Post Go back to editing

Non causal LTspice behavior on ksc3503 saturation

Category: Software
Product Number: XVII
Software Version: 17.0.35.0

Hello every body

I am new on this forum, and would to share a strange issue that I have with LTspice.

Here is the (self contained) code:

.TRAN .1n 20m 0 10n

.MODEL KSC3503 NPN IS=2.0893E-14 BF=101.5 NF=1 BR=7.655 NR=1.007 ISE=4.3652E-14 NE=1.5 ISC=1.2598n NC=2 VAF=717.25 VAR=13.16 IKF=0.2512 IKR=0.0832 RB=2.98 RBM=0.001 IRB=0.001 RE=0.5305 RC=0.9 QCO=0.05 RCO=50.1187 VO=2.476 GAMMA=1.8231E-7 CJE=6.6039E-11 VJE=0.7017 MJE=0.3253 FC=0.5 CJC=6.6072p VJC=0.5 MJC=0.2439 XCJC=0.6488 XTB=1.4089 EG=1.2129 XTI=3 Vceo=300 Icrating=100m mfg=Fairchild
.save V(5) V(3) V(4) I(Q1)
VCC 1 0 DC 40V
V1 3 0 pulse(20 0 0 30n 30n 5000u 10000u)
R1 3 4 400000
R2 1 5 10000
Q1 5 4 0 KSC3503
.end

So I mande this test with R1 = 500000 so Q1 is not saturated and with R1 = 400000 Q1 should be saturated at T = 5ms

in first case eveything is OK

In second case even at T = 0 when V1 = 0 : V(3) is 446 mv with Ib(q1) = 0 and Ic(q1) = 3.95mA

so I do not understand this strange behavior.

if I change the bjt from KSC3503 to KSC1845 or KSA1381(PNP) this behavior disapears

If somebody could explaine me, that would be great.

Regards,

Stéphane

: