SPICE gets it wrong

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