Resistance of switch component (SW) in LTspice?

[Bill Bowden]

How do I define the resistance of the SW (switch) component in the
LTspice library? Apparently it's 1 ohm, and I'm trying to simulate a
MOSFET (IRF-640 or similar) of 180 milliohms.

Thanks,

-Bill

 

[John Fields]

On Sun, 19 Jun 2011 16:36:27 -0700 (PDT), Bill Bowden
<bperryb@bowdenshobbycircuits.info> wrote:

How do I define the resistance of the SW (switch) component in the
LTspice library? Apparently it's 1 ohm, and I'm trying to simulate a
MOSFET (IRF-640 or similar) of 180 milliohms.

---
Add a directive to your schematic in the form of:

..model SW SW(Ron=1 Roff=10Meg Vt=0.5 Vh=0)

and edit it to suit.

--
JF

 

[John Fields]

On Mon, 20 Jun 2011 07:50:59 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Sun, 19 Jun 2011 16:36:27 -0700 (PDT), Bill Bowden
bperryb@bowdenshobbycircuits.info> wrote:

How do I define the resistance of the SW (switch) component in the
LTspice library? Apparently it's 1 ohm, and I'm trying to simulate a
MOSFET (IRF-640 or similar) of 180 milliohms.

---
Add a directive to your schematic in the form of:

.model SW SW(Ron=1 Roff=10Meg Vt=0.5 Vh=0)

and edit it to suit.

---
BTW, from the help:


S. Voltage Controlled Switch


Symbol Names: SW



Syntax: Sxxx n1 n2 nc+ nc- <model> [on,off]



Example:



S1 out 0 in 0 MySwitch

..model MySwitch SW(Ron=.1 Roff=1Meg Vt=0 Vh=-.5 Lser=10n Vser=.6)



The voltage between nodes nc+ and nc- controls the switch's impedance
between nodes n1 and n2. A model card is required to define the
behavior of the switch. See the schematic file
..\examples\Educational\Vswitch.asc to see an example of a model card
placed directly on a schematic as a SPICE directive.



Voltage Controlled Switch Model Parameters



Name
Description
Units
Default

Vt
Threshold voltage
V
0.

Vh
Hysteresis voltage
V
0.

Ron
On resistance
W
1.

Roff
Off resistance
W
1/Gmin

Lser
Series inductance
H
0.

Vser
Series voltage
V
0.

Ilimit
Current limit
A
Infin.




The switch has three distinct modes of voltage control, depending on
the value of the hysteresis voltage, Vh. If Vh is zero, the switch is
always completely on or off depending upon whether the input voltage
is above the threshold. If Vh is positive, the switch shows
hysteresis, as if it was controlled by a Schmitt trigger with trip
points at Vt - Vh and Vt + Vh. Note that Vh is half the voltage
between trip points which is different than the common laboratory
nomenclature. If Vh is negative, the switch will smoothly transition
between the on and off impedances. The transition occurs between the
control voltages of Vt - Vh and Vt + Vh. The smooth transition follows
a low order polynomial fit to the logarithm of the switch's
conduction.



There is also a level 2 voltage-controlled switch which is an advanced
version of the level 1 switch with negative hysteresis. The level 2
switch is never completely on or off. The conduction as a function of
control voltage Vc is



g(Vc) = exp(A * atan((Vc - Vt)/abs(Vh)) + B)



where



A = log(Roff / Ron) / pi

B = log(1 / (Roff * Ron)) / 2



Also, the transition of the level 2 switch to current limit is gradual
instead of abrupt. At a fixed control voltage, the I-V curve is giving
by the equation



I(V) = Ilimit * tanh(g(Vc) * V)



where Ilimit defaults to 10 amperes for the level 2 switch.

The level 2 switch supports the option to conduct in only one
direction by either specifying the flag "oneway" or specifying a
voltage drop with parameter Vser. The transition between forward
conduction and reverse open circuit can be specified to be a smooth
transition by specifying the parameter epsilon to be non-zero.

--
JF

 

[Bill Bowden]

On Jun 20, 5:50 am, John Fields <jfie...@austininstruments.com> wrote:

On Sun, 19 Jun 2011 16:36:27 -0700 (PDT), Bill Bowden

bper...@bowdenshobbycircuits.info> wrote:
How do I define the resistance of the SW (switch) component in the
LTspice library? Apparently it's 1 ohm, and I'm trying to simulate a
MOSFET (IRF-640 or similar) of 180 milliohms.

---
Add a directive to your schematic in the form of:

.model SW SW(Ron=1 Roff=10Meg Vt=0.5 Vh=0)

and edit it to suit.

--
JF

Yes, that works. Took awhile to find the directive option on the edit
list. I was right clicking on the switch symbol and expecting a menu
similar to the voltage symbol menu.

-Bill