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)