ref.ce-p469 04/09 thermal management of cl-l251 1. introduction the light-emitting element of an le d radiates light and heat accordi ng 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 heat sinks, is thus r equired. the heat release configuration for the c onnection portion of the external radi ator mainly uses heat conduction. regarding led packages, to contro l the junction temperature of the light-emitting element tj is important. the tj must be kept from exceeding the absolute maximum rating in the specific ations under any conditions. because direct m easurement of the junction temper ature of a light-emitting element inside a package is seldom possible, the temperature of a particular part on t he 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]. the package structure of the cl-l251 se ries minimizes the thermal resistance rj-c , and the heat generated at the light-emitting element can be conducted to the external radiator efficiently. this document describes the detailed heat release configur ation of the cl-l251 series and provides necessary data for thermal design of lighting apparatus, which leads to optimal utilization of led performance. 2. package configurati on and thermal resistance fig. 1 (a) illustrates the example of the cross-section structure where the package of the cl-l251 series is connected to an external heat sink. the package is composed of an aluminum substrate and the lami nated structure of insulating layers and conductive copper foil patterns. a distinctive point is the light-emitting element is not mounted on the insulating layer, which has low thermal conductivity, but directly on the well conductive 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). as described abov e, the heat generated in the junction section of the light-emitting element is main ly transferred as conductive heat fr om the light-emitting element via element-mount adhesive, t he aluminum substrate, and grease (adhes ive) to the heat sink. the thermal resistance from the junction section of the light-emitting element to the aluminum substrate side of the package outer shell is rj-c, which is the specific thermal resist ance value of the package. hence, the following equation makes sense. tj = rj-c x pd + tc in addition, the thermal resistance of the grease (adhesive) outside of the package is rb [deg c/w], that of the heat sink is rh [deg c/w], and the ambient temperature is ta [deg c]. fig. 1 (b) shows the equivalent t hermal resistance along the cross-sect ion diagram on fig. 1 (a). the thermal resistances rj-c, rb, and rh are connected in series bet ween the junction temperature tj and the ambient temperature ta . now the thermal resistances outside the package rb and rh can be integrated into the thermal resistance rc-a , which leads to the following equation. tj = ( rj-c + rc-a ) x pd + ta fi g . 1 ( a ) fi g . 1 ( b )
ref.ce-p469 04/09 3. thermal design outside the package the thermal resistance outside the package rc-a [deg c/w], which is the co mbination of those of the heat-dissipative grease (adhesive) and the heat sink, is limited by the input power pd [w], the ambient temperature ta [deg c], and the thermal resistance of the package rj-c [deg c/w], i.e., tj = ( rj-c + rc-a ) x pd + ta rc-a = ( tj - ta ) / pd - rj-c the formula can be convert ed into the function of tj as follows: rc-a = - ta / pd tj / pd - rj-c, which indicates the straight line with the slope of -1 / pd and the intercept of tj / pd - rj-c . fig. 2 is the chart on the cl-l251-c4 package that shows the relations hip between the ambient temperature ta and the thermal resistance outside the package rc-a with variations in the driving current, where tj is assumed to be 120c, the absolute maxi mum rating value in the specif ications. the higher the ambient temperature ta and the larger the driving current, the smaller the allowable thermal resistance outside the package rc-a = rb + rh. this means that the grease (adhes ive) and heat sink with smaller t hermal resistance (in other similar words, better heat dissipation) are required in order to keep tj from exceeding the absolute maximum rating in the specifications of 120c, if the ambient temperature becomes higher and/or the driving current is larger. therefore, use fig. 2 as a guide when selecting the external heat radiation part, and conduct thermal verification on actual devices at the end. for reference, fig. 3 shows the equiva lent chart on the cl-l251-c6 package. fi g . 3 fi g . 2
ref.ce-p469 04/09 4. simulation simulation is one of the effective means for thermal design. fig. 4 (a) and (b) indicate the simulation results under the following conditions where the cl- l251-c6 package is connect ed to a heat sink using a thermal conductive sheet. use them for reference. boundary conditions ambient temperature: ta = 25c thermal conductivity: 5 w/m k heat release coefficient of the heat sink: 0.2 connection resistance: out of consideration model conditions thermal conductivity of t he thermal conductive sheet: 1.2 w/m k thickness of the thermal conductive sheet: t = 0.12 mm material of the heat sink: aluminum (number of fins = 7) dimensions: 31.5 mm w x 12 mm h x l mm (variable) fi g . 4 ( a ) fi g . 4 ( b )
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