Junction Temperature Of Light-emitting Diodes Research Articles

High-Power LED Photoelectrothermal Analysis Based on Backpropagation Artificial Neural Networks

As an electroluminescentdevice, the coupling relationship between light-emitting diode (LED) input currents, optical power, and LED junction temperature is a complicated multiphysics process. In this paper, a simplified LED photoelectron-thermal (PET) model by artificial neural network (ANN), which can translatemultiphysics field issue into a singlephysics field problem, ismentioned to study the coupling relationship. In the first, an LED lumens, optical power, and electric power at different temperatures are monitored in a temperature controlling integrating sphere. Then, a backpropagation (BP) ANN is trained by these data to construct an LED Photo-Electron-Thermal (PET) relationship. In addition, LED luminaire thermal analyzing is performed using a finite-element method on the outputs of the BP ANN. Finally, the advantage of this method in terms of saving computing resources and computing time is analyzed by comparing the degrees of freedom in different models. The result shows that at least 6.7 times computing resource are saved by this method, which will reduce the LED thermal management system analyzing time greatly. Finally, the application and extension of this model are discussed.

Factors Influencing the Temperature Distribution of 200 W Light Emitting Diode Module Used in the Spotlight

ABSTRACTThe junction temperature of LED (light emitting diode) has a significant impact on its performance and lifetime. In this paper, a simplified model based on the finite element analysis is developed to simulate the temperature distribution of the 200 W LED module using software ANSYS. The model contains LED package, the heat pipe radiator, as well as TIM (thermal interface material) between the LED package and radiator. The temperature distribution of the simulation agrees with that of the experimental measurement. Thickness of TIM affects the heat dissipation significantly, the chips temperature and the maximum temperature difference of chips increases sharply with TIM thickness increasing. Substituting aluminum fins with copper fins cannot improve the heat dissipation performance of heat pipe radiator, and the air velocity of heat pipe radiator plays a key role in the heat dissipation. Thermal conductivity of package submount directly affects the chip temperature and the uniformity of temperature distribution of package submount.

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 450s is needed to reach steady-state under heat load of 42.78W. 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 140mm, bottom space height of 20mm and distance away from substrate surface of LED of 8mm 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.

Determining Junction Temperature of LEDs by the Relative Reflected Intensity of the Incident Exciting Light

Relative reflected intensity of the incident exciting light is proposed to measure the junction temperature of light-emitting diodes (LEDs) under test. Reflectance spectra at a wide junction temperature range are acquired. Multichannel optical fibers greatly increase the collecting efficiency of the reflected light. Lock-in technique is utilized to exclude the interference of the emitting light from LEDs under test and to increase the dynamic range greatly. The results are in good agreement with those directly tested by a microthermocouple. To avoid extra carrier absorption and modulation effect, the incident exciting light should harbor smaller bandgap than that of LEDs under test.

Improving Peak-Wavelength Method to Measure Junction Temperature by Dual-Wavelength LEDs

In this paper, the improvement of the method measuring the junction temperature of light-emitting diodes (LEDs) has been studied experimentally. A practical method is proposed with only three measurement procedures. With the consideration of indium (In) composition and blue shift, the method has a high applicability, which is practical for the LED chips vary from blue to green chips under different currents, including the packaged chips. On the other hand, according to the experimental and derived results, the junction-temperature difference and peak-wavelength shift in both blue-shift and red-shift fields show similar parabolic-like relations. To simplify the experimental processes, dual-wavelength LEDs were fabricated and measured instead of conventional single-wavelength LEDs.

