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

[Flintrock]

So I did check the cap ripple equation and the bottom line is those numbers were actually too high. Basically what I scribbled down looks like equations 24 and 25 on page 6 here:

http://www.ti.com/lit/an/slva630a/slva630a.pdf (fixed the link)

For output ripple, really only the capacitive part ends up mattering, so what phase angle to add the resistive part in (which creates the other detail in that paper really) is irrelevant. The cap term is just Q/C where Q is average I*t and the rising time is haf of 1/f and the average current is half of the max current and the max current is half of the peak to peak, so with three halves you get v_p2p = 1/8 * I_p2p/(f*C) as per above link.

But the powerlectronics equation doesn’t really agree with that for a couple of reasons, which seem to me to just be wrong, and wrong usually actually too high. I’ll use equation 25 from Ti now which you can see still errs on the high side compared to their simulation.

Components:

Ok I looked at caps a little again. I’m sticking to my 1210 recommendation. There are no 10uF caps on digikey in x7r with ratings 35V or higher in anything smaller (and the 50V ones have the lowest ESR). Furthermore, I looked at larger caps and they get REALLY expensive in those kinds of specs:

http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=209394405

That’s a cheap one. The one’s in between cost twice as much.

and it’s not clear you can do any better. For output 10uF is enough. For input ESR is really the problem anyway and above 10uF the caps are pretty much all measured only at low frequency. Even for 10uF where they’re measured at 1khz, I’m not sure I trust extrapolation to 1Mhz well, but at least I can find some cheap ($.30)

http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=209396138

and some individually specced (rare, reading through these cap sheets is a pain) up to high frequency, even if not amazingly low ESR (average).

http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=209397082

I would recommend room for two or three in parallel on input though.

So I’m done with caps. If you can find something with more capacitance, less ESR, rated better for high frequency or in a smaller package, have at it, but this is the best I got. I am probably about done with diodes too once I post what I have, so a power Mosfet will wrap up the main bits.

[Flintrock]

@Texas_Ace

Yeah, I figured they would need a large thermal pad, although since it is positive and not ground it is hard to actually transfer that heat directly to the host via copper pour. Some silicone thermal cubes are quick and easy way of doing it but also a bit more involved for most and I would prefer that not be a requirement, just optional. A small heat sink could also be glued onto it but with no contact to anything I doubt it would make a large difference. A synchronous buck IC would be great but I sure have not found anything suitable, have you?

Wait, the diode doesn't connect to ground? (or maybe just the casing doesn't?). I better review the circuit. I've seen one Ti example where everything is upside down. Suitable syncrhonous? Not really. I found some IC with like 50 pins and a billion features that yes would work in principle and could handle boost circuits too, but... And it turns out thse "Or'ing" IC's are pretty slow and yes do leak during the 1/2 microsecond when they're closing.

I'm tracking components in post 96: https://budgetlightforum.com/t/-/41130/96

But your links here don't work: https://budgetlightforum.com/t/-/41130/69 post 69.

[Texas_Ace]

Odd, my links work fine for me but none of your digikey links for me.

We can put a cap in the input, just goes against my principle of large traces for large current as it all has to pass through the small cap. Although paralleling them is an option as well. I doubt I could find anything better, the issue wasn’t the exact cap to use, it was wether it was truly needed and would make a noticeable difference in the final product.

I will look at the parts list and update the PCB when I get a chance.

[Flintrock]

OK, I'll try to understand the digikey thing (probably will have to use two browsers to figure it out, sounds like the links might be linked to cookies or something). I better add model numbers.

The input current doesn't pass through the input cap. It's in parallel to the battery contact, and the traces for the cap only need to handle the ripple current, not full DC current. I think it will help protect from all the stray field stuff you're worried about, among other things.

As for what's really needed, you thought 10uF was small, so I don't know what to say. x7r seems required, and I think 2X actual voltage is standard minimum and those specs lead you to 1210 or at least 1206. There's just nothing else. Plus that samsung one is really cheap.

