Update: Varistor/MOV Spice Modeling

"Jim Thompson" <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote 
in message news:pg24lati560f9obr5rqcfji0ce8c02h23u@4ax.com...
>>and if he sees some
>>examples, maybe he'd even admit what it is, too ;-)
>>
>>Tim
>
> How about an NCP1072?  That was requested on the LTspice List >:-}

http://www.onsemi.com/PowerSolutions/supportDoc.do?type=models&rpn=NCP1072

"Document ID" says it's an "ISpice Model", but extension says it's a 
.DWG.... WTF?

Of course, an IC is something quite a bit more involved than a passive 
two-terminal.  It would take time, especially if no good resources are 
available.  I wouldn't do that for free (nor would I guess you, either!).

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com 

On Tue, 12 May 2015 18:54:48 -0500, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>"Jim Thompson" <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote 
>in message news:pg24lati560f9obr5rqcfji0ce8c02h23u@4ax.com...
>>>and if he sees some
>>>examples, maybe he'd even admit what it is, too ;-)
>>>
>>>Tim
>>
>> How about an NCP1072?  That was requested on the LTspice List >:-}
>
>http://www.onsemi.com/PowerSolutions/supportDoc.do?type=models&rpn=NCP1072
>
>"Document ID" says it's an "ISpice Model", but extension says it's a 
>.DWG.... WTF?

That link is certainly an AutoCAD Drawing (.DWG)

>
>Of course, an IC is something quite a bit more involved than a passive 
>two-terminal.  It would take time, especially if no good resources are 
>available.  I wouldn't do that for free (nor would I guess you, either!).

A typical I/C (or hybrid, as I've been doing lately) usually takes me
3-6 weeks.

>
>Tim

There _is_ an ISpice model of the NCP1072 rummaging around, but it
apparently won't run on anything but ISpice.  I haven't tried to
decipher its syntax into Berkeley-compliant... yet.
		
                                        ...Jim Thompson
-- 
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

"Ralph Barone" <address_is@invalid.invalid> wrote in message 
news:338871053453128354.323302address_is-invalid.invalid@shawnews...
> Perhaps not the right place in the thread to say this, but what problems
> would come up if you modelled a varistor as a behavioural current source
> with I = k* V^alpha in series with a bulk resistance? I've had pretty 
> good
> luck with that model (just not in SPICE).

I've seen that advertised before in appnotes, but it doesn't make sense. 
There's no physical reason to have a power law in a semiconducting device, 
and it suggests way more leakage current than there should be (the 
breakdown region might be softer than an exponential, but there still has 
to be an exponential leakage tail in there).  Perhaps it's just a crude 
fit to the random breakdown-ESR pattern.  In much the same way as 1/f 
noise (another odd order power law) can be modeled as a stacked chain of 
noise sources of random bandwidth.

SPICE won't appreciate it, because a negative number to a random power is 
likely to result in some random complex number.  You'd at least need 
abs(V) to start, then put the sign back on the current later.  Most of the 
derivatives all disappear at V=0, which doesn't help.

A symmetrical exponential function, like tanh, would probably do a good 
job, though being a bit too sharp.  As John says, connecting some 
resistors in series with that, then cleaning it up with a few more of 
different threshold voltages and ESRs, would do; but some may balk at this 
solution using "too many lines". :)

A single-line rendering of that isn't actually possible, because a 
"resistor in series with an exponential" is a transcendental equation, and 
has to be solved iteratively by the SPICE engine.  Your alternative would 
be to build a "dulled" tanh function (say, toning down the exponential 
asymptotes by taking the sqrt or something), but that is also impossible, 
because the only thing that's "dull" enough to tame an exponential is a 
log (any polynomial or power law just becomes a constant factor to the 
exponent).  But that simply undoes the exponent entirely, giving flat 
asymptotes; and doesn't work for negative values (see 
http://www.wolframalpha.com/input/?i=ln%28tanh%28x%29%29 ).

