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.     

SPICE level implementation of physics of filamentation in ESD protection devices

In this work, we build circuit models to understand the physics of electro-thermal instability and associated thermal runway in advanced ESD protection devices under filamentation. The circuit building methodology takes into account, the instability arising out of inhomogeneous triggering of 2 − D planar bipolar and looks into inherent instability causing the 3 − D phenomenon. Subsequently, the electro-thermal coupling is analyzed to get SPICE model, as we develop a physics based methodology to comprehend the 3 − D localization in the device. Furthermore, we understand the instability through appropriate modeling of localization behavior using area factor (α) and related electro-thermal circuit models.     

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.     

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.     

A model reduction approach for constructing compact dynamic thermal models of IGBT-modules of inverters

This paper presents a model reduction approach for constructing lumped RC thermal networks of IGBT-modules of inverters for which heat and subsequent temperature increases vary with time on different scales ranging from nanosecond to second. It was observed that the time-dependent heat and temperature profiles of transistors and diodes of IGBT-modules of inverters oscillate at two frequencies, one in the range 0.1–50 Hz corresponding to the load current modulation, and the other in the range 1–20 kHz corresponding to the switching frequency. The reduction approach consisted of decomposing the module into different elements, each being described with a number of RC cells selected according to the time-constant of the element with regard to the module. The lumped RC thermal networks were found in good agreement with the continuous model by offering a considerably lower computational time on the different time scales. For simplicity, the reduction approach is presented for one-dimensional heat flow through the cross-plane direction of the module.     

Improvement of thermal management of high-power GaN-based light-emitting diodes

The thermal management of high-power light-emitting-diode (LED) devices employing various die-attach materials is analyzed. Three types of die-attach materials are tested, including silver paste, Sn–3 wt.% Ag–0.5 wt.% Cu (SAC305) solder, and SAC305 solder added with a small amount of carbon nanotubes (CNTs). The analysis of thermal management is performed by comparing the temperatures of the LED chips in use and the total thermal resistances of the LED devices obtained respectively from the thermal infrared images and thermal transient analysis. Due to the high thermal conductivity of CNT, the addition of CNTs into the SAC305 solder reduces the total thermal resistance and chip temperature of the LED device, and the thermal management of the LED devices is improved accordingly.     

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.     

Failure and stress analysis of through-aluminum-nitride-via substrates during thermal reliability tests for high power LED applications

The objective of this study is to evaluate the reliability of through-aluminum-nitride-via (TAV) substrate by comparing those experimental results with the finite element simulation associated with measurements of aluminum nitride (AlN) strength and the thermal deformation of Cu/AlN bi-material plate. Two reliability tests for high-power LED (Light emitting diode) applications are used in this study: one is a thermal shock test from − 40 °C to 125 °C, the other is a pressure cook test. Also, the strength of AlN material is measured by using three-point bending test and point load test. The reliability results show that TAV substrates with thicker Cu films have delamination and cracks after the thermal shock test, but there are no failure being found after the pressure cook test. The determined strengths of AlN material are 350 MPa and 650 MPa from three-point bending test and point load test, respectively. The measurement of thermal deformation shows that the bi-material plate has residual-stress change after the solder reflow process, also indicating that a linear finite element model with the stress-free temperature at 80 °C can reasonably represent the stress state of the thermal shock test from − 40 °C to 125 °C without considering Cu nonlinear effect. The further results of the finite element simulation associated with strength data of AlN material have successfully described those of the reliability test.     

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.     

Electro-thermal coupling analysis methodology for RF circuits

In this paper an electro-thermal co-simulation methodology suitable for RF circuits is presented. It circumvents traditional transient simulation drawbacks that arise when signals or magnitudes whose frequencies are separated orders of magnitude are present simultaneously in the simulated circuit. The accuracy of the proposed technique is verified experimentally by comparing simulation and measurements of the thermal coupling between an integrated power amplifier (PA) and a differential temperature sensor embedded in the same silicon die, using a 65 nm CMOS technology.     

Thermal characteristics and fabrication of silicon sub-mount based LED package

In this paper, the cost of a light emitting diode (LED) package is lowered by using a silicon substrate as the base attached to the chip, in contrast to the conventional chip-on-board (COB) package. In addition we proposed an LED package with a new structure to promote reliability and lifespan by maximizing heat dissipation from the chip. We designed an LED package combining the advantages of COB based on conventional metal printed circuit board (PCB) and the merits of a silicon sub-mount as a substrate. When an input current 500–1000 mA was applied, the fabricated LED exhibited the light output of approximately 112 lm/W at 29 W. We also measured and compared the thermal resistance of the sub-mount package and conventional COB package. The measured thermal resistance of the sub-mount package with a reflective film of Ag and the COB package were 0.625 K/W and 1.352 K/W, respectively.     

