Forums

diode recovered charge

Started by John Larkin September 6, 2014
In article <0a2c631c-30fa-4539-b555-601b1bf755c2@googlegroups.com>, 
bill.sloman@gmail.com says...
> or yourself, but I've never seen any actual VBIC model parameters for a real transistor. > > http://www.designers-guide.org/VBIC/ > > I raised this point here, a few years ago, when I was trying to get an LTSpice model of a Baxandall class-D oscillator built with bipolar transistors to "squeg" with a relatively high feed inductance. > > The simulations I ran made it fairly clear that it was something about the operation of a bipolar transistor in inverted mode (using the emitter as if it were a collector and the collector as if it were an emitter) that produced "squegging" but equally that the Gummell-Poon model didn't capture that particular behavior, because my LTSpice circuits always settled down to uniform oscillation where the real circuits had never settled down to a constant amplitude
oscillation.
> > -- > Bill Sloman, Sydney > >
Ltspice supports VBIC style. If you really tried to use your tools instead of blowing smoke, you'd know that. Level 9 maybe a hint. But you may need to supply a model card and that is more than likely is out side your scope. Even if you did figure out how, it still would not make any improvements to that failed oscillator. Jamie
On Monday, 8 September 2014 11:33:26 UTC+10, Maynard A. Philbrook Jr.  wrot=
e:
> In article <0a2c631c-30fa-4539-b555-601b1bf755c2@googlegroups.com>, =20 > bill.sloman@gmail.com says... >=20 > > or yourself, but I've never seen any actual VBIC model parameters for a=
real transistor.
> >=20 > > http://www.designers-guide.org/VBIC/=20 > > =20 > > I raised this point here, a few years ago, when I was trying to get an =
LTSpice model of a Baxandall class-D oscillator built with bipolar transist= ors to "squeg" with a relatively high feed inductance.=20
> > =20 > > The simulations I ran made it fairly clear that it was something about =
the operation of a bipolar transistor in inverted mode (using the emitter a= s if it were a collector and the collector as if it were an emitter) that p= roduced "squegging" but equally that the Gummell-Poon model didn't capture = that particular behavior, because my LTSpice circuits always settled down t= o uniform oscillation where the real circuits had never settled down to a c= onstant amplitude oscillation.=20
> > =20 > Ltspice supports VBIC style. If you really tried > to use your tools instead of blowing smoke, you'd know that.
If your read all of what I posted, you'd be aware that I know that. "What the manufacturers use in-house, are VBIC models, which LTSpice can ru= n if you can find the parameters." Snipping that bit of my post doesn't make it go away.
> Level 9 maybe a hint. >=20 > But you may need to supply a model card and that is > more than likely is out side your scope.
I did mention that I'd tried to get hold of a set of VBIC transistor parame= ters, and I even spent some time seeing if I could fill in the numbers to get something that might be vaguely realistic, but it was indeed more than = I could manage.
>=20 > Even if you did figure out how, it still would not=20 > make any improvements to that failed oscillator.
