Turn J-Channel FETs OFF?

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buurin

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I am starting to use FETs as switches instead of using BJTs as I have in the past.

I have been playing with them in pspice and cannot seem to get the FETs into an off state.

Notice in the following image that the circuit on the left shows 3.473V at the source pin of the FET, where I expect ~0V.

The circuit on the right is on, hence 5V at the source pin of the FET. (Not really sure why I included this... it made sense when I uploded it )

**broken link removed**

How can I get ~0V on the source pin?

Thanks
 
Look at the datasheet then learn about how Mosfets work.
It is a P-channel Mosfet so your circuit needs a negative power supply, or turn the Mosfet upside down.

If your circuit has a negative supply then the left Mosfet would be turned on and its source would be close to -5V. The right Mosfet would be turned off and its source would be 0V.

Like most Mosfets, the IRF9140 is turned on the most with 10V between gate and source.
 
Thanks for the reply audioguru.

Excuse my posting title(j-channel), I understand its a p-channel.

I had a n-channel simulating as i expected but could not get the p-channel right, I ended up flipping it upside down as you suggested and now its working correctly.

Thanks a lot.
B
 
Now the MOSFET is off but showing ~100mv @ its drain(now the FET is flipped), so its not completely off.

My BJTs would show much less, in the nV range

Updated schematic:
**broken link removed**

Also does anyone have any reccomended docs on MOSFET usage? I am less interested in the physics and more in actual usage.

I was swtiching to FETs because I thought they would be easier for switching applications in which the FET is directly controlled via microcontroller.


Thanks again
 
Last edited:
Are they enhancement mode or depetion mode MOSFETs?

N channel depletion mode MOSFETs need a negative voltage to turn them off.
 
buurin said:
p-channel enhancement
Click to expand...
This vague but correct.

I downloaded the data sheet and it is a P channel so you need a negative supply voltage (with respect to the source) and a negative gate voltage to turn it on.
 
The datasheet For the Mosfet says that its max leakage current is 25uA with 80V across it with its junction at 25 degrees C. So the one is your sim is leaking a little too much at 11uA with only 5V across it.

The IRF9140 is rated for 11A to 18A so why use it with a 10k load where its max current is only 0.5mA?

A 2N3055 is a BJT with a 15A similar rating. Its max leakage current is a whopping 200mA at its rated max voltage and its junction at 25 degrees C.
 
If you want a small signal MOSFET then try something like the BS250 but I wouldn't bother I'd use a PNP bjt.
 
The reason I am using the IRF9140 in the simulation is because its the only p-channel mosfet model I have available in Pspice (student version)

I was planning on ordering the BS250 for a actual use in my projects.

I have tried everything and the IRF9140 is just leaking way too much. I think I just need a new model.

Thanks everyone.
 
If you look at the data sheet of the IFR9140 its leakage current is specified as 25 uA max. So if you need a MOSFET that has less leakage, then you need to select a lower power one.
 
As stated above, I dont think it should be leaking 11uA @ 5V...

The BS250 would fit the bill I just cant figure out how to use model I found for it Time for a new thread
 
buurin said:
As stated above, I dont think it should be leaking 11uA @ 5V...

The BS250 would fit the bill I just cant figure out how to use model I found for it Time for a new thread
Click to expand...
I suggest you read the data sheet!

This is a high power MOSFET designed for a maximum of 18A so it has some leakage. 11uA sounds about right given that, as I said above, it has a max leakage of 25uA.
 
The datasheet that I have does not have any graphs specific to leakage current, so I will take your word for it.

Thanks
B
 
No I have not built it with MOSFETS. I currnently have it built with BJTs.

Like I said before, I never had any intention of using the IRF9140... It was simply the only model I had available for simulation. I just wanted to tinker around and figure out how to use a MOSFET, since I have never dealt with them in actual application.
 
Maybe you sim software needs to be updated so it knows more about Mosfets, transistors and linear ICs that have been made in the last 20 years.

My free SwCAD III sim software nags me that new models are available every few days.
 
I'm downloading SwitcherCad as we speak... Thanks for the tip.

The reason my pspice is limited is because its the student version.
 
buurin said:
The datasheet that I have does not have any graphs specific to leakage current, so I will take your word for it.

Thanks
B
Click to expand...
Its not in a graph. I have the SEME Data Sheet that I downloaded from www.alldatasheet.com.

In the table headed "Static Electrical Ratings" it is in the row "Idss" (the DSS is in subscript upper case) "Zero Gate Voltage Drain Current"
 
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