Probably, best would be to sit down with a spreadsheet and plug in curves 
until it works.  There's always boring old polynomials, which are probably 
quite a good idea in this case -- with the right combination of (complex) 
poles and zeroes, the function can be odd (= gives opposite current for 
negative argument) and the asymptotes can be linear or quadratic (linear 
would make sense in that it's the minimum ESR when all semiconducting 
grains are conducting).  With some tweaking, perhaps a non-geometric 
polynomial could be built that exhibits realistic leakage current, and 
approximates the V^alpha asymptote.

One can also make polynomials from other polynomials (Chebyshev and other 
named orthogonal polynomial series are typically better for building 
curve-fits than just throwing coefficients at a geometric series), or from 
other functions (e.g., the periodic polynomials in cos^n(phi) and such, 
useful for harmonic analysis).

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com 

"Tim Williams" <tiwill@seventransistorlabs.com> wrote:
> "Ralph Barone" <address_is@invalid.invalid> wrote in message 
> news:338871053453128354.323302address_is-invalid.invalid@shawnews...
>> Perhaps not the right place in the thread to say this, but what problems
>> would come up if you modelled a varistor as a behavioural current source
>> with I = k* V^alpha in series with a bulk resistance? I've had pretty 
>> good
>> luck with that model (just not in SPICE).
> 
> I've seen that advertised before in appnotes, but it doesn't make sense. 
> There's no physical reason to have a power law in a semiconducting device, 
> and it suggests way more leakage current than there should be (the 
> breakdown region might be softer than an exponential, but there still has 
> to be an exponential leakage tail in there).  Perhaps it's just a crude 
> fit to the random breakdown-ESR pattern.  In much the same way as 1/f 
> noise (another odd order power law) can be modeled as a stacked chain of 
> noise sources of random bandwidth.

I can't say whether it makes physical sense, but in the limited research
I've done into the characteristics of metal oxide varistors, the I = k*
V^alpha equation keeps popping up. In the curve matching that I have done
(in Excel, mind you), a bit of series resistance kept the curve from
flattening out too much at high applied voltages, and also made sense in
terms of the bulk resistance of the material (disregarding the grain
boundaries where all the non-linear magic occurs).

> SPICE won't appreciate it, because a negative number to a random power is 
> likely to result in some random complex number.  You'd at least need 
> abs(V) to start, then put the sign back on the current later.  Most of the 
> derivatives all disappear at V=0, which doesn't help.

Definitely, it would need a bit of a wrapper around it (sign() and abs()
functions) to make it symmetrical.  I tend to post from my phone, so
tenseness is rewarded (at least while I'm typing).

> A symmetrical exponential function, like tanh, would probably do a good 
> job, though being a bit too sharp.  As John says, connecting some 
> resistors in series with that, then cleaning it up with a few more of 
> different threshold voltages and ESRs, would do; but some may balk at this 
> solution using "too many lines". :)
> 
> A single-line rendering of that isn't actually possible, because a 
> "resistor in series with an exponential" is a transcendental equation, and 
> has to be solved iteratively by the SPICE engine.  Your alternative would 
> be to build a "dulled" tanh function (say, toning down the exponential 
> asymptotes by taking the sqrt or something), but that is also impossible, 
> because the only thing that's "dull" enough to tame an exponential is a 
> log (any polynomial or power law just becomes a constant factor to the 
> exponent).  But that simply undoes the exponent entirely, giving flat 
> asymptotes; and doesn't work for negative values (see 
> http://www.wolframalpha.com/input/?i=ln%28tanh%28x%29%29 ).
> 
> Probably, best would be to sit down with a spreadsheet and plug in curves 
> until it works.  There's always boring old polynomials, which are probably 
> quite a good idea in this case -- with the right combination of (complex) 
> poles and zeroes, the function can be odd (= gives opposite current for 
> negative argument) and the asymptotes can be linear or quadratic (linear 
> would make sense in that it's the minimum ESR when all semiconducting 
> grains are conducting).  With some tweaking, perhaps a non-geometric 
> polynomial could be built that exhibits realistic leakage current, and 
> approximates the V^alpha asymptote.
> 
> One can also make polynomials from other polynomials (Chebyshev and other 
> named orthogonal polynomial series are typically better for building 
> curve-fits than just throwing coefficients at a geometric series), or from 
> other functions (e.g., the periodic polynomials in cos^n(phi) and such, 
> useful for harmonic analysis).
> 
> Tim

