Sorry everyone that I haven't responded or provided updates - I got incredibly busy with work again, and I've had very little time to keep up my with this project. Finally got a big project finished up yesterday, so I'll have a little more time to put into it. The new circuit boards shipped yesterday, meaning I will have them on Monday.
It's my pleasure! I'm thrilled that some people are following my development of this project - I think I said it back in the beginning of the thread, but it's so much fun to develop in a public forum where others can share and comment on the progress. Most of what I do is very structured and the vast majority will never be shared with but a few people.
I see a lot of build threads around here, so I'm not so sure it's rare, but I'm happy that it's appreciated anyway. Please don't hesitate to ask questions either! There's a reason I'm posting the entire design and build experience.
Yeah, I decided from the beginning that if I was going to design something, it was going to be ground-up. The only off the shelf parts are the components on the circuit boards and the optics - and even then, I'm not using a traditional Switch-Mode controller. I instead opted for a single DSP that will control the switch gates of all 8 supplies. In an LED light-bar like this, I'm breaking new ground, for sure. In fact, there are very few products in the world right now that use a DSP to control the SMPS of even a single LED - let alone 24 high power ones.
The comment about the pros making mistakes.. Trust me, I'm fully aware.. And disappointingly, I am one of the 'pros'.. I'm not really that upset about it - it happens. I'm helping another engineer right now with a product (commercial nature) that is still not working at Rev Q (Yep, ABCDEFGHIJKLMNOPQ) - and he's a very good engineer!
I'm going to be using three different optics. They're all the CUTE-3 optics from LEDiL, but I'll be using four of the narrow optics, two of the medium optics and two of the wide. This should give me a powerful throw and a ton of wide angle spill and cover everything in between. I'm expecting a long-reaching WALL of light. In the unit, the optics will be positioned, W-M-N-N-N-N-M-W.
Each triple is a series of the three emitters (roughly 9.9V Vf). The triples are individually driven using their own SMPS controlled by the DSP, so they're independent from each other. I could run every triple at a different current, if I wanted.
I have modeled it using the very thin FR-4 circuit board with tons of vias (which performs BETTER than an MCPCB), and I modeled the emitter itself using CREE's published thermal resistance through the package. At 3A per emitter (10W, roughly), I'm looking at a DeltaT of about 31C from the emitter to the enclosure (25C through the emitter package and 6C through the PCB). at my 100C trip-point, the emitters themselves will be running at about 131C. 100C is the trip-point, though, not the expected run-temperature. My study suggests that even on a very hot 90F day, the enclosure temp should not exceed about 78C - and this is worst case scenario with almost no air-movement. Just a little air-movement from wind or from driving would reduce that temperature drastically (10s of C). Real-world, normal use temperature of the enclosure is probably about 60-65C, meaning the emitters would be running 91-96C.
When talking about the enclosure to LED temp, you mentioned 5 or 10C. An MCPCB 'might' be able to maintain 5 or 10C from enclosure to LED Package, but remember the temp that matters isn't package - it's the actual diode junction. The XM-L has a 2.5C/W thermal resistance from Packge to Junction, so a MCPCB that could maintain 10C from Enclosure to Package would be able to maintain a 35C Delta from Enclosure to Junction when the emitter is running at 10 Watts. To get an XM-L to output 100% of it's rated Lumens, one would have to mount it on an MCPCB and then keep the MCPCB at minus 5 or 10C.
CREE rates the XM-L to operate at up to 150C Junction temperature, so with 31C Delta between enclosure and junction, I'm perfectly safe cutting off at 100C enclosure temp.
The bluetooth works! I mounted the bluetooth module on the Rev.A circuit board and put the pcb in the enclsoure. I was able to make a connection to the module using my Netbook, send data to the DSP and get data back! Range is pretty good, as well - about 30 yards (meters) if I am somewhere in front of the enclosure and 18-20 yards (meters) if I'm behind it. Much better than I expected, actually - since the bluetooth module is wrapped in a 4 pound aluminum shell. I'm just using the bluetooth module as a dumb bluetooth->serial converter since I can handle the data-packet in the DSP.
