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P-Channel Mosfet VGS

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Nikos A.:
Thank you for your answers guys!! I got the point!!

magic:

--- Quote from: Psi on October 31, 2019, 07:45:22 am ---The whole point of this line is to say.
 "This mosfet is ok for 3.3V logic level gate drive"
Whether or not it is a good 3.3V mosfet is another question.

--- End quote ---
No, it doesn't say it's suitable for 3.3V drive. VGS(th) is only relevant to linear applications.

The spec you are looking for is RDS(on), and specifically, the VGS in the "test conditions" column for which RDS(on) is rated. The datasheet clearly says it's a part for 5V drive.

Siwastaja:

--- Quote from: Psi on October 31, 2019, 07:45:22 am ---All logic level mosfets can be driven at higher gate voltages for lower Rds, so it's normal for a DS to say the gate drive range is up to 25V.

--- End quote ---

Indeed, but it's worth noting that this MOSFET starts to get into the "optimize Rds(on)" region after about Vg=4V.  At Vg=3.3V, this MOSFET still cannot be modeled as a simple constant Rds(on) like we normally do for switching applications, not even approximately; the Rds(on) is a curve depending on Id.

We call the MOSFET is "fully on" after we have such high enough Vg, so that the Vds vs. Id is a straight line over the meaningful Id range, so that Rds(on) can be approximated as a "constant" independent of Id. Increasing Vg even further indeed makes sense, to reduce Rds(on) (i.e., change the slope of that straight line). That's why the front page lists Rds(on) at the recommended minimum Vgs=4.5V, and also at Vgs=10V, which gives the optimally low Rds(on).

This means, if you insist on using it as a 3.3V logic level MOSFET, yes, you can definitely do that, but you can't do the calculations based on Rds(on) like we normally do for switching applications; you need to refer to the curves with actual load current, including any peaks there might be. There is less room for error in the design. This is made especially difficult because the curve sets may be "typical", not necessarily worst case. So you need to leave a lot of margin. Ballparking from the curves, this could go up to around maybe Id=5A at Vgs=3.3V, which is only a fraction of the current this MOSFET is designed to be able to switch. Note that Fig.2 starts at Vgs=3.5V. It's very crappy at 3.3V and requires careful understanding.

No, I disagree with you. For a switching MOSFET which clearly isn't designed to be used at Vgs=3.3V, but at Vgs=4.5V and up, with only marginal specifications how it performs at Vgs=3.3V, it's irresponsible to say: "It's OK for 3.3V logic level gate drive", then later go on that it's just not good. Don't do that.

Psi:
That's a fair point.

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