I recollect from reading previous posts, warnings of confusing the Vgs value when the mofet starts to conduct value as the Vgs value to drive the mosfet to saturation. In this data sheet http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00003023.pdf, specifically what tables or charts would one find those two values? Thanks - Scotty
Vgs(th) - 1.7 to 2.5V is the voltage where it starts to conduct... see "Figure 4. Transfer characteristics" for more details. It depends on Id. 5volts will be enough for 60 Amp.
Figure 3 also applies to the analysis as it takes into consideration the drain voltage being used.
Lefty
Figure 3 is what you need.
Figure 4 has no meaning for a switching application.
dhenry:
Figure 3 is what you need.Figure 4 has no meaning for a switching application.
Yes, you are correct, sorry for my mistake.
Figure 7, "Gate charge vs. gate-source voltage" suggests the plateau voltage is pretty much on
5V, making this device a very poor choice for 5V drive - however they do give a decent value for
on-resistance at Vgs=5V.
I suspect figure 7 is wrong and the plateau is a bit lower, but the asymmetry in switching times
suggests the plateau is more like 4V than 2.5V (which would give more symmetrical rise and fall
times).
Ideally you'd want the plateau at about 1/2 the gate drive voltage, meaning that equal charge and
discharge currents flow through the gate resistor, meaning similar switching times for on and off
(and thus minimum total switching time all other things being equal). This particular device seems
to be rather a compromise (it will work at logic levels, but it works best at 10V drive). Many logic
level devices have a Vthr around 1V, and plateaus around 2 to 3V (IRF3706, IRL5602S, STB80NF55L-06
for instance).
Of course this all rather academic if not switching at high frequency
sorry for my mistake.
Nothing to be sorry about. You are just trying to help.
Also, that's not one of the best MOSFETs to use. With Rds(on) = 0.028 ohm, at 40A
[per the title], it would be dissipating 44.8 watts, and that's a lot. Fry an egg. You
have to take the marketing info with a leery gaze. You can find better devices with
10X lower Rds.
Thanks for all of your responses. Do I read it correctly from figure 3 that the mosfet doesn't reach full saturation or that 'saturation' is a ambiguous. Unlike the mosfet here http://www.sparkfun.com/datasheets/Components/General/RFP30N06LE.pdf. I can see saturation clearly in figure 7. Also, do I read it correctly (in figure 7) that saturation can be attained with a gate voltage of 3v for a drain current of 20 amps but a higher gate charge is needed if a higher current is to be passed? - Scotty
If you look closely, you'll see they both have the same characteristic, except shifted
somewhat. The 1st device only turns on fully at a higher Vgs. Your 2nd device is "logic-level",
meaning turns on fully at lower Vgs.
Your observation about > 3V is correct.
or that 'saturation' is a ambiguous.
That's correct. There is no clearly defined "saturation", or a mosfet doesn't go from "non-saturation" to "saturation" in a black-and-white fashion. Saturation is generally defined as an area where the device's d-s channel becomes ohmic - Vds is linear to Id.
Unfortunately, a real mosfet will never get to that stage: with sufficiently high Vgs, it approaches that stage. So the only way to ensure saturation is to get Vgs sufficiently high (typically > 5v for logic mosfet and >10v for others).