MOSFETs, why the price difference?

Hi DC, Well I guess the point is that was a very confusing definition :roll_eyes:

Ideally we would direct people to the characteristic curves for the device, but for newbies I think we maybe need to keep saying "turn the FET on well" by "making the Gate-Source voltage high enough", and "High Enough" is 5V for "Logic level FETS" and usually 10V for "?Regular Power FETS"..

I'll try to find a good graphic for this...

terryking228: Ideally we would direct people to the characteristic curves for the device, but for newbies I think we maybe need to keep saying "turn the FET on well" by "making the Gate-Source voltage high enough", and "High Enough" is 5V for "Logic level FETS" and usually 10V for "?Regular Power FETS"..

I agree, that should be adequate and safe for most power mosfet switching applications. I've thought about trying to explain the Ids/Vds/Vgs curves before, but it's a little hard to do in a forum post.

The other question that comes up a lot is "Is this particular mosfet logic level?". I normally suggest checking whether the Vgs at which Rds(on) is quoted is 4.5v or 5v (logic level), or 10v (not logic level).

Thanks guys, good discussion. I am a newb, but do feel I get the basics of Vgs(th), and have discovered your trick of looking at the data sheet for the fine print in the Rds(on) section to find what is reasonable to assume is the voltage required to saturate the channel. Still not sure why that's not a spec of its own.

The gate voltage needed to "saturate" the mosfet depends on the drain current - that's probably why it's not a quoted parameter. If you look at the first graph on page 4/8 of http://www.datasheetcatalog.org/datasheet/stmicroelectronics/9387.pdf you can see this very clearly. This mosfet is not logic level, and at high currents it typically needs 7v or more to ensure it is more or less fully turned on at high currents.