On Tuesday, May 12, 2015 at 8:12:16 PM UTC-4, Tim Williams wrote:
> "Ralph Barone" <address_is@invalid.invalid> wrote in message 
> news:338871053453128354.323302address_is-invalid.invalid@shawnews...
> > Perhaps not the right place in the thread to say this, but what problems
> > would come up if you modelled a varistor as a behavioural current source
> > with I = k* V^alpha in series with a bulk resistance? I've had pretty 
> > good
> > luck with that model (just not in SPICE).
> 
> I've seen that advertised before in appnotes, but it doesn't make sense. 
> There's no physical reason to have a power law in a semiconducting device, 
> and it suggests way more leakage current than there should be (the 
> breakdown region might be softer than an exponential, but there still has 
> to be an exponential leakage tail in there).  Perhaps it's just a crude 
> fit to the random breakdown-ESR pattern.  In much the same way as 1/f 
> noise (another odd order power law) can be modeled as a stacked chain of 
> noise sources of random bandwidth.

Brownian motion and diffusion can have power laws.  
(I don't know how a varistor really works.) 

The "1/f" noise in some HV opamps went as the 2/3rd's power.  
(over several orders of magnitude in frequency...)  
I have no idea where the 2/3 comes from.  

George H.  
> 
> SPICE won't appreciate it, because a negative number to a random power is 
> likely to result in some random complex number.  You'd at least need 
> abs(V) to start, then put the sign back on the current later.  Most of the 
> derivatives all disappear at V=0, which doesn't help.
> 
> A symmetrical exponential function, like tanh, would probably do a good 
> job, though being a bit too sharp.  As John says, connecting some 
> resistors in series with that, then cleaning it up with a few more of 
> different threshold voltages and ESRs, would do; but some may balk at this 
> solution using "too many lines". :)
> 
> A single-line rendering of that isn't actually possible, because a 
> "resistor in series with an exponential" is a transcendental equation, and 
> has to be solved iteratively by the SPICE engine.  Your alternative would 
> be to build a "dulled" tanh function (say, toning down the exponential 
> asymptotes by taking the sqrt or something), but that is also impossible, 
> because the only thing that's "dull" enough to tame an exponential is a 
> log (any polynomial or power law just becomes a constant factor to the 
> exponent).  But that simply undoes the exponent entirely, giving flat 
> asymptotes; and doesn't work for negative values (see 
> http://www.wolframalpha.com/input/?i=ln%28tanh%28x%29%29 ).
> 
> Probably, best would be to sit down with a spreadsheet and plug in curves 
> until it works.  There's always boring old polynomials, which are probably 
> quite a good idea in this case -- with the right combination of (complex) 
> poles and zeroes, the function can be odd (= gives opposite current for 
> negative argument) and the asymptotes can be linear or quadratic (linear 
> would make sense in that it's the minimum ESR when all semiconducting 
> grains are conducting).  With some tweaking, perhaps a non-geometric 
> polynomial could be built that exhibits realistic leakage current, and 
> approximates the V^alpha asymptote.
> 
> One can also make polynomials from other polynomials (Chebyshev and other 
> named orthogonal polynomial series are typically better for building 
> curve-fits than just throwing coefficients at a geometric series), or from 
> other functions (e.g., the periodic polynomials in cos^n(phi) and such, 
> useful for harmonic analysis).
> 
> Tim
> 
> -- 
> Seven Transistor Labs, LLC
> Electrical Engineering Consultation and Contract Design
> Website: http://seventransistorlabs.com

