is this heatsink good enough

[shrimphead]

so i want to run 1 xml and 1 xte on one heatsink at about 70% so thats about 9 watts without using a fan.

I found this: http://uk.farnell.com/fischer-elektronik/sk-100-75-sa/heat-sink-75mm/dp/4621827


it has a thermal resistance of 1.9c/w.
if i try and work out the running temp of the leds (which i'm not completely sure this is right.) then adding up the thermal resistances:
(1.9, heatsink) + (1 , grease?) + (1, mpcb rapid led?) =3.9

so 3.9 x 9(watts) = 35.1 + ambient temperature 25.

35.1 + 25 = 60.1 degrees c running temp
that temps ok isn't it?
and can anyone correct me if i've made a mistake on the calculation somewhere, thx.

I didn't know where to find the thermal resistances of the grease and the mpcb but figured they couldn't be much

thx rob

[TimmyD16]

Should be fine... though I'm not familiar with your math, seems a bit basic but still makes sense...

[mcgyvr]

almost there.. but you can see how getting the wrong values will quickly throw off your numbers..

First you forgot LED junction to case (or junction to solder pad) resistance..
And I'd bet that your MPCB value is way low..
#1-junction to solder pad on LED= 2.5 C/W
#2-MPCB = ? Probably more like= 5 C/W
#3-Thermal grease/pad= 1 C/W is about right for hand applied application (do it as THIN as possible)
#4-Heatsink to air= 1.9 C/W
So 2.5+5+1+1.9 = 10.4 C/W
Now 10.4 x 9 watts = 93.6 deg C rise over ambient.
BTW (I'm assuming your 9 watts is correct)
So assuming 25 deg ambient
25 +93.6 = 118.6 deg C

Will that work.. Yes as its under the max junction temp..
BUT will the lifespan and light output be reduced.. YES it sure will.

Personally keep junction temps at 75 deg C max to get the 50k hours.

[shrimphead]

damn, just can't seem to get the temp down. what if you had a 40 watts total, it would be impossible to find a heatsink for it as your thermal grease and mpcb would push it over the limit straight away whatever heatsink you get.

don't think my led design is going to work

[shrimphead]

just emailed rapid led what the thermal resistance of their mpcb's are and i've been looking at thermal grease resistances and they all seem to be pretty low like this one "Thermal resistance: 0.05 C/W;Thickness 0.2mm"

[mcgyvr]

yes normally the grease thermal resistance "in a perfect world" is much lower than 1.. This ain't a perfect world. Based on the pictures of users thermal grease applications I've seen on this board there is NO WAY that any of them are a consistent .2mm film (more like 5x that)

A more accurate number to use for hand applied grease is probably .25 c/w

And yes when you can't get there with natural convection you move to forced air and the resultant thermal resistance drops considerably...

Of course 118 deg C isn't really "that" bad at all and is completely doable.. but it could be better. Using 1 of those heatsinks per LED easily gets you to a long life/low temp number.

The sink/setup you proposed will more than likely be just fine.. I'm always over cautious about this stuff when it comes to DIY/hobby users.. I like to scare them a little to make sure they are as safe as they can be..

[shrimphead]

found out my estimated 9 watts in my calculation was wrong because apparently i should be saying it's 4.5 watts because half of it is converted to light and the other half to heat. If it was 9watts of heat then it would be just heat no light, makes sense i suppose.