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About thermal resistance of LEDs

R

Raymond Chou

Jan 1, 1970
0
Hi all:
I have a question about thermal resistance of LEDs.
For a single LED, is the thermal resistance a constant value?
Further, is the thermal resistance independent on the driving
current and the junction temperature?

Hope your reply, thanks a lots
 
D

Don Klipstein

Jan 1, 1970
0
Hi all:
I have a question about thermal resistance of LEDs.
For a single LED, is the thermal resistance a constant value?
Further, is the thermal resistance independent on the driving
current and the junction temperature?

Hope your reply, thanks a lots

Thermal resistance of an LED is nominally constant, but for ones without
heatsink slugs may vary somewhat.

For LEDs with heatsink slugs, the thermal resistance is from the
junction to the external surface of the heatsink slug. It is ratio of
temperature rise of junction (above temp. of heatsinkable surface) in
degrees C or K, to watts of heat being dissipated.

You should figure a worst-case amount of heat, using the datasheets for
the LEDs. Figure the worst-case maximum LED current and the worst-case
maximum voltage drop of the LED, and multiply these for worst case watts
of heat.

Do not subtract from this anything for energy input becoming light
energy, unless you know a worst-case small amount for worst-case low
output, derated for worst case junction temperature, derated for maximum
amount of degradation during the life of the LED, and minus any returned
to the LED or the heatsink by reflection and/or absorption.
Keep in mind that some (many?) LED manufacturers state photometric (and
sometimes radiometric) performance with junction temperature 25 degrees C,
rather than at a more realistic condition.
This will usually require knowing luminous efficacy of the emitted
light, and the datasheet usually does not have that - figure 400 lumens
per watt for a white LED unless you know for sure something less (this
will usually be closer to 300).
Somehow it appears to me better practice to assume that the LED is 100%
efficient at converting electrical energy to heat when doing thermal
design for anything with much more reliability requirement than most
flashlights.

As for LEDs with leads: Thermal resistance is usually stated in terms
of degrees C/K per watt of temperature rise of the junction above the lead
that the junction is mounted to, usually 5 mm from the body of the LED
for through-hole ones, with the LED in a typical circuit board in a
typical environment. This figure can be increased by impeding any of the
heat that escapes from the LED by other routes more than a "typical
environment" does.

Thermal resistance of small surface mount LEDs is usually degrees C/K
per watt for temperature rise of the junction above the environment in
typical mounting. Unusual mounting can change this, though maybe only
slightly (especially expect only slight change if it is for the better).

Keep in mind what temperature can be expected inside whatever you put
your LEDs in, especially in situations such as places without air
conditioning, outdoors in downtown Phoenix or Las Vegas, recessed
ceiling fixtures, LED "bulbs" to screw into enclosed fixtures, etc.

As for how thermal resistance may vary in a specific installation with
current and temperature: It does not unless something cracks or moves, or
ability of a heatsink to radiate changes due to surface characteristic
changing with age or dust accumulation.
Ratio of temperature rise to power input can vary, among different
degrees of "negligibly to slightly to somewhat" less than the true thermal
resistance, as variations in current, temperature, or condition related to
aging changes the efficiency of the LED (changes the LED's ability to
convert electrical energy into light rather than heat).

- Don Klipstein ([email protected])
 
V

Victor Roberts

Jan 1, 1970
0
Hi all:
I have a question about thermal resistance of LEDs.
For a single LED, is the thermal resistance a constant value?
Further, is the thermal resistance independent on the driving
current and the junction temperature?

Hope your reply, thanks a lots

No. For all materials the thermal resistance, like the
electrical resistance, is a function of the temperature of
the material. However, over the typical temperature ranges
for most LEDs, the change in thermal resistance due to
temperature will be very small compared to other factors
that must be taken into consideration.

For a good discussion of thermal design for LED applications
see the Luxeon Application Brief AB05, Thermal Design Using
Luxeon Power Light Sources:

http://www.luxeon.com/pdfs/AB05.pdf

--
Vic Roberts
http://www.RobertsResearchInc.com
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