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SPICE gets it wrong

Started by Tim Williams May 7, 2017
On 05/09/2017 02:05 AM, Tim Williams wrote:
> In related news, I've discovered this circuit drives Darlington > transistors well beyond ludicrous speed: we're into plaid speed! > > I have a 2SD560 (NEC, probably 80s or 90s) which claims 1/1.2us > (rise/fall) at hFE(sat) = 1000. > > In comparison, the fastest transistor (2SC4821) does: > turn-off delay 125ns > rise time 30ns > turn-on delay 614ns* > fall time 59ns > > (*A consequence of the Q3 base damper, see last response to bitrex.) > > The 2SD560 does: > turn-off delay 160ns > rise time 160ns > turn-on delay 750ns > fall time 230ns > > Tim >
Can u post full schematic?
On 05/09/2017 09:46 AM, bitrex wrote:
> On 05/09/2017 02:05 AM, Tim Williams wrote: >> In related news, I've discovered this circuit drives Darlington >> transistors well beyond ludicrous speed: we're into plaid speed! >> >> I have a 2SD560 (NEC, probably 80s or 90s) which claims 1/1.2us >> (rise/fall) at hFE(sat) = 1000. >> >> In comparison, the fastest transistor (2SC4821) does: >> turn-off delay 125ns >> rise time 30ns >> turn-on delay 614ns* >> fall time 59ns >> >> (*A consequence of the Q3 base damper, see last response to bitrex.) >> >> The 2SD560 does: >> turn-off delay 160ns >> rise time 160ns >> turn-on delay 750ns >> fall time 230ns >> >> Tim >> > > Can u post full schematic?
Nevermind I found the link
On 05/09/2017 06:11 AM, Tim Williams wrote:

> The "unlimited" current seems to help more than it hurts, while the > clamp is doing its job -- at turn-on, it's goosed with enough extra > charge to swing the output node down /and/ stay fully saturated for a > few hundred nanoseconds.
There isn't unlimited current for sure; even for large signals the 3904 definitely doesn't have infinite output admittance. Randomly-picked gate stoppers and ferrite beads and stuff are finally kind of a kludge; hey! I thought the whole point of a driver circuit for a BJT was to slam gobs of current in and out of the base fast-like!
> Here's the full circuit, adjustments included: > https://www.seventransistorlabs.com/Images/BakerDarlingtonSch.png > Ironically, this simulation oscillates during the /off/ period! Dunno > if it's parametric, or something accidentally biased, or what. Putting > 300 ohms across L1 seems to fix it. > > BTW, the ferrite bead L2 is a reasonable model (though a linear one), > and C4 has a tinge of ESR and ESL. No supply R or L, and no other stray > L beyond what's shown. (L3 really should have a model, since it's on a > somewhat lossy ferrite core.) > > Tim
What's the frequency response of the C3/R8/L3/R6 network in isolation? Band-reject?
Tim Williams wrote...
> > "Winfield Hill" wrote ... >> 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.
B+D has 10 of 'em still in stock, $1.97 each. -- Thanks, - Win
"bitrex" <bitrex@de.lete.earthlink.net> wrote in message 
news:uVjQA.16821$Cj6.9351@fx23.iad...
> There isn't unlimited current for sure; even for large signals the 3904 > definitely doesn't have infinite output admittance.
Yeah, in practice it's something like 100mA, which is still quite reasonable, and much more than the steady-state base drive... Ooh, Q5 and Q3 are actually 2N4401 (and, perhaps unsurprising, all the other "MMBT"s are just lies pulled from my library -- they're actually "2N"s), which means more current for the same hFE, give or take. That makes a difference. With D4 removed, everything else more or less as shown, I noticed the on-time was significantly reduced: because almost half the shunt current (via R13) came from base drive alone. That was partly, but not wholly, made up for by the whopping 8us turn-off time for the TIP31! :-) So that means about half an amp through the poor Q3. Which doesn't add up; R3 is only delivering about 2mA, so that implies a fairly high hFE, 250 -- at current, no less. (That would be reasonable for a ZTX651, but a 2N4401 maybe not as much.) Incidentally, PBSS303NX also works fantastically in place of Q6, and 2N4401 is also capable of being abused hard enough to do the job. 2N3904 still manages to switch, but it looks kind of ugly... :)
> Randomly-picked gate stoppers and ferrite beads and stuff are finally > kind of a kludge; hey! I thought the whole point of a driver circuit for > a BJT was to slam gobs of current in and out of the base fast-like! >
It does, at least in so much as C3 allows "some". (It's also rate limited by C1 + R3, hence the long turn-on delay.)
> What's the frequency response of the C3/R8/L3/R6 network in isolation? > Band-reject? >
Yes, it's parallel resonant (with lossy L and C giving a low Q), strung between two low-impedance nodes. If r_e is on the order of 1 ohm, then the LF cutoff frequency is 20kHz. Notch center at 3.8MHz, then reaching a shelf at 33MHz where the 22 ohm takes over. In practice, it seems the LF cutoff is much higher than 20kHz, seeing as Q3 is more dynamic than that (it will swing up as high as C1+R7 lets it go, then D4 pulls in). L3 may also be saturating in part, as it's on a ferrite toroid. Hmm, I estimate it's 70% inductance at 400mA, so maybe not much. It's 5 turns on a T37-43 (Fair-Rite 2643375102), FWIW. In any case, it inserts a lot of "squishiness" between Q3 and Q6, and that also allows C1+R7 to be more effective (on some transistors, they aren't necessary at all). Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
On Sunday, May 7, 2017 at 5:14:50 PM UTC-7, Tim Williams wrote:

