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SPICE gets it wrong

Started by Tim Williams May 7, 2017
On Mon, 08 May 2017 13:31:49 -0500, Tim Wescott
<seemywebsite@myfooter.really> wrote:

>On Sun, 07 May 2017 19:14:26 -0500, Tim Williams wrote: > >> Speaking of SPICE, here's a nice little circuit: >> https://www.seventransistorlabs.com/Images/BakerDarlington.png >> >> I discovered the oscillator on the left, when attempting to build a >> Darlington transistor pair with a Baker clamp. (In practice, R10 comes >> from a logic voltage input, and R2 is a load, often inductive. R14 is >> optional, but exacerbates the oscillation, making it reproducible.) >> >> The model on the left, of course, doesn't produce any oscillation: it >> reaches equilibrium within a couple hundred nanoseconds. (Which is kind >> of suspiciously fast for a TIP31 to do much of anything, I might add.) >> >> The simulation on the right, showing semi-reasonable parasitics, was >> necessary to reproduce the oscillations. The parasitics are in >> reasonable locations, but their values are completely unreasonable. >> >> The left circuit appears to be a limit cycle of an chaotic system; >> adding C1 to the real circuit introduces what looks like rising-edge >> crossover distortion, and adding a few other things causes all-out >> chaos. >> >> (Incidentally, if you'd like to volunteer solutions that stabilize the >> right hand circuit -- other than reducing and removing the parasitics, >> which apparently isn't an option -- I'd be interested to hear them.) >> >> I think the TL;DR is: >> 1. TIP31Cs suck as switching transistors to begin with, >> 2. The TIP31C SPICE model sucks, despite them having half a century to >> try and get it right. >> >> Tim > >Interelectrode capacitances aren't enough to make it pop off? You need >the lead inductances?
I think that wirebond inductances are the big contributor to transistor oscillation. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On 05/08/2017 02:31 PM, Tim Wescott wrote:

