Schottky diodes have higher V drop than PN?

Chris Carlen · Jul 31, 2005

Hi:

Are these models correct?

http://web.newsguy.com/crcarl/diode-compare.asc

The built-in BAT54 model in LTSpice has a VI curve that overtakes the
1N4148 voltage drop above about 0.28A.

The Fairchild BAT54 model also has more voltage drop above about 0.61A.

However, a 1N5817 (of course this is a much "heavier" diode) never has
more voltage drop than the 4148.

What gives? Are the models for BAT54 bonkers, or can small Schottkys
actually behave in this manner?

Thanks.

--
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5

Jim Thompson · Jul 31, 2005

On Sun, 31 Jul 2005 12:16:30 -0700, Chris Carlen
<crobc@BOGUSFIELD.sbcglobal.net> wrote:

Hi:

Are these models correct?

http://web.newsguy.com/crcarl/diode-compare.asc

The built-in BAT54 model in LTSpice has a VI curve that overtakes the
1N4148 voltage drop above about 0.28A.

The Fairchild BAT54 model also has more voltage drop above about 0.61A.

However, a 1N5817 (of course this is a much "heavier" diode) never has
more voltage drop than the 4148.

What gives? Are the models for BAT54 bonkers, or can small Schottkys
actually behave in this manner?

Thanks.

480)460-2350 | |
| E-mail Address at Website Fax

John Larkin · Jul 31, 2005

On Sun, 31 Jul 2005 12:38:18 -0700, Jim Thompson
<thegreatone@example.com> wrote:

On Sun, 31 Jul 2005 12:16:30 -0700, Chris Carlen
crobc@BOGUSFIELD.sbcglobal.net> wrote:

Hi:

Are these models correct?

http://web.newsguy.com/crcarl/diode-compare.asc

The built-in BAT54 model in LTSpice has a VI curve that overtakes the
1N4148 voltage drop above about 0.28A.

The Fairchild BAT54 model also has more voltage drop above about 0.61A.

However, a 1N5817 (of course this is a much "heavier" diode) never has
more voltage drop than the 4148.

What gives? Are the models for BAT54 bonkers, or can small Schottkys
actually behave in this manner?

Thanks.

Small Schottky's tend to be resistive.

because they're small.

John

Bob · Jul 31, 2005

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:5icqe19k2gh0he1bk093dat5hc8g79p6oj@4ax.com...

On Sun, 31 Jul 2005 12:38:18 -0700, Jim Thompson
thegreatone@example.com> wrote:

On Sun, 31 Jul 2005 12:16:30 -0700, Chris Carlen
crobc@BOGUSFIELD.sbcglobal.net> wrote:

Hi:

Are these models correct?

http://web.newsguy.com/crcarl/diode-compare.asc

The built-in BAT54 model in LTSpice has a VI curve that overtakes the
1N4148 voltage drop above about 0.28A.

The Fairchild BAT54 model also has more voltage drop above about 0.61A.

However, a 1N5817 (of course this is a much "heavier" diode) never has
more voltage drop than the 4148.

What gives? Are the models for BAT54 bonkers, or can small Schottkys
actually behave in this manner?

Thanks.

Small Schottky's tend to be resistive.

because they're small.

John

and resistive.

Bob

Chris Carlen · Jul 31, 2005

Jim Thompson wrote:

On Sun, 31 Jul 2005 12:16:30 -0700, Chris Carlen
crobc@BOGUSFIELD.sbcglobal.net> wrote:

Hi:

Are these models correct?

http://web.newsguy.com/crcarl/diode-compare.asc

The built-in BAT54 model in LTSpice has a VI curve that overtakes the
1N4148 voltage drop above about 0.28A.

The Fairchild BAT54 model also has more voltage drop above about 0.61A.

However, a 1N5817 (of course this is a much "heavier" diode) never has
more voltage drop than the 4148.

What gives? Are the models for BAT54 bonkers, or can small Schottkys
actually behave in this manner?

Thanks.

Small Schottky's tend to be resistive.

