I just started searching for schottkys. I way underestimated Vf. At 0.55V (ok 0.45 at 15A, less if it warms up well) ... schottky losses are looking very big in 1S output, about 9.5% vs 3.1 in 2S. I did find a synchronous IC that could work (bypassing this loss), but it was constant voltage, divider style, only. Might be dimmable with some hack.
There are 10uf caps available in 0805 as well, I have some that I ordered for C1 at some point.
22mm should be able to fit but would leave very little extra space for other components so we need to make sure they are all in order first.
Far as 1s vs 2s vs 4s. In the Q8 (what this driver is really meant for), it comes stock with 4x 3535 LED’s in parallel and another 4x 5050 led pads as optional, also in parallel (assuming that Thorfire uses our design).
The MCPCB design is setup for parallel only and switching it would be much harder then simply installing new DTP stars.
So with the setup it comes with the 1S 3V emitters it comes with are best driven by an FET as they will be needing 16-20 amps of current, while this buck driver could do that it would be a lot of extra cost and effort for very little gains unless the LED’s were rewired in series with individual stars, that is not worth it except to the extreme modders.
The 5050 pads are basically for XHP50’s, they are once again best driven by an FET driver due to the fact they could easily use 20+ amps. Once again this buck driver would work but if max light output is your goal we all know that the FET will win.
Now the last major mod idea is XHP35’s, they are obviously 4S voltage only and can not be driven with an FET due to the fact they like to fry like that. So that is where this buck driver comes in, it is to allow XPH35’s to be dropped into a Q8 for a high CRI 8000+ lumen monster of a light without a donut hole issue.
So 4s input to 4s output is the primary concern, the others are great to play with later and if it works with 4s to 4s it should work with any of them given component changes. The XHP35 is the only setup that requires a buck driver, the others would just be for the extreme modders that want the very best, even if they are spending as much on the driver as the light itself. Although most of them will want an FET anyways.
I had thought the PWM port on the IC should work but figured I had missed something.
I'm sorry, yeah, I misread (maybe a few times) as 4p being primary concern I see (didn't really know about xhp35's and hadn't looked yet). Well I still think 4s to 1s is nice even if 2s might be "impossible". Max output is not all that matters. The buck will give you controlled output much longer (in anything other than 4s to 4s at least). Can that light really sustain more than 50W or power without burning your hand? Turbo shine is neat, but for long use at say 30 to 50 W without battery sag getting in the way, a buck still seems useful. I'll have to get my hands on the light. It's hard for me to imagine how it could be so hard. Worst case you resolder the LED's with their bottom pads grounded down and use the top contacts to connect wires, unless their is really no space for a wire. Mildly complex mod but so is assembling one of these drivers.
Anyway, I will add 4s to my scenarios. I put in component specs and get out performance specs for as many use cases as I want. Still, all these scenarios have different difficulties. I don't think any one makes all the rest optimized. In 4S the diode loss won't matter at all for example, in 4:1 it's a big deal.
As for the caps, yes, I've found many in 0805. I didn't find any with high voltage rating. I guess they should be 30V minimum, but I definitely could have missed them.
This is the proposed MCPCB for the Q8, as you can see it would be very difficult to convert to series, although not impossible. That said the work to convert would be more then simply dropping in new stars, which is what I would do if converting but that is something I would only do for a show light.
The buck does not have to run at 4s, it can also be run at 2s2p levels.
Naturally the light will not handle that level of output long term, heck even stock it won’t handle turbo long term. We don’t care about that here at BLF, we want usable modes for using the light and then turbo modes for play time. lol
Driving 1s or 2s LED’s is nice but honestly unless they are in series you won’t gain a whole lot over the Texas Avenger driver I already have for the Q8 with the bank of 7135’s. Not nearly enough to be worth the price or effort IMHO.
Now if you convert the LED’s to series as well, then things get more interesting but that is far too much work unless going for max possible lumens (not something I chase much anymore).
Now the smaller versions of this driver will only need to worry about the ~6 amp range which is much more reasonable.
I have to find a place to post pics. My first idea goes like this. Isolate the cathode with a vertical cut to their left. If cutting from top to bottom, stop when you hit anode trace and turn right, cutting across the two through holes and stopping above led 3.
You can mount led 1 and 3 backwards and jumper the cathode to the area north of led pad 7.
That requires reflow. With just a bit more cutting (basically continue the horizontal cut out to both edges) and two jumpers you can do it without reflow. All this could easily be accomplished with a few cuts in the trace, zero ohm resitors and/or an extra solder pad prior that would allow reconfiguration. It's really unfortunate in a light intended to be moddable.
2S isn't just about max lumens. It's about keeping say 2A per led (or whatever it can sustain) from being affected fairly quickly by voltage sag. It gives you a setup where light performance has nothing to do with the battery voltage sag/depeleting, so long as it's capable of putting out the power required. But yes, you get max lumens too, so everyobody should love it. Anyway, I didn't understand about xph35's and I should have stopped to understand what you were saying there. I will try to post a pic later.
