Question for the experts

Hi,

This is probably a really stupid question, as I am
relatively new to this field but I was thinking the
other day that if a 5W luxeon consists of 4 dies that
are 1.25 W each, why can't they come up with a 12W
luxeon that has 4 3W dies.

Would this be down to heat-sinking issues?

Even if it was under-driven, it could output a
tremendous amount of light, and due to the whiter
tints of 3W, should retain a relatively white tint
even when underdriven.

the three watt is a five watt with one die, from what I've heard. the difference between the five watt and the one watt is thicker wires and a different heat transfer-er because of current and heat issues, so the three watt is a one watt with the thicker wires and the different heat transfer-er to handle the current. so the 12watt probably isn't possible in a five watt package without some super efficient heat transfer unit. sorry to make it sould so confusing; it isn't.

Ok - so what you're saying is that each of the 4 dies
in the 5W have thicker wires than those of a 1W die,
more like those of a 3W die.

But if that's the case then one would expect a higher
potential output than 5W for these 4 dies together,
unless the heat-sinking is the sole determinating factor.

In any case, if each of the dies on the 5W had the potential
to be driven to 3W, surely it would mean that the
5W leds are automatically underdriven, so then why do
they have such poor longevity in comparison with
indidual 1W and 3W?

easy,
It is a heatsinking issue for the most part. It dumps the heat through 4 dies but the dies are next to each other. This creates a hotspot that is quite hard to cool.
Is it just the white Luxeon V that is rated at 1,000 hours? It has been awhile, but I think my 5W Cyan is rated much, much higher.

It is purely an issue of heat.

Each Luxeon 3 die is rated for 1A of current. If you were to run a 5W at the same current/die rating, then you'd end up with a 2A, 7.4V (approx) luxeon. That's 14.8W of power. The junction-slug thermal resistance of a 5W is 8C/W at best. That's a 118C difference junction-slug. Assuming the slug is kept at 25C, the junction temp is at 143C, almost 10C over the absolute maximum temperature. All that is assuming the best possible thermal characteristics (transfer and heat sink size).

Now for the real world, you're looking at like 10C/W to the heatsink, which will slowly rise in temperature. At 10C/W, the junction temperature is initially at 173C, far exceeding the maximum junction temperature (and this whil the heat sink is still at 25C). LED life would be very very short.

OK thanks for explaining that so thoroughly evan9162
(& BentHeadTX).

The Lux5 works pretty good at 1.5amps and about 10 watts as in the MR-X, tho, with the Hotlips heatsink.

Hi RussH - excuse my ignorance but as a newbie I didn't
see much about the Hotlips. What is the principle behind
its design.

If only I could get hold of an MR-X!

Hotlips heatsink

Most LEDs will take a lot more than the specified current as long as it's well heatsinked . But each additional watt you pump through it gets you less and less extra brightness in return, so you cant just keep using bigger sinks and more current.