Awright:
Heatsink - sounds good. 60-70°C is perfectly acceptable for long and reliable life. The boards made on aluminum are really good for high power surface mount since they spread heat way better than anything that can be done with the foils on a standard epoxy-glass substrate.
Eyesight - Ha! I used to be very near-sighted but now have plastic eyeballs (just plastic lenses, actually) and absolutely fixed focus eyes. It has a lot to do with why I don't actually do electronics anymore. My brother used to be a competitive marksman (bench rest, mostly).
I've done a lot of switcher design and used those green and blue cores extensively. They are quite a good compromise of performance and cost.
Board differences - a problem with rather a lot of stuff from China is that something decent gets copied - sort-of, and then maybe the copy gets copied, and often the copies are second-rate approximations. Until you get something, examine it and test it, you just have to hope for the best. From what I can see in photos, your board looks pretty good, but looks only count for a little. For example, for ceramic capacitors of the same nominal value and voltage there is often a choice of two or three or even more types, differing in the ceramic used, which affects the electrical behavior. The better type might cost 2 cents more, so someone who doesn't understand the difference buys the cheaper part that is inadequate for the task. (One of my clients once showed me a bunch of caps they bought to use on a board designed - I called them crapacitors.)
It takes so bloodly long for things to arrive from China that buying something and deciding you want to try a similar something is really really frustrating. I order a few things from Amazon marketplace vendors early last year and they were still drifting in three months later.
Stuff on perfboard: It may not be a problem, but I really do recommend some extensive testing with the original trimmer on the PCB. I wish we could scare up a schematic for your board - it would be very helpful in assessing sensitivity of that bit of circuitry to noise. Often the part of the circuit that measures the current is operating at millivolt levels, so it doesn't take much noise coupling in to scramble things. It's usually surrounded by circuits that switch high currents and voltages at high speed. I often say of switchers that everything is in conflict with everything else. It isn't much of an exaggeration.
I've seen and spent many many hours tracking down and fixing noise problems in switchers and may seemingly trivial changes that made things much better or much worse.
FET failure - I have no reasonable explanation or suspicion; on paper that FET looks very very good for the job
FETs (most semiconductors, actually) almost always fail short-circuit, though if that leads to really high current for long enough they'll blow open. I've had lots in packages much bigger than your FET blow the plastic paclage apart with a loud bang. But that was work on a 1200 watt AC powered switcher. They know lots of ways to frighten you. I'm like a nervous cat when I work on some of that stuff. I literally used blast shields sometimes - acrylic or plywood to shield me from bits that might go flying.
Best way to drive the LED: I think what you are trying to do is quite reasonable given what is available off-the shelf. There are some commercial high-power LED drivers that can be pulse-width modulated for dimming, but that isn't something you want for photography, since the PWM frequency is usually low enough that you could have a whole exposure at full brightness, a whole exposure at zero brightness, or shot with half & half if you're using a focal plane shutter. One drawback to changing the current to change brightness is that it does have a very small effect on color temperature, but precious few eyeballs on this planet would notice. (little aside on that - I used to do photography a long time ago. I was doing some GUI design about 3 years ago and set up what was supposed to be a neutral grey background. It looked too brownish to me. I dug out an old Kodak neutral grey test card, looked at it and concluded my memories of neutral grey were wrong, since it also looked brownish. After I had my eyes repaired, the brownish cast was gone. I'm actually really surprised color memory could be that good. For awhile I had one fixed eye and one bad eye. The color differences between the two were astounding.)
Dynamic response: For a fixed load like a LED and with a well regulated input supply, slow response shouldn't be any kind of an issue. Dynamic response comes into play for rapidly changing loads and/or source supplies.
From what I've seen, most boards like this kind of cobble the constant current circuitry into what is primarily a constant voltage regulator. Sometimes the CC circuity is made extra-slow, either because the designer isn't very good or there really is no practical choice that isn't quite expensive and complicated. If you turn up your brightness pot and the LED overshoots in brightness and then settles, or similarly if you turn the brightness down, it does suggest response that is slower than it needs to be. But for your ap it shouldn't be a concern, though it might be somewhat annoying.
I'm new here and not sure I'll hang around for long. It's been kind of fun, but I could sink an enormous amount of time here. If I go away I'll try to remember to drop in for a look at this thread from time to time until you have achieved success or frustration has sent you to climb a clock tower with a ...
Good luck!
