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CL-L251 Datasheet(PDF) 1 Page - CITIZEN ELECTRONICS CO., LTD. |
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CL-L251 Datasheet(HTML) 1 Page - CITIZEN ELECTRONICS CO., LTD. |
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1 / 1 page Ref.CE-P469 04/09 Thermal Management of CL-L251 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 to the atmosphere. An external radiator, such as heat sinks, is thus required. The heat release configuration for 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, which is nearly equal to the input power Pd [W]. The package structure of the CL-L251 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-L251 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-L251 series is connected to an external heat sink. The package is composed of an aluminum substrate and the laminated 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 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, the aluminum substrate, and grease (adhesive) 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 resistance 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 thermal resistance along the cross-section diagram on Fig. 1 (a). The thermal resistances Rj-c, Rb, and Rh are connected in series between 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 Fig. 1 (a) Fig. 1 (b) |
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