Thanks for the (brief) reply ;-)

What do you make of the difference between the BAT54 models?

Oh, I am not sure if you have LTSpice, though surely you must by now
just for it's utility as the "PDF" of electronics forum discussions.

Good day!

--
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5

Jim Thompson · Jul 31, 2005

On Sun, 31 Jul 2005 13:21:14 -0700, Chris Carlen
<crobc@BOGUSFIELD.sbcglobal.net> wrote:

Jim Thompson wrote:
On Sun, 31 Jul 2005 12:16:30 -0700, Chris Carlen
crobc@BOGUSFIELD.sbcglobal.net> wrote:

Hi:

Are these models correct?

http://web.newsguy.com/crcarl/diode-compare.asc

The built-in BAT54 model in LTSpice has a VI curve that overtakes the
1N4148 voltage drop above about 0.28A.

The Fairchild BAT54 model also has more voltage drop above about 0.61A.

However, a 1N5817 (of course this is a much "heavier" diode) never has
more voltage drop than the 4148.

What gives? Are the models for BAT54 bonkers, or can small Schottkys
actually behave in this manner?

Thanks.

Small Schottky's tend to be resistive.

Thanks for the (brief) reply ;-)

What do you make of the difference between the BAT54 models?

Oh, I am not sure if you have LTSpice, though surely you must by now
just for it's utility as the "PDF" of electronics forum discussions.

Good day!

Sure I have LTspice, though I use PSpice for work.

Schottky's are rarely modeled properly, because the model framework is
built around junction-style devices, and the model makers don't know
how to twiddle them to fit Schottky slope differences ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice

480)460-2350 | |
| E-mail Address at Website Fax

Spehro Pefhany · Jul 31, 2005

On Sun, 31 Jul 2005 20:22:54 GMT, the renowned "Bob"
<nimby1_notspamm_@earthlink.net> wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:5icqe19k2gh0he1bk093dat5hc8g79p6oj@4ax.com...
On Sun, 31 Jul 2005 12:38:18 -0700, Jim Thompson
thegreatone@example.com> wrote:

On Sun, 31 Jul 2005 12:16:30 -0700, Chris Carlen
crobc@BOGUSFIELD.sbcglobal.net> wrote:

Hi:

Are these models correct?

http://web.newsguy.com/crcarl/diode-compare.asc

The built-in BAT54 model in LTSpice has a VI curve that overtakes the
1N4148 voltage drop above about 0.28A.

The Fairchild BAT54 model also has more voltage drop above about 0.61A.

However, a 1N5817 (of course this is a much "heavier" diode) never has
more voltage drop than the 4148.

What gives? Are the models for BAT54 bonkers, or can small Schottkys
actually behave in this manner?

Thanks.

Small Schottky's tend to be resistive.

because they're small.

John

and resistive.

Bob

But not as leaky as if they were big and semiconductory.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

Joerg · Jul 31, 2005

Hello Chris,

Small Schottky's tend to be resistive.

Thanks for the (brief) reply ;-)

What do you make of the difference between the BAT54 models?

BAT54 are resistive but not a whole lot more than a 4148. Bottomline, if

you get much more than 450mV typical at 25C and 100mA for the BAT54
something must be wrong with the model.

Remember, this kind of diode isn't meant for several hundred mA. They
can stomach non-repetitive peaks of half an amp but not several amps.

Regards, Joerg

http://www.analogconsultants.com

Chris Carlen · Jul 31, 2005

Spehro Pefhany wrote:

On Sun, 31 Jul 2005 20:22:54 GMT, the renowned "Bob"
nimby1_notspamm_@earthlink.net> wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:5icqe19k2gh0he1bk093dat5hc8g79p6oj@4ax.com...
On Sun, 31 Jul 2005 12:38:18 -0700, Jim Thompson
thegreatone@example.com> wrote:

On Sun, 31 Jul 2005 12:16:30 -0700, Chris Carlen
crobc@BOGUSFIELD.sbcglobal.net> wrote:

Hi:

Are these models correct?

http://web.newsguy.com/crcarl/diode-compare.asc

The built-in BAT54 model in LTSpice has a VI curve that overtakes the
1N4148 voltage drop above about 0.28A.

