Forums

SPICE gets it wrong

Started by Tim Williams May 7, 2017
On 05/08/2017 08:14 AM, bitrex wrote:
> On 05/07/2017 08:14 PM, Tim Williams 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 >> > > I found this interesting paper from back in the day about stability > issues when using BJTs as emitter followers/switches in them newfangled > computer circuits. Just digitized in 2013: > > <https://archive.org/stream/emitterfollowers134moto#page/n19/mode/1up>
Page 8 seems to have a pertinent section: "In order to achieve some type of compensation network the effect of Re and Lc, the collector inductance, was studied. There is nearly no problem associated with the collector inductance. However, if Lc becomes large fast pulses will yield voltage drops, which in turn cause Cc (collector depletion layer capacitance) to be modulated. But this effect is beneficial, since Cc would tend to increase, resulting in a decrease of the negative real part magnitude. But a side effect also occurs: If several collectors are tied together through a common power supply wire and marginal stability exists, instability of one may cause instability of all. ...the possibility of relaxation oscillations exists and has been verified expeimentally"
Add 100 Ohms in series with the base of the 3904.

m
On 05/08/2017 11:15 AM, makolber@yahoo.com wrote:
> Add 100 Ohms in series with the base of the 3904. > > m >
I was going to say a ferrite bead "gate stopper" in the base lead of the TIP31
"bitrex" <bitrex@de.lete.earthlink.net> wrote in message 
news:PtZPA.72168$4a5.13940@fx01.iad...
>> <https://archive.org/stream/emitterfollowers134moto#page/n19/mode/1up> > > Page 8 seems to have a pertinent section: >
<snip> Thanks, I'll give that a try. I'd dabbled a bit with adding collector impedance, but didn't seem to get far. Maybe some additional C-B impedance (R+C?) will help. That also implies there's something in series, though: the base-to-output diode is a pretty low impedance when it pulls in. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
On Mon, 8 May 2017 08:15:45 -0700 (PDT), makolber@yahoo.com wrote:

>Add 100 Ohms in series with the base of the 3904.
That's (50 ohms) the classical fix. Note that ECL is *full* of emitter followers and works rather well.
"bitrex" <bitrex@de.lete.earthlink.net> wrote in message 
news:CG0QA.54511$xN7.20878@fx04.iad...
> On 05/08/2017 11:15 AM, makolber@yahoo.com wrote: >> Add 100 Ohms in series with the base of the 3904. >> > > I was going to say a ferrite bead "gate stopper" in the base lead of the > TIP31
It's not that simple: I already tried a couple of things there: 10 ohm R Ferrite bead (100 ohms @ 100MHz, about 0.6uH at low frequencies) 10uH || 100 ohm (and other R values) The tell is this: the oscillation is low frequency, typically 5-10MHz. If it can be squeezed high enough that it goes away, that would work. Most attempts to stabilize it, cause the frequency to drop, and don't do much to the amplitude. Varying the power transistor, has the most interesting effect. I've tried: TIP31C (ST, early 2000s date): spiky oscillation. Reasonable switching performance, considering. FJP13009 (Fairchild, 2010s?): average oscillation, slow switching. 2SD401-Y (SEC, 1998?): somewhat worse oscillation, very slow. KTC4368 (KEC, 1999?): spikier than TIP31C, seems faster in the linear range (is actually rated fT 100MHz typ), slow turnoff. And just for kicks, a 2SC4821, which oscillates massively in the 30MHz range, and switches as fast as I can push it (turn-off delay time 40ns, no drool). :) Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
On 05/08/2017 01:28 PM, Tim Williams wrote:
> "bitrex" <bitrex@de.lete.earthlink.net> wrote in message > news:CG0QA.54511$xN7.20878@fx04.iad... >> On 05/08/2017 11:15 AM, makolber@yahoo.com wrote: >>> Add 100 Ohms in series with the base of the 3904. >>> >> >> I was going to say a ferrite bead "gate stopper" in the base lead of the >> TIP31 > > It's not that simple: I already tried a couple of things there: > 10 ohm R > Ferrite bead (100 ohms @ 100MHz, about 0.6uH at low frequencies) > 10uH || 100 ohm (and other R values) > > The tell is this: the oscillation is low frequency, typically 5-10MHz. > If it can be squeezed high enough that it goes away, that would work. > Most attempts to stabilize it, cause the frequency to drop, and don't do > much to the amplitude. > > Varying the power transistor, has the most interesting effect. I've tried: > > TIP31C (ST, early 2000s date): spiky oscillation. Reasonable switching > performance, considering. > FJP13009 (Fairchild, 2010s?): average oscillation, slow switching. > 2SD401-Y (SEC, 1998?): somewhat worse oscillation, very slow. > KTC4368 (KEC, 1999?): spikier than TIP31C, seems faster in the linear > range (is actually rated fT 100MHz typ), slow turnoff. > > 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). :) > > Tim >
I rigged up a sim in LTSpice using just a simple hybrid-pi model with your model of your collector load and parasitics, only including the current generator, C_pi, R_pi, emitter inductance, Miller capacitance, and Z_o of the previous stage, with values that seemed reasonable for a slowpoke BJT like the TIP31. It was enough to show a huge spike in gain in the low MHz where the gain comes up from well below 0 dB to around 10dB, and the phase rockets down from around 160 degrees (where it hovered around for most of the Bode plot prior) to 0 degrees.
On 05/08/2017 02:08 PM, bitrex wrote:
> On 05/08/2017 01:28 PM, Tim Williams wrote: >> "bitrex" <bitrex@de.lete.earthlink.net> wrote in message >> news:CG0QA.54511$xN7.20878@fx04.iad... >>> On 05/08/2017 11:15 AM, makolber@yahoo.com wrote: >>>> Add 100 Ohms in series with the base of the 3904. >>>> >>> >>> I was going to say a ferrite bead "gate stopper" in the base lead of the >>> TIP31 >> >> It's not that simple: I already tried a couple of things there: >> 10 ohm R >> Ferrite bead (100 ohms @ 100MHz, about 0.6uH at low frequencies) >> 10uH || 100 ohm (and other R values) >> >> The tell is this: the oscillation is low frequency, typically 5-10MHz. >> If it can be squeezed high enough that it goes away, that would work. >> Most attempts to stabilize it, cause the frequency to drop, and don't do >> much to the amplitude. >> >> Varying the power transistor, has the most interesting effect. I've >> tried: >> >> TIP31C (ST, early 2000s date): spiky oscillation. Reasonable switching >> performance, considering. >> FJP13009 (Fairchild, 2010s?): average oscillation, slow switching. >> 2SD401-Y (SEC, 1998?): somewhat worse oscillation, very slow. >> KTC4368 (KEC, 1999?): spikier than TIP31C, seems faster in the linear >> range (is actually rated fT 100MHz typ), slow turnoff. >> >> 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). :) >> >> Tim >> > > I rigged up a sim in LTSpice using just a simple hybrid-pi model with > your model of your collector load and parasitics, only including the > current generator, C_pi, R_pi, emitter inductance, Miller capacitance, > and Z_o of the previous stage, with values that seemed reasonable for a > slowpoke BJT like the TIP31. > > It was enough to show a huge spike in gain in the low MHz where the gain > comes up from well below 0 dB to around 10dB, and the phase rockets down > from around 160 degrees (where it hovered around for most of the Bode > plot prior) to 0 degrees.
Needz a lil' negative Miller capacitance rather than positive
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? -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
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. -- Thanks, - Win