Re: 42W 12Led Cree Torch + Lamp (updated with thermal design)
Thermal Design
Heat is always a problem in high powered torches which are run for more the a few minutes. Even the most efficient leds dissipate almost all their power as heat. So the thermal design is an important consideration. From my previous builds I had some idea of how much finning would be needed. To do the detail checks I used Natsink, (
http://www.frigprim.com/frigus_natsink.html). The demo version models a simple finned heatsink with a fixed depth of 75mm. To estimate the performance of the torch head I first set up a simple model that has the same fin height, fin thickness and pitch as the torch head and determine the heat dissipation for a given temperature rise. Then the results are scaled up to the available surface area on the real torch head using the circular fins. Previous builds and tests have shown this scaling gives a good indication of the final result.
This torch design has a nominal voltage of 14.4V and a design current of 3Amps. This means it has to dissipate approximately 43Watts.
The Cree XR-E specifications project 70% for light output after 50,000 hours provided the led junction temperature is kept below 80C. When running at a junction temperature of 100C the output of the leds drops to 80% of the output at 25C. The maximum allowable junction temperature is 150C. When running at this junction temperature (150C) the output of the leds reduces to less the 70% of their 25C value.
As an initial design, I choose 80C as the operating junction temperature of the leds (which results in about a 15% drop in output). The thermal resistance of the led from the junction to the solder point is 8C/W. At 1Amp the led are dissipating about 3.6Watts, so the junction temperature will be about 29C above the temperature of the copper mounting disk the led is soldered to. This implies I want to keep the copper disk at or below 51C.
The Natsink model takes the thermal resistance of heat conduction throught the bulk aluminium into account and as a first approximation I am ignoring the thermal resistance of bolting the copper disk to the aluminium disk (with Arctic silver) and ignoring the thermal resistance of the press fit of the aluminium disk into the head of the torch as these resistances will be swamped by the thermal resistance between the fins and the air. For example the Arctic silver has a thermal resistance of <0.0045C-in^2/watt for a 0.001 inch layer. The copper disk is 3.04in^2 which implies a thermal resistance of 0.00148C/W for the Arctic silver layer. That is less than 0.06C rise for 43Watts. The modelling done here is not that accurate.
So the parameters for the simplified Natsink model are:- 3mm thick heat sink base (the wall thickness of the torch head); width 70mm, the length of the torch head. With an ambient air temperature is 25C and the heatsink temperature should be no more than 51C, so the temperature difference between the heatsink and the air should be less than 26C. A single heat source 10mm wide and covering the depth of the simplified heatsink is specified as being 20mm from one end of the heatsink. This is approximately where the 10mm thick aluminium disk will be pressed in to the torch head.
The final value that needs to be specified is the height of the fins to match the fin height on the actual torch. On the torch head I intend to taper the fins from 155mm dia. at the back of the head to 125mm at the front. So for the simplified model I started by using the mean fin dia. of 140mm. That is a fin height 40mm above the 60mm dia head tube that the fins are pressed. The error introduced using this mean will be check later.
There are a number of design questions the need to be answered
- What is the optimal number and spacing of the fins?
- How thick should the fins be?
- How will the performance change with changes in ambient temperature?
- What will the dissipation of the Torch head be?
- What is the error associated with using the mean height of the torch fins?
1. What is the optimal number and spacing of the fins?
The graph below show the temperature rise of the heat sink above an ambient of 20C versus the pitch of 2mm fins for a heat source of 9.3W. A finer pitch means more fins in the 70mm. Natsink says optimal number of fins is 8off x 2mm thick fins spaced at 9.7mm pitch which will dissipate 9.3W for a temperature rise of 26C.
2. How thick should the fins be.
Now lets investigate the effect of fin width on the temperature rise. Looking at 1mm, 1.5mm and 2mm width fins gives the following results. Again with a heat source of 9.3W
The 1mm fins are better, however I chose 1.5mm fins as being more robust and almost at good as 1mm fins. For the front fin I choose a 2mm thick fin for extra strength.
So far the design is 8 fins, each 1.5mm (except the front one) on a pitch of about 10mm. Which will give a rise of about 25.4C from an ambient of 20C when dissipating 9.3W.
3 How will the performance change with changes in ambient temperature?
As you can see air temperature variations from 20C to 60C have almost no effect on the temperature rise at the 10mm pitch point. Of course if the ambient at 40C then the temperature rise of 25.4C means the heatsink, and hence the copper disk, will have a temperature of 65.4C and the junction of the leds will be at about 94.4C. This is well above my target design temperature but still well below the maximum allowable led junction temperature of 150C.
4. What will the dissipation of the Torch head be?
This is the big question. To estimate the torch dissipation I calculate the fin area of the simplified Natsink model and then scale up the wattage dissipated to the actual torch fin area. The final torch will the same number of fins, fin pitch and fin thickness as the simplified model. Only the fin height and area will change.
A mean fin height was used in the Natsink model (see discussion below for the effect of changing the fin height)
The total area of fins in the Natsink model is 8fins x 75mm x 40mm x 2sides = 48000mm^2. In the actual torch, where the fins start at 155mm at the back and graduate down to 125mm at the front fin, the total area when they are pressed onto the 60mm tube, is 202235mm^2. That is the area of the torch head is 4.21 times the area of the simplified model. That implies the torch head will dissipate about 9.3Wx4.21 = 39W for 25.4C temperature rise above ambient.
This is a bit below the required 43W. So using the ration of 4.21 in reverse implies that the Natsink model needs to dissipate 10.2W
Updating the heatsource wattage figure in the Natsink model and re-running the simulation gives temperature rise of 27C instead of the previous 25.4C. This is only only one degC more then desired temperature rise implying the leds junction temperature will be about 81C instead of 80C. This design is acceptable.
5. What is the effect of using the mean height of the torch fins?
Finally since the size of the fins on the torch are going to taper from 155mm at the back of the head to 125mm at the front, I checked how the change in height of the fins would effect the heat dissipation in the simple model. So far the design checks have used a mean dia of 140mm (ie.40mm height above the 60mm dia head tube). However the back fins will be 155mm dia, that is 47.5mm high while the front fin will be only 125mm dia, that is 32.5mm high. The table below summarizes the results
This shows that the larger fins are marginally less effective in getting rid of the heat, but it is clear that using a mean height of 40mm will give an adequate estimate of the torch head's performance.
Of course this is only an estimate which needs to be confirmed by measuring the final torch's performance.