The Fairchild BAT54 model also has more voltage drop above about 0.61A.

However, a 1N5817 (of course this is a much "heavier" diode) never has
more voltage drop than the 4148.

What gives? Are the models for BAT54 bonkers, or can small Schottkys
actually behave in this manner?

Thanks.
Small Schottky's tend to be resistive.
because they're small.

John

and resistive.

Bob

But not as leaky as if they were big and semiconductory.

Don't you mean Semiconductish?

--
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5

John Larkin · Jul 31, 2005

On Sun, 31 Jul 2005 20:22:54 GMT, "Bob"
<nimby1_notspamm_@earthlink.net> wrote:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:5icqe19k2gh0he1bk093dat5hc8g79p6oj@4ax.com...
On Sun, 31 Jul 2005 12:38:18 -0700, Jim Thompson
thegreatone@example.com> wrote:

On Sun, 31 Jul 2005 12:16:30 -0700, Chris Carlen
crobc@BOGUSFIELD.sbcglobal.net> wrote:

Hi:

Are these models correct?

http://web.newsguy.com/crcarl/diode-compare.asc

The built-in BAT54 model in LTSpice has a VI curve that overtakes the
1N4148 voltage drop above about 0.28A.

The Fairchild BAT54 model also has more voltage drop above about 0.61A.

However, a 1N5817 (of course this is a much "heavier" diode) never has
more voltage drop than the 4148.

What gives? Are the models for BAT54 bonkers, or can small Schottkys
actually behave in this manner?

Thanks.

Small Schottky's tend to be resistive.

because they're small.

John

and resistive.

Bob

Good point. Thanks.

John

John Larkin · Jul 31, 2005

On Sun, 31 Jul 2005 12:16:30 -0700, Chris Carlen
<crobc@BOGUSFIELD.sbcglobal.net> wrote:

The Fairchild BAT54 model also has more voltage drop above about 0.61A.

I hope you're talking pulsed current here. Fairchild specs theta at
430k/w.

John

Chris Carlen · Jul 31, 2005

Joerg wrote:

Hello Chris,

Small Schottky's tend to be resistive.

Thanks for the (brief) reply ;-)

What do you make of the difference between the BAT54 models?

BAT54 are resistive but not a whole lot more than a 4148. Bottomline, if
you get much more than 450mV typical at 25C and 100mA for the BAT54
something must be wrong with the model.

Remember, this kind of diode isn't meant for several hundred mA. They
can stomach non-repetitive peaks of half an amp but not several amps.

That's why you should look at the LTSpice file I posted which models a
typical ESD scenario. It gives 9-10A surge currents through the diodes.
What can you do to protect from that except make some assumptions
about the surge capability of the diodes at very short pulse widths.

So the question becomes, as the time duration of the surge current
becomes smaller, how much more current can be tolerated.

I was given some very old Philips databook images for HC logic, which
showed a graph of this. Now I don't know where it got to.

With any sort of input resistance that still allows fast data rates, one
has little choice but to exceed the 1 second ratings of the diode.
Although at least for the MMBD4148, they give a 1us rating of 2A.

But by putting a resistor in between the external diodes and the
internal, as I said before, one can ensure that the chip's ratings
aren't exceeded.

I sure wish the diode datasheets included more detailed surge ratings,
such as a graph.

Good day!

--
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5

Chris Carlen · Jul 31, 2005

John Larkin wrote:

On Sun, 31 Jul 2005 12:16:30 -0700, Chris Carlen
crobc@BOGUSFIELD.sbcglobal.net> wrote:

The Fairchild BAT54 model also has more voltage drop above about 0.61A.

I hope you're talking pulsed current here. Fairchild specs theta at
430k/w.

John

It's about ESD.

