Thanks for the warm welcome DavidEF and everyone here :). Thanks for the heads up for DTP heatsinks!
I reflowed the old XML LED off the original heatsink and replaced it with a XHP50 but as I've found, it gets very hot very quickly. As a result I've in fact ordered a few 20mm copper 'Sinkpads' for additional testing. I did some reading up and it appears that Noctigon is another good brand? These should perform better and I plan to do a quick thermal comparison test to try to quantify the improvements. Would appreciate additional suggestions for good heatsinks (short of just making my own! Though I'd like to keep to the spirit of this project which was to do a simple modification of an existing low-cost flashlight).
For reflow - I do reflow in an over on a regular basis, but I didn't send stencils to be made for this project just yet! I suppose if this project is successful enough I'd make a stencil so I can do a small run perhaps ^_^.
Again thanks on the input for 17mm, I'll definitely be trying to see how I can optimize the design for 17mm. One thing I'm concerned about is the drivers thermal performance and I tried to take as much care as possible during layout to ensure that the power components have good thermal sinking to the ground planes (and eventually to the outside ring to the host). For larger driver sizes I think it makes more sense to optimize it for even better components for less loss and to add some nice to have features such as larger packages for easier soldering and programming. But I'll like to focus on the GXB20 first, then perhaps 'GXB17' :)
Finally, with very minimal work (or in fact... none at all!), this driver can certainly drive 12V LEDs! This is a constant current driver so it will easily generate 12V to match the desired load current. I'll have to re-check the components (e.g. the output capacitor needs to be replaced with a higher voltage one, safety protection features need to be adjusted etc) but making it a 12V driver will certainly work. The reason I went for 6V was due to the fact that (1) commonly found XML heat-sinks have footprints compatible with XHP50 in 6V configuration and (2) I did some quick simulations on the boost drive and it's a little bit more efficient at 6V output compared to 12V output (across the range).
Will do - I'll try out with some Sinkpad IIs I purchased earlier (http://www.sinkpad.com/datasheet-sinkpad-2.php(link is external)) though it also seems like Noctigon is great too! I'll also document the spring bypass and additional methods I can do to improve efficiency. Speaking of springs, does anyone have any recommendation for good springs I can use which are not too large? For example the one I'm using now has a fairly large 8.5mm base spring which limits PCB space. I was looking at some springs like the 5mm or 7mm base springs on (http://www.mtnelectronics.com/index.php?route=product/category&path=25_81) - are these good or do you know of better ones?
Finally, I've been reading a little bit more about the different modes in flashlights and it made me wonder - would a 'moonlight mode' be something useful? And if so, what the typical brightness that people like?
Thanks again to everyone for the encouragement and kind words and for following this project. I do appreciate your time