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AB23

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Thermal Design Considerations for Luxeon 5 Watt Power Light Sources
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  1 application brief AB23 Thermal DesignConsiderations for Luxeon5 Watt Power LightSources Luxeon 5 Watt Power Light Sources generate about four times the power dissipationof Luxeon 1 Watt Power Light Sources, making effective thermal design especiallycritical. In this Application Brief we discuss the thermal performance of two compactheat sink designs tested with the Luxeon 5 Watt Power Light Sources. These designsprovide the Luxeon 5 Watt Power Light Source with a reasonable operatingtemperature range. This Application Brief extends the discussion started in AB05, Thermal Design Using Luxeon Power Light Sources , regarding thermal designconsiderations, heat sinks and specific procedures.Luxeon Power Light Sources provide the highest light output with the smallest footprintof any Light Emitting Diodes (LEDs) in the world. With the introduction of the Luxeon 5Watt Power Light Source, Luxeon significantly extends its strong lead in LED lumenoutput. A single 5 Watt white emitter package can output up to 120 lumens in thesame footprint as a 30 lumen 1 Watt Luxeon Emitter. (Competing LEDs typicallyoutput only 2–3 lumens per emitter.) The small form factor and groundbreaking lightoutput of the Luxeon 5 Watt Power Light Source provides lighting designers with evengreater flexibility to explore unique product and design concepts. As with the Luxeon 1Watt Power Light Sources, lighting designers need to become familiar with the impactof temperature rise on optical performance in LED devices. All Luxeon Emitterpackages must be kept below the rated operating temperature through appropriatethermal design.We recommend taking the time to develop a thermal model for your applicationbefore finalizing your design. The Luxeon Custom Design Guide  providesimportant details about operating temperatures for each Luxeon emitter package.Once you determine your target temperature, a thermal model will allow youto consider the impact of factors such as size and type of heat sink, and airflowrequirements. Application Brief AB05 provides more information about developinga thermal model for your application.™  2 Table of Contents Minimum Heat Sink Requirements3Thermal Modeling3  Thermal Resistance Model 3  Thermal Resistance of 5 Watt Luxeon Light Sources 5Heat Sink Characterization5  Test SetUp 5 Definition of Heat Sink Size 6 Heat Sink Comparison Table 6 Additional Resources7  3 Minimum Heat Sink Requirements  As a rule, product applications using LuxeonPower Light Sources must be mounted to aheat sink for proper thermal management underall operating conditions. The Luxeon Star 5 Wattproduct consists of an LED mounted on a PCBand aluminum heat spreader (Alheatspreader). The Alheatspreader is designed to mount to aheat sink, while the PCB provides the electricalconnections. We do not recommend lightinga 5 Watt Luxeon Power Light Source for morethan a few seconds at its rated current withoutfirst mounting to an appropriate heat sink.Use caution. Thermal Modeling  A. Maximum Junction Temperature  The purpose of thermal modeling is to predictthe junction temperature (T  Junction  ). The word“junction” refers to the pn junction within thesemiconductor die, where the light is generatedand emitted. The maximum junction temperature for a5 Watt Luxeon Power Light Source is 135 ºC. The next section describes the componentsin a thermal model 5 Watt Luxeon Light Source. B. Thermal Resistance Model One of the primary mathematical tools used inthermal management design is thermalresistance (R Θ  ). Thermal resistance is definedas the ratio of temperature differenceto the corresponding power dissipation. Theoverall R Θ JunctionAmbient (JA) of a Luxeon 5 Watt PowerLight Source plus a heat sink is defined inEquation 1: Equation 1 Definition of ThermalResistance.   Where: Vf VoltaeForward*If CurrentForward Pd (W) DissipatedPower  Pd C)( AmbientTJunctionT amb.- junct.  ∆ T   ==°−= d  Ambient  Junction Ambient  Junction  P T  R −− ∆=Θ  4  A simplified model of the thermal path is aseriesthermal resistance circuit, as shown inFigure 1A. The overall thermal resistance (R Θ JA   ) of anapplication using a 5 Watt Luxeon Star can beexpressed as the sum of the individualresistances of the thermal path from junction toambient (Equation 2). The correspondingcomponents of each resistance in the heat pathare shown in Figure 1B. The physicalcomponents of each resistance lie between therespective temperature nodes.Where:    R Θ  JunctionSlug   = R Θ of the die, die attach epoxy and the slug.    R Θ SlugBoard    = R Θ of the adhesive and the aluminum heat spreader.    R Θ BoardAmbient    = R Θ  of the adhesive between the heat sink and thealuminum heat spreader and the heat sink. P= V f  * I f  T Junction T Slug T Board T  Ambient R Θ J-S R Θ S-B R Θ B-Ad T Junction T Slug T Board T Ambient Al-heatspreader Heat sink DieAdhesiveDie Attach EpoxyPCB Figure 1A. Series Resistance Thermal CircuitFigure 1B. Emitter Cut-Away  Ambient  Board  Board Slug Slug  Junction Ambient  Junction  R R R R −−−− Θ+Θ+Θ=Θ Equation 2 Overall ThermalResistance. Figure 1A  Series ResistanceThermal Circuit. Figure 1B Emitter Cut-Away.
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