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

Flashlight Modding and DIY Parts
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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.

77

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.

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

79

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.

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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.

81

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.

82

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.

83

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.

84

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.

85

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.

86

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.

87

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.

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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.

89

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.

90

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.

91

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.

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I don’t have time to read all of that but your summery is making sense. Using another small footprint FET to PWM the IC is an option, although it does add more components to the cost/build and complexity.

If we selected the right FET we could hopefully adjust the footprint to allow for a jumper to be used if someone did want moon mode lower then what the IC can do on it’s own (I still think it will be enough).

22uh is the best bang for the space I have come across, lower I don’t think will work at all. Higher is great but they quickly get too large to be practical. even at 22uh we are looking at close to 1mhz switching frequency according to the spreadsheet (I didn’t make it BTW, just found it online, no idea if it is correct).

Now for total current, I think we need to aim for 15 amps to start out with, it can always be lowered easily later but higher is much harder. Plus if it is stable at 15 amps then we know it will work fine at 10 amps.

For the Q8 we are going to be stuck with parallel LED’s on the stock MCPCB unless a drastic change was made and this driver became the stock driver for it (don’t see that happening). So high amps is a must. with 15 amps that is enough to drive 4x xhp35’s at around 3 amps + a bit of headroom. Or 4x xhp50’s at 3.75 amps (pretty low for them).

Like I said we can always go lower but higher is not so easy. For example with a custom 4S LED setup you would only need ~5-8 amps per LED which would be easy.

I would much rather design it for overkill and work backwards. Tis how I do things. Honestly if I thought it was possible I would be aiming for 20 amps on the Q8 driver. Just don’t think that we have enough room for that though.

93

Ripple kills LEDs so it’s a good idea to think about dealing with it otherwise you have to set current max lower.

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Yeah, it is something to be dealt with. It is mostly an issue of parts selection though as this particular IC allows for basically whatever ripple you want if you select the correct parts.

That said space quickly becomes an issue, as to eliminate ripple you need large components and/or very high switching frequencies. both of which provide their own challenges.

With the numbers I am working with right now, it looks like ripple should be able to be kept under ~250 mA, which is less then 2%. What it will do in the real world is yet to be seen though.

95

I think we can get one thing fixed (and that helps).

So I have a full caclulator setup now using that reference I posted. I did make it, but I didn't make the equations. I did check some of them and looking into others.

I'm not sure I agree now that 22uH is a must. I actually cannot find anything in 22uh above 12A rated (you can find higher saturation, so maybe good cooling can push one beyond spec) smaller than 22mm. I did find this comprimise: http://www.digikey.com/product-detail/en/vishay-dale/IHLP6767GZER150M11/541-1287-6-ND/ at 15uH 14A rated 17.15mm 12A saturation shielded inductor. I'm not worried about the saturation. At high current you can live with less inductance. This might be the part. (darn expensive though)

But I think we need smaller than 22mm. Basically everything above 10uH above 12A, shielded, below 22mm is 17.15 *17.15 or close enough to not matter. So I would say that is the footprint. What do you think? There are options here. You can get a 19A 10Uh if you want high power or a 12A 22uH if you want better low power efficiency, so the end user can decide.

Now as for 10uH, Plugging some numbers through my calculator, at 10uH you get 2 to 4% inductor ripple current at high power (15A) at 1MHz or 8 to 15% at 300 khz, but lowside cap of 10 or 20uF easily shields the output voltage from this. The problem is in low power, 0.25A per LED you start to get in the the neighborhood of 100 to 300% ripple current or more in 2S depending on frequency (200 means entering discontinuous mode I believe), 3 times less still in 1S. But that's not the end of the world. Ok, so at low power you enter discontinuous mode. We know that and have discussed how to deal with it. I'm still only getting 7% total power loss (my gut says it will be twice that from realities not calculated, edit: gut was right, had an error in inductor loss equation.) at the threshold to DCM. The question is how low and at what frequency? From the equations there I'm not seeing big switching losses, but I want to review that more. 1Mhz makes me concerned we'll see unpredictable losses from many parasitics we can't easily predict, but this probably isn't a footprint issue anyway.

As for the diode it does look like it will be the biggest loss (edit: in some situations), but it's ok and hardly an issue at all in 4S to 2S.

I'm not worried about 4p personally. Rewiring the LED board is simple, but I see your point.