On 3 Feb 2017 09:20:19 -0800, Winfield Hill <hill@rowland.harvard.edu> wrote:>Jim Thompson wrote... >> >> I work on the basis that my customers are happy with the models >> I provide, not on any basis that requires your approval. > > Are you primarily selling models as an end product, > or IC designs, in which you use your models to be > confidant of the result?I left out, in my previous answer... I do, often, use behavioral modeling to mock up a system (SOC) and check out the desired function before converting it to device-level. Allowed me to catch, for a recent example, a video DC restorer that clamped the sync tip to ground with a switch is NOT the way to go. What a switch does is drops the input impedance, requiring a very low source impedance to drag the clamped signal back positive... otherwise video distortion. The proper way is an active loop that uses a current pull-up to stop the sync tip at the desired level. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Thinking outside the box... producing elegant solutions.
Spice Diode Modeling of Forward Overshoot & Reverse Recovery
Started by ●January 30, 2017
Reply by ●February 3, 20172017-02-03
Reply by ●February 3, 20172017-02-03
On Wed, 01 Feb 2017 15:37:34 -0800, John Larkin wrote:> > Well, helping you sure turns out to be a stupid thing to do.Sigh. What a shame. For a minute there I thought you two were finally going to get along.
Reply by ●February 3, 20172017-02-03
"Phil Hobbs" <pcdhSpamMeSenseless@electrooptical.net> wrote in message news:yb2dna0H3qF7NwnFnZ2dnUU7-V3NnZ2d@supernews.com...> And since it doesn't model actual diffusion behaviour, I have no reason to > suppose that it would work in more complicated and realistic situations > such as tone bursts in wideband noise.Phil, Do you have any example data that illustrates the necessity of solving the transport equation exactly? Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Reply by ●February 3, 20172017-02-03
>Phil,>Do you have any example data that illustrates the necessity of solving the >transport equation exactly?>TimWell, JL's Grekhov diode pulser is a good one, as are normal SRDs. Even the lowly 1N914 will get charge density waves set up in it if you hit it with a fast ringy pulse, and the details of the response will depend on carrier diffusion. If I wanted to really model photodetection accurately, I'd need to take into account the 2-1/2 D (i.e. cylindrically symmetric) diffusion problem of carrier transport and light absorption vs depth and wavelength. If the PD had only a single contact rather than a ring, it would be a full 3D problem. This matters a lot, because delay and bandwidth are functions of position, charge distribution, and lateral voltage drops. And it all depends on the amount of reverse bias. I've measured all those effects, but not in nearly adequate detail for modelling. Of course if you know the analytic solution to enough special cases, you might possibly be able to cobble up something acceptably general for typical use cases. Optics and radar folks use the Geometric or Physical Theory of Diffraction (GTD/PTD), which works sort of like that. (Stealth technology is based on it, which is why the F-117A is all made of flat facets--you could simulate that on a 1970s computer.) However, being a transport problem, carrier dynamics is way more complicated than vanilla electromagnetics--it's more like plasma physics. It's simpler because the ions can't move, but more complicated because of traps and defects and stuff. So doing the GTD thing would be hard. Cheers Phil Hobbs
Reply by ●February 3, 20172017-02-03
On Tue, 31 Jan 2017 08:23:31 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:>On Mon, 30 Jan 2017 17:33:05 -0800, John Larkin ><jjlarkinxyxy@highlandtechnology.com> wrote: > >>On Mon, 30 Jan 2017 17:37:44 -0700, Jim Thompson >><To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote: >> >>>Nothing like a PhD telling me something can't be done to get my solver >>>genes working double-time... >>> >>>Over baby-back ribs at Firebirds this afternoon I realized how to >>>parameterize my method. >>> >>>But all the information I have is from "peer-reviewed" semi-BS IEEE >>>papers. >>> >>>Would someone be so kind as to take data on something common like a >>>1N914... forward overshoot and reverse recovery at various slew rates >>>and current levels? >>> >>>Then I can fit a known object. >>> >>>Thanks! >>> >>> ...Jim Thompson >> >>I posted this previously: >> >>https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_a.JPG >> >>https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_b.JPG >> >>https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_c.JPG >> >>https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_d.JPG >> >> >>That's a 1N914 from the middle of a 50-ohm transmission line to >>ground. You can see the pulse drive level on the P400 display; that's >>the unloaded generator output voltage... divide by 2 for the 50 ohm >>load. >> >>The diode is effectively driven by 25 ohms. >> >>Generator rise time is about 1 ns. >> >>In that last pic, you can see the diode holding up the falling edge of >>the pulse for a couple of ns until it runs out of stored charge. >> >>I wouldn't expect various "1N914" parts to be super similar. > >John, > >Could you repeat the experiment at different risetimes?Sorry, that would be a lot of work, and I'd need a sub-ns highish voltage pulse generator, which I don't have. The 1N914 isn't very interesting, and I'd never use it in a nanosecond or picosecond application. Heck, we rarely use 1N914s at all! Maybe I can kluge a resistor and a 1N914 onto the flipflop test rig that I have set up. It can make a 3.3 or maybe 5 volt step, apparently very fast. Barely possible.> >The theory (and gut) says the peak drops and "unsharpens" at slower >risetimes and eventually goes away...the rate at which it does this is >a measure of charge storage.Sure, when the pulse risetime is much slower than the recombination time, the recovery things vanish, and then LT Spice becomes accurate. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●February 3, 20172017-02-03
>> >>The theory (and gut) says the peak drops and "unsharpens" at slower >>risetimes and eventually goes away...the rate at which it does this is >>a measure of charge storage.>Sure, when the pulse risetime is much slower than the recombination >time, the recovery things vanish, and then LT Spice becomes accurate.I don't think it has to be that slow--in Si carriers can diffuse a long way before recombining. Cheers Phil Hobbs
Reply by ●February 3, 20172017-02-03
On Fri, 3 Feb 2017 18:31:16 -0600, "Tim Williams" <tiwill@seventransistorlabs.com> wrote:>"Phil Hobbs" <pcdhSpamMeSenseless@electrooptical.net> wrote in message >news:yb2dna0H3qF7NwnFnZ2dnUU7-V3NnZ2d@supernews.com... >> And since it doesn't model actual diffusion behaviour, I have no reason to >> suppose that it would work in more complicated and realistic situations >> such as tone bursts in wideband noise. > >Phil, > >Do you have any example data that illustrates the necessity of solving the >transport equation exactly? > >TimParticularly given the ability, in Spice, of creating a voltage-and-current-and-charge variable capacitor ?>:-} Then Spice can solve the so-called "transport equation". ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Thinking outside the box... producing elegant solutions.
