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

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leroycp
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Please let us know when these boards are available.

Texas_Ace
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The PCB is basically done but it is useless without the parts list to fill it so waiting till we get that knocked out before releasing the PCB. I don’t want people spending money on something that doesn’t work, or at least have the possibility of working.

leroycp
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Hope we find out soon. Like!!

Flintrock
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A little quick searching does look like there are better fet alternatives to solve this, need to compromise a little on rdson probably.  Might require a minor footprint/pad change. (maybe soic-8, so pretty close I think)  No time to pin it down right now.

Flintrock
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And yeah, like TA says, drawing a basic board that will get electricity in one side and out the other, well probably isn't easy at all actually, but the details of how well all the parts really work together are MUCH more complicated in a buck than a DD or linear driver.  We probably wont predict all the issues, which is a reason at some point to get on with it to see, but when it doesn't work you want to have some handle on as much as possible too so it's not just time to shrug.  Some of these things are easily predictable (probably even some of the ones we'll miss), and staring at a board that doesn't make light will only give so much insight into it.  Although it's been a little quiet here, I have been making progress on a few things slowly.   We just never really looked into the FET in any significant detail at all until now.  Not a big deal, but I have about 70 rows of calculations or inputs, none of which are a big deal.  Just takes time to get around to thinking about them all.  It will get there, probably.

Texas_Ace
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No problem, I have been swamped myself so I understand.

Flintrock
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A couple of Mosfets:

http://www.digikey.com/short/3bjdqf

http://www.digikey.com/short/3bjd8h

Both are very similar, same footprint to the previous one (better double check the pins)

Both about 20x less gate capacitance and about 10x more Rdson, which works out as a big win I think. (Notice the 74W one quotes gate charge at 4.5V and the 100W one at 10V, so they’re pretty similar once that's corrected).  I kind of like the 100W one but the difference is miniscule, so whichever costs you less.  Don't know what I was reading.  They are pretty similar but in fact the 75W one has lower gate charge and I'd pick it probably. Plus it's cheaper.

 

There are some other options with better numbers for rdson and gate charge, but either bigger, less standard, or worse thermal performance as far as I found.  I think these are a good compromise.  I don’t swear the the best possible, but they bring down a 2.2W low power drain to now about 0.2W.  

 

There is an Rdson hit at very high power, 5.3% loss at 20A, regardless of duty cycle.  That's 14W in 1:1 (but that's in a 280W output mode, lol).  It maybe a little less crazy to see this happening in 4:1 where that's only 3.7W because 20A 4:1 is only 70W total.  Either way, it's still a 5.3% loss at 20A though.

At a more reasonable 10A 1:1 output it's 4x less loss (I^2 R) so 3.5W, not so bad, especially since we saved 2W in gate drive (and maybe more in other fet capacitance and speed issues I suspect; these are just lighter on their toes).  I definitely think this is a good tradeoff.  Unless you only ever run your light on meltdown mode, these fets are better, but the buyer can choose.

 

I've got new numbers about ready.  I want to revisit input cap selection still.  I was wrong aout Coff charging.  On balance of all the numbers, we should leave it alone.  Further improvement can only be done by selecting the RC for the desired output voltage.

We're close.

Texas_Ace
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Those both look good, I agree either should work and just pick one based on cost and availability.

Flintrock
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Well I got confused (too many digikey windows, dizziness sets in).  They're both quoted at 4.5V, so I'd pick the cheaper 75W one then with lower gate charge: STL30P3LLH6

Flintrock
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If there's any competition, it's maybe this:

http://www.digikey.com/short/3bj1h8

STD37P3H6AG

 

Half the Rdson, but twice as much space.

Texas_Ace
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Those are only sold in 2500 increments, so I am gonna say those are not real practical due to that lol.

Although while it may be able to fit I think that keeping to the powerpak footprint is wiser.

Flintrock
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oops:  http://www.digikey.com/short/3bpccw

 

There are a ton of options (probably more actually, only stmicro seems to do the powerpaks) in these TO-252's as well (but this is the only one that's better on digikey at least), so still leaves choices, but I'm still leaning to the powerpak too for some reason.

 

I updated post 96, marked things as selected, and added model numbers in case links break.

Flintrock
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 I guess one mark against the powerpak is the same thing as the LDO drama, that people have powerpaks that look like these already here for other drivers (including possibly confusingly, some of yours), but trying to just use what you've got laying around is going to not work at best.  Packages are just packages though.  Mostly the powerpak is smaller, so can be used better when downsizing.

Texas_Ace
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Yeah, anything larger then the powerpak would have to be changed for a smaller driver anyways. Might as well start out with something that at least has a shot of being used in a smaller version.

Flintrock
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Ok, the price of that is about 2.5W of heat at 15A 1:1 from the extra Rdson.  I'm ok with that.  And it's already drawn.

Texas_Ace
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2.5W more then the larger FET?

If heat is an issue we can always upgrade later as well.

Flintrock
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Ok, here are the right numbers.  have to slow down a minute to remember what my own numbers do.

 

It's actually worse than that once an estimate of heat dependence is included.  By hand it's about 2.8W difference ( 15^2*0.015 *14/16.8)but the spreadsheet estimates roughly the effect of increased resistance at high temperature, 150C  junction temp (not case).

 

at 150C junction,  15A 1:1 the 30mohm powerpak becomes about 49 mohm(based on some generic curve, but it seems to be reasonable) giving 9.6W

the 15mohm TO-252 is well half of that so 4.8W for a 4.8W difference when hot.  

 

So to bracket it, between 2.8W and 4.8W difference at 210W total output, depending on heat sinking and how long you leave it in turbo mode.

 

Update: so for the powerpak 31 C/W  junction to PCB    2C/W junction to case.    At Tj 150C the Rdson is actually over 2x higher than at 25 so I'm still underestimating significantly for that particular fet.

 

The To-252 quote 50 C/W and 2.5 C/W for PCB and case but only a 50% rise in Rdson at 150C, and with fewer watts dissipated from the start so generating less heat anyway.

 

I don't really understand why there's so much thermal resistance to the PCB.   Anyway, it's called turbo mode for a reason, won't last long at all. 

 

Flintrock
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Another nice option:

http://www.digikey.com/short/3bprcz  TPN4R712MD

4.7mohn rdson

 

the 20V max is a bit scary though ( bucks ring)

http://www.digikey.com/short/3bpr78     (I'm liking this)

with 30V max and 8.4 mOhm Rdson

 

Twice the gate charge of both will mean about 0.4W  0.22 W  (updated because it's quoted at 10V) minimum draw at 1Mhz.  (Higher switch over to PWM helps optimize this out)

 

But at 3.5 x lower rdson compared to the previous powerpak, (2.8W 1:1 15A, when hot) a good option for a turbo oriented build.  Total power dissipation handling is less, but it won't need nearly as much either.

 

(These thermal resistances are hard to reconcile.  This one states 50C/w junction to ambient.  Same package as previous one, so if I assume still 30 C/w j to case, that's more delta T between metal/silicon to metal than insulated metal to air.  That's hard to believe. It's hard to really nail things down at this level, just have to choose one.)

 

 I'm liking this one.

Texas_Ace
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Good numbers, that last one is the best so far IMO. It not only has the best numbers but is also the cheapest interestingly. This driver is going to cost enough as is, anywhere we can save money and get better performance is a very good thing.

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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.

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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.

Flintrock
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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)

Texas_Ace
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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”.

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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.

Texas_Ace
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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.

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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. 

Flintrock
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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.

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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.

Flintrock
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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.

 

 

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Texas_Ace wrote:
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.

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!

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