MOSFET as a switch and Rds(on)

[eliben]

Hello,

I'm having a slight confusion over what Rds(on) means for a MOSFET.
In the book it says that Rds(on) is the resistance (Vds / Ids) in the
linear (triode) region. After the FET enters saturation, the
resistance obviously changes as the current stays the same for
increasing voltage.

However, people I ask say that they only use FETs for switching (i.e.
either cutoff or saturation), and for them Rds(on) is the resistance
in saturation.

Which is right? Can you point to a lucid explanation of this topic?
Thanks in advance

 

[John Larkin]

On Tue, 14 Jul 2009 19:33:51 -0700 (PDT), eliben <eliben@gmail.com>
wrote:

Hello,

I'm having a slight confusion over what Rds(on) means for a MOSFET.
In the book it says that Rds(on) is the resistance (Vds / Ids) in the
linear (triode) region. After the FET enters saturation, the
resistance obviously changes as the current stays the same for
increasing voltage.

However, people I ask say that they only use FETs for switching (i.e.
either cutoff or saturation), and for them Rds(on) is the resistance
in saturation.

Which is right? Can you point to a lucid explanation of this topic?
Thanks in advance

There are two common uses of the word "saturation" with respect to
mosfets. Some people (the more academic types) use it to refer to the
higher-voltage constant-current region. Some (more likely to be
working engineers) sometimes use it to refer to the low-voltage ohmic
region, like the bipolar transistor saturation region. That may
explain the confusion.

Rds_on is the slope of Vd/Id for small excursions near zero volts,
with some specified amount of gate turn-on voltage applied. It's less
ambiguous to refer to this region as the "ohmic" region, and the
higher-voltage part of the drain curve as the "constant-current"
region.

John

 

[default]

On Tue, 14 Jul 2009 19:33:51 -0700 (PDT), eliben <eliben@gmail.com>
wrote:

Hello,

I'm having a slight confusion over what Rds(on) means for a MOSFET.
In the book it says that Rds(on) is the resistance (Vds / Ids) in the
linear (triode) region. After the FET enters saturation, the
resistance obviously changes as the current stays the same for
increasing voltage.

However, people I ask say that they only use FETs for switching (i.e.
either cutoff or saturation), and for them Rds(on) is the resistance
in saturation.

Which is right? Can you point to a lucid explanation of this topic?
Thanks in advance

The Rds(on) when seen in a one line description of the part or on the
first page of a data sheet - is "how low" in resistance "it can go"
(like bragging) while saturated.

Then they give you curves to modify that figure based on temperature
Vds/Ids, etc. with more realistic expectations.


--

 

[gearhead]

On Jul 14, 10:33 pm, eliben <eli...@gmail.com> wrote:

Hello,

I'm having a slight confusion over what Rds(on) means for a MOSFET.
In the book it says that Rds(on) is the resistance (Vds / Ids) in the
linear (triode) region. After the FET enters saturation, the
resistance obviously changes as the current stays the same for
increasing voltage.

However, people I ask say that they only use FETs for switching (i.e.
either cutoff or saturation), and for them Rds(on) is the resistance
in saturation.

There's nothing wrong with your understanding. "Saturation" has two
different meanings, depending on whether you're dealing with bipolars
or mosfets.