True. I'm not quite sure which oscillator you imagine has failed, but the q= uestion I was addressing was why the bipolar version of the Baxandall Class= -D oscillator can squeg. Working out why this happens probably wouldn't imp= rove the oscillator one iota - it works fine in areas where it's appropriat= e (and has done since Baxandall first publicised it in 1959). The "squeggin= g" is a trifle mysterious, but if you want to use a high value feed inducto= r, you can avoid the "squegging" by using MOSFET switches (which weren't ar= ound when Baxandall invented the circuit). Congratulations on making a bigger prat of yourself than I'd have imagined = possible, even for you. --=20 Bill Sloman, Sydney
John Larkin wrote:
> > I got curious about the amount of reverse-recovery charge in PN > diodes, as a function of forward current (and time of fwd bias) and > diode size/type. > > Question is, are the LT Spice diode models realistic? We'll have to > test some diodes to see. Since diode recovery for a given part number > depends a lot on the manufacturer, we should stick to sole-source > parts and tweak the Spice model to align with reality. > > The ES1D below, straight from the LT Spice diode list, seems to have a > definite step-recovery behavior, which probably isn't realistic. It > stores 115 nC when biased to 1 amp forward. The recovered charge is > not a function of ON time, also unrealistic. > > > Version 4 > SHEET 1 1104 680 > WIRE 128 80 16 80 > WIRE 336 80 208 80 > WIRE 336 112 336 80 > WIRE 16 128 16 80 > WIRE 336 224 336 176 > WIRE 416 224 336 224 > WIRE 640 224 496 224 > WIRE 784 224 640 224 > WIRE 944 224 784 224 > WIRE 1008 224 944 224 > WIRE 1056 224 1008 224 > WIRE 16 256 16 208 > WIRE 336 256 336 224 > WIRE 496 256 496 224 > WIRE 416 272 416 224 > WIRE 448 272 416 272 > WIRE 640 272 640 224 > WIRE 784 272 784 224 > WIRE 944 288 944 224 > WIRE 448 320 416 320 > WIRE 336 368 336 336 > WIRE 416 368 416 320 > WIRE 416 368 336 368 > WIRE 336 400 336 368 > WIRE 496 400 496 336 > WIRE 640 400 640 336 > WIRE 784 400 784 352 > WIRE 944 400 944 352 > FLAG 16 256 0 > FLAG 336 400 0 > FLAG 496 400 0 > FLAG 640 400 0 > FLAG 784 400 0 > FLAG 944 400 0 > FLAG 1008 224 Qd_nC > SYMBOL diode 352 176 R180 > WINDOW 0 -69 44 Left 2 > WINDOW 3 -84 14 Left 2 > SYMATTR InstName D1 > SYMATTR Value ES1D > SYMBOL voltage 16 112 R0 > WINDOW 0 51 57 Left 2 > WINDOW 3 -16 -79 Left 2 > WINDOW 123 0 0 Left 2 > WINDOW 39 0 0 Left 2 > SYMATTR InstName V1 > SYMATTR Value PULSE(-10 100 10u 5n 5n 20u) > SYMBOL res 112 96 R270 > WINDOW 0 -45 58 VTop 2 > WINDOW 3 -53 57 VBottom 2 > SYMATTR InstName R1 > SYMATTR Value 1 > SYMBOL res 320 240 R0 > WINDOW 0 -76 43 Left 2 > WINDOW 3 -79 78 Left 2 > SYMATTR InstName R2 > SYMATTR Value 1m > SYMBOL g 496 240 R0 > WINDOW 0 46 61 Left 2 > WINDOW 3 23 107 Left 2 > SYMATTR InstName G1 > SYMATTR Value 1000 > SYMBOL cap 624 272 R0 > WINDOW 0 57 22 Left 2 > WINDOW 3 62 54 Left 2 > SYMATTR InstName C1 > SYMATTR Value 1n > SYMBOL res 768 256 R0 > WINDOW 0 56 40 Left 2 > WINDOW 3 60 70 Left 2 > SYMATTR InstName R3 > SYMATTR Value 1g > SYMBOL diode 960 352 R180 > WINDOW 0 -53 48 Left 2 > WINDOW 3 -55 17 Left 2 > SYMATTR InstName D2 > SYMATTR Value Dx > TEXT 664 112 Left 2 !.tran 25u > TEXT 600 64 Left 2 !.model DX D(Vfwd=0) > TEXT 552 -32 Left 2 ;Diode Reverse Charge Tester > TEXT 592 16 Left 2 ;J Larkin Sep 6, 2014 > TEXT 936 176 Left 2 ;1 volt per nC >
For the analysis,dig out your Linvill (Transistors and active circuits).