That's probably not even the most clever way (almost the least) just first things I saw. I think can get a solution with either no jumpers, only cutting, or certainly can get one with no reversing of led's and probably still only requiring one jumper. I'll try post pics later.
Bettter yet, you can use the extra led pads for the jumper(s). It might be possible to make this fully reversible/reconfigurable just by re-configuring 0-ohm leds on thee extra pads. Splitting the wire pads gives even more options. Now it's like a game. Is there a way to cut traces so that xml's can then be configured in 4p, 2s or 4s only by jumping large led pads, and changing wire connection points, without even resoldering leds? The thermal sensors might get in the way, not quite sure how they are wired.
Oh, you can mod the MCPCB to work in series, anything CAN be done. I said it was easier to install individual stars and I still say that is the case. With stars you simply install them, line them up and glue them down. All bolt in parts with no custom work needed. Even for me I prefer bolt in whenever possible. Why makes things harder then they have to be.
Personally I would have preferred the light be setup for 4s LED’s run by 4S cells from the start (maybe 2s2p LED’s). The overwhelming majority said they wanted 4s cells and LEDs though so that is what we have at this point.
Got to work with what we have.
To me this all seems MUCH easier than assembling a driver (which you still have to do but...) and it's quite a bit cheaper and faster than ordering new stars. If a few cuts in traces, and existing jumper points (the extra led pads, or just add proper jumper pads) can make this reconfigurable is it really too late to push it through?
(are they heat sink pads case grounded? do they need to be avoided electrically?)
Yes, the MCPCB is DTP. Like I said it could be converted but I personally hate cutting traces, I have always had massive issues getting a clean cut that doesn’t short out.
The $4 for new stars would be well worth the cost for me but to each their own.
Solved all that:
http://imgur.com/gallery/ebS7c (well solved in theory, I'd have to learn dip trace to actually solve it)
4p as shown, positive is always to the left (or negative if you prefer).
Can do 4p, 2s, or 4s. No cutting, only jumpers (4s requires jumpering two of the wire pads, although there is room to route a trace for a zero-ohm resistor instead for that too, but jumping the wire pads seems ok). LED's do not need to be reversed. This should work as a stock board. (I deteted the thermistors because they were in my way at the time, but this is just a concept version obviously)
I had considered this, actually had a design half made up for something like this but decided against it as people would complain that the traces were not big enough.
If Miller says he is ok with trying to change it at this point I can finish my first design and see what the Q8 guys think of it.
Either way it doesn’t effect this driver much. The goal is to design it for overkill and then work backwards for smaller versions. I figure I would like to work down to at least a 26mm and maybe get a 20mm if possible since the mtnmax is not available in that size. Although that would be limited to the tiny13 which is not a great option but thats for a later design.
What's wrong with the one above? The places where traces are narrow, it's obvious how to improve it, just didn't for the concept drawing. Random addressing is fine, but that plus laying it out for short jumpers for the most used paths without ever needing LED reversal is a bit more of a trick. I did try to keep the central pad/hole design the way it was because I didn't know the constraints. If I could put pads anywhere I want, traces get a little better yet, but I'd still do it like this basically.
Anyway, if I inspired movement on it, that's great to me.
There is nothing wrong with it, but if going for a fully customizable board, might as well go all out IMHO. I tossed together another version and posted it in the Q8 thread that allows any setup you could want using jumpers.
The issue is practicality of thorfire actually using it or any of our designs.
Get it to production. There are a number of us that wish it………………when the Q8 is ready. Hope we get an update from TH soon!
Edit.. moved my comments there. This is was buck driver thread.
Ok, I felt if the board had a chance it had to be now. Done all we can on that, so back to the buck.
So I'm mostly finding power diodes with about 500mV Vf. I'll get a summary of something soon. That produces 10% resistive power loss in the diode alone for all 1S configurations, regardless of current level. I don't like that.
But I did find a couple of gems like this:
Which is really slow (and can't find just how slow) and just not meant for this. It's half tempting to put one in parallel with a faster diode though, so it can kick in when it catches up, if it ever even does though.
I think without finding a current regulated synchrounous IC though, that's just reality. We could try to parallel two diodes (or more), but it's not guaraunteed to help much since whichever takes more power, gets hotter.. lowers Vf, and takes even more power. So I'll try to round up the best of the bunch, mostly already done probably.
I was also thinking about parallel diodes.
Keep in mind that the primary usage of this driver will be for 4S setups or 2s setups. For example if you were running 1s 3V LED’s then you would run the driver as 2s voltage. If using 2s LED’s then at 4s voltage and obviously 4s voltage for 4s LED’s.
I don’t see a reason to use 4S input to 1S output in any case. Although having the option is great naturally.
I tend to agree. There's not much reason for 4:1. 2:1 is showing about 3.4% loss for the diode, and 1:1 (close to it anyway) well I haven't run the numbers yet but it will be next to nothing, so it's not so bad.