysis of LED configurations with and without the dome. The heat paths de-embedding procedure proposed significantly increases accuracy of the LED thermal characterization compared to a typical singular heat path approach. The method is demonstrated with a representative mid-power LED. The results are validated with steady-state FEA. Suppressed estimation errors of the heat path evaluation are indicated.

Rapid growth of Solid-State Lighting market has been supported by progress in Light Emitting Diode (LED) technology. Strong competition requires that lighting product development cycle should be shorter and more effective. Optimal design of a LED luminaire in major degree depends on thermal-electro-optical properties of LEDs in use. These properties are required to be accurately characterized and well predicted for various operating conditions of the LEDs within luminaire in close interaction with other parts of the system. Delphi4LED project [1] aims at developing standardised method to create multi-domain (thermal, electrical, and optical) compact models from the measurement data. Boundary-independent, accurate and well junction temperature-controlled iso-thermal electro-optical characterization procedure for LED samples is required to obtain highly reliable and representative data sets as an input for the compact model. Often, thermal and optical characterization experiments are performed separately at dedicated equipment. In this paper, we report the results of our evaluation and experimental verification of various sources for uncertainties and inaccuracies originated from separation of thermal and optical measurement procedures.