Showing papers on "Junction temperature published in 2008"

Accelerated Life Test of High Brightness Light Emitting Diodes

Abstract: Short-term accelerated life test activity on high brightness light emitting diodes is reported. Two families of 1-W light-emitting diodes (LEDs) from different manufacturers were submitted to distinct stress conditions: high temperature storage without bias and high dc current test. During aging, degradation mechanisms like light output decay and electrical property worsening were detected. In particular, the degradation in light efficiency induced by thermal storage was found to follow an exponential law, and the activation energy of the process was extrapolated. Aged devices exhibited a modification of the package epoxy color from white to brown. The instability of the package contributes to the overall degradation in terms of optical and spectral properties. In addition, an increase in thermal resistance was detected on one family of LEDs. This increase induces higher junction temperature levels during operative conditions. In order to correlate the degradation mechanisms and kinetics found during thermal stress, a high dc current stress was performed. Results from this comparative analysis showed similar behavior, implying that the degradation process of dc current aged devices is thermal activated due to high temperatures reached by the junction during stress. Finally, the different effects of the stress on two families of LEDs were taken into account in order to identify the impact of aging on device structure.

Forced convective interlayer cooling in vertically integrated packages

Abstract: The heat removal capability of area-interconnect-compatible interlayer cooling in vertically integrated, high-performance chip stacks was characterized with de-ionized water as coolant. Correlation-based predictions and computational fluid dynamic modeling of cross-flow heat-removal structures show that the coolant temperature increase due to sensible heat absorption limits the cooling performance at hydraulic diameters les 200 mum. An experimental investigation with uniform and double-side heat flux at Reynolds numbers les 1000 and heat transfer areas of 1 cm2 was carried out to identify the most efficient interlayer heat-removal structure. Parallel plate, microchannel, pin fin, and their combinations with pins using in-line and staggered configurations with round and drop-like shapes at pitches ranging from 50 to 200 mum and fluid structure heights of 100 to 200 mum were tested. A hydrodynamic flow regime transition responsible for a local junction temperature minimum was observed for pin fin inline structures. The experimental data was extrapolated to predict maximal heat flux in chip stacks with a 4-cm2 heat transfer area. The performance of interlayer cooling strongly depends on this parameter, and drops from >200 W/cm2 at 1 cm2 and >50 mum interconnect pitch to <100 W/cm2 at 4 cm2.

High power light-emitting diode junction temperature determination from current-voltage characteristics

Abstract: Optical and electrical characteristics of power light-emitting diodes (LEDs) are strongly dependent on the diode junction temperature. However, direct junction temperature determination is not possible and alternative methods must be developed. Current-voltage characteristics of commercial high power LEDs have been measured at six different temperatures ranging between 295 and 400 K. Modeling these characteristics, including variation in the bandgap with temperature, revealed a linear temperature dependence of the forward voltage if the drive current is chosen within a rather limited current range. Theoretically, the voltage intercept can be deduced from the bulk semiconductor bandgap. However, accurate junction temperature determination is only possible if at least two calibration measurements at a particular drive current are performed. The method described in this paper can be applied to calculate the thermal resistance from the junction to any other reference point for any particular LED configuration.

Comprehensive system-level optimization of thermoelectric devices for electronic cooling applications

Abstract: Advanced cooling solutions are needed to address the growing challenges posed by future generations of microprocessors. This paper outlines an optimization methodology for electronic system based thermoelectric (TE) cooling. This study stresses that an optimum TE cooling system should keep the electronic device below a critical junction temperature while utilizing the smallest possible heat sink. The methodology considers the electric current and TE geometry that will minimize the junction temperature. A comparison is made between the junction temperature minimization scheme and the more conventional coefficient of performance (COP) maximization scheme. It is found that it is possible to design a TE solution that will both maximize the COP and minimize the junction temperature. Experimental measurements that validate the modeling are also presented.