>> I think the TL;DR is: >> 1. TIP31Cs suck as switching transistors to begin with, >> 2. The TIP31C SPICE model sucks, despite them having half a century to >> try and get it right. >> >> Tim > > Interelectrode capacitances aren't enough to make it pop off? You need > the lead inductances?
I suspect the steadily rising impedance of the inductive load in the TIP31's collector plus the increasing value of its already large C-B capacitance with frequency is the primary culprit here, getting fed back as positive feedback through the depletion capacitance of the clamp diode to the input transistor
On 05/07/2017 08:14 PM, Tim Williams wrote:
> Speaking of SPICE, here's a nice little circuit: > https://www.seventransistorlabs.com/Images/BakerDarlington.png > > I discovered the oscillator on the left, when attempting to build a > Darlington transistor pair with a Baker clamp. (In practice, R10 comes > from a logic voltage input, and R2 is a load, often inductive. R14 is > optional, but exacerbates the oscillation, making it reproducible.)
Cool 5 MHz power oscillator, bro http://imgur.com/a/o8Cg2
>"Tim Williams" wrote in message news:oeod3g$id1$1@dont-email.me...
>Speaking of SPICE, here's a nice little circuit: >https://www.seventransistorlabs.com/Images/BakerDarlington.png
>I discovered the oscillator on the left, when attempting to build a >Darlington transistor pair with a Baker clamp. (In practice, R10 comes >from a logic voltage input, and R2 is a load, often inductive. R14 is >optional, but exacerbates the oscillation, making it reproducible.)
>The model on the left, of course, doesn't produce any oscillation: it >reaches equilibrium within a couple hundred nanoseconds. (Which is kind of >suspiciously fast for a TIP31 to do much of anything, I might add.)
>The simulation on the right, showing semi-reasonable parasitics, was >necessary to reproduce the oscillations. The parasitics are in reasonable >locations, but their values are completely unreasonable.
Just had a little look so far... What is the tf in the model? What is unreasonable about the 100p? I checked the device data sheet. Its ft=3MHz so tf needs to be around 50n. This is a large value Cbe = 40.IC/2pi.ft so, at 100ma, its 0.2uf or so! 100p is small in comparison. I will look in more detail later, once I get time... -- Kevin Aylward http://www.anasoft.co.uk - SuperSpice http://www.kevinaylward.co.uk/ee/index.html
"Kevin Aylward" <kevinRemovAT@kevinaylward.co.uk> wrote in message 
news:ZcedndVdObm2Vo3EnZ2dnUU7-RvNnZ2d@giganews.com...
> Just had a little look so far... > > What is the tf in the model?
http://www.onsemi.com/PowerSolutions/supportDoc.do?type=models&rpn=TIP31C 2.062n BF is stupendously unreasonable as well, but I forget how the other parameters work off against that. Eugh, VAF is probably better than that, too.
> What is unreasonable about the 100p? >
Physically speaking, I can't account for more than 10pF and 20nH in any connections on this circuit.
> I checked the device data sheet. Its ft=3MHz so tf needs to be around 50n. > This is a large value > > Cbe = 40.IC/2pi.ft > > so, at 100ma, its 0.2uf or so! 100p is small in comparison.
Ah, the Miller equivalent input, more or less? Yeah, that would be quite large.
> > I will look in more detail later, once I get time...
Cheers! Progress: PBSS303NX works /fantastically/ well, being both fast and well-behaved. In fact, with 470p + 100R from Q3 base to GND, it's quite stable, just a few blips at the beginning of a cycle. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
"John Larkin" <jjlarkin@highland_snip_technology.com> wrote in message 
news:vhf1hcltlt23aqpts4stfqq50eb5hb41q5@4ax.com...
>>Interelectrode capacitances aren't enough to make it pop off? You need >>the lead inductances? > > I think that wirebond inductances are the big contributor to > transistor oscillation. >
At 5MHz? With a TIP31C? Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
"Winfield Hill" <hill@rowland.harvard.edu> wrote in message 
news:oeqdq902ukm@drn.newsguy.com...
> Tim Williams wrote... >> >> And just for kicks, a 2SC4821, which oscillates massively >> in the 30MHz range, and switches as fast as I can push it >> (turn-off delay time 40ns, no drool). :) > > Hey, that's a GHz power transistor! > Or should I say, ex-transistor.
They're rather hard to find nowadays, outside of scrapped CRT monitors, which is where I found them. Rest in peace, little transistors. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
On 05/08/2017 05:22 PM, Tim Williams wrote:

> Ah, the Miller equivalent input, more or less? > > Yeah, that would be quite large.
Yes, combine the fact that beta is in reality a complex-valued function of frequency (that giant capacitor), the C-B Miller effect, and the inductive load, and a transistor like the TIP31 in that structure seems to pretty much oscillate on principle. ;-)
"bitrex" <bitrex@de.lete.earthlink.net> wrote in message 
news:sq5QA.66310$se.33217@fx38.iad...
> On 05/08/2017 05:22 PM, Tim Williams wrote: > >> Ah, the Miller equivalent input, more or less? >> >> Yeah, that would be quite large. > > Yes, combine the fact that beta is in reality a complex-valued function of > frequency (that giant capacitor), the C-B Miller effect, and the inductive > load, and a transistor like the TIP31 in that structure seems to pretty > much oscillate on principle. ;-)
Then why doesn't it oscillate when I strap a schottky from its own B to C? :^) It's very interesting that even the stupid-fast transistor oscillated at only a modest frequency. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
On Mon, 8 May 2017 16:25:09 -0500, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>"John Larkin" <jjlarkin@highland_snip_technology.com> wrote in message >news:vhf1hcltlt23aqpts4stfqq50eb5hb41q5@4ax.com... >>>Interelectrode capacitances aren't enough to make it pop off? You need >>>the lead inductances? >> >> I think that wirebond inductances are the big contributor to >> transistor oscillation. >> > >At 5MHz? > >With a TIP31C? > >Tim
Probably not, although they might contribute at high currents. Didn't the sim need nanohenry inductances to get it to oscillate? The base lead of even a big transistor can have a skinny wirebond. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com