Thermal System Options

1. Existing fixture housing

The lowest-cost option is to reuse the fixture housing of an existing design as the housing and heat sink for the LED luminaire.
Obviously, this is not an option for new luminaire designs. Also, most existing housings are made of steel, which is a poor thermal conductor. Generally, a steel housing will be a bad choice for a heat sink.

2. Off-the-shelf heat sink

Another option is to buy an off-the-shelf heat sink. This heat sink will be a proven design and come with full specifications
from the manufacturer. However, it may not be optimized in performance, size or shape for the target application.

3. Custom heat sink

A custom solution provides the best opportunity to optimize the heat sink for the application but has several drawbacks.
This option requires the designer to have access to thermal simulation software or access to a third party with thermal
design expertise. Tooling and manufacturing fees may drive the per-unit cost of the custom heat sink higher than an off-the-shelf design.

Target luminaire cost, available heat sink development time, and target maximum ambient temperature will usually drive the decisions for the thermal system. In general, Option 2 is better for situations where low cost is more important than maximum ambient temperature. Option 3 is better when maximum ambient temperature is more important (e.g., outdoor lighting or indoor lighting in unconditioned spaces).

The example LED luminaire will use an off-the-shelf heat sink with a thermal resistance of 0.47°C/W. With the heat sink thermal resistance value, the maximum ambient temperature can be calculated with the following formula:

Tj = Ta + ( Rth b-a x Ptotal ) + ( Rth j-sp x PLED ) Tj = LED junction temperature
Ta = Ambient temperature
Rth b-a = Heat sink thermal resistance
PLED = Single LED power consumption
= (Operating current) x (Typical Vf @ Operating current)
Ptotal = Total power consumption = (# LEDs) x PLED
Rth j-sp = LED package thermal resistance

Example luminaire values:
Tj MAX = 80°C
Rth b-a = 0.47°C/W
PLED = 0.35 A x 3.3 V = 1.155 W
Ptotal = 16 x 1.155 W = 18.48 W
Rth j-sp = 8°C/W

Ta MAX = Tj MAX – ( Rth b-a x Ptotal ) – ( Rth j-sp x PLED )
Ta MAX = 80°C – ( 0.47°C/W x 18.48 W ) – ( 8°C/W x 1.155 W )
Ta MAX = 80°C – 8.6856°C – 9.24°C
Ta MAX = 62°C

A maximum ambient temperature of 62°C for the example luminaire is acceptable for this indoor application. For an operating environment needing higher maximum ambient temperature, either the maximum junction temperature should be raised (which may impact lifetime) or the thermal system (Rth b-a) improved (e.g., better heat sink).

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