Improvement of High Heat Flux Measurement Using a Null-Point Calorimeter

Abstract: K ENNEDYet al.first proposed transient heatfluxmeasurements using swept null-point calorimetry in 1972 [1]. They published a copper sensor design, the form of which is still in use. It is shown in Fig. 1. The interior design of the sensor is shown in Fig. 2. The heat flux is estimated from a temperature measurement. The location of the temperature measurement is chosen such that the null-point scenario is given [2,3]. This means that the measured temperature history at the backside, i.e., the null point, is assumed to be identical to the surface temperature history. Then, the temperature at the null point can be inserted into a one-dimensional inverse heat conduction problem for a semi-infinite solid to determine the heat flux at the surface. The theoretical assumption of one-dimensional and linear heat conduction is strengthened by an appropriate sensor design (see Fig. 2). Because of the high enthalpy flow condition, the essentially uncooled sensors have to be passed very quickly across the flow diameter. The sensors in the current configuration are moved with a velocity of 1 m=s across the flow. Considering the plasma flow diameter of about 30mm, one heat fluxmeasurement is performed in 0.03 s, which is about half the time the sensor is supposed to be used [1,4]. However, the recorded temperature profile allows to deduce the heat flux along the measured direction, i.e., perpendicular to the flow axis, which is the radial profile. For the temperaturemeasurement itself, there are historically three different approaches: coaxial surface thermocouples, thin-film resistance thermometers, and null-point calorimeters. As mentioned, the fundamental assumption in the heat flux estimation using such devices is that of linear one-dimensional heat conduction. If a homogeneous temperature between the probe surface and the measurement location during the experiment can be assumed, the thin-film theory is applied and the heat diffusion problem can be solved analytically [5]. In contrast, the thick-film theorymeans that it is assumed that the temperature at the opposite end of the surface which is exposed to the heat flux rests constant throughout the experiment, which corresponds to semi-infinite behavior and leads also to an analytical solution. Usually, the thin-film theory is applied to resistance thermometers and the thick-film theory to surface thermocouples and null-point calorimeters. To characterize the plasma flow, the radial profile of heat flux has to be measured, which is only applicable to sensors according to the thick-film theory [4]. However, as will be shown later, the especially short measurement times lead to the conclusion that the one-dimensional conditions are no longer valid. Moreover, the thermocouple’s junction never reaches a homogeneous temperature level during exposure time, which means that the measured signal, that is a voltage drop, cannot be related to the junction temperature using the thermoelectric calibration as it is proposed up to now. Hence, this well-known measurement technique has to be further investigated to adapt it to Received 27 March 2007; revision received 8 October 2007; accepted for publication 22 October 2007. Copyright © 2007 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0022-4650/08 $10.00 in correspondence with the CCC. ∗Research Engineer, Laboratoire TREFLE, Esplanade des arts et metiers, 33405 Talence; stefan.loehle@dlr.de. Member AIAA. Professor, Laboratoire TREFLE. Research Engineer, Avenue du General Niox. Research Engineer, Route des Gargails. JOURNAL OF SPACECRAFT AND ROCKETS Vol. 45, No. 1, January–February 2008

Junction temperature prediction method and apparatus for use in a power conversion module

Abstract: A method and apparatus for predicting junction device temperature of at least a first switching device in a power conversion module that includes the first switching device and at least a second switching device, the method comprising the steps of identifying a cross thermal impedance value indicative of how the temperature of the second switching device effects the first switching device temperature an using the cross thermal impedance value to predict the temperature of the at least a first switching device.

Power cycling induced failure mechanisms in the viewpoint of rough temperature environment

Abstract: Applications of power electronic devices in hybrid electric cars demand a higher allowed junction temperature of the power devices, if compact inverters close to the engine which use the combustion engine cooling circuit (with water temperatures up to T=115deg C) are required. Main challenge is the packaging technology. This paper reports on different power cycling tests, failure mechanisms and possible improvements.

Maximizing performance of an electronic device by maintaining constant junction temperature independent of ambient temperature

Abstract: Methods and apparatus are disclosed for maximizing performance of an electronic device, such as a display device. Aspects of the exemplary embodiments include operating the electronic device by adjusting power to the device to maintain a predetermined junction temperature of the device independent of the ambient temperature at which the device is operating.