[Flintrock]

Wow, I see, yes, paste the link in a different browser, and they don't work!

[Flintrock]

I updated the digikey links in post 96. To get a good link you have to find the share button on the digikey product page. For my own links.. sometimes I have to re-enter the product number in the search to get the share button back.

[Texas_Ace]

@Flintrock

OK, I’ll try to understand the digikey thing (probably will have to use two browsers to figure it out, sounds like the links might be linked to cookies or something). I better add model numbers.

The input current doesn’t pass through the input cap. It’s in parallel to the battery contact, and the traces for the cap only need to handle the ripple current, not full DC current. I think it will help protect from all the stray field stuff you’re worried about, among other things.

As for what’s really needed, you thought 10uF was small, so I don’t know what to say. x7r seems required, and I think 2X actual voltage is standard minimum and those specs lead you to 1210 or at least 1206. There’s just nothing else. Plus that samsung one is really cheap.

Ah, ok now things clicked (been distracted recently and never really took the time to think this aspect through). For some reason I was picturing the input cap inline with the mosfet instead of parallel. Now everything makes perfect sense and 10uf should indeed work fine.

[Texas_Ace]

@Flintrock

I updated the digikey links in post 96. To get a good link you have to find the share button on the digikey product page. For my own links… sometimes I have to re-enter the product number in the search to get the share button back.

Interesting, I will keep this in mind.

I will be placing an order with arrow in the next few days due to the free shipping they have going right now, so I might see if they carry these items and grab a set to have on hand.

[Flintrock]

@Texas_Ace

@Flintrock

Ah, ok now things clicked (been distracted recently and never really took the time to think this aspect through). For some reason I was picturing the input cap inline with the mosfet instead of parallel. Now everything makes perfect sense and 10uf should indeed work fine. :+1:

It will work. Fine, well, you can get about a 3% efficiency boost for using 3 instead of 1 maybe more if real ESR at 1MHz is worse than predicted, or less if it's better. That's why it wouldn't hurt to add places for a few if there's no downside. $.30 for a percent more efficent? Enh, you now, I might do it, just because.

[leroycp]

LOL…………just do it!

[Flintrock]

@leroycp

LOL…just do it!

Not if you do it first.

[Texas_Ace]

@Flintrock

@Texas_Ace

@Flintrock

Ah, ok now things clicked (been distracted recently and never really took the time to think this aspect through). For some reason I was picturing the input cap inline with the mosfet instead of parallel. Now everything makes perfect sense and 10uf should indeed work fine. :+1:

It will work. Fine, well, you can get about a 3% efficiency boost for using 3 instead of 1 maybe more if real ESR at 1MHz is worse than predicted, or less if it’s better. That’s why it wouldn’t hurt to add places for a few if there’s no downside. $.30 for a percent more efficent? Enh, you now, I might do it, just because.

Yeah, in parallel is easy and there should be plenty of room, could even go with a larger form factor if there was a better option vs paralleling a few 10uf.

I will rearrange everything with the latest component list and see what the space looks like in the next few days.

[Flintrock]

Send a working link to that mosfet and diode you found if you get a chance.

[Texas_Ace]

remember these were just the first things I found and I didn’t peruse them much at all.

The P-channel version of the SIR800: http://www.digikey.com/short/39nh5h

This diode was literally the first thing I clicked on that had specs that should work, simply to see the form factor: http://www.digikey.com/short/39nh50

[Flintrock]

Great.

I still want to look over diode choice more soon. There might be things in more standard packaging without losing much.

Haven't searched for MOSFETs but on first glance that one looks pretty good. Still need to run reasonable numbers on switching losses when I get a minute for it.

[Flintrock]

Ok, I updated post 96 with a bunch of diode research. Basically there are good options in POWERDI5060-8, and in TO-263 (D2PAK). These can probably fit on the same pads if done right. Tiny heat sinks exist for the TO-263 which also need footprint and space consideration if used.

