Thermal characterization of multicolor LED luminaire

The LED based dynamic lighting scheme, require compact and thermally efficient luminaire. This paper presents the thermal investigation on the conceptual design of 36 W multicolor light emitting diode (LED) luminaire. The developed prototype design includes configuration and placement of the multichip LED package, RGBW and single die amber LEDs in a 5 × 3 array on the heat sink. LED configurations with low power input are placed between the LEDs having the high power input. The proposed configuration and placement of LEDs reduces the local heat concentration in the centre region of the heat sink. The temperature of 72 °C at LED chip base plate is reduced to 32.1 °C on the heat sink surface. The numerical results are experimentally validated. The proposed method contributes to a reduction in the size of the luminaire and also enhancement of heat dissipation for improving the longevity of the multicolor based LED luminaire.     

Optimizing n-type contact design and chip size for high-performance indium gallium nitride/gallium nitride-based thin-film vertical light-emitting diode

The effects of the n-contact design and chip size on the electrical, optical and thermal characteristics of thin-film vertical light-emitting diodes (VLEDs) were investigated to optimize GaN-based LED performance for solid-state lighting applications. For the small (chip size: 1000×1000 µm²) and large (1450×1450 µm²) VLEDs, the forward bias voltages are decreased from 3.22 to 3.12 V at 350 mA and from 3.44 to 3.16 V at 700  mA, respectively, as the number of n-contact via holes is increased. The small LEDs give maximum output powers of 651.0–675.4 mW at a drive current of 350 mA, while the large VLEDs show the light output powers in the range 1356.7–1380.2 mW, 700 mA, With increasing drive current, the small chips go through more severe degradation in the wall-plug efficiency than the large chips. The small chips give the junction temperatures in the range 51.1–57.2 °C at 350  mA, while the large chips show the junction temperatures of 83.1–93.0 °C at 700  mA, The small LED chips exhibit lower junction temperatures when equipped with more n-contact via holes.     

Simulation of LED based luminaires by using multi-domain compact models of LEDs and compact thermal models of their thermal environment

Fast and accurate prediction of hot lumens of LEDs installed in luminaires is an important step in the design of robust and reliable products. A possible approach to this is to create a multi-domain circuit model of a complete LED chip + package + luminaire system that can be simulated by any Spice-like circuit simulator with electro-thermal capabilities. Many LED chip and LED package models and modeling techniques have been published recently, but compact thermal modeling of luminaires as multi heat-source system was not yet dealt with in the literature. This paper aims to fill this gap be describing a systematic approach for system (luminaire) level analysis aimed at solving the combined thermal, electrical and light output simulation problem consistently by describing a method for creating a compact thermal model of LED luminaries with an approach borrowed from the layout based electro-thermal simulation of analog ICs. The applicability of the described method is demonstrated with a real life example, including the validation of the results with thermal measurements.     

Thermal characterization of high power LED with ceramic particles filled thermal paste for effective heat dissipation

The next generation packaging materials are expected to possess high heat dissipation capability. Understanding the needs for betterment in the field of thermal management, the present study aims at investigating the package level analysis on a high power LED. In this study, commercially available thermal paste was heavily filled with ceramic particles of aluminium nitride (AlN) and boron nitride (BN) in order to enhance the heat dissipation of the device. Different particle sizes of AlN and BN fillers were incorporated homogenously into the thermal paste and applied as a thermal interface material (TIM) for an effective system level analysis employing thermal transient measurement. It was found that AlN TIM achieve less LED junction temperature by a difference of 2.20 °C compared to BN filled TIM. Furthermore, among D₅₀ = 1170 nm, 813 nm and 758 nm, the AlN at D₅₀ = 1170 nm was found to exhibit the lowest junction temperature of 38.49 °C and the lowest total thermal resistance of 11.33 K/W compared to the other two fillers.     