10 kV SiC MOSFET Based Boost Converter

Abstract: W kV SiC MOSFETs are currently under development by a number of organizations in the United States with the aim to enable their applications in high voltage high frequency power conversion applications. The aim of this study is to demonstrate their high frequency high temperature operation capability in the application of a DC/DC boost converter. A DC/DC boost converter based on a 10 kV 10 A SiC MOSFET and a 10 kV 5A Junction Barrier Schottky (JBS) diode is designed and tested for continuous conditions up to a switching frequency of 25 kHz, an output voltage of 4 kV, an output power of 4 kW and a junction temperature of 174degC for the SiC MOSFET. In the steady state of the 20 kHz boost converter operation, the input power is 4335 W, the output power is 4030 kW and the efficiency is 93%. The power loss analysis shows the total power loss in the 30.45 mm2 SiC MOSFET is 115 W, and the operating junction temperature of the SiC MOSFET is 140degC at the 20 kHz switching frequency. The power losses and the junction temperature of the SiC MOSFET as a function of the switching frequency, load current and input voltage in the boost converter are investigated extensively. The fast switching, low loss and high temperature operation capability of 10 kV SiC MOSFETs demonstrated in the DC/DC boost converter make them attractive in high frequency high voltage power conversion applications.

Experimental and Numerical Investigation of a Microjet-Based Cooling System for High Power LEDs

Abstract: The optical extraction efficiency and reliability of light emitting diodes (LEDs) relies heavily on successful thermal management due to their inherit dependence on the low junction temperature of LED chips. In this paper, a microjet-based cooling system is proposed for the thermal management of high power LEDs. Experimental and numerical investigations on such an active cooling system were conducted. Thermocouples were packaged with LED chips to conduct an online measurement of the temperature and evaluate the cooling performance of the proposed system. The experimental results demonstrate that the microjet-based cooling system has good cooling performance. For a 2 × 2 LED chip array, when the input power is 5.6 W and the environmental temperature is 28°C, the temperature of the 2 × 2 LED chip array reaches 72°C within 2 minutes and continues to increase sharply if no active cooling technique is applied. By using the proposed cooling system to cool the LEDs, however, the maximum LED temperature measured ...

Measurement of the junction temperature in high-power light-emitting diodes from the high-energy wing of the electroluminescence band

Abstract: By using pulsed driving currents with a small duty cycle, the high-energy wing of the electroluminescence band in AlGaInP and InGaN high-power light-emitting diodes (LEDs) was calibrated to measure the junction temperature in the range of 223–358K. In a red AlGaInP LED with a thick active layer, an accuracy of 2% was achieved for the junction temperature derived from the high-energy slope in the spectral range free from parasitic absorption by taking into account the three-dimensional density of band states. Meanwhile, the far high-energy region of the slope distorted by parasitic absorption can be used for the extraction of the junction temperature by using only an appropriate linear correction procedure (∼7% accuracy). In a blue InGaN LED with multiple-quantum-well active layers, the junction temperature can be determined with an accuracy of 2% from the inverse derivative of the spectra in a narrow spectral region ∼150meV above the peak energy by using a linear correction.

CMOS temperature sensors - concepts, state-of-the-art and prospects

Abstract: The paper reviews the state-of-the-art in IC temperature sensors. It starts by revisiting the semiconductor theory of thermodiodes and thermotransistors, continues with the introduction of IC temperature sensors, the concepts of VPTAT - voltage proportional to absolute temperature and IPTAT (current proportional to absolute temperature) and discusses the possibility of use of parasitic bipolar transistors as temperature sensors in pure CMOS technology. The next section demonstrates the very high operating temperature of a dasiaspecialpsila thermodiode, well beyond the typical IC silicon junction temperature. This is achieved with a diode embedded in an SOI CMOS micro-hotplate. A discussion on the temperature limits of integrated temperature sensors is also given. The final section outlines the prospects of IC temperature sensors.