TO-220 through hole also has many options, in 2 or 3 pin. Those would certainly require some kind of heat sink, but are also easy to attach to one. If the surface mounts can handle the heat, it's probably better to use those instead. I see your point about the thermal pad being on the wrong side of the diode.

I think your SO-8 mosfet is almost certainly a fine footprint. So I think we're about all set for major components.

[Texas_Ace]

Great, I am real busy over the next few days but I will get around to updating the PCB before too long.

[Flintrock]

Now, about your circuit, I don't know about how C1in and R1Sense are arranged in the top right.

Edited: Never mind... I see Ti is actually intentionally reading ripple current, so it makes sense as is.

I'll start reading the manual more now. The constant off time and peak current control are interesting. It doesn't seem they change any of the fundamentals, just changes how frequency and current get controlled, so for instance in my table a particular frequency at one voltage output should probably imply a different particular frequency at another. I'll probably try to get familiar with it and re-arrange my tables instead of by frequency as by offtime RC constant. That will give a little better picture, but shouldn't impact parts selection I think.

[Flintrock]

Learning all the dirty laundry about this IC.

So if we use full input range for current adjust, 1.24V max on IADJ corresponds to 248mV of sense voltage. At 15A, VI is nearly 4 watts across the sense resistor. We could use half the range for control (lose resolution a little). Or add an amplifier (seems a bit drastic to save 5% power maybe).

However there is also this funny business where the peak current to turn off the switch jumps up and down because they keep flipping the voltage comparator every two cycles. See figure 24 page 15 and the discussion. To me this seems nuts. They claim it makes a more accurate average current, but accurate compared to what? It seems keeping the fixed target would be just as consistent but with less ripple and without all the annoying caveats that come after it. Actual inductor ripple will be higher than what I calculate because of this effect.

Enough grumbling, the point is they claim you should maintain a minimum ripple current (see equation 11), bigger than the variation in their sensing setpoint (that they caused), which they esitmate as maybe 24mV. Using full scale 248mV sense range that's already 10% at full current, so going to a much smaller sense resistor and smaller voltages is maybe not a great idea. Note this is not output ripple voltage though. Mostly we care about this because if we try to keep ripple high at full current, it's even higher at low current, and goes into discontinuous mode earlier. We'd rather not have high ripple current.

You have to read details there to understand it. Actually though I think it's ok to have ripple lower than what they say, at high power and I'm not sure I'd worry about the degraded current control they say it causes. Peak current still won't ever go above the max cuttoff(s) and the net effect seems to be you'll just get an effectively double off-time followed by twice as long of an on time, in other words, higher ripple current anyway (kind of by way of effectively lower frequency), but no need to target higher ripple and damage the low current performance at the same time in my opinion. Basically I would ignore the constraint in equation 11. It could make ramping uneven as you cross the threshold for double or tripple off-time, creating jumps in ripple current, since it's peak current that's controlled, not average and higher ripple for same peak lowers the average. But any given mode should still stay pretty steady.

I suspect when all is said and done, there's about 5 more places with 1W losses that are just too detailed to find, and all this brings 7V output down to 80% efficient at high power. It's a good reason to fight a percent or two here and there where possible though, like the input caps.

[Flintrock]

for footprint, 5W, 7 to 15mohm resistors are available in D2Pak's (like the diodes I listed) or in sizes from 2818 up to 4527. Bigger pads, lose more heat anyway, so. Could also make it through hole and try to sink it to the case. The D2PAK's are about $2.00 and the standard flat resistors are about $1.35 for the cheapest. This driver won't be cheap. The D2Pak's are rated at very high power, like 35W so might stay cooler.

In a 4 by xp-l setup this is only heading toward about 1.5W at 6A per led if tuned for 248mV sense at max current give or take (1S or 4S doesn't matter, so long as you tuned the resistor that way for either, duty factor cancels the current change). It's just driving 4 x 12V xph35's that pushes it close to 4W.