Why doesn't the mosfet simulate correctly?

[dbur]

Hi,

I got this P channel mosfet model from Fairchild  FQT3P20.  I try to simulate it with tansient analysis in a simple circuit.  The simulation runs but does not produce a resonalble result.  How do I get this model to work with MS 10.1?

Thanks,
David B

The MS file is attached with the part inside, you should be able to open it and see the model, save to your database, etc.



Message Edited by dbur on 06-11-2008 01:53 PM

[lacy]

To me it aqppears the model is working. The problem seems to be the 2 voltage sources you have in the source and drain circuits. I am not suure what purpose you may have had for them, but I do not believe that you can use them as loads. Try eliminating them and move the source resistor to the drain and take the voltage measurement at the junction of the resistor and drain.

See if that helps anything.

[dbur]

The sources are there because that gives me the option to specify the cuurent in those legs for output display.  Other than that they have no function.  Removing them didi not help the simulation to run correctly.
Thanks anyway,
David B

[lacy]

My bad, I should have said it seemed to be simulating correctly (without error messages) but that doesn't mean correct operation. I will look into this more deeply and see what I can come up with.

[lacy]

Yes, I do see it now. This is definitely a model problem. The bad part is I do not know what is needed to correct it. I did find a 200v Mosfet in the database that seemed to work. I tried comparing the 2 models and try to find out why this one is not working, but not knowing exactly what I am looking for in the model file I was not successful.

I will have to let someone else with more modeling experience take this one. Multisim is supposed to be somewhat compatible with PSpice models and there are some that do work very well, but most of the them are just not 100% compatible and that creates problems when trying to use model from the manufacturer especially if they create the model for a specific software.

Sorry about my misunderstanding your first post. I was looking for the simulator to return errors and when it didn't I naturally assumed the model was working and didn't look close enough at your drain voltage source  and compare the ouput of the Mosfet to that. I basically got confused.

[RnDMonkey]

Maybe I'm totally wrong here, but is this even what an enhancement-mode P-channel MOSFET is supposed to do? See attached file and tell me if that is right. Shouldn't the voltage on the drain be equal to your supply voltage when your gate voltage is zero. Isn't the gate voltage supposed to increase conduction in enhancement-mode devices, meaning a voltage on the gate will increase conduction, pulling the drain toward ground in the case of this circuit?  It seems more like this is acting as a depletion-mosfet, and a weird one at that. Basically, in enhancement-mode P-FETs, isn't a high gate-source voltage an ON signal (gate closed, conducting), while no gate-source voltage is an OFF signal (gate open, non-conducting)? Somebody slap me, because I can't believe I'm right here.

Message Edited by morgol on 06-11-2008 09:51 PM

[dbur]

The channel is off when Vgs=0 V.  It's on when Vgs < ~-4V.  The fet in your circuit is always on.  You have it configured as a source follower.  The source just follows the gate voltage minus the threshold.  In my original circuit the gate source is configured with an offset so that at the peak it is the same as the supply, and so the mosfet is off.  As the gate voltage drops the mosfet should turn on and the drain voltage should appear as the inverse of the gate voltage times some gain factor.

Replace the FQT3P20 in my circuit with the BSS110, change the supply to 50V (It's a 50V mosfet) and the gate source offset to 45V and you will see the correct result for the simulation.

David B

[RnDMonkey]

I was doomed to failure from the start, since I had my drain and source confused. Anyway, I had to go into the operation and timing cycles of the BSS110 and BSS 119 to make sure I was figuring things right. I made a nice little circuit that has a bunch of switches that make it cluttered, but it gives really pretty o'scope readings. I then replaced the BSS 110 with your problematic FQT3P20 and noted that the Vgs is dead (possibly a short in the model?) compared to the BSS110 which I made swing between 0 and -10. On that note, shouldn't the nomenclature be Vsg (read: Voltage from source to gate) since we are talking about the gate voltage relative to the source?

[dbur]

shouldn't the nomenclature be Vsg (read: Voltage from source to gate) since we are talking about the gate voltage relative to the source?

That's why Vgs is specified as a negative voltage for a p channel mosfet.  No one calls it Vsg.
DavidB

[RnDMonkey]

Yes, but isn't the gate voltage negative relative to the supply voltage? The way Vgs is written, wouldn't the supply voltage be negative relative to the gate voltage (read: gate voltage positive relative to supply voltage). I read Vgs as "Voltage from gate to source" or "Voltage at source, referenced to gate"