# Mosfet saturation vs linear. Does it Correlate?

#### JeanMichelComprendRien

Joined Jun 30, 2016
1
Maybe a stupid question.

In a MOSFET, does the linear mode rdson always correlate with the saturation current ?

Assume 2 MOSFETs, geometrically identical. If MOSFET A has higer Rdson than MOSFET B. Does it mean it will have also a lower saturation current than MOSFET B ?

Thks,

JMCR

#### crutschow

Joined Mar 14, 2008
27,015
I don't think there's any particular correlation between the two.

#### Bordodynov

Joined May 20, 2015
2,843
Rdson=Rds_MOS+Rd+Rs
Rds_MOS~1/KP and id_sat=F(KP,Rd,Rs,Vgs,Vds)

#### MrAl

Joined Jun 17, 2014
8,154
Maybe a stupid question.

In a MOSFET, does the linear mode rdson always correlate with the saturation current ?

Assume 2 MOSFETs, geometrically identical. If MOSFET A has higer Rdson than MOSFET B. Does it mean it will have also a lower saturation current than MOSFET B ?

Thks,

JMCR
Hi,

I am not so sure your question is properly worded so i will word it according to the question it appears you are trying to ask.

It appears that you are trying to ask if there is a relationship between the current rating and the Rdson of a MOSFET. Note i am saying the 'rating' here, which means the max current for the device really, and of course the on resistance at full current.

For the same package type, there would have to be some relationship but to find out how strong that relationship is you could look at several data sheets and see for yourself.
For example, if we had a TO220 package and the MOSFET was rated at 5 amps and Rdson=0.2 ohms, then the voltage drop at full current (5 amps) would be 1v, and more importantly that would be 5 watts. If we had the same package and the MOSFET was rated at 50 amps, would could not get away with having Rdson equal to 0.2 ohms because that would mean a drop of 10 volts and at 50 amps that would be 500 watts which would not be possible. So there must be some correlation, although at the low end we have more flexibility where a 1 amp MOSFET can have a more varied resistance value. A 50 amp device with Rdson=0.02 ohms would see 1 volt drop and at 50 amps that would mean about 50 watts which is actually possible.
The limiting factor is how fast you can get heat away from the package with a heatsink, and the heat transfer rate is limited by the surface area of the bottom of the MOSFET and how the die diffuses the heat to the bottom. This is similar to how a wire diameter limits the max current in a wire. With a limited bottom surface area there is a limited rate of heat transfer, and therefore there is a limit on the Rdson*Imax, and since this would be some constant K we have K=Rdson*Imax, so there is no wiggle room in that if there is a certain Imax then there can only be a certain Rdson or else the package would not be able to achieve it's full Imax current level, in that package.
Also, it is not as likely that a manufacturer would create a die that has 0.001 ohms Rdson and only rate it at 5 amps, although it is possible i guess. That would be a waste of die size really.
Also, going to a bigger die, as the die size increases the cross sectional area will increase and therefore the current rating goes up and the resistance goes down.

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