John Larkin wrote:
> On Sat, 6 Sep 2014 19:12:00 -0500, "Tim Williams" > <tiwill@seventransistorlabs.com> wrote: > >> SPICE is notoriously bad at reverse recovery. I've heard of some models >> which claim to produce realistic results, but not seen. > > Looks that way. I may have an application where an accurate power > diode reverse recovery model really matters. We may wind up > characterizing candidate diodes and then making some weird nonlinear > capacitor behavioral model, nothing like a normal diode model. > >
Here is where Transistors and Active Circuits by Linvill will give the edge you are looking for..
>"John Larkin" wrote in message >news:du7p0ad9fmfik7f8utc9e419paiklu6uod@4ax.com...
>Playing with the standard 1N914 in LT Spice, it shows no forward >recovery (ie, no turn-on delay). It does store reverse charge, but it >snaps off instantly, probably just junction capacitance limited, when >the charge is exhausted. Power diodes do that in Spice, too.
>This is interesting:
>https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_Spice.jpg
>https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_d.JPG
>Spice has no turn-on delay and gets the reverse charge wrong by maybe >3:1 or so. Could be worse, I guess.
.MODEL D1N914 d(is=100f Rs=2 CJO=10p Tt=4n Bv=100 ) I had a play with the model values. The above gives a much better match to your oscilloscope graph than the LTSpice version. The LTSpice graph shows excessive delay in turn off. The CJO here is larger than stock, but gives a more rounded turn off like the measurement. Turn on overshoot is not modelled. TT is the key parameter to set diffusion capacitance. Kevin Aylward www.kevinaylward.co.uk www.anasoft.co.uk - SuperSpice
"Kevin Aylward"  wrote in message 
news:wcydnUoc3IdqdZDJnZ2dnUVZ7rWdnZ2d@bt.com...

>"John Larkin" wrote in message >news:du7p0ad9fmfik7f8utc9e419paiklu6uod@4ax.com...
>Playing with the standard 1N914 in LT Spice, it shows no forward >recovery (ie, no turn-on delay). It does store reverse charge, but it >snaps off instantly, probably just junction capacitance limited, when >the charge is exhausted. Power diodes do that in Spice, too.
>>This is interesting:
>>https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_Spice.jpg
>>https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_d.JPG
>>Spice has no turn-on delay and gets the reverse charge wrong by maybe >>3:1 or so. Could be worse, I guess.
>.MODEL D1N914 d(is=100f Rs=2 CJO=10p Tt=4n Bv=100 )
>I had a play with the model values. The above gives a much better match to >your oscilloscope graph than the LTSpice version. The LTSpice graph shows >excessive delay in turn off. The CJO here is larger than stock, but gives a >more rounded turn off like the measurement. Turn on overshoot is not >modelled.
>TT is the key parameter to set diffusion capacitance.
