James T. Petroski

Thermal Management of LEDs: Package to System

Light emitting diodes, LEDs, historically have been used for indicators and produced low amounts of heat. The introduction of high brightness LEDs with white light and monochromatic colors have led to a movement towards general illumination. The increased electrical currents used to drive the LEDs have focused more attention on the thermal paths in the developments of LED power packaging. The luminous efficiency of LEDs is soon expected to reach over 80 lumens/W, this is approximately 6 times the efficiency of a conventional incandescent tungsten bulb. Thermal management for the solid-state lighting applications is a key design parameter for both package and system level. Package and system level thermal management is discussed in separate sections. Effect of chip packages on junction to board thermal resistance was compared for both SiC and Sapphire chips. The higher thermal conductivity of the SiC chip provided about 2 times better thermal performance than the latter, while the under-filled Sapphire chip package can only catch the SiC chip performance. Later, system level thermal management was studied based on established numerical models for a conceptual solid-state lighting system. A conceptual LED illumination system was chosen and CFD models were created to determine the availability and limitations of passive air-cooling.

Thermal challenges in the future generation solid state lighting applications: light emitting diodes

Light emitting diodes, LEDs, historically have been used for indicators and produced low amounts of heat. The introduction of high brightness LEDs with white light and monochromatic colors have led to a movement towards specialty and general illumination applications. The increased electrical currents used to drive the LEDs have focused more attention on the thermal paths in the level-1 packages and developments in LED power packaging. The luminous efficiency of LEDs is expected to reach over 80 lumens/watt, that is approximately 6 times more than one tungsten bulb. The thermal challenges of these products in many applications will open new research areas for engineers from chip level to system level thermal management.

Optimization of Piezoelectric Oscillating Fan-Cooled Heat Sinks for Electronics Cooling

Piezoelectric fans have been investigated for electronics cooling over the last decade. The primary usage or method has been to place the vibrating fan near the surface to be cooled. The piezofan used in the current study is composed of a piezo actuator attached to a flexible metal beam. It is operated at up to 120-VAC and at 60 Hz. While most of the research in the literature focused on cooling bare surfaces, larger heat transfer rates are of interest in the present study. A system of piezoelectric fans and a heat sink is presented as a more efficient method of system cooling with these fans. In this paper, a heat sink and piezoelectric fan system demonstrated a cooling capability of 1 C/W over an area of about 75 cm2 where electronic assemblies can be mounted. The heat sink not only provides surface area, but also flow shaping for the unusual 3-D flow field of the fans. A volumetric coefficient of performance (COPv) is proposed, which allows a piezofan and heat sink system volume to be compared against the heat dissipating capacity of a similar heat sink of the same volume for natural convection. A piezofan system is shown to have a COPv of five times that of a typical natural-convection solution. The paper will further discuss the effect of nozzles in flow shaping obtained via experimental and computational studies. A 3-D flow field of the proposed cooling scheme with a piezofan is obtained via a flow visualization method. Velocities at the heat sink in the order of 1.5 m/s were achieved through this critical shaping. Finally, the overall system characterization to different heat loads and fan amplitudes will be discussed.

Chip to System Levels Thermal Needs and Alternative Thermal Technologies for High Brightness LEDS

Light emitting diodes (LEDs) historically have been used for indicators and produced low amounts of heat. The introduction of high brightness LEDs with white light and monochromatic colors has allowed them to penetrate specialty and general illumination applications. The increased electrical currents used to drive the LEDs have resulted in higher heat fluxes than those for average silicon integrated circuits (i.e., ICs). This has created a need to focus more attention on the thermal management engineering of LED power packages. The output of a typical commercial high brightness, 1 mm 2 , LED has exceeded 100 1m at drive levels approaching 3 W. This corresponds to a heat fiux of up to 300 W/cm 2 . Novel thermal solutions need to address system architectures, packaging, phosphors for light color conversion, and encapsuianfs and fillers for optical extraction. In this paper, the effect of thermal management on packaging architectures, phosphors, encapsulants, and system design tire discussed. Additionally, discussions of microscopic defects due to packaging problems as well as chip active layer defects are presented through experimental and computational findings.