I started cobbling together a bit of android code, but god, I'm definitely no Android developer. If anyone here is good at writing Android code, I'll trade you some hardware engineering/design for some Code :)
New circuit boards showed up yesterday. They look great. Since I was re-spinning them anyway, I made a few subtle changes. The biggest change was adding small dots of solder mask to the back side of the via's in the thermal pads of the XM-Ls. This will prevent solder from wicking down through the via's and getting all over the gold on the back-side of the PCB. Of course, the new FET Gate drivers now have a home and I moved the one inductor that was just a bit too close to the optics for comfort. Overall, I'm very pleased with the new boards.
Front Side:
Back Side:
I started soldering down some of the components, and I was really hoping to get further along, but it has just been too long of a day - I'm exhausted, and an abundance of coffee has made my hands as wobbly as a one-legged table. The DSP is installed, 6 XM-L emitters, and a bunch of the supporting parts. Tomorrow I'll finish up the caps, resistors, fets and gate-drivers. Should be making lumens by tomorrow.
I have to remove the bluetooth radio from the Rev.A board as well - but that's no big deal, so that part will be on the new board as well.
Haha.. trust me, no-one's expectations are higher than mine - just as it sits now, that's $60.00 worth of XM-L's ready to burst into flames if something isn't right :)
I continue to be amazed at the ingenuity that I see here. If you were in DK or even Europe I'd have to go see you real life to co-develop some ideas I have for industrial LED lighting in EX zones on ships. That would be awesome! Maybe not as awesome as this but still...
It works, and it works quite great, in fact. Well, it did work anyway :) I smoked 3 XM-Ls, but it was my fault - not the electronics. Everything was working great, and 6 XM-Ls were burning away at 1.5Amps each - looking good...
So I started cranking up the input voltage to see how the DSP handled the larger Delta-V from input to output.. Got to 60V and it was doing great - The DSP kept backing off the Buck converter keeping a nice steady 1.5 Amps going to the emitters.. Here's me, wanting to look at the signals - Output of MOSFET Gate Driver - beautiful. FAST rise and fall times. Output of Current-Sense Op-Amp - Really Nice.. Clean.. And then.. Slip, and I shorted the high side input of the op-amp to ground with the scope probe... The output of the op-amp went low (as it should), The DSP's internal comparator went low (as it should), the DSP instantly ramped the buck-converter up to 100% duty cycle (as it should), and the XML-s (near) instantly saw 60V accross the group of 3.. 20V each.. BAM, POP, Smoke, and I'm still seeing spots. For about 1 micro-second, XML-s are REALLY bright at 20V :) .. The perils of poking around with an oscilloscope..
So, I've got a perfectly functioning board! All tests out perfectly. I placed the order for 24 brand-new XM-L emitters (going Neutral White T6 instead of Cool White U2) and they should be here Friday. I'm also going to swap out the 15uH inductors for 22uH since they're back in stock - 22uH is really what I wanted to begin with, but CoilCraft was out of stock on them, so I went with the 15s. For this load 15uH is just not quite enough - the current rises and falls a bit too quickly. It doesn't take more than 10 minutes to swap out the 8 inductors, so I'm going to get the ones I really want on there..
Other than the little mishap, I couldn't be happier. If I can keep my grubby little scope probe out of places it doesn't belong, this board will be making more than 20,000 lumens by the weekend.
Now on the the control software in the DSP. All it does right now is turn the LEDs on or off - time to starting coding up the communications routines (Bluetooth) and the dimming routines. Luckily, now that the DSP is properly controlling current (the hard part), there's nothing I can really do to "blow things up".
I really can't explain how bright those three emitters were for a fraction of a second - it was like someone cut off a small piece of the sun and dropped it on the desk in my lab. I wasn't looking anywhere near the emitters and I was still absolutely blinded. I don't recommend feeding XM-L LEDs 20V - it's truly dangerous to the eyes.