Thermal Analysis of LED Arrays for Automotive Headlamp With a Novel Cooling System

Abstract: In this paper, we report the thermal performance of a light-emitting diode (LED) headlamp module with a novel cooling system. An air-circulating cooling system was design for the LED headlamp module. The precise fluid field modeling and heat transfer analysis using computational fluid dynamics were performed according to the practical working conditions for the headlamp. The junction temperatures of LEDs were found to decrease by using the air-cooling system and, thus, improved the heat dissipating capability of the LED array. The junction temperature of the LED array was decreased from 70.6degC to 30.25degC when the circulating speed of the air increased from 0 to 120 km/h. Also, the temperature decrease of 2degC ~ 4degC was obtained by using fins. By thermal analysis, the cooling system of LED arrays for the headlamps was found to be feasible. Also, the reliability of the headlamp with LED arrays can be improved with a good cooling system.

Diamond Heat Spreader Layer for High-Power Thin-GaN Light-Emitting Diodes

Abstract: A diamond-coating layer is studied as a heat spreading layer for a thin-GaN light-emitting diode (LED) chip. Our results show that this diamond layer can effectively spread the heat and improve the temperature uniformity of a thin-GaN LED chip, which enhances the heat dissipation efficiency down to the heat sink. With the diamond heat spreading layer, the junction temperature of the thin-GaN LED was reduced by 20 C at 1-A current input and the uniformity of the temperature distribution is also greatly improved.

Junction Temperature Measurements and Thermal Modeling of GaInN/GaN Quantum Well Light-Emitting Diodes

Abstract: We present a junction temperature analysis of GaInN/GaN quantum well (QW) light-emitting diodes (LEDs) grown on sapphire and bulk GaN substrate by micro-Raman spectroscopy. The temperature was measured up to a drive current of 250 mA (357 A/cm2). We find better cooling efficiency in dies grown on GaN substrates with a thermal resistance of 75 K/W. For dies on sapphire substrates we find values as high as 425 K/W. Poor thermal performance in the latter is attributed to the low thermal conductivity of the sapphire. Three-dimensional finite-element simulations show good agreement with the experimental results, validating our thermal model for the design of better cooled structures.

Devices And Methods For LED Life Test

Abstract: A life test device comprises an oven, a current source, a voltage meter, a control module, and a process module. A light-emitting diode (LED) is disposed in the oven. The temperature of the oven is gradually changed in a first period and remains at a set temperature in a second period. The current source provides a first current and a second current to the LED. The voltage meter measures forward voltages of the LED. The control module controls the current source to output the first or second current to the LED and controls the voltage meter to measure the forward voltages of the LED. The process module calculates a junction temperature of the LED according to the forward voltages and a variation relationship formula between the forward voltages and the temperature of the oven.

A Physics-of-failure based Prognostic Method for Power Modules

Abstract: This paper describes a physics of failure (PoF) based prognostic method for power electronic modules. This method allows the reliability performance of power modules to be assessed in real time. A compact thermal model was firstly constructed to investigate the relationship between the power dissipation and the temperature in the power module. Such relationship can be used for fast calculation of junction temperature and the temperatures at each interface inside power modules. The predicted temperature profile was then analyzed using a rainflow counting method so that the number of thermal cycles with different temperature ranges can be calculated. A reduced order thermo-mechanical model was also constructed to enable a fast calculation of the accumulated plastic strain in the solder material under different loading conditions. The information of plastic strains was then used in the lifetime prediction model to predict the reliability of the solder interconnect under each regular loading condition. Based on the linear damage rule and the number of cycles calculated from the rain flow counting algorithm, the accumulated damage in the power module over the whole period of usage can be predicted. As a demonstration, this method has been applied to a typical IGBT half bridge module used in aircraft applications.