Numerical Simulation and Experimental Validation for the Thermal Analysis of a Compact LED Recessed Downlight with Heat Sink Design

Light emitting diode (LED)-recessed downlighting is currently mainly used for indoor lighting of residential and commercial buildings. Maintaining a low junction temperature and increasing the lifetime and reliability of LED lighting is desirable. This study investigated designed heat sinks’ heat dissipation and capability of maintaining a low junction temperature, as well as increases in the lifetime and reliability of the lighting. This paper presents a designed traditional trapezoidal aluminum finned heat sink (ALFHS) and annular open cell copper foam heat sink (CUFHS) mounted to a 10 W compact LED-recessed downlight (CLRDL) and individually installed in a simulation 105 mm × 105 mm × 100 mm (L × W × H) test box. The purpose was to evaluate the temperature performance by testing the downlight in a small enclosed space with high ambient temperature while the LED is in operation. The downlight exhibited a long lifetime at normal use temperature and functioned according to Arrhenius’ law. Numerical simulation was performed first and followed by experimental validation. The heat sink design was the main factor in the heat management of the CLRDL. The heat sinks height was determined using numerical simulation and experimental validation before and after installation (INST) of the ALFHS and CUFHS. The CUFHS height was initially selected based on a larger heat dissipation capacity, and then by determining the ALFHS height. Both the ALFHS and CUFHS with the same height of 17 mm exhibited a similar capacity of heat dissipation before INST. Subsequent to INST, the temperature of the solder point of the ALFHS was higher than that of the CUFHS with an identical height of 17 mm. To compare the heat dissipation behavior of the two heat sinks, the ALFHS height was increased to 23 mm, which is 6 mm higher than that of the CUFHS. The results of a stationary simulation study for junction temperature coincides with the experimental results tested obtained using J-type thermocouples and a data acquisition system. The temperature of the solder point of the CLRDL with both the ALFHS and CUFHS was 91.7 °C after INST, corresponding to an LED junction temperature of 121.7 °C, which is lower than specified 135 °C LED limit temperature. The temperature distributions of specified points under natural convection were evaluated. The CLRDL installed in a small enclosed space withstood the lifetime requirements according to the results of an accelerated life test prediction. The CLRDL lumen maintenance was tested to be more than 25,000 h. The experimental results demonstrate compliance with the Energy Star® requirements for indoor LED lighting fixtures.

Thermal performance of direct illumination high-power LED backlight units with different assembling structures

This work presents a detailed study about the heat dissipation performance of direct illumination high-power light emitting diode (LED) backlight units with two different assembling structures, one of which is traditional and the other is new. The traditional structure, referred to by structure-1, consists of multiple LEDs being directly welded to the printed circuit board (PCB), where the PCB is used as a physical support, an electrical connector and also as a heat dissipation medium. The new structure, referred to by structure-2, places the LEDs directly on the cooling boss; in this case the PCB plays mainly the role of an electrical connector. Thermal characteristics related to the two backlight units are analyzed in terms of thermal resistance network, numerically simulated and experimentally tested. The obtained results by different methods accord with each other reasonably well and all indicate that both structures can meet the requirements of heat dissipation for backlight units at an ambient temperature of 30 °C. Among the two structures, the LED junction temperature of structure-1 backlight unit is 7–8 °C higher and the temperature distribution in the back plane of the backlight unit is also more uniform.

헤드업 디스플레이 휘도 증가를 위한 LED 합성비율과 영상잡음에 대한 연구

본 논문은 디스플레이 휘도를 증가시키기 위하여 적색, 녹색, 청색 고휘도 Light Emitting Diode(LED)를 광원으로 사용하고, Digital Micro Device(DMD) 패널을 적용한 헤드업 디스플레이에서 세 개의 LED 광원 합성 비율과 합성에 따른 양자화 노이즈를 최소화시키는 LED 합성 비율을 연구하였고, 밝기에 영향을 미치는 녹색 LED 구동 비율과 접합 온도에 대하여 분석하였다. 이를 통하여 39%의 LED 중첩 방법을 통하여 황색과 청록색의 2차원 색을 합성하여 33.3%의 휘도 증대 효과를 얻을 수 있었다. 또한 양자화노이즈를 억제하고 영상 화질 측면에서 우수한 색정밀도를 얻을 수 있었다. This paper studies a light emitting diode(LED) overlapping method of a head-up display that consists of a digital micro device(DMD) panel and a red, green, blue LED in order to increase the brightness of display system and optical output power. This optimization overlapping method removes a quantization noise which occur due to LED overlapping too excessive and stabilizes the junction temperature of LED. In order to reduce junction temperature of LED, the a correlation between a green duty and LED overlapping ratio is studied. Throughout this study, the brightness of head-up display exhibited high increasement ratio of luminance around 33.3 percent at 39 percent overlapping method.