See these:

http://web.newsguy.com/crcarl/esd-HC-input-pn-vs-Shottky.asc
http://web.newsguy.com/crcarl/diode-compare.asc

--
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5

Joerg · Aug 1, 2005

Hello Chris,

That's why you should look at the LTSpice file I posted which models a
typical ESD scenario. It gives 9-10A surge currents through the diodes.
What can you do to protect from that except make some assumptions about
the surge capability of the diodes at very short pulse widths.

Ok, just did. V(N004)-V(N005) (1N4148) gives 7V peak while the
V(N010)-V(N011) (BAT54) results in 22V. Whether the BAT54 model is any
good or not, with that 1.5k up front chances are that either diode will
be toast. With such low resistance you need something larger. Even via
the extra 100Ohms towards the substrate diode it'll still see 70mA. Too
close for comfort for me.

So the question becomes, as the time duration of the surge current
becomes smaller, how much more current can be tolerated.

If you need this much you'd have to ask the mfg but they most likely
won't endorse it.

But by putting a resistor in between the external diodes and the
internal, as I said before, one can ensure that the chip's ratings
aren't exceeded.

But I'd go higher than 100Ohms there.

I sure wish the diode datasheets included more detailed surge ratings,
such as a graph.

We are lucky we have those graphs at all. Try to coax that kind of data

out of uC manufacturers. I tried but it was like kicking a big oak tree.

Regards, Joerg

http://www.analogconsultants.com

Aug 1, 2005

On Sun, 31 Jul 2005 17:21:58 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wroth:

What gives? Are the models for BAT54 bonkers, or can small Schottkys
actually behave in this manner?

Thanks.

Small Schottky's tend to be resistive.

because they're small.

John

and resistive.

Bob

But not as leaky as if they were big and semiconductory.

And small resistive Schottky's are a lot faster than big semiconductory
devices. The 1N4000 series have junction capacitances of around 15 pF and Trr
times of 2 uS or more.

Jim

Jim Thompson · Aug 1, 2005

On Sun, 31 Jul 2005 17:16:27 -0700, Chris Carlen
<crobc@BOGUSFIELD.sbcglobal.net> wrote:

Joerg wrote:
Hello Chris,

Small Schottky's tend to be resistive.

Thanks for the (brief) reply ;-)

What do you make of the difference between the BAT54 models?

BAT54 are resistive but not a whole lot more than a 4148. Bottomline, if
you get much more than 450mV typical at 25C and 100mA for the BAT54
something must be wrong with the model.

Remember, this kind of diode isn't meant for several hundred mA. They
can stomach non-repetitive peaks of half an amp but not several amps.

That's why you should look at the LTSpice file I posted which models a
typical ESD scenario.

480)460-2350 | |
| E-mail Address at Website Fax

jake of center · Aug 1, 2005

Schottkys are built differently, more doped layers.
None of the idiots on this form apparently know that.

Bob · Aug 1, 2005

"jake of center" <nospam@invalid.com> wrote in message
news:42eda018$0$60147$892e7fe2@authen.white.readfreenews.net...

Schottkys are built differently, more doped layers.
None of the idiots on this form apparently know that.

We were hoping that somebody like you would show up and enlighten us. Thanks
for the informative response to this form (forum?).

Bob

Chris Carlen · Aug 1, 2005

Jim Thompson wrote:

On Sun, 31 Jul 2005 17:16:27 -0700, Chris Carlen
crobc@BOGUSFIELD.sbcglobal.net> wrote:

Joerg wrote:
Hello Chris,

Small Schottky's tend to be resistive.
Thanks for the (brief) reply ;-)

What do you make of the difference between the BAT54 models?

BAT54 are resistive but not a whole lot more than a 4148. Bottomline, if
you get much more than 450mV typical at 25C and 100mA for the BAT54
something must be wrong with the model.

Remember, this kind of diode isn't meant for several hundred mA. They
can stomach non-repetitive peaks of half an amp but not several amps.

That's why you should look at the LTSpice file I posted which models a
typical ESD scenario.

