Choosing MOSFET - datasheet

[Port88]

Hi!
I have a question concerning n-channel logic level MOSFETs. Suppose that VGS=VDS=5V (SAT). Is there a way just by looking at the MOSFET datasheet to be sure whether the current that i get is lower than ID maximun rating? all this temperature stuff is confusing. Take this datasheet for example...

Thanks in advance!

 

[audioguru]

If you have a Vgs of 5V then that Mosfet is turned on with a drain-source resistance of 0.35 ohms or less when it is cool. Then a drain-source current of 1.1A (a max of 0.86A when the gate-source voltage is only 4.5V) will cause it to overheat since its drain-source voltage will be 0.42V. Did you notice that 1.1A squared x 0.35 ohms= 0.42W which is higher than the max allowed power of 0.41W (when it is cool somehow)?

Why do you say that the drain-source voltage is 5V? When it is turned on with a load then the Vds voltage is very low so that the load gets most of the power supply voltage.

 

[Port88]

First of all, thank you for answering! I'm totally new at this ...

So , why are you saying there'll be a current of 1.1 A, given Vgs=5? Is that obvious somehow according to the datasheet ? Or maybe , you're referring to the minimum current so that power begins to exceed the max allowed... (?)

Yes you're right about this. There is a load of 5 Ohms , so given I want to achieve a current of 500 mA , Vds will be reduced to 2.5 V if I'm right...

 

[audioguru]

So , why are you saying there'll be a current of 1.1 A, given Vgs=5?
The datasheet spec's the max on-resistance to be 0.35 ohms when the Vgs is 4.5V and the mosfet is cool. Its max allowed power is 0.41W and its max allowed current is 0.86A.
I guessed that the max on-resistance is 0.338 ohms when Vgs is 5.0V so the max power dissipation is 0.41W.

There is a load of 5 Ohms , so given I want to achieve a current of 500 mA , Vds will be reduced to 2.5 V if I'm right...

With a max on-resistance of 0.338 ohms and a current of 500mA then the max heating in the Mosfet is only 0.0845W which is much less than the max allowed dissipation of 0.41W. Vds will be a max of 0.338 ohms x 500mA= 0.169V, not anywhere near 2.5V.

 

[Port88]

Well , that was very helpful! Thank you!
I'd like to be more specific. Suppose I have the circuit given below. If I want to achieve 500 mA with the specific load of 5 ohms, should I choose a MOSFET with an RDSon=5 ohms?
(Ignore the PWM, for now...)

 

[audioguru]

Mosfets are made to be a switch with a very low resistance. Their on-resistance is a typical or maximum resistance so you don't know what the actual resistance will be.
Use the PWM to reduce the average current.

 

[crutschow]

I'd like to be more specific. Suppose I have the circuit given below. If I want to achieve 500 mA with the specific load of 5 ohms, should I choose a MOSFET with an RDSon=5 ohms?
(Ignore the PWM, for now...)

If you want to achieve 500mA without using the PWM then you would add an additional 5 ohms in series with the load. You always want the MOSFET to be fully on or off when using it as a switch. You don't use the MOSFET on-resistance to limit the current, since the on-resistance can vary significantly depending upon operating conditions and unit-to-unit variation. It also causes the MOSFET to dissipate heat unnecessarily.

 

[Port88]

Thank you! Now I suppose I shall choose a MOSFET that its Id max is higher than the current calculated when it is fully ON given the specific circuit. That's about 1A. I think that will do.

 

[audioguru]

Thank you! Now I suppose I shall choose a MOSFET that its Id max is higher than the current calculated when it is fully ON given the specific circuit. That's about 1A. I think that will do.

You also need to look at how much gate voltage will cause how much on-resistance.