Enhancement of heat dissipation of LED module with cupric-oxide composite coating on aluminum-alloy heat sink

A composite coating composed of cupric oxide (CuO) and silicon-based resin was applied to an aluminum-alloy heat sink for a light emitting diode (LED) module. The purpose of the composite coating is to improve the heat dissipation performance of heat sink by enhancing thermal radiation emission. The heat dissipation performance was investigated in terms of LED junction temperature and thermal resistance using a thermal transient method. The CuO and silicon-based resin composite coating showed higher emissivity, and the lower junction temperature and thermal resistance of the heat sink was achieved. In addition, a continuous operation test of the LED chip with the heat sink revealed that the surface treated with the CuO composite coating stably dissipated heat without degradation. In conclusion, the composite coating proposed here showed a significant improvement of the heat dissipation performance of the aluminum-alloy heat sink due to the enhanced thermal radiation property.

<italic>In-Situ</italic> Measurement of Junction Temperature and Light Intensity of Light Emitting Diodes With an Internal Sensor Unit

This study designed and tested an innovative light-emitting diode (LED) chip with a built-in sensor. Two electrically isolated units, the LED (for light emission) and the sensor (for monitoring junction temperature and light intensity), were integrated on a single chip. The sensor unit determines the junction temperature by measuring the forward voltage; the light output power of the LED unit can be precisely extrapolated with a polynomial function based on the photocurrent and junction temperature. This novel structure enables the in-situ real-time monitoring of the LED junction temperature and light output power, which allows a highly detailed and/or in-field LED reliability analysis and provides valuable feedback information for smart LED lighting systems.

Roll-to-roll printed and assembled large area LED lighting element

Novel continuous and mass customizable light-emitting diode (LED) lighting foil system, capable to produce adequate lighting levels for general lighting, was designed, processed, and characterized. Lighting element substrate was processed by roll-to-roll (R2R) printing using silver ink and automatic bonding of LEDs and current regulators on polyethylene terephthalate (PET) substrate using isotropic conductive adhesive (ICA). Demonstrator consisting of two basic lighting elements contained 98 LEDs and produced 860 lm when running with 25 mA operational current through the LEDs when using total electrical driving power of 8.4 W. Measured power conversion efficiency of the demonstrator was 31 % and efficacy 102 lm/W. Element produced 460 lx illumination level measured by an illumination level meter at element’s central axis at distance of 1 m. At a distance of 2 m, illumination level was 110 lx, respectively. Temperature measurements with T3Ster thermal characterization instrument showed that when driving LED with maximum nominal driving current of 100 mA, LED junction temperature was about 120 °C, when lighting element was in air in room temperature. Accelerated environmental stress tests consisting of 500 cycles from −40 … +80 °C in aging oven and 1000 h in +60 °C/95 % RH climate chamber were performed to test samples without any failures. In addition, over 700 bending cycles using 20 mm bending radius were performed to test samples without any failures, so bonding of LEDs were shown reliable according to these tests. Achieved results proved that thin, flexible, and large area high luminous flux lighting elements and systems can be processed based on plastic foil manufactured using R2R silver ink printing and R2R automatic bonding of LEDs and regulator components using ICA on that foil.