Heatsink - sounds good. 60-70°C is perfectly acceptable for long and reliable life. The boards made on aluminum are really good for high power surface mount since they spread heat way better than anything that can be done with the foils on a standard epoxy-glass substrate.
Eyesight - Ha! I used to be very near-sighted but now have plastic eyeballs (just plastic lenses, actually) and absolutely fixed focus eyes. It has a lot to do with why I don't actually do electronics anymore. My brother used to be a competitive marksman (bench rest, mostly).
I've done a lot of switcher design and used those green and blue cores extensively. They are quite a good compromise of performance and cost.
Board differences - a problem with rather a lot of stuff from China is that something decent gets copied - sort-of, and then maybe the copy gets copied, and often the copies are second-rate approximations. Until you get something, examine it and test it, you just have to hope for the best. From what I can see in photos, your board looks pretty good, but looks only count for a little. For example, for ceramic capacitors of the same nominal value and voltage there is often a choice of two or three or even more types, differing in the ceramic used, which affects the electrical behavior. The better type might cost 2 cents more, so someone who doesn't understand the difference buys the cheaper part that is inadequate for the task. (One of my clients once showed me a bunch of caps they bought to use on a board designed - I called them crapacitors.)
It takes so bloodly long for things to arrive from China that buying something and deciding you want to try a similar something is really really frustrating. I order a few things from Amazon marketplace vendors early last year and they were still drifting in three months later.
Stuff on perfboard: It may not be a problem, but I really do recommend some extensive testing with the original trimmer on the PCB. I wish we could scare up a schematic for your board - it would be very helpful in assessing sensitivity of that bit of circuitry to noise. Often the part of the circuit that measures the current is operating at millivolt levels, so it doesn't take much noise coupling in to scramble things. It's usually surrounded by circuits that switch high currents and voltages at high speed. I often say of switchers that everything is in conflict with everything else. It isn't much of an exaggeration.
I've seen and spent many many hours tracking down and fixing noise problems in switchers and may seemingly trivial changes that made things much better or much worse.
FET failure - I have no reasonable explanation or suspicion; on paper that FET looks very very good for the job
FETs (most semiconductors, actually) almost always fail short-circuit, though if that leads to really high current for long enough they'll blow open. I've had lots in packages much bigger than your FET blow the plastic paclage apart with a loud bang. But that was work on a 1200 watt AC powered switcher. They know lots of ways to frighten you. I'm like a nervous cat when I work on some of that stuff. I literally used blast shields sometimes - acrylic or plywood to shield me from bits that might go flying.
Best way to drive the LED: I think what you are trying to do is quite reasonable given what is available off-the shelf. There are some commercial high-power LED drivers that can be pulse-width modulated for dimming, but that isn't something you want for photography, since the PWM frequency is usually low enough that you could have a whole exposure at full brightness, a whole exposure at zero brightness, or shot with half & half if you're using a focal plane shutter. One drawback to changing the current to change brightness is that it does have a very small effect on color temperature, but precious few eyeballs on this planet would notice. (little aside on that - I used to do photography a long time ago. I was doing some GUI design about 3 years ago and set up what was supposed to be a neutral grey background. It looked too brownish to me. I dug out an old Kodak neutral grey test card, looked at it and concluded my memories of neutral grey were wrong, since it also looked brownish. After I had my eyes repaired, the brownish cast was gone. I'm actually really surprised color memory could be that good. For awhile I had one fixed eye and one bad eye. The color differences between the two were astounding.)
Dynamic response: For a fixed load like a LED and with a well regulated input supply, slow response shouldn't be any kind of an issue. Dynamic response comes into play for rapidly changing loads and/or source supplies.
From what I've seen, most boards like this kind of cobble the constant current circuitry into what is primarily a constant voltage regulator. Sometimes the CC circuity is made extra-slow, either because the designer isn't very good or there really is no practical choice that isn't quite expensive and complicated. If you turn up your brightness pot and the LED overshoots in brightness and then settles, or similarly if you turn the brightness down, it does suggest response that is slower than it needs to be. But for your ap it shouldn't be a concern, though it might be somewhat annoying.
I'm new here and not sure I'll hang around for long. It's been kind of fun, but I could sink an enormous amount of time here. If I go away I'll try to remember to drop in for a look at this thread from time to time until you have achieved success or frustration has sent you to climb a clock tower with a ...
Good luck!