Stop wondering, and don't try it. It probably wouldn't have been quite as bad if I had been using 5 18650's in series - They would somewhat self current-limit at somewhere between 4 and 10 amps.. The supply I was hooked to though can deliver 100 Amps - and I'm confident that it did for the fraction of a second before the bond wires vaporized. Because of the Voltage-Current curve on LED's (diodes), at 20V they would conduct essentially infinite current. I'm just glad the bond-wires in the emitter vaporized before the traces on the circuit board did! I'm tired of soldering 64 pin fine-pitch QFN's by hand :)
I can really really see the design and thought that has gone into this design!, something that is very rarely seen these days. I have many ideas in my head of what I want to be achieving, but my gosh I don't have anywhere near your ability!, such as Hand soldering QFN packages (OMG!), enough said!
I don't have a distinct need for one, but my gosh, if you do down the line consider putting another one together, I'm very much interested! Even if its just to appreciate the engineering and thought that has gone into this!
Thanks very much - your comments and compliments are very much appreciated. There has definately been a lot of thought put into this thing - and thus far, it's been a lot of fun. I never get the chance to design something for myself, and I finally decided it was time.
Surprisingly, soldering qfn's by hand isn't really that 'difficult' - it's just time consuming and tedious. Maybe I'll post a video of how to do it some day - Most people could do it, it just takes a bit of patience.
If I ever decide to build more - I'll make sure to drop you a note.
Yes, the optics can easily be switched even after the whole thing is fully assembled. Remove the front bezel and pop the optics right out.
To my knowledge, the only thing that will fit my emitter arrangement is the CUTE-3 Optics from LEDiL, so I'm stuck with those, but I could Use Wide, Medium and Narrow is different ratios.
yeah, that should do the job :) I didn't even realise that you can't get Cute-3 elliptical optics, which kind of makes sense as you'd have to use the triple in a certain orientation for it to work, but it's still a bit of a shame because those wides are going to be putting a lot of their light up into the sky (and mediums and narrows to lesser and lesser extents). I'm more used to 20mm optics where there's a bit more variety.
that's a really neat bit of engineering. Varying current to different triples would be handy if you wanted to vary the beam within a certain thermal envelope too.
I don't doubt that you've done your modelling and the thermal paths look exceptional, I'm only wondering if the efficiency losses from running the LEDs at higher currents are worth it, when you could get a similar light output at a lower current and LED temperature. A guy called Troutie on the mtbr DIY light forum has done some tests of light output vs. temperature for the XM-L and it was pretty instructive, even if I can't find it any more :(
Both of my lights (~10W and ~20W, so a shadow of yours) have their thermal trip set at 60C, so that the LEDs should never run at more than 90C, although that is more to protect the driver. Their real world temperature is most likely a lot lower than that. I just think that a trip point set so high doesn't give you much of a margin, that's all.
I would be really happy if the CUTE-3 Optics were available in an elliptical, but sadly, they're not. Thankfully, although I'm going to be pointing a bit of the beam up into the sky and down into the dirt, I'm pretty confident that I have enough raw lumens that it just won't matter.
Not only does it help keeping thermals under control, but because I have the different optics, it also allows me to vary the beam pattern. I can turn on All flood, All Spot, or any combination that I want.
I have little doubt that you're quite right - Almost as many lumens will be produced at 2 amps as will be at 3 because of thermal sag. That's why the dimming function exists. Most of the time, I can run at 66% (2 Amps) which will be more than adequate for most situations - when I really need max power though - it's available. Unless my modeling is way off, the 100C trip point is still 20C shy of being destructive to the emitters. For me, that's plenty of margin. Keep in mind also that my thermal model suggests that 100C would be almost impossible to reach - 78C is max expected even under really extreme circumstances (90F Ambient with practically zero air-flow). 100C is the Safety Shut-off point, it has nothing to do with lumen efficiency. (edited - It's actually not a "Shut-Off" point, it's a back off point. The light will not go off, it will just self-dim to maintain < 100C)
Dimming Code Finished. Small demo - The bluetooth communications routines are not done yet, so this isn't being 'controlled' by anything - the DSP is auto-looping from max bright to zero and back again. Right now, the current limit is set to 1.2Amps instead of the full 3 because I don't have any heat-sinking behind the circuit board.