Now that I have actually checked the dc response with a data sheet, er...ahh... .MODEL D1N914 d(is=4n N=1.9 Rs=0.5 CJO=2p Tt=4n Bv=100 ) Is quite a good fit. Note the lack of the many digits of precision that are usually worthless and attempt to give the user a false sense of security. Teaser: To get N, first use any "is", from the data sheet, measure the Vbe difference difference from any decade change in current, in the lower current region. N=(delta Vbe)/60mv Why? Then set "is" by trial and error on simulation runs of V against I at one low current point. Set R from one high current point. Oh... I found a tutorial on turn on time. The equation is V = Vt.ln(1 + If/Io(1-exp(-t/tp))) Kevin Aylward www.kevinaylward.co.uk www.anasoft.co.uk - SuperSpice
On Tue, 9 Sep 2014 18:21:33 +0100, "Kevin Aylward"
<ExtractkevinRemove@kevinaylward.co.uk> wrote:

>"Kevin Aylward" wrote in message >news:wcydnUoc3IdqdZDJnZ2dnUVZ7rWdnZ2d@bt.com... > >>"John Larkin" wrote in message >>news:du7p0ad9fmfik7f8utc9e419paiklu6uod@4ax.com... > >>Playing with the standard 1N914 in LT Spice, it shows no forward >>recovery (ie, no turn-on delay). It does store reverse charge, but it >>snaps off instantly, probably just junction capacitance limited, when >>the charge is exhausted. Power diodes do that in Spice, too. > >>>This is interesting: > >>>https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_Spice.jpg > >>>https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_d.JPG > >>>Spice has no turn-on delay and gets the reverse charge wrong by maybe >>>3:1 or so. Could be worse, I guess. > >>.MODEL D1N914 d(is=100f Rs=2 CJO=10p Tt=4n Bv=100 ) > >>I had a play with the model values. The above gives a much better match to >>your oscilloscope graph than the LTSpice version. The LTSpice graph shows >>excessive delay in turn off. The CJO here is larger than stock, but gives a >>more rounded turn off like the measurement. Turn on overshoot is not >>modelled. > >>TT is the key parameter to set diffusion capacitance. > >Now that I have actually checked the dc response with a data sheet, >er...ahh... > >.MODEL D1N914 d(is=4n N=1.9 Rs=0.5 CJO=2p Tt=4n Bv=100 ) > >Is quite a good fit. Note the lack of the many digits of precision that are >usually worthless and attempt to give the user a false sense of security. > >Teaser: > >To get N, first use any "is", from the data sheet, measure the Vbe >difference difference from any decade change in current, in the lower >current region. N=(delta Vbe)/60mv > >Why? > >Then set "is" by trial and error on simulation runs of V against I at one >low current point. Set R from one high current point. > >Oh... I found a tutorial on turn on time. The equation is > >V = Vt.ln(1 + If/Io(1-exp(-t/tp))) > > >Kevin Aylward >www.kevinaylward.co.uk >www.anasoft.co.uk - SuperSpice
Thanks, Kevin. In this particular application, I don't care about turn-on time, as I have microseconds to forward bias my diode. I do care about being able to control the amount of stored charge, and I somewhat care about the nature of the turn-off; LT Spice seems to treat diodes as SRDs, snapping off when the charge is used up. Real power diodes have softer turnoff, especially high voltage ones. I have a ton of annoying stuff to do this week, so I'll have to wait for the weekend at least to play with your stuff. Our potential customer is playing the increasingly common big-company-pummels-little-company game, wanting to own everything and make us do "open costing" to guarantee that we'll lose money. We may just tell them to drop dead. If P.C. gets rational and lets us do this, I may just hire someone to find us a diode and make a good model of it. -- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
"John Larkin"  wrote in message 
news:59eu0a521ld1frmg2596rrpbtksg65mgdj@4ax.com...

On Tue, 9 Sep 2014 18:21:33 +0100, "Kevin Aylward"
<ExtractkevinRemove@kevinaylward.co.uk> wrote:

>"Kevin Aylward" wrote in message >news:wcydnUoc3IdqdZDJnZ2dnUVZ7rWdnZ2d@bt.com... > >>"John Larkin" wrote in message >>news:du7p0ad9fmfik7f8utc9e419paiklu6uod@4ax.com... > >>Playing with the standard 1N914 in LT Spice, it shows no forward >>recovery (ie, no turn-on delay). It does store reverse charge, but it >>snaps off instantly, probably just junction capacitance limited, when >>the charge is exhausted. Power diodes do that in Spice, too. > >>>This is interesting: > >>>https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_Spice.jpg > >>>https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_d.JPG > >>>Spice has no turn-on delay and gets the reverse charge wrong by maybe >>>3:1 or so. Could be worse, I guess. > >>.MODEL D1N914 d(is=100f Rs=2 CJO=10p Tt=4n Bv=100 ) > >>I had a play with the model values. The above gives a much better match to >>your oscilloscope graph than the LTSpice version. The LTSpice graph shows >>excessive delay in turn off. The CJO here is larger than stock, but gives >>a >>more rounded turn off like the measurement. Turn on overshoot is not >>modelled. > >>TT is the key parameter to set diffusion capacitance. > >Now that I have actually checked the dc response with a data sheet, >er...ahh... > >.MODEL D1N914 d(is=4n N=1.9 Rs=0.5 CJO=2p Tt=4n Bv=100 ) > >Is quite a good fit. Note the lack of the many digits of precision that are >usually worthless and attempt to give the user a false sense of security. > >Teaser: > >To get N, first use any "is", from the data sheet, measure the Vbe >difference difference from any decade change in current, in the lower >current region. N=(delta Vbe)/60mv > >Why? > >>Then set "is" by trial and error on simulation runs of V against I at one >>low current point. Set R from one high current point. > >>Oh... I found a tutorial on turn on time. The equation is > >>V = Vt.ln(1 + If/Io(1-exp(-t/tp)))
>Thanks, Kevin.
