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

Just discussion for fun. I think we'll have a traditional diode this round and 4% diode loss in 2:1 is ok with me, but the larger inductor doesn't actually help efficiency until ripple current saturates, so below 3.5W as 200% ripple means you hit zero current. Just means you use PWM from a little higher mark.

As for the cap, it's only handling the ripple current, and at very high frequency, so low charge (I*t) per cycle. This isn't 60hz AC. I'll check the numbers again by hand though. I did just kind of bash in the equation from that article for that one without much thought, other than following the idea about at the detail I just stated. Still I think there's a tendancy to think like the output cap is supporting power during off time, but the inductor is doing the bulk of that work, at least under optimal conditions.

Oh, one problem with diode loss, as they hot they start behaving more like resistors. That means Vf drops, that's good, but reverse leakage current also increases. That's bad. I have not even tried to estimate how much this matters if at all, but the point is yes, heat sinking the diodes well is probably needed.

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?

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.

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.

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.

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.

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

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.

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:

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.

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.

LOL…………just do it!

Not if you do it first.

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.

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

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

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.

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.

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