The Texas Buck driver series, Q8 / Skyray King 2S/4S buck driver RELEASED!

Yeah, in the end, it may be fun to see if we can estimate the losses, but there probably aren't that many choices of components really that come close, and you just pick the best from the ones that can fit, if any and hope for the best.

I found a couple of small buck converters on amazon that looked pretty nice. Maybe a couple were upward of 10 A. They were on the right size scale, single board, but not the right shape. Maybe it's worth seeing what they use? Then again, they might not really handle what they say they handle, or not well. I doubt they use any magic component that you don't see on digikey. Another possibility is your standards are too high. Maybe you can squeeze by with less than you think?

And you checked the Coilcraft line-up on the inductors? Arrow Industries is a good place to look as well as DigiKey and Mouser, probably not telling you anything…

WIthout trying hard (first click) I found a mosfet with lower capacitance, but higher Rdson. I don't know, maybe you can push harder on switching frequencies with lower capacitance? Might require running numbers to see what's a good tradeoff.

"In practice, switching frequencies higher than 1MHz may be difficult to obtain due to gate drive limitations, high input voltage, and thermal considerations."

Ooh, "thermal considerations" sounds like power loss to me.

Well a flintrock makes a little more light than a Flinstone at least.

Very possible, I was looking for low resistance when I searched as the calculator was showing something like 30w of power would be dissipated by the default MOSFET at our drive currents.

Thermal considerations are something to think about but I was more worried about the gate drive limitations myself. Thermal can be dealt with, gate drive can not.

Like I said, picking parts from scratch is NOT my strong suite, I am simply trying to find footprints at this point.

This is the closest inductor I could find to our specs: SRP1770TA-220M Bourns Inc. | Inductors, Coils, Chokes | DigiKey

Still only rated for 12A but I figure we could push it a bit to ~15A since the saturation current is 18A.

The issue is it is huge at 17mm, it is also pricey at 3.50 each.

Seriously though, maybe the first component to check is the driver IC.

http://www.ti.com/lit/an/slyt358/slyt358.pdf

"A synchronous converter with an integrated low-side MOSFET offers benefits such as reduced size, lower parts count, and easier design. "

I don't know why.

Look at that figure 3. Those moonlight efficiencies in the synchronous bucks look a bunch better. Synchronous means the diode is replaced with a switch. There seem to be drawbacks at high curent though and that's probably not a good trade. I haven't had time to dig into it.

I was just about to suggest searching on price, highest to lowest. I'd aim this project at quality.

I wonder if the tradeoff from low to high current is Vf*I vs I^2*Rds That would make sense. At low current I^2Rds wins. At high current, Vf*I wins. So.. why not put a switch and a diode in parallel (other than space and price obviously), then you get both? A smart controller would turn the switch off at high current too.

If you have a suggestion for a better buck IC I am all ears, like I said finding parts from scratch is not my strong suite.

Here is a VERY early first draft of the PCB, just put the components onto a 46mm pcb to see how fitment looked. All the small components are 0603 and I am sure some will need to be enlarged so as you can see it fits but there is not a lot of room to spare once you start running traces.

Once we nail down at least the footprints for the component list I will start routing traces.

Not sure what y’all are finding wrong with the Vishay SIR800DP but it’s proven to be extremely hardy, Richard used a single one of these to run 12 XHP-50’s at 34,000 lumens. Just sayin.

I’m using one 17mm FET driver with Zener mod to run 4 of the 9V MT-G2’s at 15,000 lumens. Also running 3 9V MT-G2’s in a BTU Shocker at around 10,000 lumens. Same driver, again, for 3 XHP-70’s at 14,455 lumens in another BTU Shocker. Pretty sure this MOSFET works, tried and true.

The sir800 is great for a driver that switches the ground but you can’t use an n-channel mosfet to switch the positive side, that is where the P-channel comes into play.

The one I linked to above is the P-channel version of the sir800 because we know that one works good. It has really low resistance which is great but the capacitance is higher then some which lowers the max frequency that you can run the buck driver at. This is turn means you have to use larger parts.

It is a trade off both ways, just a matter of figuring out the best options.

Ok, I could not leave it alone, first run at running traces and not tweaked at all but the circuit itself should be good.

Lots of copper in the ground pour, I tried to get it between most traces to minimize interference.

Not yet, spare time is tightening up. But a bunch of those thoughts came from Ti docs, so I guess they make something, but don't know the specs. Given enough time, I'm sure I'll take an interest to look (and understand the high current trade offs). At the rate you're going it may be irrelevant, which is great.

http://intl-outdoor.com/ld4b-24a-17mm-buck-driver-3v16v-p-817.html

hard to see exactly what's going on there. It's "only" 2.4A. But it seems the inductor is lofted over some of the components, and they made use of the hole in the middle. That might even be a nice driver.

