# Understanding of ltspice models

Discussion in 'Electronic Basics' started by PierreJ, Jan 29, 2007.

1. ### PierreJGuest

Some people here kindly helped with a silly question. I have perhaps
another.

Trying to understand spice models to make a new model for ltspice. Not
understand so looked at existiing ones. Spice model to Fairchild
datasheet for 2N3904 is

NPN (Is=6.734f Xti=3 Eg=1.11 Vaf=74.03 Bf=416.4 Ne=1.259 Ise=6.734
Ikf=66.78m Xtb=1.5 Br=.7371 Nc=2 Isc=0 Ikr=0 Rc=1 Cjc=3.638p Mjc=.3085
Vjc=.75 Fc=.5 Cje=4.493p Mje=.2593 Vje=.75 Tr=239.5n Tf=301.2p Itf=.4
Vtf=4 Xtf=2 Rb=10)

Spice model to ltspice for same transistor is

NPN(IS=1E-14 VAF=100 Bf=300 IKF=0.4 XTB=1.5 BR=4 CJC=4E-12 CJE=8E-12
RB=20 RC=0.1 RE=0.1 TR=250E-9 TF=350E-12 ITF=1 VTF=2 XTF=3 Vceo=40
Icrating=200m mfg=Philips)

Some parameters are not in both. Even ones that are in both are
different in each one. Not understand what any mean but anyway eg
Ise=6.734 in Fairchild yet Ise not exist in ltspice model. Is=6.734f
in Fairchild yet Is=1E-14 in ltspice.

If I get model to data sheet how to make into ltspice model?

Spice model for MPSA42 from Fairchild is this

NPN (Is=34.9f Xti=3 Eg=1.11 Vaf=100 Bf=2.65K Ne=1.708 Ise=16.32p
Ikf=23.79m Xtb=1.5 Br=9.769 Nc=2 Isc=0 Ikr=0 Rc=7 Cjc=14.23p Mjc=.5489
Vjc=.75 Fc=.5 Cje=49.62p Mje=.4136 Vje=.75 Tr=934.3p Tf=1.69n Itf=5
Vtf=20 Xtf=150 Rb=10)

How to make into ltspice model?

How it works?

P.

2. ### john jardineGuest

For the same device, the numbers vary a bit from maker to maker.
You'll usually get a bit more accurate results using the maker's models.
LTspice seems to have a list of default numbers that will be inserted if
your model does not supply that particular parameter.

To make your new device part of the normal component selection list when you
run LTspice you need to edit a particular LTspice model file.

The MPSA42 model text needs adding to the file LTspice/LIB/CMP/
"standard.bjt". This is the store for all the PNP NPN transistors. Open it
using 'Notepad', select and 'copy' the model text (I usually do this from
the manufacturers web site) and paste onto the bottom of the list. If more
than one line add a "+" at each new line start.
Then add " mfg=Fairchild" and " Ic rating =123ma" (say).
Philips and Fairchild models seem to correspond well with the real devices.

Run LTspice and your added model will turn up as a normal component.
john