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Optoelectronics/Displays??

High-power video projection with liquid-cooled HB LEDs

Posted: 16 Sep 2007 ?? ?Print Version ?Bookmark and Share

Keywords:high-power video projection? liquid-cooled HB LEDs? high-brightness LED?

By Girish Upadhya and Fred Rebarber
Cooligy Inc.

High-power/brightness LEDs are an increasingly attractive solution for projection applications due to their enhanced color range, high performance over the life of the end product and lower cost of ownership than current arc lamp technology. Still, a few technical hurdles remain to be cleared before the market will see a more rapid adoption of LED technology in high-power projection applications.

The inherently high power density of the LED poses a significant thermal challenge for conventional cooling schemes. With the advent of reliable liquid-cooling systems in recent years for applications such as high-end workstations, there is now a viable approach to solving the thermal challenges of cooling high-brightness LEDs in the high-end display industry.

Until now, growth in LED applications has been limited by performance (brightness) and reliability (service life) issues that are well within the capability of Emerson Network Power's maintenance-free Cooligy Active Micro-Structure liquid cooling system to solve. This article will discuss the three main areas impacting projection applications!thermal performance, cooling reliability and total "cost of ownership" for the end user!based on a comparison of arc lamp technology and the newer high power/high bright LEDs.

LEDs for projections applications
One of the challenges of implementing LEDs in projection applications is dealing with the problem of excessive heat caused by the high power required to generate sufficient brightness for LED applications, including:

? Rear projection TVs
?Home theater projectors
? Business projectors
?Large venue projectors

A key advantage of LEDs is that their service life may be up to 30 times longer than typical arc lamps (up to 60,000hrs versus up to 2,000hrs, respectively). Therefore, enabling LEDs not only positively impacts overall system reliability, but also lowers total cost of ownership due to the cost of replacing arc lamps over the service life of the unit. For less than the cost of changing an arc lamp, the consumer can have a maintenance-free LCS.

Figure 1: Schematic of the microstructure liquid cooling system loop
Click image to view figure.

Liquid cooling
Successful implementation of LED technology is not only achievable but cost-effective using existing liquid-cooling methods. The superior thermal management afforded by liquid cooling optimizes projection system performance by ensuring a continuously bright light source with a much longer service life. In terms of cost, as many as ten or more arc lamps, costing hundreds of dollars each, may be required over the normal service life of one liquid-cooled LED device which, in comparison, represents a significantly lower installed cost. As a technical challenge, the tremendous heat generated by LED power densities approaching 1,000W/cm?, require a thermal management solution that can only be provided through liquid cooling.

Cooligy's microstructure liquid cooling system
Figure 1 shows the schematic of Cooligy's microstructure closed-loop liquid cooling system for a typical device-cooling application. As shown, cold liquid enters the micro-heat exchanger at a specific volumetric flow rate driven by the mechanical pump, and picks up the heat from the LED device before exiting the micro-structure heat exchanger. The warm liquid then enters a radiator, which is air-cooled by a fan. The pressure drop for the liquid flow through the system is managed by an appropriate fluid-delivery mechanisms built into the design of the individual components.

Thermal performance
Several factors contribute to the superior thermal performance of the micro-scale liquid cooling system, including:

Micro-structure heat exchanger design: Although cooling systems using microstructure heat exchangers have been a topic of considerable research in the past two decades, the application of the technology to commercial products has not been widespread, due to several key technological advancements that needed to occur. First, the difficulty of driving flow through the micro-structure heat exchanger has limited the realized thermal performance of these devices.

Figure 2 shows the nature of the dependence of heat transfer efficiency and pressure drop on channel width. Though thermal performance can improve as the channel dimensions narrow, the resulting pressure drop also increases. To reduce pressure drop, Cooligy has developed patented fluid-delivery mechanisms that enable high flow rates with lower pressure drop. Cooligy's microstructure heat exchangers are mass produced using high-volume manufacturing techniques.

Figure 2: Dependence of thermal performance and pressure drop on channel width
Click image to view figure.

Unique attachment mechanism: The liquid-cooling system described in this paper utilizes a patented heat exchanger-mounting mechanism that enables consistent thermal performance in a typical manufacturing set up.

High-efficiency radiator design: The radiator design for the applications described in this paper has been optimized to deliver the best possible performance for a given airflow. The optimization process utilized various numerical simulation techniques and analytical models. The experimental validation, for example, was accomplished using a thermal test vehicle consisting of controlled heat input using a copper block, and controlled airflow using wind tunnels.