> 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.
> 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.
In the right application, TIP31 is quite a nice switch (it makes a good driver for a low-dropout regulator, for instance). Power transistors usually don't
On Sunday, May 7, 2017 at 5:14:50 PM UTC-7, Tim Williams wrote: 

> 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.
> 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.
In the right application, TIP31 is quite a nice switch (it makes a good pass transistor for a low-dropout regulator, for instance). Power transistors usually don't saturate as nicely. That's no design error, it's not easy to guard against second breakdown without gumming up the ON operation of a transistor. (obligatory lowpass negative regulator...) <https://www.digikey.com/schemeit/project/neg-regulator-low-dropout-TVGG5BO301S0/> The TIP31C spec was SO loose because it was a pioneering plastic part ( the 'C' is a voltage breakdown bin, it's that old...) which was intended to be cheap with high yield. Various manufacturers who co-opted the part number (to compete on the cheap part) weren't making the same part, just meeting the spec. It's hardly surprising that the SPICE models are junk. The manufacturer tries hard not to spend extra test time on the low-end product. At a guess, these parts will not be similar batch-to-batch, nor manufacturer-to-manufacturer. TIP31 and 1N914 are similar, that way.
On Tue, 9 May 2017 14:27:59 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

>On Sunday, May 7, 2017 at 5:14:50 PM UTC-7, Tim Williams wrote: > >> 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. > >> 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. > >In the right application, TIP31 is quite a nice switch (it makes a good >driver for a low-dropout regulator, for instance). Power transistors >usually don't
I rarely use bipolar transistors any more. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Tue, 09 May 2017 16:32:11 -0700, John Larkin
<jjlarkin@highland_snip_technology.com> wrote:

>On Tue, 9 May 2017 14:27:59 -0700 (PDT), whit3rd <whit3rd@gmail.com> >wrote: > >>On Sunday, May 7, 2017 at 5:14:50 PM UTC-7, Tim Williams wrote: >> >>> 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. >> >>> 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. >> >>In the right application, TIP31 is quite a nice switch (it makes a good >>driver for a low-dropout regulator, for instance). Power transistors >>usually don't > >I rarely use bipolar transistors any more.
They're cheap.
On Tue, 09 May 2017 20:06:35 -0400, krw@notreal.com wrote:

>On Tue, 09 May 2017 16:32:11 -0700, John Larkin ><jjlarkin@highland_snip_technology.com> wrote: > >>On Tue, 9 May 2017 14:27:59 -0700 (PDT), whit3rd <whit3rd@gmail.com> >>wrote: >> >>>On Sunday, May 7, 2017 at 5:14:50 PM UTC-7, Tim Williams wrote: >>> >>>> 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. >>> >>>> 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. >>> >>>In the right application, TIP31 is quite a nice switch (it makes a good >>>driver for a low-dropout regulator, for instance). Power transistors >>>usually don't >> >>I rarely use bipolar transistors any more. > >They're cheap.
A 2N7002 or FDV301 costs us 2 cents. MTP10N10 is 100 volts, 10 amps, TO220, 37 cents. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com