A heated issue

27 November, 2009

LED lights are one of the most efficient sources of light available, and generate proportionately less heat than halogen or incandescent lamps in particular. The diodes at the heart of an LED light are tiny and can see considerable heating, despite the lower power consumption characteristic of LED lights. Keeping the junction temperature of the diode at, or ideally well below the rated maximum improves the service life, enhances efficiency and ensures that it delivers the correct colour temperature.

LED equivalents to the MR16 halogen light are now available offering over four times better efficiency, mainly by reducing the heat output. Such an LED delivers some 290 lumens of light output from 4.5W, an impressive figure in comparison. But at Typ 9.3V, the diode will be experiencing a current of 0.48A. given the small size of the diode, this is quite enough to cause considerable heating if the LED assembly isn't designed with appropriate thermal management characteristics in mind.

Performance characteristics of LED light sources are specified for a rated current and 25°C LED die junction temperatures. Since most LEDs operate well above 25°C, these values should be considered for reference only and the light output should be based on the anticipated operating temperatures.

Higher temperatures of the LED light sources can result in reduced lumen maintenance and shorten useful life. When designing a new system, a heat sink should be selected with sufficient cooling capacity to keep the die junction below 120°C. Even within this limit, LED die junction temperatures can affect dominant and peak wavelength and also cause slight shifts in colour temperature for LED white light sources.

The power output ratio

The light-emitting element of an LED radiates light and heat according to the input power. However, the surface area of an LED package is quite small, and the package itself is expected to release little heat to the atmosphere. An external radiator, such as a heat sink, is therefore required.

The junction temperature of the light-emitting element Tj must be kept below the absolute maximum rating in the specifications under all conditions to be encountered in service. Direct measurement of the junction temperature of a light-emitting element inside a package is seldom possible, so the temperature of a particular part on the package outer shell (the case temperature) Tc [deg C] is normally measured. Tj [deg C] is calculated from the thermal resistance between the junction and the case Rj-c [deg C/W] and the amount of emitted heat, which is nearly equal to the input power Pd [W]. A well designed LED package is composed of an aluminum substrate and laminated structure with insulating layers and conductive copper foil patterns.

A key point is that the light-emitting element is not mounted on the insulating layer, which has low thermal conductivity, but directly on the aluminum substrate. Thus, the heat generated at the light-emitting element can be efficiently conducted to the outside of the package. The aluminum substrate side of the package outer shell thermally connects to the heat sink via heat-dissipative grease (or adhesive). The heat generated in the junction section of the light-emitting element is thus transferred as conductive heat via the element-mount adhesive, the aluminum substrate, and grease (adhesive) to the heat sink.

Design process

Using current simulation software can achieve thermal designs for LED packages that balances good heat removal with maintaining the minimum size and weight of heat sink. Thermal analysis using SolidWorks COSMOS 2007 software gives the temperature distribution and heat flowing in the product - as well as the heat exchanged between the product and its environment.

The movement of heat through the component and heat sink into the environment can be pictured by plotting heat flux vectors. The investment in software is amply repaid by the results it achieves. All prototypes are subjected to tests in a temperature chamber, and the measured temperatures are normally within 3-4° of the simulation result. Using this process, Marl has never had to redesign a product following the building of the prototype.

LEDs with a power dissipation of 35W or more are now available. Such lights are so bright that they are uncomfortable if not dangerous to look into directly, but with increasing power there is increased thermal load and more heat to dissipate. Adherence to sound thermal design principles will ensure best performance and long life.