GaN HEMT thermal behavior and implications for reliability testing and analysis

Abstract: GaN HEMT reliability evaluation in a typical Arrhenius manner requires establishing peak junction temperature for a particular stress condition. Several new techniques have yielded promising results toward establishing peak temperature for these devices in combination with detailed physical modeling, particularly micro-Raman imaging. This paper compares results from finite element modeling to measurements by infrared imaging and micro-Raman imaging. The limitations of IR imaging were confirmed similar to earlier reports. Two techniques for establishing temperature from micro-Raman measurements were used to reveal excellent correlation to the model, and also provide insight into the relationship between temperature and structural change in the device. Temperature modeling data is reported for base plate temperature from 85°C to 250°C for practical GaN HEMT devices. Implications of the measurements for GaN HEMT reliability stress testing and analysis will be discussed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Power electronic module igbt protection method and system

Abstract: A power electronics device with an improved IGBT protection mechanism is provided. More specifically, systems and methods are provided for reducing the switching frequency of an inverter module based on the junction temperature variation of the IGBT.

Comparison Among Power LEDs for Automotive Lighting Applications

Abstract: The first LED diode was created in 1969. Since then, they have been experiencing a continuous improvement in luminous efficiency and power dissipation, achieving current AIInGaP and InGaN devices for yellow to red, and blue to green, including white, colours respectively. The improvement experienced in both luminous efficiency and power dissipation makes them interesting for lighting application in automotive industry, where the advantages offered are numerous, from a lower time response in brake lights to an operating life much higher than that of the vehicle itself [1], [2]. In the other hand, the improvement experienced in luminous efficiency and power dissipation allows the construction of most of the signal devices using just a few emitters [1]. Due to the points stated above, a comparison among the most efficient red, amber/yellow and white Power LEDs currently available from the leading manufacturers is proposed, offering a comparison in terms of luminous efficiency in nominal test conditions, thermal resistance between the junction and the slug and also giving a comparison about the performance depending on junction temperature between the two most efficient LEDs for each colour.

Thermal analyses and measurements of low-Cost COP package for high-power LED

Abstract: The high-power light emitting diode (LED), which features low-power consumption, longer life time and shorter response time, has a potential to replace the conventional general lighting, such as incandescent and fluorescent lamps. However, the LED issues, associated with high cost, high junction temperature, low luminous efficiency, and low reliability, have to be solved before gaining more market penetration. With special features of low-junction- temperature and low-cost design, a novel package for high- power LED, so called COP (chip on plate) package, is proposed in this study. The thermal behaviors of the COP package with and without a heat sink are investigated by experimental measurements (with LED junction temperature tester and thermal couples), a thermal resistance circuit (TRC) method, a finite element method (FEM) and a computational fluid dynamics (CFD) approach. The junction temperature (Tj) of the COP package was measured by the junction temperature tester and found to be comparable with those from commercial products, such as Cree's, and Lumiled's packages. Furthermore, the TRC and FEM were used for addressing the thermal fields of the COP package with and without a heat sink. The results of the thermal fields including the Tj from the experiments, FEM and TRC were found to be reasonably consistent under various input powers for the COP package, but not for the package with a heat sink. Moreover, the under-estimated thermal fields of the package with a heat sink from both FEM and TRC analyses were evaluated again by the CFD approach. The results indicate that the heat convection coefficients on the heat sink used the FEM and TRC analyses are higher than those calculated from the CFD. Finally, the reasonable and validated FEM and TRC models were used for parametric studies and their results show that the thermal conductivities of the die attach, chip substrate and package substrate (rather than the heat sink, chip, thermal grease and encapsulant) have an obvious effect on the Tj. In addition, for reducing the Tj, increasing the radius of the heat sink was found to be more beneficial than increasing the height.

Analysis of Thermal Performance of High Power Light Emitting Diodes Package

Abstract: This paper reports on the thermal characteristics of the high power LED package. The increment of input power generates more heat in the chip, decreasing the luminance and life span of LEDs. To enhance the efficiency of high power LEDs, challenges related to thermal management need to be addressed. In this research, a detailed finite element model of the high power LED package with proper input power and boundary condition is established using the ANSYS@ finite element analysis program. The applied input power is 1W on GaN, and the convection coefficient is adopted from William's experimental results. Radiation is also included in the FEM model. Additionally, forward voltage methods used to indirectly measure the junction temperature are also performed to validate the finite element model with predicted input power. The simulation results closely match the experimental data, with only 5% error. Various thermal performances under different design parameters of the high power LED package are developed following verification of the simulation analysis. Five design factors including (a) the substrate of the chip, (b) the thickness of the die attach material (c) the electro-optical conversion efficiency (d) the thickness of the copper slug and (e) the area of the copper slug are chosen to determine themost dominant factor in this study. The factorial design provides a guide line for the compromise between thermal enhanced design and manufacturing process in the future.

