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Reading datasheets, it seems that maximum efficiency is attained when
leds are driven at 50% of the nominal current. Still the thermal
resistance expressed in Celsius degree/W is hardly understandable to
me: if a led with 6degree/W of thermal resistance runs at 1W, what
does it imply ? How could I figure out the passive cooling system to
be added to the led in order to keep the junction temp below the
typical one, considering that the typical temperature of my room is
30 degrees ?
By the way, I noticed that colour leds are far more efficient than
white ones. Is it difficult to render white light using primary
colours leds ?
The thermal resistance tells you how much temperature rise you can
expect across the LED junction from the amount of power flowing across
it. In your case that means if you have 1 watt of power flowing through
the LED then you junction temperature will be 6° higher than the contact
point of the LED to it's mounting board. You want to keep the LED's
junction temperature lower than it's maximum rated junction temperature
to enable it to survive. If you want long lifetime performance it is
better to design for a lower temperature like 80°C or so depending on
the LED type etc.
Once the heat leaves the LED, you will need to transfer it away from the
LED to keep the temperature low by conduction. If you leave it in air
for example, the air does not effectively transfer heat away by
conduction and the LED will get very hot very quickly. To decide how
big of a heat sink you need, you need to know how much power you need to
dissipate and what temperature you are targeting. You can characterize
that by the heatsink thermal resistance. For example this is a standard
heat sink that you can buy that would give you close to 5°C/W
http://www.aavid.com/products/extrusion-heatsinks/72930 in a small
package. Also, you need to have a circuit board to power the LED which
is another thermal interface that will have it's own thermal resistance.
That's while you'll see metal core printed circuit boards used so
often with LEDs rather than standard FR-4 boards because they FR-4 board
can have rather high thermal resistances.
Putting it altogether 1 led mounted to a metal core circuit board
(assume thermal resistance of 2.5°C/W for example) which is then mounted
to a heat sink and your ambient temperature was 30°C and you were
running the LED at 1W your junction temperature would be 43.5°C
(30+5+2.5+6+2.5). That's assuming you had good thermal contact between
your LED, circuit board and the heatsink(usually requires high pressure,
thermal grease or thermal adhesive).
In the above example 43.5°C is pretty low, so the heatsink size is
probably overkill. You could reduce it's size or choose another profile
and get one with a larger thermal resistance that would still allow you
to run your LED's cool enough.
As for your other question, It can be difficult to render satisfactory
white with color leds because their characteristics (color and
efficiency) can vary significantly with both current and temperature so
it can require complex circuitry to establish and maintain a particular
shade of white.