Reply by ●February 3, 20172017-02-03
>Particularly given the ability, in Spice, of creating a >voltage-and-current-and-charge variable capacitor ?>:-}>Then Spice can solve the so-called "transport equation".I thought you didn't want to talk about it any more. But you're entirely wrong once again. Any sort of capacitor--nonlinear, time varying, whatever you like--still obeys an ODE. Transport doesn't. Cheers Phil Hobbs
Reply by ●February 3, 20172017-02-03
On Fri, 3 Feb 2017 18:24:38 -0800 (PST), pcdhobbs@gmail.com wrote:>>Particularly given the ability, in Spice, of creating a >>voltage-and-current-and-charge variable capacitor ?>:-} > >>Then Spice can solve the so-called "transport equation". > >I thought you didn't want to talk about it any more. > >But you're entirely wrong once again. Any sort of capacitor--nonlinear, time varying, whatever you like--still obeys an ODE. Transport doesn't. > >Cheers > >Phil Hobbs >Please explain _why_ you think Spice can't solve it. Stop thinking explicit solutions... Spice is a numeric machine. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Thinking outside the box... producing elegant solutions.
Reply by ●February 4, 20172017-02-04
On 02/03/2017 10:10 PM, Jim Thompson wrote:>>> Particularly given the ability, in Spice, of creating a >>> voltage-and-current-and-charge variable capacitor ?>:-} >> >>> Then Spice can solve the so-called "transport equation". >> >> I thought you didn't want to talk about it any more. >> >> But you're entirely wrong once again. Any sort of >> capacitor--nonlinear, time varying, whatever you like--still obeys >> an ODE. Transport doesn't.> Please explain _why_ you think Spice can't solve it. Stop thinking > explicit solutions... Spice is a numeric machine.A particular kind of numeric machine, but not a universal one, however clever a modeller you may be. SPICE is a pretty capable solver for sparse systems of nonlinear, coupled, ordinary differential equations, and suffices for most circuit purposes as well as a large class of other problems that can be reduced to such a system. A general system of N variables leads to an NxN matrix that has to be solved on each time step. The effort required goes as N**3, which gets old fast. Sparse systems have many fewer nonzero elements, typically just a few near the main diagonal. For instance, FDTD codes such as my EM simulator lead to a heptadiagonal matrix (seven entries per row, corresponding to the current volume element and six nearest neighbours. That kind is pretty simple to solve, in theory, taking a few times N operations per time step. (FDTD is a bit slower because you have to shrink the time step as you refine the mesh, so it goes like N**(4/3).) However, circuits aren't that regular, leading to sparse systems whose nonzero entries can be anywhere, but which typically have no more than five or so connections per node, which helps a lot. Like all (or nearly all) solvers for systems like that, SPICE uses iterative methods that take more steps but avoid filling in the zero entries. These methods aren't guaranteed to converge unless the system is numerically stable and the initial guess is sufficiently close, which is why SPICE needs all that help with convergence. It's a good tool all round, but it can only solve ODE systems unless the code is hacked up to add features (such as transmission lines). Transport problems such as gas motion are not in general expressible as systems of ODEs. Classical nonequilibrium gas transport is governed by the Boltzmann equation (see e.g. <https://en.wikipedia.org/wiki/Boltzmann_equation>) which is an irreducible integrodifferential equation. Carrier motion is more complicated because there are two kinds of carriers that move and recombine according to quantum rules, and because electric and magnetic fields are coupled to the phase space density, but it's still a set of coupled integrodifferential equations, and so can't be solved by SPICE. The difference is basically that integrodifferential equations have a lot of internal state that ODEs don't, and they can be nonlocal in time, i.e. the result of the next step doesn't depend merely on the observables from the previous step. For instance, a transmission line in SPICE is also an integral equation, because what comes out one end is a delayed replica of what went in the other end. You can't express that as a differential equation, because it has internal state that isn't visible at the terminals. You have to express it as a convolution integral with a shifted Dirac delta function (perhaps with a high frequency cutoff). ODE systems have a very rich set of possible responses, so (as you know better than I) it's possible to dream up ways to mimic the behaviour of many IDEs with SPICE models in restricted situations. For instance, if you know that the transmission line will only have pulses going into it, you can use a delayed source to mimic that. However, because these models don't correctly represent the internal state of the solutions of the transport equation, a change in conditions is very likely to make the mimicry fail. Sending charge density waves into a 1N914 by hitting it with a step with a big 2-GHz ring is an example. A model based only on the response to a clean step is unlikely to get that right, because it has no way of expressing the wave behaviour, which will have excursions in both density and momentum. Transmission lines are sufficiently important that it's worth hacking up the solver to handle them as a special case, which fortunately is quite simple. Transport is another kettle of fish entirely. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
