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

So when y’all get to where you need it idiot proof tested, I’ll try to build one. :wink:

Richard says if there’s a tiny miniscule chance of something going wrong, I’m the guy that stumbles into it. lol So I hereby volunteer to be a guinea pig for this project.

LOL, I will keep that in mind. I think these have a good chance of working well but not of being cheap.

Once we nail down a parts list and design I will release everything open source.

Now all the numbers: (the third image is my favorite if you only look at one, but the component assumptions are in the top of the first image)

This time I’m showing everything across the board at 70W and 3.5W for apples to apples. That’s a bit low for 1:1 (5A) and very much pushing the limits of 4:1 (20A), but is a pretty good high power number for 4 xp-l’s in any configuration.

I’ve set the first image up with a fixed Coff RC for all voltages, tuned for 300khz and 1Mhz at 7V output. So this is what happens if you use the same setup at different voltages. However, I have changed the sense resistor so that max is 70W for all voltages (so not the same resistor). A 200W max setup will have less sense loss than this at 70W, but may have worse control at 3.5W if it can go that low at all.

You can see that this implies much lower frequencies (see “coff derived frequency”) for both 1:1 and 4:1. That results in large ripple current. Anything over 200% is discontinuous mode (which actually might not be such a bad thing, but I haven’t tackled it yet and would prefer to avoid it for now). Anyway, the numbers become meaningless for ripple much over 200, so basically the low power 14V results there are useless. It turns out this effect actually gets mostly worse with coff charged from a fixed voltage. The moral is, a driver should maybe be optimized for one configuration.

The next image is back to fixed frequencies, so a different Coff RC in each case. That’s more of a fair comparison of what each voltage output is capable of:

Finally here are configurations setup for constant high power current, 12A across the board. That seems a little too civilized for 4p xp-l at 3.5V (but may be all we can get anyway), is living on the edge for 14V xhp35, and is probably great for 2s xp-l at 7V if you’re a little brave, but anyway much closer to reality for all three. For low mode I still stuck with 3.5W because low mode doesn’t have power constraints and if you only want 3.5W of light, then that’s what you want.

This is my subjectively most real-world relevant set of values.

Some observations…

Frequency:

Mostly low frequency increases inductor ripple, important at low current/power, entering DCM sooner, and increases input cap voltage ripple at high current/power. However lower frequencies greatly reduce gate drive losses, improving low power efficiency, assuming things stay under control otherwise.

Cap ripple:

Output cap ripple and losses still look great after much checking of the math. (Note RMM said he had stability problems with such high output capacitance. I hope this is related to the hysteretic control on the Max, we’ll see.)

Input cap ripple. Fixed some bugs here. It’s not that bad, but could be a little better. Power loss is now fine, but there is some significant voltage ripple at 300khz. This assumes that the battery provides a constant average current and the cap provides the full pulsed ripple to the fet, possibly true (assumption used many places including by Ti). I don’t know what impact this has on output ripple voltage. I would think not too much actually. I guess it will drop max output voltage by 0.1V.

Since ESR is no longer a big concern, I could look for bigger caps, but they’ll have larger size. Maybe one large one is better than two small ones. None of these larger caps are tested/rated at high frequency though, and I just don’t have experience to guide this. I’ll see what I can figure out though.

Mosfet:

The higher rdson of the new mosfet shows its teeth in the 20A 4:1 (because it’s 20A not because it’s 4:1). Gate drive losses are down to 0.22W minimum at 1Mhz though, 10x lower than our original choice. Notes on mosfet losses are long and complex. This is just an estimate to help choose the best fet we can find.

Biggest effects:

The biggest differences from 14V to 7V to 3.5V are still from the inductor resistance and diode Vf*I A major change in performance requires cramming in through hole inductors, finding a suitable synchrounous IC, and using some crazy high side fet driver and rsense options that are expensive, more complex, and possibly open up other cans of worms, things for another version, if at all. I’m also finding some interesting low-side switched IC’s, but at this point will save that discussion until this version is finished off.

So the main thing if any to polish off is the input cap (it’s not so bad though), and then maybe just getting a systematic listing of caps and resistors for some desired setup.

(edited for spacing only)

Numbers look good. We could always meet in the middle for frequency with something like 500khz to try to get the best of both worlds as well but that should only change values and not component footprints.

So if we can nail down a final parts list and layout changes I can release this thing and see if it works!

Thanks for all the hard work Flint, I would not have had remotely enough time to sort all of the parts out, I franky don’t enjoy it and as such don’t find much time for it lol. PCB layout is not so bad, it is “direct interaction”.

I'd say it's good. There might be tad room for improvement with different input caps, and that probably would require bigger pads, but it's not a big deal.

Ok, in that case I guess we need to build a cart with all the parts to make it easy for people to order the right stuff so I can put it in the OP with the driver.

I if you have a list of all the components and their values I can try to put it together, I only saw the main components listed, not all the resistors and caps ect.

Or naturally you are welcome to put it together as well and might be a better idea since you are aware of all the little things that need to be taken into account with the components.

And yes, we're going to need different coff resistance for different voltage outputs probably anyway. I think rsense and the Roff are going to have to be user configured. Also the Mtn-Max has rsense as user configured. I give values in the tables for Roff for some frequencies assuming 470pf Coff (this is how Ti's example did it, start with 470pf and derive Roff as needed). You can interpolate pretty much linearly. I see absolutely no need for 1% resistors here as Ti uses.

