LT Spice diode C-V graph

"Steve Wilson" <none@nospam.com> wrote in message 
news:XnsA35CD5685BD8idtokenpost@69.16.179.21...
> No simulation is perfect. Are you dissatisfied with the results of the
> LTspice simulations, or does the spherical cow reference apply to all
> simulations?

Yes.

All models / simulations are approximations.  No one really knows if 
there's anything beneath quarks and such -- Standard Model stuff -- but 
that just means, at the energy level and precision that atom smashers 
currently have, we can't tell if there's anything more.  It's the most 
exact we can determine at this time.

That said, computational models (FEA I suppose?) can evaluate stuff up to 
the level of atoms.  Not terrifically well I guess, but I don't know what 
the literature is up to on that.

Presumably, one can do a complete three-of-four-fundamental-forces 
simulation up to the level of atoms or even molecules, at which point, hot 
damn is your model ever going to be slow, you really need to start making 
approximations because you're calculating a lot of redundant 
information -- on the time and space scale of electron orbitals, the 
nucleus doesn't change or move at all (unless you're waiting for that rare 
moment when an unstable nucleus poops something out, but there are better 
ways of playing with that sort of thing).

Theoretically possible, but computationally intractible, to do anything 
more -- say, simulating a transistor from the atomic level to the 
electrical and thermal bulk level.

So from the lowest level to the highest level, you pick the most important 
part and go with it for your problem at hand.  Just be careful that you're 
taking the right parts, and that the other parts are sufficiently small 
not to mind.

This sort of thing has always been in physics.  While putting together an 
equation you have to integrate, you can tell which terms will be more or 
less important under different situations, temperature or scale or 
whatever.  Then instead of doing one intractible integral, you do two (or 
more) hard integrals, and say "for this range we use this, in that range 
we use the other one, and in this middle range, funny stuff happens". 
Which really is usually just interpolation, but sometimes not so nice 
(e.g. the discontinuity in Cp on 1st order phase transitions, a 
notoriously hard problem; which I don't know offhand if it's considered a 
solved problem now, and to what level, or what).

Tim

-- 
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com 

On 2014-07-01, RobertMacy <robert.a.macy@gmail.com> wrote:
> On Mon, 30 Jun 2014 15:49:00 -0700, Phil Hobbs <hobbs@electrooptical.net>  
> wrote:
>
>>> ...snip...
>> https://en.wikipedia.org/wiki/Spherical_cow
>>
>> I've been talking about LTspice's spherical cow universe on and off for  
>> the last couple of weeks.
>>
>> Cheers
>>
>> Phil Hobbs
>>
> Larkin posted a Wikipedia URL for the round cow, but now I seem to have  
> missed how that applies to LTspice ?? as a round cow universe ?? Arrrggg I  
> think I'll give up subtle thought.

spherical cows are easy to model.  as are diodes that doen't break down,

In the real world it hard to grow round cows* and petavolt diodes.




(*) cows start out spherical but by the time they're born they are
considerably less ideal.

-- 
umop apisdn

On Monday, June 30, 2014 5:07:03 PM UTC-4, Robert Macy wrote:
> On Mon, 30 Jun 2014 12:04:39 -0700, Phil Hobbs <hobbs@electrooptical.net>  
> 
> wrote:
> 
> 
> 
> >>> ...snip...
> 
> > Well, the Earth is nearly spherical already, which helps. ;)
> 
> >
> 
> > Cheers
> 
> >
> 
> > Phil Hobbs
> 
> >
> 
> 
> 
> 
> 
> Hmmm....let's see Mount Everest(sp?) is around 28,000 ft above sea level,  
> 
> and the earth's diameter approx 8,000 miles
> 
> That's like having a ball bearing 1 cm in diameter polished to 6 microns  
> 
> or so. Yep, pretty good sphere.
> 
> 
> 
> Turn that around a 1 cm ball polished to 1 micron is like having an earth  
> 
> with the highest peak around 4200 ft.  hmmm

This is a bunch of highlights, interesting, brief and easy to understand.

http://web.ics.purdue.edu/~ecalais/teaching/geodesy/Elements_of_geodesy.pdf

On Mon, 30 Jun 2014 08:34:16 -0700, RobertMacy <robert.a.macy@gmail.com>
wrote:

>On Mon, 30 Jun 2014 07:56:18 -0700, Jim Thompson =20
><To-Email-Use-The-Envelope-Icon@on-my-web-site.com> wrote:
>
>> ....snip...
>> Setting options to not show unnecessary data makes it easy to
>> separate.  Obviously you know what you titled it, so a good text
>> editor (like UltraEdit) easily finds it in the .OUT file.
>> 	=09
>>                                         ...Jim Thompson
>
>
>This comment is not aimed at you but myself... Amzingly just can't let =
go =20
>of what works to take on unknown methods that work even better, just are=
 =20
>not enticing enough to pursue.
>
>My primary example is using python, it works better all around for =20
>everything you want to do, but I just can't stop using octave to learn =20
>python.