That is a common way of mounting the inductors but it adds to the height and also those are not shielded coils. Both of which would cause issues that I think would be better avoided on the 46mm version of the driver. Once we have this size working that is an option to consider for the smaller versions if they are deemed a good option.

I haven't had much time but I did get to review that synchronous buck article more closely and the related IC's. It seems not so exciting. I had the right idea with I^Vf vs I^2Rdson as the main losses to compare, but that didn't give the picture. The main difference, and it makes the synchronous worse, not better( saw that wrong), seems to be in the afterthought there, that in discontinuous mode, the inductor still reverses flow through the low side FET, but not through the diode. So that dumps energy and makes at least a too simplistic synchronous buck bad in moonlight.

At reasonably higher current, I^2Rdson is apparently a better deal than IVf and the synchronous buck can win a bit. I suspect at some even more higher current this flips the other way again but that may be irrelevant.

Anyway, maybe kind of obvious, but diode losses (or low side FET) matters twice as much when driving 1S as driving 2S just because you spend twice as much time freewheeling for the low voltage output.

I guess the synchronous issues will get worked out with smarter switching, but anyway, the IC's I found for that are also much more complicated to implement and appear to be quite a big larger. So this lead nowhere new.

I found an in interesting article about the importance of the input capacitor and having it very close to the FET. I guess the IC doc covers that though.

Right now I am only worried about high current performance, that is what we have always had issues doing with buck drivers. Low current is easy and can always be taken care of with a 7135 if needed.

When you say input cap, you are talking about input for the LED or for the IC? Not sure we would have room for an input cap large enough to make any difference for the LED.

Not 7135, now you're following my mispeaks. If a 7135 reduces current from the buck, then it reduces current out of the buck and the buck has to operate at low current, and we're current regulating it anyway. You've only raised the output voltage slightly, still with the same current, and dumping the extra dVI into resistive loss while not improving anything.

But I made the same slip on the last page. What you mean to say is for lower current you do LED PWM. That needs and extra FET, but not a big one because you start with low current. This was brought up by a couple of people in the "more efficient driver" thread.

Anyway, the cap I was talking about was input to the FET, parallel to the source. In principle that does nothing since it's hard tied to the source (and the batteries probably have pretty enourmous capacitance themselves) but this note mentioned stray inductance and needing this cap very close to the FET because of that. The claimed effect was very impressive. I lost track of the link. I'll try to find it again.

I started (not completed yet) my own calculator based mostly on this:

http://powerelectronics.com/site-files/powerelectronics.com/files/archive/powerelectronics.com/mag/606PET25.pdf

A very practical summary of many of the basics. It's probably redundant with your spreadsheet but I'll compare later.

Anyway started looking at inductors. It seems to me there's quite a bit in 22mm * 22mm * 22uH. My initial impression is that 22uH would be nice, but size is still a pretty big problem there. It looks like you've started with a bit smaller footprint, which is probably sensible if it can work. Going shielded certainly takes more space. Aparently shielding above 200khz can be conductive shielding (thin metal, maybe tape) and I don't think you need that much space to get away from the fields. The irony is by requiring shielding you force things well up over 200khz anyway likely by not being able to get as much inductance. An open inductor with shielding tape, maybe extra added, at 300 to 500 khz might not be a bad way to go.

Anyway, I've only just started looking, and just started seeing how actual numbers fit in the math. Math aside I've seen 22uH actually used in a cheap 10W driver. So it's not an absurd number, maybe just absurd for 15A. I've noticed though that some of these inductors, like Vishay can keep going well above their saturation current. They just don't go as well, so things will get more wild and less efficient, but this is pushing up into turbo modes anyway, so so what.

12A (plus ripple) will run 2S 2P at 6A per diode. That's already a BUNCH. And it will run 4p at 3A per diode which is also plenty high really and you can still crank higher with some slop and heat. I've been aiming for 15 saturation so far, but it's probably higher than needed. This is why I would buck a Q8 at 4s batt 2s LED. It's the only way that makes sense to me. Many factors to trade off though.

http://www.eetimes.com/document.asp?doc_id=1273212

That meets more with my intuitive understanding of input capacitance, basically saying it shouldn't matter for a perfect source. I'll try to hunt down the other note.

But even my link in the post above, page 49, bottom, calculates a value for the input cap somehow without any reference to the source impedance or other source qualities. That's a bit puzzling to me but similarly to the other reference that I presently can't find.