Connecting plugs of high-powered GaN-based lighting-emitting diodes prepared by electroplating

We investigate the fabrication and the characteristics of a gallium nitride-based light-emitting diode (GaN-based LED) with a connecting plug. The connecting plug was prepared by electroplating, connecting the front and back side of the GaN-based LED via a through hole formed by a laser driller to improve the heat dissipation and the yield loss that was caused by the disconnection between the front and the back sides of the GaN-based LEDs because of the edge coverage effect. The junction temperature of the GaN-based LEDs with the connecting plug increased from 19 to 54 °C when the injection current was increased from 100 to 500 mA.     

Light output stabilisation of LED based streetlighting luminaires by adaptive current control

Temperature dependence of LED operation is often not fully considered during the design of solid state lighting products. If temperature dependence is not carefully considered, solid-state lighting products are typically overdesigned to be too robust enough to fulfil the requirements under any possible environmental conditions. Temperature dependent nature of LEDs though, could even be a new benefit if properly considered. Overdesign means designing for the worst case that is the highest possible environmental temperature when LED efficiency/efficacy is low. With a control scheme resulting in constant emitted total luminous flux significant electrical power saving can be achieved since at lower temperatures, due to increasing efficiency/efficacy less electrical power, thus, lower forward current levels are sufficient. This paper describes different methods to specify the so called iso-flux control of LEDs' operating point, in which effect of temperature changes on light output characteristics is compensated by adjustment of the forward current. Parameters for an automated temperature compensation can be identified with the help of multi-domain LED models. This paper describes our LED multi-domain model based approach applied to the design of the light output control of an existing street-lighting luminaire. During the design of the control scheme real, archived meteorological temperature data set was considered. Based on our model we implemented the temperature compensated iso-flux control of a luminaire and the planned operation was validated by actual measurements. The verified luminaire model was further investigated with multi-domain models of aged LEDs obtained during an LM-80 standard compliant aging of a set of LEDs, characterizing LEDs up to 6000 + h of operating life time.     

Life prediction of LED-based recess downlight cooled by synthetic jet

This paper details the adaptation and implementation of a proposed hierarchical model to the reliability assessment of LED-based luminaires. An Edison base ? 6 in., compatible can, downlight ? LED replacement bulb, cooled by active synthetic jets, is used as the test vehicle. Based on the identified degradation mechanisms and the experimentally obtained degradation rate of the cooling device, the reduction in the heat sink enhancement factor, and thus the increase in the LED junction temperature, is determined as a function of time. The degradation mechanisms of the dual-function power electronics – providing constant power to the LEDs and to the drivers of a series of synthetic jets – are also analyzed and serve as the basis for a hybrid model which combines these two effects on the luminaire lifetime. The lifetime of a prototypical luminaire is predicted from LED lifetime data using the degradation analyses of the synthetic jet and power electronics.     

Study on thermal performance of high power LED employing aluminum filled epoxy composite as thermal interface material

This paper elucidates the thermal behavior of an LED employing metal filled polymer matrix as thermal interface material (TIM) for an enhanced heat dissipation characteristic. Highly thermal conductive aluminum (Al) particles were incorporated in bisphenol A diglycidylether (DGEBA) epoxy matrix to study the effect of filler to polymer ratio on the thermal performance of high power LEDs. The curing behavior of DGEBA was studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The dispersion nature of the Al fillers in polymer matrix was verified with Field Emission Scanning Electron Microscope (FESEM). The thermal performance of synthesized Al filled polymer composite as TIM was tested with an LED employing thermal transient measurement technique. Comparing the filler to polymer ratio, the rise in junction temperature for 60 wt% Al filled composite was higher by 11.1 °C than 50 wt% Al filled composite at cured state. Observed also from the structure function analysis that the total thermal resistance was 10.96 K/W higher for 60 wt% Al filled composite compared to 50 wt% Al filled composite. On the other hand, a significant rise of 9.5 °C in the junction temperature between cured and uncured samples of 50 wt% Al filled polymer TIM was observed and hence the importance of curing process of metal filled polymer composite for effective heat dissipation is discussed extensively in this work.     

