What does Rds(on) in a MOSFET mean?

[millwood]

(mos) lower rds(on) mosfet

This whole thread is about MOSFETs, not BJTs...

i know. I think they just reversed the terms on mosfets, vs. those on bjts, to confuse you.

 

[varunkant2k]

Re: rds on mosfet wiki

if you want to look at your chart, the saturation region is one where the traces are going up: where Vds is very low and Ids goes up with Vds, given a Vgs. the left most region where you put the "on" state symbol.

the linear region is where Vds is significant and does NOT vary with Id. Instead, Id goes up and down with Vgs. That's the middle area where the traces are flat.

the "off" area is where Vgs is < Vgs(th).

Added after 4 minutes:

another way to look at the same thing is to look at its Id vs. Vgs chart: from irfp240.

the left most region is where Vgs<Vgs(th), so the device remains off (Id is very small).

as Vgs goes above Vgs(th), the mosfet starts to conduct and its Id goes up with Vgs. this is the linear region.

As Vgs continues to go up, Id stops increasing as the mosfet is "saturated".

the same thing holds for bjt as well.



Added after 34 seconds:

and you can find the same relationship in ALL mosfet datasheets.

Hi Please see the attached modified MOSFET characteristic.
Question: Switch works in which region: Answer: Triode/Linear region.
(Please do not confuse with BJT. There are another region called active region where only CE made in forward bias.)
In Switch vout waveform, the slpoe between low voltage to high voltage region is the saturation region but that is very small. and hence preferred to made negligible.
Hope this will find helpfull.
Triode region RDs On is always very low (few tens of ohm or less). but in saturation ron (compare it with RDS on) is hundreds of kohms.

 

[xaccto]

@varunkant2k, what reference are you using for you information.

this topic got me worried that I mis-understood some things, then again maybe not.
Good attempt at getting ones ideas confuted though.....:razz:

 

[varunkant2k]

HI, no worry.
See I can't recall the reference specifically. But it is a very basic thing need to read any standard book again.
If you say, I can try again to elaborate more.
Take one n-ch MOSFET with known VDS of less than VDD. Now If you you increase Vgs from 0V to VDD, Untill Vgs =Vth, MOSFET will be OFF. As Vgs>Vth, transitor is on ( but what is the region?). Check if Vds > Vgs-Vth, this is operating in Saturation region. So After on transitor comes in saturation. Now As soon Vgs = Vds + Vth, it touches the triode region.
So Id/Vgs characteristics, have three regions as shown in the above graph.

 

[nepdeep]

Re: rdson wiki

i think milliwood is totally wrong about it...Rds on is the constant resistance in linear region...at saturated condition...mosfet resistance if you say is considerably higher..

 

[walkura]

Re: rdson wiki

At school they thought me that a transistor is saturated when a further increase of base current or gate-voltage won't result in opening the transistor or mosfet any further.
The linear range is between Vth and Vsat.
Have a look at the following "book" from the IRF website and look under on-resistance (figure 6).
http://www.irf.com/technical-info/appnotes/mosfet.pdf

I hope this clarifies it.

 

[varunkant2k]

The linear range is between Vth and Vsat.

Hi Walkura, first you should not confused with BJT characteristics with MOSFET.
In MOSFET no term is like active region as in BJT. Active region in BJT off course occurs between OFF and SAT when in ID, VGS curve.
But Id vs Vgs curve for MOSFET, triode region comes ( miss spelt by linear region) after SAT when Vgs increases further towards VDD. So for example, 5 volts NMOS devices and application, less than VTh devices are off (or sub threshold) after Vth upto Vgs = 3-4V saturation region, and then triode region starts upto Vgs =5V.
-------
I did not get any thing what you are telling in the paper attached by you.

 

[walkura]

@Varunkant2
There is no reason for confusion.
A transistor is saturated when a further increase of base current won't result in a lower VCE.
A mosfet is saturated when a further increase of gate voltage won't result in a lower VDS.
In some country's its called triode area, some other country's call it ohmic or linear region.
(although You can use mosfets with a lower gate voltage (above Vth) which just means You accept higher losses due to a higher RDSon with the gatevoltage given)
I think this discussion drags on because of the "cultural" difference between the terms used,
but all those names apply to the same behavior which is very well documented by more or less every company producing these parts.

Best regards,
Walkura.

 

[crutschow]

I believe the confusion is the term "saturation". For BJTs is is when the transistor is fully on (collector-emitter voltage equal or less than the base-emitter voltage). In FETs it is technically defined as when the drain current is little affected by changes in drain voltage. But commonly, saturation is sometimes considered to be when the FET is fully on with minimum drain-source resistance (in the linear region) based upon similarity with BJT saturation .

So it just depends upon whether you want to use the technical description for FET saturation or the common (but not rigorous) description.

To avoid confusion I always refer to the FET being "fully on" rather than using the term saturation.

 

[FvM]

But commonly, saturation is sometimes considered to be when the FET is fully on with minimum drain-source resistance (in the linear region) based upon similarity with BJT saturation.

What's "commonly" in this regard? You won't find the term saturation with this meaning (fully on) used in any MOSFET datasheet or text book. MOSFET datasheets mostly avoid the term saturation at all, but in text books, it's exclusively used for the constant current or "pentode" region of the MOSFET characteristic.

Interestingly, IGBT literature also uses the BJT terminology (according to the nature of the device). This can cause additional confusion because IGBT and MOSFET are used in similar circuits, but the effect described as "desaturation" of IGBTs should not be designated with the same term for MOSFETs. Using it though might be an example of "common language" usage for MOSFET.

 

[learning_curve]

Re: fet switch high current rdson

It operates in the ohmic region(linear) when employed as a switch.
Say the switch is nmos switch and is "on", this means Vds across it is going to be really small..therefore Vds < (Vgs -Vth)
where Vth is the threshold voltage of the device.
Since Vds < (Vgs-Vth), the switch is operating in the linear region.

 

[leonzheng]

Hi Walkura, first you should not confused with BJT characteristics with MOSFET.
In MOSFET no term is like active region as in BJT. Active region in BJT off course occurs between OFF and SAT when in ID, VGS curve.
But Id vs Vgs curve for MOSFET, triode region comes ( miss spelt by linear region) after SAT when Vgs increases further towards VDD. So for example, 5 volts NMOS devices and application, less than VTh devices are off (or sub threshold) after Vth upto Vgs = 3-4V saturation region, and then triode region starts upto Vgs =5V.
-------
I did not get any thing what you are telling in the paper attached by you.

https://www.doe.carleton.ca/~tjs/21-mosfetop.pdf