Energy Efficiency of Low Form Factor Cooling Devices

With increasing attention to the energy efficiency of consumer and commercial products, thermal engineering and science community is devoting greater effort and attention to the design and implementation of energy-efficient cooling solutions. This study focuses on the cooling potential and Coefficient of Performance, (COP), achievable with three distinct meso-scale cooling technologies, applicable to a wide range of electronics cooling challenges. The thermo-fluid and thermodynamic characteristics of synthetic jets, piezo-driven vibrating blades, and compact muffin fans will be addressed. We are dedicating this paper to Prof. Kakac for his contributions to heat transfer science and technology, developing young scientists, writing highly valuable heat transfer textbooks, and most importantly for his kindness and friendship.Copyright

Advanced Natural Convection Cooling Designs for Light-Emitting Diode Bulb Systems

The movement to light-emitting diode (LED) lighting systems worldwide is accelerating quickly as energy savings and reduction in hazardous materials increase in importance. Government regulations and rapidly lowering prices help to further this trend. Todays strong drive is to replace light bulbs of common outputs (60 W, 75 W, and 100 W) without resorting to compact fluorescent (CFL) bulbs containing mercury while maintaining the standard industry bulb size and shape referred to as A19. For many bulb designs, this A19 size and shape restriction forces a small heat sink which is barely capable of dissipating heat for 60 W equivalent LED bulbs with natural convection for todays LED efficacies. 75 W and 100 W equivalent bulbs require larger sizes, some method of forced cooling, or some unusual liquid cooling system; generally none of these approaches are desirable for light bulbs from a consumer point of view. Thus, there is interest in developing natural convection cooled A19 light bulb designs for LEDs that cool far more effectively than todays current designs. Current A19 size heat sink designs typically have thermal resistances of 5–7 °C/W. This paper presents designs utilizing the effects of chimney cooling, well developed for other fields that reduce heat sink resistances by significant amounts while meeting all other requirements for bulb system design. Numerical studies and test data show performance of 3–4 °C/W for various orientations including methods for keeping the chimney partially active in horizontal orientations. Significant parameters are also studied with effects upon performance. The simulations are in good agreement with the experimental data. Such chimney-based designs are shown to enable 75 W and 100 W equivalent LED light bulb designs critical for faster penetration of LED systems into general lighting applications.

Piezoelectric Fans: Heat Transfer Enhancements for Electronics Cooling

Piezoelectric fans have been investigated for electronics cooling over the last decade. The primary usage or method has been to place the vibrating fan near the surface to be cooled. The piezofan used in the current study is composed of a piezo actuator attached to a flexible metal beam. It is operated at up to 120VAC and at 60 Hz. While most of the research in the literature focused on cooling bare surfaces, larger heat transfer rates are of interest in the present study. A proposed system of piezoelectric fans and heat sink is presented as a more efficient method of system cooling with these fans. In this paper, a heat sink and piezoelectric fan system demonstrated a capability of cooling an area of about 75 cm2 (about 1 C/W) where electronic assemblies can be mounted. The heat sink not only provides surface area, but also flow shaping for the unusual three-dimensional flow field of the fans. A volumetric coefficient of performance (COPv ) is proposed, which allows a piezofan and heat sink system volume to be compared against the heat dissipating capacity of a similar heat sink of the same volume for natural convection. A piezofan system is shown to have a COPv of five times of a typical natural convection solution. The paper will further discuss the effect of nozzles in flow shaping obtained via experimental and computational studies. A three-dimensional flow field of the proposed cooling scheme with a piezofan is obtained via laser Doppler anemometry (LDA) flow visualization method. Velocities at the heat sink in the order of 1.5 m/s were achieved through this critical shaping. Finally, the overall system characterization to different heat loads and fan amplitudes will be discussed.Copyright