Pump chip and phosphor reliability of broadband light-emitting diodes

We present a study of the degradation of phosphor-based broadband (~ 90 nm spectral peak width) colour and white LEDs. Specifically, our study looked at the reliability of the blue-emitting GaN/InGaN pump chip and the overlying phosphor in these LEDs. We have investigated thermal degradation arising from heat generation in both the pump chip and the colour-converting phosphor. The robustness of the pump chip in 1 W broadband power LEDs was examined by driving them with various dc and pulsed waveforms at different temperatures. Both catastrophic and long term degradation of pump chips was investigated. Long term degradation behaviour of phosphors was studied by both ex situ and in situ heating of phosphors for hundreds of hours while their total light output was monitored. Optical energy storage in phosphors and its bearing on phosphor degradation is also discussed.     

Thermal design and simulation of automotive headlamps using white LEDs

With an urgent need for energy conservation and pollution reduction, the trend of replacing traditional incandescent or fluorescent lamps with high-power LEDs is growing more and more popular. In this research, high power white LED chips are used in automotive headlamp low beam system design. Several different cooling devices are designed for headlamp cooling, the heat dissipation performances are simulated and analysed both by the finite volume method (FVM) in FloEFD and experimental measurements. The simulation and experimental results show that nature convection cooling is not an effective method for LED headlamp cooling. Heat sink combined with heat pipes technology can greatly improve the heat dissipation performance. When the liquid filled ratio is 10%, heat pipes with evaporator length 30 mm, adiabatic section length 40 mm and condenser length 50 mm have the best cooling performance. Cooling device with heat pipes placed dispersedly makes the junction temperature lower than cooling device with heat pipes isometric placed in the same plane. The liquid filled ratio of heat pipes can influence the equivalent heat transfer coefficient significantly, and the optimal filling rate is 30% in our study.     

GaN-based LEDs with Ar plasma treatment

The authors propose a simple Ar plasma treatment method to selectively damage the area underneath p-pad electrode of GaN-based light-emitting diodes (LEDs). It was found that we could form a highly resistive area so that the injected carriers will be forced to spread out horizontally for the LED. Under 20 mA current injection, it was found that the output powers were 16.0, 17.9 and 17.3 mW while the forward voltages were 3.17, 3.19 and 3.20 V for conventional LED and LED with SiO₂ layer, respectively. Moreover, the LED with Ar plasma treatment is superior to the other LEDs while operating at a higher injection current.     

Improving the light output power of GaN-based light-emitting diodes through the use of SiO2 cones

We introduced a simple wet-etching process to form SiO₂ cones and investigated the effect of the size and coverage of the SiO₂ cones on the output power of GaN-based light-emitting diodes (LEDs). The diameter of the cones varies from 2.8 to 17.1 μm and the height from 0.6 to 2.0 μm. It is shown that regardless of the sizes of the cones, all of the LEDs exhibit a same forward-bias voltage of 3.31 V at an injection current of 20 mA. As the size of the cones increases, the light output increases, reaches maximum at cone #3 (12.4 μm in diameter and 2.0 μm in height), and then decrease slightly. For example, the LEDs fabricated with different SiO₂ cones exhibit 11.4–35.9% higher light output power (at 20 mA) than do the LEDs without the cones. The electroluminescence (EL) intensity (at 20 mA) also exhibits cone size dependence similar to that of light output power. For example, the LEDs fabricated with different cones exhibit 7.7–36.3% higher EL intensity than the LEDs without the cones.     

Phase-change immersion cooling high power light emitting diodes and heat transfer improvement

A new cooling method of ethanol direct-contact phase-change immersion cooling was proposed in the thermal management of high power light emitting diodes (LED) and the feasibility of this cooling method was investigated. The heat generated by LED was measured firstly using two types of power systems: DC power and LED driver. Then the heat dissipation performance was evaluated under different experimental conditions. The results indicate that startup process of the cooling system is quick and only 450 s is needed to reach steady-state under heat load of 42.78 W. The minimum thermal resistance of 1.233 °C/W is obtained when liquid filling ratio is 33.14%. The junction temperature of LED under different absolute pressures is much lower than the limited value of 120 °C. Baffle with total height of 140 mm, bottom space height of 20 mm and distance away from substrate surface of LED of 8 mm improves heat transfer performance best due to ethanol self-circulating in the cooling receiver. Overall, the ethanol phase-change immersion cooling is an effective way to make sure high power LED work reliably and high efficiently.     

