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update: I edited question so that it will be mainly related to LTspice

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OK, now I have used irfz44n for an example model, I realized that, by using the directive above(nmos), I am not able to modify Vto value. However if I use the second one(vdmos), I can control the Vto value. But while choosing the mosfet from component selector, I get it from "nmos" option. I wonder why this is the situation? Should I use vdmos instead of nmos all the time?

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  • \$\begingroup\$ Knowing nothing about the model, that could be perfectly reasonable. \$\endgroup\$ – Matt Young Dec 10 '19 at 1:52
  • \$\begingroup\$ @MattYoung the model is ltspice default mosfet model, I am changing only the vto value. \$\endgroup\$ – muyustan Dec 10 '19 at 2:32
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    \$\begingroup\$ You miss almost everything. People are not going to consult spice or guess the version or ... . Look at specs for a real world device or full specs for the LTS one, calculate what you'd expect and then comment if you find that your calculations are correct and SPICE's are wrong. \$\endgroup\$ – Russell McMahon Dec 10 '19 at 7:31
  • \$\begingroup\$ @RussellMcMahon please see my edit. \$\endgroup\$ – muyustan Dec 10 '19 at 8:19
  • \$\begingroup\$ IRFZ44N datasheet here At Vgs = 5V based on the datasheet I'd expects RDSon to be under 0.1 Ohm (probably worst case), so VDS a few mV. \$\endgroup\$ – Russell McMahon Dec 10 '19 at 9:15
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The problem is the vdmos model IRFZ44N already exists in the component databases.

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Adding a .model IRFZ44N nmos(..) card will not change the type of the IRFZ44N to a monolithic mosfet.
You are therefore not able to manipulate the parameters of the IRFZ44N using .model nmos(..), but you can change them using .model vdmos(..)

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  • \$\begingroup\$ I have to say that probably this is not correct too, because I can edit the behaviour of an npn for example 2n2222. And also I can edit irfz44n by .model irfz44n vdmos(.....). \$\endgroup\$ – muyustan Dec 12 '19 at 20:09
  • \$\begingroup\$ @muyustan You're right again. With your comment, it is not hard the draw the conclusion in the updated answer. \$\endgroup\$ – Huisman Dec 12 '19 at 20:33
  • \$\begingroup\$ I don't get what you are meaning by updated answer. \$\endgroup\$ – muyustan Dec 12 '19 at 20:35
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    \$\begingroup\$ @muyustan Next time, I'll update before commenting :) \$\endgroup\$ – Huisman Dec 12 '19 at 20:40
  • \$\begingroup\$ hahahah, thanks. I have quickly looked to all mosfet definitions which are premade in ltspice. They were all defined with vdmos. Are all those really vdmos? For example when I look datasheet of irfz44n it does not mention something as vdmos. \$\endgroup\$ – muyustan Dec 12 '19 at 20:43

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