I see a bunch of MOSFETs listed like this:
Logic Level Gate, 4V Drive.
The "logic" part seems to tell me they are designed to switch on at 5v; does the "drive" mean the actual voltage they will switch at?
Any suggestions for a small MOSFET to drive no more than about 200mA?
Logic means that they are fully "on" with 5volt between source and gate.
Their Vgs(th) is usually <= 2.5volt.
2N7000.
Leo..
I see a lot of logic FETs with rds of 10v. Doesn't that imply they aren't fully on at 5v?
Like this one, that maybe is SMT version of the 2n7000.
RDS is 5 Ohm @ 500mA, 10V
So what do we know about RDS at 5v?
This one seems better to me for 20v, 200ma, logic gate.
Rds On (Max) @ Id, Vgs 1.6 Ohm @ 50mA, 5V
Comments?
Vgs(th) (Max) @ 2.4V @ 250µA // the voltage it just starts conducting.
Rds On (Max) @ Vgs 1.6 Ohm @ 500mA, 10V // minimum "on" resistance when driven hard.
I think practically there isn't much difference between the two.
On a 3.3volt supply, I would probably choose the BSS138, and on a 5volt supply the 2N7002.
If I had to use hundreds of them, I would study all the graphs in the datasheet.
Leo..
So as I see it, the downside here is that it takes 10v to saturate, and get low resistance.
At at 5v logic level driving the gate, how much resistance is there? We don't know?
If I am driving LEDs, I'm wasting a lot of power in a FET that's only partially turned on.
At 5v the BSS138 is fully on, and RDS is 1.6 ohms. That seems a lot more efficient, but I'm definitely open to comments.
db2db:
So as I see it, the downside here is that it takes 10v to saturate, and get low resistance.
At at 5v logic level driving the gate, how much resistance is there? We don't know?If I am driving LEDs, I'm wasting a lot of power in a FET that's only partially turned on.
At 5v the BSS138 is fully on, and RDS is 1.6 ohms. That seems a lot more efficient, but I'm definitely open to comments.
Yeah, there's a graph in the datasheets for most mosfets.
There are a lot of unscrupulous sellers who promote MOSFETS that don't turn on fully with logic level drive (defined as 4.5v, unless otherwise specified) for use with 5v drive (Like Arduino outputs); they barely work at low current, and aren't usable for higher current (despite said sellers claims otherwise). Do not be fooled, read the datasheet.
I default irf3708pbfs, since they'll work even with 3.3v logic level, and come in a through hole package
http://www.nxp.com/documents/data_sheet/2N7002P.pdf
https://www.fairchildsemi.com/datasheets/BS/BSS138K.pdf
I don't know what you want to switch.
Do your homework.
All the information is in the graphs.
Leo..
Both the BSS138K and 2N7002P look about the same to me.
So the question is if 1.6 Ohm does the job, it would cause a voltage drop of 320mV at 200mA which will dissipate 64mW of heat, which is safe since they can dissipate 350mW.
The Diodes DMN3404L is another option if you want low RDSon and a low gate threshold. At 200mA, it will only drop (.2A*0.05Ohm) 10mV.
DrAzzy:
Yeah, there's a graph in the datasheets for most mosfets.
Graphs for "typical values" aren't a promise of anything, whereas the Rds(on) max spec is
a more legally binding part of the datasheet.
Gate threshold values vary a lot between devices, and vary as a device ages(*), so the "typical"
graphs have to be taken with a liberal dose of salt - if the typical graph shows a plateau at 4V,
you basically know it cannot be logic level since actual plateaus will be in the 3 to 5V range.
(*) ion migration in the gate oxide occurs over time due to the extreme electric fields across
it, and this changes the arrangement of charge that biases the threshold voltage. The fields are
extreme because the gate oxide thickness is measured in nanometres. This is also why MOSFETs
are very static sensitive.
Nomenclature:
"theshold voltage" - the point the device switches (nearly) fully off, you must
drive the gate to well below this (normally 0V) to switch off quickly.
"plateau voltage" - the point at which the channel is opening up as the device switches on
and the gate charges up (gate charge mirrors channel space-charge). The plateau shape
depends on the drain-source current level.
"on voltage" - as specified in the Rds(on) specification, a voltage for which there is a guaranteed
on-resistance, typically quoted for 1/2 the maximum current (more than you normally ever run a MOSFET
at BTW)
Typically you'd see the plateau voltage is about 50 to 60% of the on voltage, the threshold voltage is
20 to 30% of the on-voltage.
As I mentioned you don't run a MOSFET near its maximum current rating, because that is the
thermal limit with unlimited heatsinking (usually), so basically you'd treat it as a peak pulse rating,
never a continuous one.
Watch out for certain manufacturers 'lying' about their current spec - they quote something like
270A, and then in a footnote say the package limit is 120A (which is also a thermal limit you'd
never go near in practice, A TO220 package is best kept to 20A or below).
You choose MOSFETs by their on-resistance, and the power dissipation you can tolerate. You also
have to make sure (particularly for logic level devices) that the drain-source voltage is small when on,
less than 0.5V is appropriate, because if parts of the channel are at a higher voltage the device isn't
guaranteed to turn on fully - the voltage difference that matter is that between the gate and the
channel