On Wed, 13 May 2015 06:13:26 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Tuesday, May 12, 2015 at 8:12:16 PM UTC-4, Tim Williams wrote:
>> "Ralph Barone" <address_is@invalid.invalid> wrote in message 
>> news:338871053453128354.323302address_is-invalid.invalid@shawnews...
>> > Perhaps not the right place in the thread to say this, but what problems
>> > would come up if you modelled a varistor as a behavioural current source
>> > with I = k* V^alpha in series with a bulk resistance? I've had pretty 
>> > good
>> > luck with that model (just not in SPICE).
>> 
>> I've seen that advertised before in appnotes, but it doesn't make sense. 
>> There's no physical reason to have a power law in a semiconducting device, 
>> and it suggests way more leakage current than there should be (the 
>> breakdown region might be softer than an exponential, but there still has 
>> to be an exponential leakage tail in there).  Perhaps it's just a crude 
>> fit to the random breakdown-ESR pattern.  In much the same way as 1/f 
>> noise (another odd order power law) can be modeled as a stacked chain of 
>> noise sources of random bandwidth.
>
>Brownian motion and diffusion can have power laws.  
>(I don't know how a varistor really works.) 
>
>The "1/f" noise in some HV opamps went as the 2/3rd's power.  
>(over several orders of magnitude in frequency...)  
>I have no idea where the 2/3 comes from.  
>
>George H.  
>> 
>> SPICE won't appreciate it, because a negative number to a random power is 
>> likely to result in some random complex number.  You'd at least need 
>> abs(V) to start, then put the sign back on the current later.  Most of the 
>> derivatives all disappear at V=0, which doesn't help.
>> 
>> A symmetrical exponential function, like tanh, would probably do a good 
>> job, though being a bit too sharp.  As John says, connecting some 
>> resistors in series with that, then cleaning it up with a few more of 
>> different threshold voltages and ESRs, would do; but some may balk at this 
>> solution using "too many lines". :)
>> 
>> A single-line rendering of that isn't actually possible, because a 
>> "resistor in series with an exponential" is a transcendental equation, and 
>> has to be solved iteratively by the SPICE engine.  Your alternative would 
>> be to build a "dulled" tanh function (say, toning down the exponential 
>> asymptotes by taking the sqrt or something), but that is also impossible, 
>> because the only thing that's "dull" enough to tame an exponential is a 
>> log (any polynomial or power law just becomes a constant factor to the 
>> exponent).  But that simply undoes the exponent entirely, giving flat 
>> asymptotes; and doesn't work for negative values (see 
>> http://www.wolframalpha.com/input/?i=ln%28tanh%28x%29%29 ).
>> 
>> Probably, best would be to sit down with a spreadsheet and plug in curves 
>> until it works.  There's always boring old polynomials, which are probably 
>> quite a good idea in this case -- with the right combination of (complex) 
>> poles and zeroes, the function can be odd (= gives opposite current for 
>> negative argument) and the asymptotes can be linear or quadratic (linear 
>> would make sense in that it's the minimum ESR when all semiconducting 
>> grains are conducting).  With some tweaking, perhaps a non-geometric 
>> polynomial could be built that exhibits realistic leakage current, and 
>> approximates the V^alpha asymptote.
>> 
>> One can also make polynomials from other polynomials (Chebyshev and other 
>> named orthogonal polynomial series are typically better for building 
>> curve-fits than just throwing coefficients at a geometric series), or from 
>> other functions (e.g., the periodic polynomials in cos^n(phi) and such, 
>> useful for harmonic analysis).
>> 
>> Tim
>> 
>> -- 
>> Seven Transistor Labs, LLC
>> Electrical Engineering Consultation and Contract Design
>> Website: http://seventransistorlabs.com

The Varistor/MOV is nicely fitted with Hyperbolic functions... just
not my usual TANH... that's best for bounding.
		
                                        ...Jim Thompson
-- 
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.