Thermal/luminescence characterization and degradation mechanism analysis on phosphor-converted white LED chip scale packages

According to the requirements on minimizing the package size, guaranteeing the performance uniformity and improving the manufacturing efficiency in LEDs, a Chip Scale Packaging (CSP) technology has been developed to produce white LED chips by impressing a thin phosphor film on LED blue chips. In this paper, we prepared two types of phosphor-converted white LED CSPs with high color rendering index (CRI > 80, CCT ~ 3000 K and 5000 K) by using two mixed multicolor phosphor materials. Then, a series of testing and simulations were conducted to characterize both short- and long-term performance of prepared samples. A thermal analysis through both IR thermometry and electrical measurements and thermal simulation were conducted first to evaluate chip-on-board heat dissipation performance. Next, the luminescence mechanism of multicolor phosphor mixtures was studied with the spectral power distribution (SPD) simulation and near-field optical measurement. Finally, the extracted features of SPDs and electrical current-output power (I-P) curves measured before and after a long-term high temperature accelerated aging test were applied to analyze the degradation mechanisms. The results of this study show that: 1) The thermal management for prepared CSP samples provides a safe usage condition for packaging materials at ambient temperature; 2) The Mie theory with Monte-Carlo ray-tracing simulation can be used to simulate the SPD of Pc-white LEDs with mixed multicolor phosphors; 3) The degradation mechanisms of Pc-white LEDs can be determined by analyzing the extracted features of SPDs collected after aging.     

Experimental study on the influence of junction temperature on the relationship between IGBT switching energy loss and device current

Accurate determination of power losses in semiconductor devices is important for optimal design and reliable operation of a power converter. The switching loss is an important component of the total device loss in an insulated-gate bipolar transistor (IGBT) in a voltage source inverter. The objective here is to study experimentally the influence of junction temperature on the turn-on switching energy loss E_on and turn-off switching energy loss E_off. More specifically E_on and E_off are both related to device current I_c; the influence of junction temperature on the relationship between E_on and I_c and that between E_off and I_c is studied. As the operating environmental conditions and load conditions of power converter vary widely, a wide range of junction temperatures between − 35 °C and + 125 °C is considered here. The experimental data enable precise determination of the switching loss in each device in a high-power converter at any practical operating condition. This leads to precise estimation of total device loss and optimal thermal design of the converter. This further helps off-line and/or on-line estimation of device junction temperatures required for study of thermal cycles and reliability.     

Optimization and validation of thermal management for a RF front-end SiP based on rigid-flex substrate

This paper mainly presents a new 3D stacking RF System-in-Package (SiP) structure based on rigid-flex substrate for a micro base station, with 33 active chips integrated in a small package of 5cm × 5.5cm × 0.8cm. Total power consumption adds up to 20.1 Watt. To address thermal management and testability difficulties of this RF SiP, a thermal test package is designed with the same package structure and assembly flow, only replacing active chips with thermal test dies (TTDs). Optimization and validation of thermal management for the thermal test package is conducted. Effects of the structure, chip power distribution, and ambient temperature aspects on the thermal performance are studied. Thermal vias designed in the organic substrate provide a direct heat dissipation path from TTDs to the top heatsink, which minimizes junction temperature gap of the top substrate from 31.2 °C to 5.3 °C, and enables junction temperatures of all the chips on the face to face structure to be well below 82 °C. Chip power distribution optimization indicates placing high power RF parts on the top rigid substrate is a reasonable choice. The ambient temperature optimizes with forced air convection and cold-plate cooling method, both of which are effective methods to improve thermal performances especially for this micro base station application where environment temperature may reach more than 75 °C. The thermal management validation is performed with a thermal test vehicle. Junction temperatures are compared between finite-volume-method (FVM) simulation and thermal measurement under the natural convection condition. The accordance of simulation and measurement validates this thermal test method. Junction temperatures of typical RF chips are all below 80 °C, which shows the effectiveness of thermal management of this RF SiP.     