>In this particular application, I don't care about turn-on time, as I >have microseconds to forward bias my diode. I do care about being able >to control the amount of stored charge, and I somewhat care about the >nature of the turn-off; LT Spice seems to treat diodes as SRDs, >snapping off when the charge is used up. Real power diodes have softer >turnoff, especially high voltage ones.
>I have a ton of annoying stuff to do this week, so I'll have to wait >for the weekend at least to play with your stuff.
>Our potential customer is playing the increasingly common >big-company-pummels-little-company game, wanting to own everything and >make us do "open costing" to guarantee that we'll lose money. We may >just tell them to drop dead.
>If P.C. gets rational and lets us do this, I may just hire someone to >find us a diode and make a good model of it.
It is interesting that the most basic semiconductor, is actually quite complex. On the surface,it would seem that an inductor in series would model the increase in voltage during turn on, which it does, but it messes up the turn off waveform. It looks like it needs to be a nonlinear inductor with other stuff. It will have to wait till the weekend though. Kevin Aylward www.kevinaylward.co.uk www.anasoft.co.uk - SuperSpice
On Wed, 10 Sep 2014 18:53:39 +0100, "Kevin Aylward"
<ExtractkevinRemove@kevinaylward.co.uk> wrote:

>"John Larkin" wrote in message >news:59eu0a521ld1frmg2596rrpbtksg65mgdj@4ax.com... > >On Tue, 9 Sep 2014 18:21:33 +0100, "Kevin Aylward" ><ExtractkevinRemove@kevinaylward.co.uk> wrote: > >>"Kevin Aylward" wrote in message >>news:wcydnUoc3IdqdZDJnZ2dnUVZ7rWdnZ2d@bt.com... >> >>>"John Larkin" wrote in message >>>news:du7p0ad9fmfik7f8utc9e419paiklu6uod@4ax.com... >> >>>Playing with the standard 1N914 in LT Spice, it shows no forward >>>recovery (ie, no turn-on delay). It does store reverse charge, but it >>>snaps off instantly, probably just junction capacitance limited, when >>>the charge is exhausted. Power diodes do that in Spice, too. >> >>>>This is interesting: >> >>>>https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_Spice.jpg >> >>>>https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_d.JPG >> >>>>Spice has no turn-on delay and gets the reverse charge wrong by maybe >>>>3:1 or so. Could be worse, I guess. >> >>>.MODEL D1N914 d(is=100f Rs=2 CJO=10p Tt=4n Bv=100 ) >> >>>I had a play with the model values. The above gives a much better match to >>>your oscilloscope graph than the LTSpice version. The LTSpice graph shows >>>excessive delay in turn off. The CJO here is larger than stock, but gives >>>a >>>more rounded turn off like the measurement. Turn on overshoot is not >>>modelled. >> >>>TT is the key parameter to set diffusion capacitance. >> >>Now that I have actually checked the dc response with a data sheet, >>er...ahh... >> >>.MODEL D1N914 d(is=4n N=1.9 Rs=0.5 CJO=2p Tt=4n Bv=100 ) >> >>Is quite a good fit. Note the lack of the many digits of precision that are >>usually worthless and attempt to give the user a false sense of security. >> >>Teaser: >> >>To get N, first use any "is", from the data sheet, measure the Vbe >>difference difference from any decade change in current, in the lower >>current region. N=(delta Vbe)/60mv >> >>Why? >> >>>Then set "is" by trial and error on simulation runs of V against I at one >>>low current point. Set R from one high current point. >> >>>Oh... I found a tutorial on turn on time. The equation is >> >>>V = Vt.ln(1 + If/Io(1-exp(-t/tp))) > > > >>Thanks, Kevin. > >>In this particular application, I don't care about turn-on time, as I >>have microseconds to forward bias my diode. I do care about