Thermal Management of CL-824
1. Introduction 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 into the atmosphere. An external radiator such as heat sinks is thus required. The heat release configuration to the connection portion of the external radiator mainly uses heat conduction. Regarding LED packages, to control the junction temperature of the light-emitting element Tj is important. The Tj must be kept from exceeding the absolute maximum rating in the specifications under any conditions. Because direct measurement of the junction temperature of a light-emitting element inside a package is seldom possible, 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 that is nearly equal to the input power Pd [W]. The package structure of the CL-824 series 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 configuration of the CL-824 series and provides necessary data for thermal design of lighting apparatus, which leads to optimal utilization of LED performance.
2. Package configuration and thermal resistance Fig. 1 (a) illustrates the example of the cross-section structure where the package of the CL-824 series is connected to an external laminated circuit board. The package structure is composed of a light-emitting element mounted on a substrate that has conductive copper foil patterns and through holes. A distinctive point is the heat generated at the light-emitting element can be efficiently conducted via through Fig. 1 (a) Fig. 1 (b) holes to the outside of the package. The electrode section of the package outer shell is electrically conductive and connects via solder to the electrode on the external circuit board, which also has the function of a heat sink. As described above, the heat generated in the junction section of the light-emitting element is mainly transferred as conductive heat from the light-emitting element via element-mount adhesive, through holes, electrodes on the outer shell, and solder to the external circuit board, which doubles as the heat sink. The thermal resistance from the junction section of the light-emitting element to the electrode side of the outer shell is Rj-c, which is the specific thermal resistance value of the package. Hence, the following equation makes sense. Tj = Rj-c x Pd + Tc In addition, the thermal resistance of the solder outside of the package is Rs [deg C/W], that of the electrodes with the heat sink function is Re [deg C/W], and the ambient temperature is Ta [deg C]. Fig. 1 (b) shows the equivalent thermal resistance along the cross-section diagram on Fig. 1 (a). The thermal resistances Rj-c, Rs, and Re are connected in series between the junction temperature Tj and the ambient temperature Ta. Now the thermal resistances outside the package Rs and Re can be integrated into the thermal resistance Rc-a, which leads to the following equation. Tj = (Rj-c + Rc-a) x Pd + Ta
Ref.CE-P473 04/09
3. Thermal design outside the package The thermal resistance outside the package Rc-a [dig C/W], which is the combination of those of the solder and the electrodes with the heat sink function, 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 converted 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-824-U1 package that shows the relationship between the ambient temperature Ta and the thermal resistance outside the package Rc-a with variations of driving current, where Tj is assumed to be 120C, the absolute Rj-c=175 (deg C/W) maximum rating value in the specifications. The higher the ambient temperature Ta and the larger the driving current, the smaller the allowable thermal resistance outside the package Rc-a = Rs + Re. This means that the external heat release mechanism with smaller thermal resistance (in other similar words, better heat dissipation) is required in order to keep Tj from exceeding the absolute maximum rating in the specifications of 120C, if the ambient temperature becomes higher Fig. 2 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 in the end.
Ref.CE-P473 04/09
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