Ummmm, no.  Comparing octave to Python is similar to comparing a
spreadsheet to Python; or apples to lamb chops.  Just not the same class
of thing.  Octave is more of a functional language and python is an
imperative language.

That said, i have seen some damn strange bits of software: like an
extension to Lotus 123 to do WYSIWYG word processing, an extension to
Dbase 3 to do financial calculations including compound interest and PV,
an extension to WordPerfect 5.1 (DOS mode) to do presentation graphics,
and there are a couple others that i forgot while typing this up.

?-)
=20

On Tuesday, 1 July 2014 11:36:35 UTC+10, Phil Hobbs  wrote:
> On 6/30/2014 8:58 PM, Steve Wilson wrote:
> > Phil Hobbs <hobbs@electrooptical.net> wrote: 
> > 
> >> https://en.wikipedia.org/wiki/Spherical_cow 
> > 
> >> I've been talking about LTspice's spherical cow universe on and off for the last couple of weeks. 
> > 
> > No simulation is perfect. Are you dissatisfied with the results of the LTspice simulations, or does the spherical cow reference apply to all simulations?
> 
> There are two general kinds of simulations, I think.  The first kind is 
where the physics is well known but the boundary conditions are 
complicated.  Most kinds of electromagnetic simulations are like that,  and so are lens designs, FEM simulations of mechanical systems made of rigid parts, static stellar structure calculations, and stuff like that.
>  
> Most of my simulators have been like that.  I've written a large-scale 
optimizing clusterized FDTD simulator, as well as simpler ones for thermal transfer and ray tracing.
>  
> The second kind is much squishier, and includes circuit simulations,  oceanography, weather, climate, dynamic stellar structure, predator/prey 
studies, yada yada.  Those require an enormous amount of ground truth to 
calibrate them, and even then you can't take anything for granted.
>  
> IC fabs work pretty hard to make their models match reality. 
> 
> Semiconductor companies don't take anything like that level of care in 
making models for board-level designs.

I suspect that they do, but only for the VBIC the models they make for their own use. The Gummell-Poon models they make available on their web-sites aren't a good.

Specifically, they don't model inverted mode operation of bipolar transistors very well - as I found out when I tried to model "squegging" in the Baxandall Class-D oscillator.

http://sophia-electronica.com/0344_001_Baxandal.pdf

Baxandall only refers to "squegging" in a footnote at the bottom of page 752, and the name is misleading - it is reported that it doesn't happen if you use MOSFET switches, though that creates its own problems.

http://sophia-electronica.com/Baxandall_parallel-resonant_Class-D_oscillator5.htm

I'd love to see whether a VBIC model of a bipolar transistor would squeg in an LTSpice simulation - there must a non-proprietary VBIC mdoel around somewhere, but I've yet to come across one.

<snip>

-- 
Bill Sloman, Sydney

On Tuesday, 1 July 2014 03:50:02 UTC+10, John Larkin  wrote:
> On Sun, 29 Jun 2014 21:21:20 -0700, RobertMacy 
> <robert.a.macy@gmail.com> wrote: 
> >On Sun, 29 Jun 2014 14:20:22 -0700, Jim Thompson   
> ><To-Email-Use-The-Envelope-Icon@on-my-web-site.com> wrote: 
> >
> >>>>>>> ...something about CJ0 being 4pF
> >>>>>>
> >>>>>> The PSpice model has the same CJ0. 
> >>>>>> 		
> > 
> > from my old PSpice .LIB not later than 92, possibly before
> >.MODEL D1N914	D(IS=0.1P RS=16 CJO=2P TT=12N BV=100 IBV=0.1P) 
> >.MODEL D1N914A	D(IS=0.1P RS=4 CJO=2P TT=12N BV=100 IBV=0.1P) 
> > 
> >isn't that 'initial' CJO like with zero volts across the junction?  arrrggg! can't remember anything about diode models tonight.
>  
> Yes. The LT Spice model has a Cjo of 4 pF. No apparent breakdown, as
they run happily at -100 KV.
>  
> Let the simulator beware!
>  
> Now, imagine the accuracy of climate modeling...

Since all that John Larkin knows about climate modelling is what he has learned from denialist propaganda planted in the Murdoch media, his imagination probably is the best data source he has got, not that he knows enough to appreciate this.

Actually, I'm being unfair here. John Larkin has actually posted links to denialist propaganda posted in the UK Daily Telegraph which isn't a Murdoch newspaper, though owned by people who are no less right-wing.

-- 
Bill Sloman, Sydney