Effect of ethyl cellulose on thermal resistivity of thixotropic ZnO nano-particle paste for thermal interface material in light emitting diode application

A large amount of heat trapped inside Light Emitting Diode (LED) is the consequence of large thermal resistance between the heat source and the heat sink. Zinc oxide (ZnO) thick film was screen-printed from thixotropic paste that consisted of binder, filler and solvent to act as thermal interface material. Structural, surface morphology, vibrational and thermal properties of the samples were studied by means of Field Emission Scanning Electron Microscopy (FESEM), Atomic Force Measurement (AFM), Fourier Transform Infrared Spectroscopy (FTIR) and Thermal Transient Tester (T3ster). XRD analysis revealed that the formation of hexagonal wurtzite ZnO powder, which is free of hydroxide. FESEM results indicated that 50 wt% of filler loading in the thick film had created longer thermal transportation chain. The surface roughness of thick film displays variation in the range of from 64.8 to 218 nm. The presence of ZnO and binder were confirmed by FTIR spectrum at 518 cm⁻¹ and 668 cm⁻¹ to 2974 cm⁻¹, respectively. Thermal characterization reveals a drop in film’s resistivity with the higher content of filler loading of 50 wt% and 55 wt%. The lowest rise in junction temperature of tested LED is reported to be 14.7 °C of 50 wt% of filler loading.     

Development and characterization of the thermal behavior of packaged cascode GaN HEMTs

This study investigates the heat generation behavior of packaged normally-on multi-finger AlGaN/GaN high electron mobility transistors (HEMTs) that are cascoded with a low-voltage MOSFET (LVMOS) and a SiC Schottky barrier diode (SBD). By foremost carrying out electro-thermal simulation and related thermal measurements with infrared thermography and Raman spectroscopy for basic 5 mm GaN HEMTs, the location of hot spot in operating device can be obtained. Based on the outcome, further packaged cascode GaN HEMT is analyzed. A hybrid integration of the GaN-HEMT, LVMOS, and SiC SBD are assembled on a directly bonded copper (DBC) substrate in the four-pin metal case TO-257 package. The metal plate is used as both the source terminal and heat sink. The analytical results of thermal investigation are confirmed by comparing them with the infrared thermographic measurements and numerical results obtained from a simulation using Ansys Icepak. For a power dissipation of less than 11.8 W, the peak temperature of the GaN HEMTs is 118.7 °C, obtained from thermal measurements.     

A compact broadband MMIC sub-harmonic mixer using quasi-lumped transmission lines

A compact broadband monolithic microwave integrated circuit (MMIC) sub-harmonic mixer using an OMMIC 70 nm GaAs mHEMT technology is demonstrated for 60 GHz down-converter applications. The present mixer employs an anti-parallel diode pair (APDP) to fulfill a sub-harmonic mixing mechanism. Quasi-lumped components are employed to broaden the operational bandwidth and minimize the chip size to 1.5×0.77 mm². The conversion gain is optimized by a quasi-lumped 90° phase shift stub. Experimental results show that from 50 GHz to 70 GHz, the conversion gain varies between −12.1 dB and −15.2 dB with a LO power level of 10 dBm and 1 GHz IF. The LO-to-RF, LO-to-IF and RF-to-IF isolations are found to be greater than 19.5 dB, 21.3 dB and 25.8 dB, respectively. The second harmonic component of the LO signal is suppressed. The proposed mixer has an input 1 dB compression point of -2 dBm and exhibits outstanding figure-of-merits.     

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.     

Thermoelectrical modelling and simulation of devices based on VO2

Limits of development of conventional silicon-based integrated circuits get closer. More and more effort is done to develop new devices for integrated circuits. A promising structure is based on the semiconductor-to-metal phase change of vanadium-dioxide at about 67 °C. In these circuits the information is carried by combined thermal and electrical currents. For device modelling and circuit design, accurate distributed electro-thermal transient simulation is mandatory. This paper is the first one to present an electro-thermal transient simulation method for VO₂ devices operating in real-world conditions. The paper presents three VO₂ material models, the algorithmic extension of an electro-thermal field simulator to be able to handle hysteresis and the transient simulation issues of VO₂ and the modelling of VO₂ based devices. The paper compares measured and simulated device characteristics.     