Rsens=0.248/Imax For this you want 1% definitely.

I'm kind of liking 10k and 3.3k for Rf2 and Rf3 and a 10uf Coff cap Cf2 cap.

This gives you less than 1% control ripple and about 1% minimum output if that 5uA iadj current is accurate.

I'll revisit my notes on this and might re-optimize a little, but this should work.

All that mcu hardware is up to you but I guess is the same as your other drivers.

Come to think of it, maybe a through hole for Roff would be nice, to allow easy changes? Or just a big footprint for easy soldering? You could always modify max current then just in software. But as before, I guess through holes are difficult. I haven't done much of this smt soldering yet. My hot air station is in the mail. I have modified about 3 tiny smt components before with an iron and just scraped through by holding my breath.

I find SMD components to be FAR easier to solder then through hole but I also have a reflow station. It is WELL worth the investment if you plan to do SMD work.

I like to release things in a complete all-inclusive fashion so that new or experienced user alike know what they need in order to do and complete the project.

Naturally the sense resistor will need to be configured per driver, an easy to follow calculator for that is ok. or just a list of resistors and the corresponding current they will give.

Roff is something that should be able to have a simple table that says if you want X setup you need Y Roff.

Basically I am trying to put together a nice list of components that is easy to follow with a pre-made shipping cart with them included for the most common setups.

I am acting like anyone building these is a complete noob with no prior parts selection skills at all. Just a soldering station and the will to give building a driver a try.

Even I am having a hard time figuring out exactly what the final parts list is going to look like when it comes down to actually placing the order. I could figure it out naturally but I don’t want people to have to do that, I want them to have a list and simply click a link to have 90%+ of the components pop up for them.

This is nothing against you, you have done amazing work. I am just trying to find a final list that I can use to create a shopping cart out of and a clear explanation in the OP of exactly what the builder needs and needs to do to get this working.

I understand your point entirely. It's just not there right now. Only just figured out what mosfet to use yesterday and only worked out the leakage current issue on iadj a couple of days ago (before that I wanted 0.5s RC for soft transition as the premium touch, but without a huge cap it probably isn't happening. I may still look into the huge cap options). Mostly though, just have to sit down now and write up a short list mostly done above, but will make it more organized shortly.

This is exactly me! I need the list you’re trying to make for this reason. Thanks to both of you for working on this buck driver!

My hot air reflow station bit the dust Sunday. Something in the iron handle shorted, the handle got hot, and turning it off didn’t solve the smoking issue at that point, had to yank the plug. The main unit smells bad so I know it’s fried. Just when I was really getting dependent on it, too!

Any recommendations as to a good combination hot air flow station? I have an Hakko 888 soldering iron, so perhaps just the hot air station?

Anyone know if this X-Tronic 4040 is worth anything?

Looks to be decent, for a limited time they’re even throwing in a 12” gooseneck magnifier light……

Y’all have me wanting to try to reflow my own stuff now. I don’t know if I want to get that involved though.

Yeah, I totally understand and no pressure. I was simply saying to let me know when you get it all figured out so I could get it all put together into a nice easy-as-possible to understand post for people to try out.

You are doing good work, keep it up. :+1:

This is the reflow station I have: YIHUA 852D 2in1 Latest Soldering Rework Station for sale online | eBay

Well one like it, the first one that came up in ebay anyways.

It has served me very well. Not an issue with it to date. It works good, is stable and temperature control is good.

Only real complaints are it is not very high wattage so it takes a minute for it to heat up the iron and the iron is a normal china knock off without the best tips to heater fit (so it temp sags more on big things).

For the price though I am fully satisfied. A hakko is great but not 4x+ greater.

I find that the soulder you use makes a far larger differnce then the station. I recently got some new solder, after a buttload of research I settled on kester 63/37 #66/245 Flux in .6mm. It was hard to find but it was worth it. Flows super good, the iron stays clean and makes a great joint.

When I first learned that I could actually solder it was because of the right solder and the Hakko 888 station. After that, I bought the big roll of Kester and now I stick with it, wont’ buy anything else. Kester solder paste too, have tried others and the dynamics of the Kester just hold it head and shoulders above everything else I’ve used.

Kester is good indeed, just got to make sure you get the right stuff. Kester sells many many (way more then I first thought) types of solder and they all work differently.

The stuff I listed above is a no-clean solder with high flux content to allow easy hand soldering and good wetting.

The classic 44 kester works fine but you have to clean up the residue or it can screw up the parts over time.

…and I too.

I will tell of my own stupidity in the hopes that someone may learn something I obviously did not.

My cheap Yihua reflow/solder station has an on/off switch to power up the transformer in the rear of the unit. Individual on/off switches and temperature settings with digital readouts for each station on the front. I found that when I went to use the soldering iron Sunday the unit was in standby mode with - - showing on the screen. I fired it up anyway, and it overheated and smoked and died. Opening it up, the transformer overheated and melted down. This was obviously a result of my leaving it powered up overnight and then using it without a cool down. Now, I will say that there are 2 wires within the soldering handle that have all the insulation burned away, so something shorted in the handle and this may be the real reason it died. The handle of the iron was very cheaply made, with a plastic male/female thread connection that wouldn’t stay together.

I’m leary of purchasing another el-cheapo unit, but how do I know that a more expensive unit won’t still have these issues?

For the record, I didn’t buy this reflow station. It was in the modding kit I bought from Old-Lumens when he sold out.