let's say I have this nmos
it says the Vgs,th is between 2V-4V
I believe the higher drive voltage will reduce my Rds,on.... so.... do I just want to drive this as high as makes sense for my circuit? this fet has a Vgs max of 30V.... and a Vgs,th of 2V-4V... how do you figure what to drive it at? I would just do 10V and move on but I'm wondering what a more thoughtful approach might be
thanks
From Figure 1, VGS > ~5.5V has very little improvement in Rds_on
How serious are your needs for low Rds_on? ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
=A0 =A0 ...Jim Thompson
=A0 =A0| =A0 =A0mens =A0 =A0 |
=A0 | =A0 =A0 et =A0 =A0 =A0|
=A0|
=A0 =A0 =A0 |
not very serious more just trying to understand does it matter how high I go with my Vgs? 12V better than 9V ? that kinda thing
[snip]
Depends on your definition of "better" ;-)
Minimal improvement in Rds_on. More gate swing, if done at a high rate equals more drive power consumed. ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Those are the range of what to expect from lot to lot and condition of operations. If you're using this as a switch, I'd be well above the MAX Vgs(Thr) voltage to turn it on. Also, gate charge and your driving device has a lot to do with it so to not hover to long over the threshold point, otherwise, this can cause heating of the FET due to it being in the linear region for too long.
That part there is best used in a 5v or more logic switch signal, I don't think it would be a good idea to use that in a 3.3V logic level system, unless of course, you plan on using a gate driver with a charge pump in it.
Jamie
--- From the data sheet's "On Characteristics", Vgs(th) - the gate threshold voltage - is defined as that voltage which, when applied across the drain and source, with the gate and drain tied together, will cause 250 microamperes of current to flow through the MOSFET.
That voltage is specified as being anywhere between 2 and 4 volts, and is the voltage required to get the MOSFET to just start to turn on with 2-4 volts across it and the gate and drain shorted.
More meaningful with respect to your drive voltage question is the Rds(on) spec, which states that with 10 volts between the gate and the source and 3.9 amperes of drain current, the drain-to-source resistance will be a maximum 0f 360 milliohms ohms.
That's just a single point, of course, but it serves to state that with 10 volts between the gate and the source, the resistance between the drain and the source will be at its minimum.
Study figures 1,2, and 3 on the data sheet in order to get a better feel for what happens with other gate voltages and drain currents.
Just as an aside, the active silicon area between the drain and the source is called the "channel", and when a small increase in gate voltage no longer elicits a correspondingly small decrease in the channel resistance, it's said to be "fully enhanced".
For most MOSFETS, the channel will be fully enhanced at somewhat less than 10V, but for "logic-level" devices that voltage will be somewhat less than 5V.
-- JF
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.