being able >>to control the amount of stored charge, and I somewhat care about the >>nature of the turn-off; LT Spice seems to treat diodes as SRDs, >>snapping off when the charge is used up. Real power diodes have softer >>turnoff, especially high voltage ones. > >>I have a ton of annoying stuff to do this week, so I'll have to wait >>for the weekend at least to play with your stuff. > >>Our potential customer is playing the increasingly common >>big-company-pummels-little-company game, wanting to own everything and >>make us do "open costing" to guarantee that we'll lose money. We may >>just tell them to drop dead. > >>If P.C. gets rational and lets us do this, I may just hire someone to >>find us a diode and make a good model of it. > > >It is interesting that the most basic semiconductor, is actually quite >complex. On the surface,it would seem that an inductor in series would model >the increase in voltage during turn on, which it does, but it messes up the >turn off waveform. It looks like it needs to be a nonlinear inductor with >other stuff. It will have to wait till the weekend though. > > >Kevin Aylward >www.kevinaylward.co.uk >www.anasoft.co.uk - SuperSpice
Here are four cases: https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_a.JPG https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_b.JPG https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_c.JPG https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_d.JPG The overshoot areas are sorta similar. Even more fun is the DSRD (Grekhov drift step-recovery diode) power diode effect, where the time of forward bias affects the amount and especially the distribution of charge. We biased one diode to +48 volts for a couple of hundred ns, then reverse biased it at about 100 amps, and then it snapped, giving us a 2KV, 3 ns pulse. It wouldn't snap off fast if the forward bias had been DC. HP discovered, in the 1960's, that an SRD snaps off faster if the forward bias is only applied for a few ns. -- John Larkin Highland Technology, Inc jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On 09/10/2014 11:51 PM, John Larkin wrote:
> On Wed, 10 Sep 2014 18:53:39 +0100, "Kevin Aylward" > <ExtractkevinRemove@kevinaylward.co.uk> wrote: > >> "John Larkin" wrote in message >> news:59eu0a521ld1frmg2596rrpbtksg65mgdj@4ax.com... >> >> On Tue, 9 Sep 2014 18:21:33 +0100, "Kevin Aylward" >> <ExtractkevinRemove@kevinaylward.co.uk> wrote: >> >>> "Kevin Aylward" wrote in message >>> news:wcydnUoc3IdqdZDJnZ2dnUVZ7rWdnZ2d@bt.com... >>> >>>> "John Larkin" wrote in message >>>> news:du7p0ad9fmfik7f8utc9e419paiklu6uod@4ax.com... >>> >>>> Playing with the standard 1N914 in LT Spice, it shows no forward >>>> recovery (ie, no turn-on delay). It does store reverse charge, but it >>>> snaps off instantly, probably just junction capacitance limited, when >>>> the charge is exhausted. Power diodes do that in Spice, too. >>> >>>>> This is interesting: >>> >>>>> https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_Spice.jpg >>> >>>>> https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_d.JPG >>> >>>>> Spice has no turn-on delay and gets the reverse charge wrong by maybe >>>>> 3:1 or so. Could be worse, I guess. >>> >>>> .MODEL D1N914 d(is=100f Rs=2 CJO=10p Tt=4n Bv=100 ) >>> >>>> I had a play with the model values. The above gives a much better match to >>>> your oscilloscope graph than the LTSpice version. The LTSpice graph shows >>>> excessive delay in turn off. The CJO here is larger than stock, but gives >>>> a >>>> more rounded turn off like the measurement. Turn on overshoot is not >>>> modelled. >>> >>>> TT is the key parameter to set diffusion