favorite Spice speedups

If we can't reasonably sim it, we'll build it. I can imagine the three secondary switchers, negative impedance loads, making the first one oscillate. That's not a risk we want on our new delay generator.

Each reg needs output caps of unknown value, and feedforward caps in its fedback divider. May as well get all that right.

I wish TI knew all that.

I suspect they give their modelers tight budgets for how many hours they can spend per model.

Cheers

Phil Hobbs

One of my guys managed to get the TI switchers to run in LT Spice.

Then managed to speed it up. As in, add a milliohm of ESR to the bootstrap cap. Things like that.

Sounds super useful. Get him to post it at the LTspice group.

Cheers

Phil Hobbs

I'll clean it up some and post here at least.

The TPS54302 sim runs at bearable speed, but efficiency is mediocre. It's pulling 6T amps on the +12 input.

What might break the deadlock for computation is add a small series resistance to the capacitance after the first regulator to low pass filter it. The trick will be to find something modest enough to not affect the predictions much but sufficient to compute it more easily.

You might have to do something like that IRL too.

I expect they never expected you to daisy chain them back to back like that and the output of the first one really doesn't like facing the negative impedance dynamic load. Classic way to make an oscillator.

And that they never considered the output of one driving the inputs of several others.

Yup.

The misbehaviour there happens at the timescale of the loop bandwidth, so it shouldn't run pathologically slowly on that account.

Cheers

Phil Hobbs

Here's what I have so far. The first switcher sees a complex load, some of which is negative impedance. We'll split the board into noisy and quiet "Q" halves with ferrite beads bridging the power pours. That makes the impedances yet more interesting.

This being 2022, the design is dominated by what we can get.

We have managed to get the TI switchers to work in LT Spice at bearable speeds, tens of minutes per run instead of overnight. I don't trust the results, especially for the first switcher, the TPS54302.

Switchers driving switchers is common. Nobody likes going from 24 volts to 1 volt, even if you can get the parts.

Go to the TI or ADI web site and look at New Products. It's mind boggling. How can they support, and make good models for, all those parts? How long will they keep making them?

The sim has C7 1 nF to ground, not in the circuit diagram?

Well walwarts go from 230 V to 6 V etc, ratio even more So do my Meanwells from 230 V to 7 V Maybe the word is 'transformer' here..

Ringcores are cool:

That was to keep my drone airborne via a thin coax carrying high voltage transformed back to 7V at the drone:

I think that is just to get the sim un-stalled. As is the 100m ESR of the bootstrap cap.

Yes. High ratio transformers help.

I am struggling for every square mm of board area.

Parts seem to be available more lately, especially direct from TI.

I tested a 30 ga twisted pair of magnet wire. It failed at 1400 volts. Wire-wrap wire must be 10 KV-ish. You'd need a clever HV downconverter to use really light wire.

How about a single wire to power a drone?

It might be worth building one to see if it really is inclined to squeg in real circuits. The sim could be telling you something important.

Yes.

I have to disagree on the claim about stiff equations.That's not often the reason in my experience. Pretty much all practical circuits are stiff, and sim ok.

Very typical reasons for insanely short time steps are that the modal derivative is discontinuous. The math algorithm thus simply fails.

"If else" type constructs will do this.

One should use functions such as

y = v(1).tanh(gain.v(test)) + v(2).(1-tanh(gain.(v(test))

to select between one voltage or the other smoothly with a continuous derivative, gain sets how sharp the tanh "switches"

One need to make sure that there are no fast edges going directly into large caps with no series resistor, resulting in extremely large currents. Put a resistor across inductors. All switching edges should be RC filtered.

There is a list of dos and don't such as only span 12 orders for resistance,

The problem is that, and lets face the reality, most "pro" model makers don't know diddly squat about them....

The design of a good model is as hard as a good analog circuit design is. There aren't many good analog designers.... :-)

Model making requires some knowledge of how spice calculates to avoid the obvious problems.

I haven't looked at the model in question, but I do know that I have a basic functional current mode controller (CurrentModeController.sss) in SuperSpice running in around 5 secs, thus I find it difficult to hold that some more advanced controller model has to take many hours to run.

One of longest running examples in SuperSpice is SuperHetRadioFrontEnd.sss. It takes 2 minutes on my laptop which does around 90 GFlops.

For those of you that know about 60s/70s 6 transistor radios.

It has an AM input driving an oscillating mixer into 3 stages of IF at

The "stiff" nature of the circuit in that it needs to have the 1 MHz running many cycles and deal with the LF modulation.

Kevin Aylward

Well... :-) ......

I have truly run millions of sims.......so..... (Cadence, ASIC design, not SuperSpice)

Gear v Trap in practice really don't have much between them in terms of speed or convergence statistically.

I might use trap when there is the characteristic triangular numerical error. Generally always use trap for oscillators to avoid correct damping.

The main determiner of simulation time for a given circuit is reltol, tratol max, step time.

If these are not set "appropriately", results can be totally garbled.

The Spice3 default for tratol is 7. To ensure that off the bat, SMPS sim correctly trtol needs to be set to 1, however trtol=7 can run 3 times as fast as trtol=1.

Tests need to be done on the circuit to see if intermediate values can be reliably used.

The fundamental problem in these models is generally complete le lack of model design knowledge.

A model needs to be designed just as a real circuit. On has to know what causes slowdowns due to convergence problems.

Kevin Aylward

The power-supply guys I knew had no problem getting cascaded switchers (and linear final regulators) to work. They simulated the whole combined assembly of voltage regulators, and paid a lot of attention to loop filters and cascade filters. Not to mention cables between, if present. The whole circuit. All this being simulated using LTspice, but with proprietary locally developed models.

By extensive cut-and-paste reuse?

Joe Gwinn

Stiff systems are only one variety of ill-conditioned simulations, sure. As the wise man said, "you can't make anything foolproof because fools are so ingenious." ;)

Cheers

Phil Hobbs

Yes. We just decided to do a quick-turn 4-layer board with all four switchers. TI's spice models are insane and admittedly don't fully model the devices.

I just need a 12 volt 9 tera-amp bench supply to run it.

On a sunny day (Thu, 27 Oct 2022 09:33:12 -0700) it happened John Larkin snipped-for-privacy@highlandSNIPMEtechnology.com wrote in snipped-for-privacy@4ax.com:

How would that work? Maybe optical fiber for a high power laser? Never tried that, do have some optical fiber here.. These coaxes I use have teflon based insulation and are really good for some hundreds of volts, very light at that, The idea was to make as little RF as possible (am using about 100 kHz), put a small antenna on the drone and send the amplified received RF down the same coax (many MHz), or even use the whole coax as shortwave antenna, balloon would work too of course, just playing and investigating :-)

High frequency AC, working against the capacitance of the drone to the world. Some tuned circuit on each end.

Power over fiber is done, but it's expensive.

Tried Micro-coax? Really tiny stuff.

On a sunny day (Fri, 28 Oct 2022 04:38:54 -0700) it happened John Larkin snipped-for-privacy@highlandSNIPMEtechnology.com wrote in snipped-for-privacy@4ax.com:

Did not know about that, did some web searching now but on the Mouser link I see max 30V rms

Such a small drone wants about 70 W for flying:

So using high voltage AC to get the current in the coax down, and transforming and rectifying back at the drone side:

That's silly. Nothing breaks down at 30 volts. I expect it's good for a few KV at least. When I was a kid I ran RG59 at 12KV. That's another story.

Send DC up to avoid rectifying and filtering. There is probably some cute little switcher chip to go from, say, 150 DC down to 7.