Sapphire substrate-transferred nitride-based light-emitting diode fabricated by sapphire wet etching technique

In order to investigate the influence of a sapphire substrate on the GaN-based light-emitting diode (LED) performance, sapphire-etched vertical-electrode nitride-based semiconductor (SEVENS) LEDs are fabricated by a sapphire wet etching technique. The performance of SEVENS-LEDs is substantially dependent on the presence of sapphire substrate. The light-output power of a SEVENS-LED with a microroughened surface structure and without a sapphire substrate (type-A) is not saturated up to a junction current as high as 300 mA, constituting a notable improvement relative to that (250 mA junction current) of SEVENS-LEDs with a 5 μm-thick sapphire substrate (type-B). The 200 mA light-output power of type-A SEVENS-LED is 1.8 times stronger than that of type-B SEVENS-LED. With increasing junction current, the variation of the peak wavelength is less for the type-A SEVENS-LED than for the type-B SEVENS-LED. These results imply that even a thin sapphire substrate on the SEVENS-LED affects the heat dissipation characteristic at high injection levels.     

Substrate temperature effect on structural,optical and electrical properties of vacuum evaporated SnO2 thin films

Tin oxide (SnO₂) thin films were deposited on glass substrates by thermal evaporation at different substrate temperatures. Increasing substrate temperature (T_s) from 250 to 450 °C reduced resistivity of SnO₂ thin films from 18×10⁻⁴ to 4×10⁻⁴▒ Ω ▒cm. Further increase of temperature up to 550 °C had no effect on the resistivity. For films prepared at 450 °C, high transparency (91.5%) over the visible wavelength region of spectrum was obtained. Refractive index and porosity of the layers were also calculated. A direct band gap at different substrate temperatures is in the range of 3.55−3.77 eV. X-ray diffraction (XRD) results suggested that all films were amorphous in structure at lower substrate temperatures, while crystalline SnO₂ films were obtained at higher temperatures. Scanning electron microscopy images showed that the grain size and crystallinity of films depend on the substrate temperature. SnO₂ films prepared at 550 °C have a very smooth surface with an RMS roughness of 0.38 nm.     

Effects of silicone coating degradation on GaN MQW LEDs performances using physical and chemical analyses

This work presents a physics of failure (POF) methodology coupling failure signatures with physico-chemical analyses. The aim is to work out electro-optical failure signatures located in packaged InGaN/GaN Multiple Quantum Wells Light Emitting Diodes (MQW LEDs). Electrical and optical characteristics performed after accelerated ageing tests (30 mA/85 °C/1500 h), confirm a 65% drop of optical power and an increase of one decade of leakage current spreading at the silicone oil/chip interfaces. Through measurements of silicone coating fluorescence emission spectra, we demonstrate that the polymer enlarges the LED emission spectrum and shifts central wavelength. This shift is related to silicone oil spectral instability and the central wavelength of packaged LED appears to be temperature insensitive. In this paper, we discriminate the degradation of bulk silicone oil responsible for optical losses from the polymer/chip interface inducing larger leakage current.     

Electrical and optical characterization of GaN-based light-emitting diodes fabricated with top-emission and flip-chip structures

We investigated the electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) fabricated with top-emission and flip-chip structures. Compared with top-emission LEDs, flip-chip LEDs exhibited a 0.25 V smaller forward voltage and an 8.7 Ω lower diode resistance. The light output power of the flip-chip LED was also larger than that of the top-emission LED by factors of 1.72 and 2.0 when measured before and after packaging, respectively. The improved electrical and optical output performances of flip-chip LEDs were quantitatively analyzed in terms of device resistance and ray optics, respectively.