capacitance. >>> >>> Now that I have actually checked the dc response with a data sheet, >>> er...ahh... >>> >>> .MODEL D1N914 d(is=4n N=1.9 Rs=0.5 CJO=2p Tt=4n Bv=100 ) >>> >>> Is quite a good fit. Note the lack of the many digits of precision that are >>> usually worthless and attempt to give the user a false sense of security. >>> >>> Teaser: >>> >>> To get N, first use any "is", from the data sheet, measure the Vbe >>> difference difference from any decade change in current, in the lower >>> current region. N=(delta Vbe)/60mv >>> >>> Why? >>> >>>> Then set "is" by trial and error on simulation runs of V against I at one >>>> low current point. Set R from one high current point. >>> >>>> Oh... I found a tutorial on turn on time. The equation is >>> >>>> V = Vt.ln(1 + If/Io(1-exp(-t/tp))) >> >> >> >>> Thanks, Kevin. >> >>> In this particular application, I don't care about turn-on time, as I >>> have microseconds to forward bias my diode. I do care about being able >>> to control the amount of stored charge, and I somewhat care about the >>> nature of the turn-off; LT Spice seems to treat diodes as SRDs, >>> snapping off when the charge is used up. Real power diodes have softer >>> turnoff, especially high voltage ones. >> >>> I have a ton of annoying stuff to do this week, so I'll have to wait >>> for the weekend at least to play with your stuff. >> >>> Our potential customer is playing the increasingly common >>> big-company-pummels-little-company game, wanting to own everything and >>> make us do "open costing" to guarantee that we'll lose money. We may >>> just tell them to drop dead. >> >>> If P.C. gets rational and lets us do this, I may just hire someone to >>> find us a diode and make a good model of it. >> >> >> It is interesting that the most basic semiconductor, is actually quite >> complex. On the surface,it would seem that an inductor in series would model >> the increase in voltage during turn on, which it does, but it messes up the >> turn off waveform. It looks like it needs to be a nonlinear inductor with >> other stuff. It will have to wait till the weekend though. >> >> >> Kevin Aylward >> www.kevinaylward.co.uk >> www.anasoft.co.uk - SuperSpice > > > Here are four cases: > > https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_a.JPG > > https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_b.JPG > > https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_c.JPG > > https://dl.dropboxusercontent.com/u/53724080/Diode_TurnOn/1N914_d.JPG > > > The overshoot areas are sorta similar. > > Even more fun is the DSRD (Grekhov drift step-recovery diode) power > diode effect, where the time of forward bias affects the amount and > especially the distribution of charge. We biased one diode to +48 > volts for a couple of hundred ns, then reverse biased it at about 100 > amps, and then it snapped, giving us a 2KV, 3 ns pulse. It wouldn't > snap off fast if the forward bias had been DC. > > HP discovered, in the 1960's, that an SRD snaps off faster if the > forward bias is only applied for a few ns. > >
ISTM that it's intrinsically hard to model carrier diffusion problems in SPICE, because SPICE is an ODE solver, and diffusion is a transport problem. Transport problems require integral equations, which in general aren't reducible to systems of ODEs. It's pretty reasonable that the turn-off behaviour would be a function of how long the forward bias is applied--you want a nice sharp front edge to the carrier distribution, so that the edge arrives back at the contact all at once. The distribution gets flatter and flatter as time goes on. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net