MOSFET ohmic region, needs to be explained

jpanhalt

Joined Jan 18, 2008
11,087
in those Figs, after the curve turns almost flat line then with increasing in the Vds, why the current (Id) doesn't increase though? so Rds also increases with increasing the Vds? is that correct?
That is correct. The flat part is "non-ohmic" meaning the current doesn't increase with increasing Vds difference. Of course, a resistor, which is ohmic, would show the expected change.

The significance of the non-ohmic region is that the mosfet junction is carrying all of the current it can at that particular gate voltage. It is "saturated." As you increase the gate voltage, more current is allowed until further increases in Vgs cause no further increases in current. At that point, the mosfet is "fully on." The maximum current when fully on often exceeds the current capacity of the device package.

When not fully on, more heat is produced as Vds increases and current doesn't increase. For that reason, mosfets doing PWM or any other switching, are usually operated fully on to reduce heating effects. However, in low power situations, you will see mosfets operated somewhat below fully on.
 

Thread Starter

farzad latifeh

Joined Oct 3, 2017
82
That is correct. The flat part is "non-ohmic" meaning the current doesn't increase with increasing Vds difference. Of course, a resistor, which is ohmic, would show the expected change.

The significance of the non-ohmic region is that the mosfet junction is carrying all of the current it can at that particular gate voltage. It is "saturated." As you increase the gate voltage, more current is allowed until further increases in Vgs cause no further increases in current. At that point, the mosfet is "fully on." The maximum current when fully on often exceeds the current capacity of the device package.

When not fully on, more heat is produced as Vds increases and current doesn't increase. For that reason, mosfets doing PWM or any other switching, are usually operated fully on to reduce heating effects. However, in low power situations, you will see mosfets operated somewhat below fully on.
can someone control the resistance between the source and the drain by a MCU like Arduino or Raspberry and with changing the duty cycle of the PWM the Vgs changes and so the ohmic region expand then? so it resembles an ohmic resistor though?
 

jpanhalt

Joined Jan 18, 2008
11,087
Like BobTPH and others have said, PWM is not a variable resistance. Continuing to call it that will only increase confusion.

You cannot change the ohmic region. It is part of the device.

Yes, if you properly integrate the PWM, in theory, any MCU can control the ohmic resistance/linear response of a mosfet, but that is probably not what you want to do. If that is what you want to do, consider switching to a BJT (ordinary transistor).
 

Thread Starter

farzad latifeh

Joined Oct 3, 2017
82
Like BobTPH and others have said, PWM is not a variable resistance. Continuing to call it that will only increase confusion.

You cannot change the ohmic region. It is part of the device.

Yes, if you properly integrate the PWM, in theory, any MCU can control the ohmic resistance/linear response of a mosfet, but that is probably not what you want to do. If that is what you want to do, consider switching to a BJT (ordinary transistor).
why BJT?
 

jpanhalt

Joined Jan 18, 2008
11,087
Perhaps that was a poor analogy. It is said that mosfets are voltage controlled and BJT are current controlled. Thus, if you control the base current, you control the collector current:
1612873123694.png
Source: http://fourier.eng.hmc.edu/e84/lectures/ch4/node3.html

In fact, some tutorials refer to them as "current controlled resistors:"
1612874064507.png
Source:
1)http://rsdacademy.net/textbooks/sol...ent flows into the base,flows out of the base.
2)Horowitz and Hill, The Art of Electronics, 3rd. ed., 2015, p.75.

If you want a PWM-controlled voltage, simply filter the output to give a steady DC voltage.
 

crutschow

Joined Mar 14, 2008
34,285
In fact, some tutorials refer to them as "current controlled resistors:"
That is not an accurate description of a BJT.
It's much closer to a current-controlled current-source, since its resistance is quite high in the active region (note that the curves shown in post #25 are nearly horizontal to the right of the saturation region, indicating a high resistance).
 

jpanhalt

Joined Jan 18, 2008
11,087
Corrections can be mailed to Professor (emeritus) Paul Horowitz at:

Paul Horowitz, Ph.D.
Harvard University
Jefferson Lab 353
17 Oxford Street
Cambridge, MA 02138
 

crutschow

Joined Mar 14, 2008
34,285
Corrections can be mailed to Professor (emeritus) Paul Horowitz at:

Paul Horowitz, Ph.D.
Harvard University
Jefferson Lab 353
17 Oxford Street
Cambridge, MA 02138
So I assume he's the one that published that questionable definition of a BJT?

In my experience Professors don't like to be corrected, but you are welcome to try.
 

jpanhalt

Joined Jan 18, 2008
11,087
But you the one that seems to think he should be contacted.
I, for one, could give a rat's rear about whether some professor wrote some incorrect technical info.

Are you trying to be a troll?
Sorry if I wasn't clear. Paul Horowitz is a well recognized teacher, writer, and emeritus Professor at a major university. You said he was wrong by correcting a direct quote from his textbook (both 2nd and 3rd editions). I believe the quote is accurate. You provided no critical evidence other than your opinion. What have you published on the subject to qualify as an SME?

So, I suggested that if you thought it was an error in a very widely used textbook that you should argue your point with him. Absent your ability or willingness to do that, I accept the accuracy of his comment.

Perhaps you ought to re-read what I wrote. More important, read Professor Horowitz's textbook.
 

crutschow

Joined Mar 14, 2008
34,285
Paul Horowitz is a well recognized teacher, writer, and emeritus Professor at a major university.
Sorry I'm not familiar with him.
You provided no critical evidence other than your opinion.
It's not opinion, it's fact.
The output resistance of a BJT is very high in the active region.
The input base current does not vary that impedance, it varies the output current (current source).

I don't see that as being difficult to understand.
And I don't need to read some professor's book to tell me something I know is obviously not correct.
 

sparky 1

Joined Nov 3, 2018
756
I think possibly too much overthinking and to many possible methodologies will keep you from actual hands on building.
Being proactive with a simple demonstration allows you to see that it agrees with the tutorial. first do this then do this then do this ect. FOLLOW directions stop over thinking it.
 

ElectricSpidey

Joined Dec 2, 2017
2,758
Calling a simple follower circuit a regulator is a bit of a stretch.

Although there is some load regulation, there is no line regulation.

Power control is a better description.
 

sparky 1

Joined Nov 3, 2018
756
I think possibly too much overthinking and to many possible methodologies will keep you from actual hands on building.
Being proactive with a simple demonstration allows you to see that it agrees with the tutorial. first do this then do this then do this ect. FOLLOW directions stop over thinking it.
The R load can be on either source or drain.
image_2021-02-09_150040.png
 
Last edited:

Audioguru again

Joined Oct 21, 2019
6,674
The simple circuit in the video an in post #38 will destroy the Mosfet when the pot is turned up higher than 20V because it is missing a resistor.
With some IRFZ44 Mosfets having a threshold voltage of 4V then the load will get no voltage until the pot is turned higher than about 4V.
The datasheet also shows that the Mosfet is fully turned on when the gate-source voltage is only 10V.
If the load is from the source to ground then the maximum voltage across the load in post #38 will be only about 22V for some IRFZ44 Mosfets.
 

DickCappels

Joined Aug 21, 2008
10,152
There have been reports of crankiness of late.

From section 1 of the User Agreement:
Appropriate conduct. Debates should be a civil activity and can be both enlightening and entertaining, but always keep discussion to the facts and the opinions. Ad hominem tactics and directed abuse are always "out-of-bounds".

We should be treating each other respectfully just as we would or did with colleagues at work.
 
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