Junction temperature measurement of light-emitting diodes using temperature-dependent capacitance

Junction temperature, as one of the most important properties of light-emitting diodes (LEDs), has great impact on LEDs’ power efficiency, luminosity, reliability, life-time, and so on. Precise measurement of junction temperature for LED device is quite important in the research of chip’s fabrication, device packaging and related applications. In this paper, we propose a new approach to measure the junction temperature of LEDs by using temperature-dependent capacitance. The capacitance of white LEDs at room temperature is measured and found to be decreased first and then increased with an increasing reverse bias. Equivalent model using vertical and horizontal capacitances connected in parallel is proposed to qualitatively explain the variation of capacitance under different reverse bias. Result obtained from the model fitting agrees well with the experimental result. The capacitance-temperature (C-T) curve of white LEDs under different reverse bias is measured and analysed. Results show that the capacitance of LEDs is sensitive to temperature at all biases. Under a reverse voltage of 0.5 V, the capacitance has the maximal response of 1.971 pF/℃ and a good linear temperature-dependent property. The C-T curve is used as the calibration for the measurement of junction temperature. By monitoring the change of capacitance of the working LEDs and comparing it with the C-T curve, the junction temperature of the LED device is successfully measured. The junction temperature of a white LED obtained by the proposed C-T method is compared with that by tranditional forward voltage method, and they are in good agreement. The C-T method is also used to measure the real-time junction temperatures of white LEDs under a constant current of 350 mA and a constant voltage of 3.2 V, respectively. In both conditions, the junction temperature of an LED needs approximately 110 sec to rise from room temperature to a steady value, and subsequently needs approximately 500 sec to fall back to room temperature after the LED is turned off. Compared with traditional methods, C-T method only needs to measure one calibration and this calibration can be applied to LEDs working at any current and voltage. Therefore, C-T method is a simple and flexible alternative to the existing technique of temperature measurement in electronic device.

Measurement of junction temperature of light-emitting diodes in a luminaire

There are vast numbers of light-emitting diode (LED) luminaires to choose from, but not all LED luminaires perform reliably. More than half of the input electrical power to a LED luminaire is wasted in the form of heat. Heat management is the one of the most critical issues faced by LED luminaire designers. The long term reliability of a LED luminaire is mainly dependent on the junction temperature of the LEDs. Hence, accurate junction temperature information during luminaire operation is critical for monitoring and assessing the health of the luminaire. In practice, it is extremely difficult to measure the junction temperature of LEDs in modern day luminaires. With the optical system and heat sink surrounding the tiny LED, measurement of junction temperature with direct methods like infrared cameras and thermocouples becomes more complex. This paper explores the possibility of monitoring and measuring the junction temperature of LEDs in a luminaire by making use of the strong correlation between the forward voltage drop at the LED junction and the temperature of that junction. Results of thermal investigations of a LED downlight are presented. The results suggest that the inherent forward voltage/ junction temperature dependency of LEDs can be used to measure and monitor LED junction temperature in a luminaire under operational conditions.

An engineering method to estimate the junction temperatures of light-emitting diodes in multiple LED application

Acquiring the junction temperature of light emitting diode (LED) is essential for performance evaluation. But it is hard to get in the multipleLED applications. In this paper, an engineering method is presented to estimate the junction temperatures of LEDs in multiple LED applications. This method is mainly based on an analytical model, and it can be easily applied with some simple measurements. Simulations and experiments were conducted to prove the feasibility of the method, and the deviations among the results obtained by the present method with those by simulation as well as experiments are less than 2% and 3%, respectively. In the final part of this study, the engineering method was used to analyze the thermal resistances of a street lamp. The material of leadframe was found to affect the system thermal resistance mostly, and the choice of solder material strongly depended on the material of the leadframe.

Numerical simulation of high power LED heat-dissipating system

In this paper, thermal analysis of the heat dissipation under different heat sink for high-power white Light Emitting Diode (LED) is presented. Junction temperature of LED is elevated as the power of LED increases, which brings up deterioration of light efficiency and other side effects. Heat dissipation is another design concern other than material and illumination efficiency. The purpose of this paper is to investigate the cooling of high-power LED chips and modules for design of heat sinks. Three types of heat sinks are designed for a tandem 12-chip module and an extensive numerical investigation of the heat sink design performance is conducted by Computational Fluid Dynamics software Fluent. The effects of heat sink geometry and adhesive material are also investigated. Design variables are the thickness of sink base, number, thickness and length of fins. The total wetted area is the dominant factor to the junction temperature. The objective of design regarding the junction temperatures around 50°C is easily achieved. However, its effect is limited at high values of these parameters, furthermore an excessive number of fins incurs reverse consequence due to problem of ventilation also waste of material.