Heat sinks for cooling LEDs in projectors

Abstract: According to certain embodiments, an apparatus for cooling light emitting diodes (LEDs) in projectors includes one or more first LEDs, one or more first heat sinks, one or more second LEDs, and one or more second heat sinks. The first LEDs are configured to generate light, and each first LED has a first desired junction temperature. The first heat sinks are configured to dissipate heat generated by the first LEDs. The second LEDs are configured to generate light, and each second LED has a second desired junction temperature that is higher than the first desired junction temperature. The second heat sinks are configured to dissipate heat generated by the one or more second LEDs. A cooling fan directs air flow over the first heat sinks and the second heat sinks, and an exhaust vent enables the air flow over the first heat sinks and the second heat sinks.

Fabrication and Thermal Analysis of Wafer-Level Light-Emitting Diode Packages

Abstract: Wafer-level packaged light-emitting diodes (LEDs) are useful for the high-power applications such as back light unit and general solid-state lighting due to the compactness and integrated fabrication process. In this letter, wafer-level packaged LEDs with red, green, and blue multichips were fabricated, and the thermal characteristics of wafer-level packaged LEDs with multichips such as thermal resistance and junction temperature are investigated using both serial and matrix measurement methods.

Design consideration of high temperature SiC power modules

Abstract: SiC power semiconductors can safely operate at a junction temperature of 500degC. Such a high operating temperature range can substantially relax or completely eliminate the need for bulky and costly cooling components commonly used in silicon-based power electronic systems. However, a major limitation to fully realizing the potential of SiC and other wide band-gap semiconductor materials is the lack of qualified high-temperature packaging systems, particularly those with high-current and high-voltage capabilities required for power conversion applications. This paper proposes a new hybrid power module architecture that allows wide bandgap semiconductor power devices to operate at a junction temperature of 300degC. The concept is based on the use of double metal or DCB leadframes, direct leadframe-to-chip bonding, and high temperature encapsulation materials. The leadframes, serving as both the external leads and the internal interconnect to the semiconductor chips, need to provide excellent high temperature stability, adequate electrical and thermal conductivity, and a coefficient of thermal expansion (CTE) closely matching that of SiC. The SiC chips are sandwiched between and bonded to the top and bottom leadframes using a brazing or adhesion process. Extensive electrical, thermal, and mechanical modeling has been performed on this new concept. Several prototypes are fabricated, and a finite element model is evaluated. Packaging architecture and materials considerations are discussed.

Dependence of Emission Spectra of LEDs upon Junction Temperature and Driving Current

Abstract: In this work, an empirical model for the emission spectra of LEDs is proposed to describe some crucial optical properties of LEDs. As well known, the junction temperature and the electric driving current are two most important factors of all. According to the phenomenal observation, the empirical model for the emission spectra of LEDs is established to simultaneously include the effects from both the junction temperature and the electric driving current. As a result, the empirical model can be applied for the prediction of the chromatic shifts due to the drift of the operating junction temperature. In addition, the stabilization of the chromatic characteristics is obvious.

Dynamic Thermal Analysis of High-Power LEDs at Pulse Conditions

Abstract: In this letter, the thermal evaluation of high-power LED packages at pulse conditions was reported. A theoretical calculation model was proposed based on the analogy between the thermal and electrical RC circuits. The thermal performance of LED packages driven by pulse input was calculated using the RC network extracted from transient thermal measurement. The junction temperature fluctuation band decreases with the frequency at certain duty cycles. The saturated average junction temperature rise linearly increases with the duty cycle at certain frequencies. These predictions were verified by the real-time junction temperature measurement using the peak shift method at pulse conditions. The theoretical model was found to be effective and applicable to the evaluation of the thermal performance of LEDs working at pulse conditions.