The influence of microwave pulse width on the thermal burnout effect of a PIN diode limiting-amplifying system

This paper analyzes the influence of the microwave pulse width on the thermal burnout effect of a PIN diode limiting-amplifying system. Based on theoretical analyses and simulation, the relationship of the burnout effect on a PIN diode limiter and a PIN diode limiting-amplifying system is obtained first. By adopting an absorption efficiency factor, the theoretical model of the relationship between the microwave pulse width and the burnout power threshold for the PIN diode limiting-amplifying system is obtained. The proposed theoretical formulas can be determined by using at least two sets of simulation or measurement results to fit the constant coefficients, which can greatly reduce the simulation or experimental costs. The results obtained by the theoretical formulas are in good agreement with the numerical simulation results obtained by our self-designed device–circuit joint simulator, which verifies the correction of the theoretical analyses and modeling. The available microwave pulse width range for the proposed theoretical formulas is from 10 ns to 10 μs. In consideration of the potential threat of microwave pulses, the system-level study results obtained in this paper will be helpful for the design of the radio frequency receivers.     

A broadband and compact asymmetrical backward coupled-line coupler with high coupling level

A new wideband asymmetric microstrip coupled-line coupler with 3 dB coupling value and quadrature phase difference is presented. Compared with the conventional edge-coupled couplers, this structure, consisting of two different transmission lines (interdigital and conventional microstrip transmission lines) as coupled lines, achieves wider operating bandwidth and larger coupling level. The coupled-line length of the proposed structure is approximately λ_g/4. To characterize the structure, an equivalent circuit model has been established. A 3 dB designed and fabricated coupler with 0.2 mm spacing between coupled lines exhibits an amplitude balance of 2 dB from 2.2 GHz to 4.2 GHz. Good agreements between the full-wave simulation and equivalent circuit model results has been achieved and verified the effectiveness of the proposed circuit model. Also, measurement results have been presented.     

Design of a highly PAE class-F power amplifier using front coupled tapered compact microstrip resonant cell

This article presents the design and implementation of a class-F power amplifier (PA) with a low voltage pHEMT, using a novel Front Coupled Tapered Compact Microstrip Resonant Cell (FCTCMRC) for obtaining a high-efficiency performance. The FCTCMRC is used as a harmonic control circuit, which is short and open circuit for the second and third harmonics, respectively. The required dc-supply voltage is low due to application of a low-voltage pHEMT in the circuit implementation. Therefore, the class-F power amplifier is designed with a high power added efficiency (PAE) and compact circuit size. To verify the method, the designed class-F PA is fabricated using a pHEMT at 1.1 GHz. The proposed class-F power amplifier using the FCTCMRC has obtained 86%PAE under 10 dBm input power, which achieves 16% improvement, also, the circuit size including the harmonic control circuit and output matching is decreased about 25%, all in comparison with the designed PA using the conventional CMRC. The measurement results of the fabricated power amplifier are in good agreement with the simulation results, which verifies the proposed design methodology.     

Fluid–solid coupling thermo-mechanical analysis of high power LED package during thermal shock testing

The virtual design by numerical simulation to model various accelerated reliability testing conditions is adopted to validate and improve the reliability of the high power LED package. In this study, the reliability of the high power LED package during thermal shock testing is investigated by fluid–solid coupling thermo-mechanical modeling by considering nonlinear time and temperature dependent material properties. Through fluid–solid coupling transient thermal transfer analysis, it is found that the maximum thermal gradient exceeds 75 K during the rapid cooling process and 91 K during the rapid heating process of the thermal shock testing which is ignored in the traditional isothermal assumption. The calculation results indicate that the equivalent plastic strain range of the bonding wire within the LED package with consideration of the temperature gradient is much higher than that with the isothermal assumption. The assumption of the isothermal condition is not appropriate which will lead to overestimation of the predicted lifetime. The viscoelastic behaviors of the silicone have significant influences on the lifetime prediction of the bonding wire and silicone with low elastic modulus and coefficient of thermal expansion (CTE) can significantly enhance the reliability of the bonding wire under the thermal shock loading. The results in this study could provide a guideline on design for reliability in the high power LED packaging.