Experimental and numerical approach on junction temperature of high-power LED

The understanding of thermal resistance and junction temperature is important in the area of designing efficient, long-lasting high-power Light Emitting Diodes (LEDs) and diode stacks. This paper developed a systematic evaluating program for investigating the effect of location and thickness on the thermal resistance and junction temperature of LED on an aluminum substrate. Structure function measurements were implemented by Thermal Transient Tester (T3ster) and Integrating Sphere on LED placed on an aluminum plate. The temperature distribution of LED was analyzed to understand the relationship between thermal resistance and location of the LED on the aluminum base. Meantime, to evaluate the validity of the test, the simulation is developed by considering structure properties. The simulation curve basically has a similarity with the experimental curve in the overall. It implies that the evaluating method can provide guidance in understanding thermal reliability of LED lamps and designing thermal management techniques.

Design Methodology for Light-Emitting Diode Systems by Considering an Electrothermal Model

The knowledge about thermal, electrical, and photometrical characteristics of Light-Emitting Diodes (LEDs) is essential to achieve a good performance to the lighting system. This paper presents a methodology for designing LED systems. The main purpose is to provide an optimal operating point, considering the electrothermal design to achieve the maximum luminous flux for a given LED system. Based on two design routines, the proposed methodology brings the possibility to consider driver output current, heatsink size, and LED junction temperature in the system design. A mathematical analysis taking into account an LED electrothermal model is presented. Computational fluid dynamics simulation employing finite element method and experimental results validate the proposed methodology.

Proposal of New Measurement Method for Internal Quantum Efficiency of Light-Emitting Diodes

A new evaluation method for effective internal optical power (IOP) and internal quantum efficiency (IQE) of light-emitting diodes (LEDs) is demonstrated. This method is based on the optical and thermal properties of LEDs. By using this proposed method, the effective IOP and the IQE of LEDs could be directly extracted from the measurements of external optical power (EOP) and junction temperature of LEDs. This method needs no assumption of the injection efficiency of carriers in the LEDs and no measurement-condition limitation of low current-injection level.

Feasibility analysis of junction temperature measurement for GaN-based high-power white LEDs by the peak-shift method

Transient thermal impedance of GaN-based high-power white light emitting diodes (LEDs) is created using a thermal transient tester. An electro-thermal simulation shows that LED junction temperature (JT) rises to a very low degree under low duty cycle pulsed current. At the same JT, emission peaks are equivalent at pulsed and continuous currents. Moreover, the difference in peak wavelength when a LED is driven by pulsed and continuous currents initially decreases then increases with increasing pulse width. Thus, selecting an appropriate pulse width decreases errors in JT measurement.

Effect of Voiding in Solder Joints on Thermal Performance of the LED

Light-Emitting Diode (LED) lighting has potential to fundamentally alter lighting by replacing incandescent and fluorescent technologies because it offers significant energy saving, longer operational life, and many other advantages. Excellent thermal management and low-cost is vital for LED lighting to become the market standard for general lighting. The packaged LED is typically soldered to the metal-core printed circuit board (MCPCB). However, as the size of the packaged LED increases, voiding in solder joints becomes a major problem since voiding increases the thermal resistance. The research questions are: what is the effect of flux in solder paste and the LED size on the voiding? What is the relationship between the voiding and the thermal performance of the LED? In this study, packaged LEDs with three different sizes were assembled to MCPCBs using solder paste with two different types of fluxes. The voiding in solder joints was imaged by an X-Ray machine. The detailed void characteristics were analyzed. The LED's junction temperature was measured based on the diode's forward voltage junction temperature measurement method. The results show that both flux in solder paste and the LED size have significant effect on the voiding. There is weak positive correlation between the voiding and the thermal performance when the voiding is over 25% coverage. However, the effect of voiding on the thermal performance is insignificance when the voiding is less than 25% coverage.