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

Ok, found the big components from arrow, they are actually a fair amount cheaper here as well:

Inductor: https://www.arrow.com/en/products/ihlp6767gzer150m11/vishay

Diode: https://www.arrow.com/en/products/stps30l30djf-tr/stmicroelectronics

Mofet: https://www.arrow.com/en/products/si7157dp-t1-ge3/vishay

Buck IC: https://www.arrow.com/en/products/lm3409mynopb/texas-instruments

First a minor thing: I have actually polished off some thoughts on Iadj, but no time to write it up. Basically the diode bias current is going to cause a voltage offset at high resistances, preventing to reach zero, but you get ripple using low resistances. None of it is terrible, but may we may need to live with some compromise between the two. It's mostly unrelated to the two resistor issue.

Now the bad news. For the first time I looked closer at the mosfet. That mosfet has 625nC of gate charge at 10V. I hadn't really noticed that before. That seemed a bit giant to me, and well, it kind of is. That's 63nf of gate capacitance!

Both the potential and charging energy is lost every cycle so just P=CV^2*f (no 1/2). Switching voltage is 6V, fixed in the IC (Vin-Vcc).

so 63E-9*6^2*1E6 = 2.268 W! (I hope I made a mistake here, but I don't think so)

That doesn't sound sooo aweful, but that's happening all the time. So at 2W power output, you've got 2.3W of switching loss. No fancy coff correction helps this. Coff, and frequency only are impacted by voltages (and are under our control anyway). This may finally be the reason PWM from full power doesn't work out so bad after all!

There are other mosfet losses that I've barely thought about. It seems very complicated actually and I likely won't ever find time or energy to deal with it all, but anyway this gives us a measure of at least part of it, and some relevant spec to improve.

I'll present the case about Coff when I get time to present it, but it's a triviality compared to this I think. Although the two will be very linked. This may drive the need to care more about how set the frequency.

Time to look carefully at what fets are available again. I read a doc, maybe Ti, exactly warning against overspecced large, low rdson fets, for this reason. We might need to look for something a little less beefy. However, I still probably don't much free time for a bit.

For comparison quickly the SIR404dp nfet has only 97 nC of gate charge. nfets are said to be better. Starting to see why. I might be giving that high side nfet driver another look, but not before looking for more suitable pfets.

Please let us know when these boards are available.

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.

Hope we find out soon. Like!!

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.

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.

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

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.

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

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

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

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

STD37P3H6AG

Half the Rdson, but twice as much space.

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.

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.

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.

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.

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.

2.5W more then the larger FET?

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

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.