[snip]

Except that 'HC' parts don't use Schottky diodes for ESD ;-)

Jim, the debate is about which is better to put external to the HC
device, or other device. My LTSpice model uses 4148 style diodes
internally. I might not mind if you had any inside tips on the models
of HC input diodes that you could share with us? I choose in my LTSpice
model whether to use Schottky vs. PN external protection diodes
depending on what the goals are (speed, etc.). The model also allows
varying the ESD charge and voltage, various resistances, inductances, etc.

The debate has now shifted to whether the BAT54 models available are any
good or not. The ones that I have tried predict much higher voltage
drops than 4148 at the large albeit brief surge currents generated
during an ESD event using the IEC ESD models.

Thanks for the input.

Good day!

--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.

Chris Carlen · Aug 1, 2005

Joerg wrote:

Hello Chris,

That's why you should look at the LTSpice file I posted which models a
typical ESD scenario. It gives 9-10A surge currents through the
diodes. What can you do to protect from that except make some
assumptions about the surge capability of the diodes at very short
pulse widths.

Ok, just did. V(N004)-V(N005) (1N4148) gives 7V peak while the
V(N010)-V(N011) (BAT54) results in 22V. Whether the BAT54 model is any
good or not, with that 1.5k up front chances are that either diode will
be toast. With such low resistance you need something larger. Even via
the extra 100Ohms towards the substrate diode it'll still see 70mA. Too
close for comfort for me.

It's >200mA with the BAT54 model, which is of course still in question.
Odd, I get only about 57mA peak with the 4148.

Anyway, the RMS current over 1us for the internal diode is 20mA, which
is the max allowed constant current. Note that the 4148 datasheet
indicates that the surge current allowed for 1us is toward 2x the surge
current allowed for 1s. Thus a safe assumption might be that the
internal diodes might handle a similar 2x there constant current rating,
for 1us. Thus, a safe assumption might be that if we get the peak
current down to 40mA, then the 1us RMS will be less than that, let's
see: With 150R before the internal diode (with 15kV and 1.2k external),
I get 38mA peak internal diode current.

To avoid making ANY assumptions about the internal diode, and limit the
current to 20mA peak, then 300R does the trick.

For the external, 3.9k limits to <2A RMS for 1us with 15kV. Would that
be safe enough?

With that resistance, the max data frequency is only about 5MHz to meet
74HC14 input threshold limits.

To get down to <2A peak current for the external diode, an external R of
7.5k is needed, bringing the data rate down to 2.5MHz.

Pretty slugish :-(

So the question becomes, as the time duration of the surge current
becomes smaller, how much more current can be tolerated.

If you need this much you'd have to ask the mfg but they most likely
won't endorse it.

Yeah.

Ok, so I'd sure like to learn a better way to avoid exceeding any max
current ratings while making no assumptions whatsoever (thus peak
currents must be <= the max surge currents listed in the data, or if
only constant max allowable currents are given, then the peak currents
must not exceed those).

Also, keep input capacitance relatively low, but actually it doens't
matter since all that really matters is bandwidth. The thresholds for
HC14 inputs are Vil<=1.0V and Vih>=3.5V from Fairchild's data.

With BAT54's 600mA surge, this makes the 4148 a winner unless a
completely different approach is taken.

I have used the Littlefuse SP724 and similar devices as well. Perhaps
they are truly superior in applications like this rather than
roll-your-own ESD protection schemes.

Good day!

--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.

Schottky diodes have higher V drop than PN?

Chris Carlen

Guest

Jim Thompson

Guest

John Larkin

Guest

Bob

Guest

Chris Carlen

Guest

Jim Thompson

Guest

Spehro Pefhany

Guest

Joerg

Guest

Chris Carlen

Guest

John Larkin

Guest

John Larkin

Guest

Chris Carlen

Guest

Chris Carlen

Guest

Joerg

Guest

Guest

Jim Thompson

Guest

jake of center

Guest

Bob

Guest

Chris Carlen

Guest

Chris Carlen

Guest

Log in

Welcome to EDABoard.com

Sponsor