Liquid-cooling reliability
Allowing junction temperatures as low as 60<C, liquid cooling ensures more than adequate luminosity for projection applications while minimizing acoustic levels to less than 30dB.

Another key consideration, reliability, is maximized by the system's leak-free joints, which prevent coolant fluid loss. With a demonstrated service life of more than 50,000hrs, Cooligy's liquid-cooling system provides a closed-loop architecture that is entirely maintenance-free. All materials are tested for compatibility to provide a particle-free environment. This is critical since the development or introduction of particles could cause system clogging.

The availability of a high-reliability, high-performance liquid-cooling system enables the widespread adoption of LED technology in high-end projection applications. Table 2 outlines a recommended suite of stress tests that a liquid-cooling system must be able to pass to ensure that its design and construction are robust enough for typical LED applications. In addition, on-going reliability testing (ORT) must be established to ensure that stable and robust manufacturing processes are being used and maintained. Cooligy's liquid-cooling system design easily meets these suggested requirements.

Cooligy has developed and implemented several technological advances that have made micro-structure liquid-cooling systems significantly more robust and reliable. These include refinements in the area of:

Table 1: System reliability tests.
Click image to view table.

Particle control: Particle control plays a crucial role in ensuring reliable long-term performance of micro-structure liquid-cooling systems. The material/fluid combination has been optimized by careful analysis, testing and characterization. The choice of material, as well as method of processing during assembly of the liquid-cooling system, significantly influences the reliability of the finished product.

Water loss control: In order to build a maintenance-free, reliable, liquid-cooling system, water loss must be controlled. This is critical to minimize water vapor permeation. Constructing robust tubing joints is necessary to prevent water or vapor loss during shipping, storage and use. In order to minimize fluid loss, several design solutions have been adopted in the microstructure liquid-cooling system. Additionally, a number of proprietary fabrication processes and assembly methods were developed to ensure long-term reliability of the completely sealed joints.

Freeze-protection technology: The working fluid used in this system is water-based and has been developed to withstand the volumetric expansion that will normally occur if the unit should freeze during shipment or storage. The liquid-cooling system described here uses proprietary technology to allow the water to freeze without causing system damage or impacting thermal performance. This has been made possible by innovative freeze-management features designed into the cooling loop.

Figure 3: Schematic of liquid-cooling system for an LED display application.
Click image to view table.

Material science control for longer life: The materials used to construct the components of the micro-structure cooling system!microstructure heat exchanger, radiator, tubing and working fluid!are carefully selected and designed to minimize corrosion and maximize long-term reliability. Extensive characterization has been performed in the selection of the materials used in the fabrication of the cooling system.

Application example
Figure 3 shows the schematic for a liquid-cooling system for an LED light engine used in a high-end display application:

In this application example, thermal loads simulated LED arrays at 35W (LED 1), 60W (LED 2) and 50W (LED 3).

The following graph plots Tcase (<C) vs. fan voltage and noise (dBA). Noise is measured at 0.5m and 1.0m.

Figure 4: Cooligy test setup with Shin-etsu thermal grease.
Click image to view table.

Figure 4 shows the data for a prototype for LED cooling system with the case temperature of the heater block being plotted as a function of fan speed.

Liquid cooling helps to maintain the low case temperatures required to enable brightness levels suitable for the application and, at the same time, greatly improve LED reliability.

Conclusion
Successful implementation of LED technology is not only achievable but cost-effective using robust liquid-cooling architectures. The superior thermal performance of a well-designed liquid cooling implementation enhances projection system performance with a continuously bright light source characterized by a longer service life. In terms of cost, as many as ten or more 1,000-2,000hr-rated arc lamps, costing hundreds of dollars each, may be required over the normal service life of one liquid-cooled LED device!representing a significantly lower cost of ownership.

As a technical challenge, the tremendous heat generated by LED power densities requires aggressive thermal-management solutions. Liquid cooling can deliver lower thermal resistance in a fraction of the volume compared to conventional solutions. With the availability of highly reliable closed-loop liquid-cooling systems like the maintenance-free Cooligy Active Micro-Structure Cooling System from Emerson Network Power, the widespread adoption of LED technology in high-power / high-brightness projection applications is now a practical reality.

About the authors
Girish Upadhya
, Ph.D. and Fred Rebarber are, respectively, director of applications engineering and director of sales and marketing at Cooligy Inc., a division of Emerson Network Power.

Reference: "Cooligy Active Micro-Structure Cooling Offers Key to Advanced Processor Performance and Quieter Systems"; Girish Upadhya, Ph.D.; Cooligy Inc., 800 Maude Avenue, Mountain View, CA, USA.




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