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
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
Fig. 1 (a)
Fig. 1 (b)
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
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 combination 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 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-L251-C4 package that shows the relationship 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 120°C, the absolute 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 = Rb + Rh.
This means that the grease (adhesive) and heat sink with smaller thermal 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 120°C, 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 equivalent chart on the CL-L251-C6 package.
Fig. 2
Fig. 3
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 connected to a heat sink using
a thermal conductive sheet. Use them for reference.
· Boundary conditions
Ambient temperature: Ta = 25°C
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 the thermal conductive sheet:
1.2 W/m·K
Thickness of the thermal conductive sheet: t = 0.12
Material of the heat sink: aluminum (number of fins = 7)
Dimensions: 31.5 mm W x 12 mm H x L mm (variable)
Fig. 4 (a)
Fig. 4 (b)
Ref.CE-P469 04/09