MOSFET on Resistance – Rds on mosfet and how to choose it in datasheet

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mosfet rds on resistance

MOSFET Rds on

Rds on stands for resistance (R), drain (D), and source (S) means resistance between the drain and source of the mosfet but still it is not clear resistance when. That is why on is used at the end to symbolize that it is resistance when mosfet is on.

when voltage at gate pin is available than Resistance between the drain and source is called mosfet on resistance

when voltage at gate pin is not available than resistance between the drain and source is called Mosfet Rds off or Mosfet off resistance.

Lets understand with an example of what is the Rds in mosfet and how to select it from Onsemi datasheet. 

If you see the datasheet of any MOSFET lets say 40V, 300Amps NVBLS0D5N04M8 Onsemi MOSFET, on going through the datasheet you will see two values of on resistance one is typical and other is maximum, the question is which value you should choose for your design to be robust.

when you run 300Amps through the MOSFET its junction temperature will increase because of the power dissipation I^2*R.

So here in the above expression what you see R is on resistance of mosfet.

mosfet rds on normalized charecteristics

If you choose the wrong Rds on then you may get wrong power dissipation and FET could probably burn.

The Rds on mentioned below is at 25degC, when the current flows through it Junction temperature (Tj) rises,

Therefore it is recommended to always choose Rds on at maximum Tj which is 175 degC a worst case value in this case.

Also Read How to choose mosfet threshold voltage   

So at 175 degC for this case is 1.7 then on resistance at 175degC = 1.7*0.57mohm = 0.969mohm, Now as you see the picture is clearly different, so for your designing you should consider Mosfet Rds on as 0.969mohm always.

Conclusion:

So now how to find Rds on at 175degC, first step is to find normalization factor at 175 degC then multiply it with max Rds on.

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