[HH] n channel mosfet questions

[Greg London]

> Do you specifically need an N-channel MOSFET?  I am fond of these:
> http://www.findchips.com/avail?part=FAN3217TMX

according to the spec: Supply Current:  0.75 mA

The reason I'm using a transistor was to turn off a 100kohm voltage
divider circuit to try and save 50 microamps of lost current when
I don't need the divider.

A gate driver that uses 750 micro amps won't put me in the right direction.

I could use an NPN BJT, but I'd need some current into the base
to turn the transistor on, and I was thinking, hey, mosfets basically
use zero current into the gate, they're voltage controlled current
sources. If I turn the voltage on, there's no current other than the
AC switching stuff, and whatever the load itself uses. Hey, great
opportunity to learn something.

But the digikey catalog seems to be thwarting me.
And googling around for
"n channel mosfet switches examples tutorials"
keeps failing to come up with the 2n2222 equivalent for the
nchannel mosfet that will turn on at 5v.

You want to switch something with an NPN BJT?
use the old workhorse 2n2222.

What is the n channel mosfet switching workhorse that turns on at 5v?

I can't find one. They all seem to mostly turn on at 10v.
Some very judicious filtering in digikey found me the mosfet
I linked to below. It's all the way on at 4.5 volts or so.
It just seemed like a lot of work to find
it and I thought maybe I'm just doing it wrong.

But maybe its just the physics of how a mosfet works that
10v or 20v is the more common "on" voltage.

I figured it would be worth asking.


> A _lot_ of people use high-current buffers rather than N-channel MOSFETs
> for low loads.  Or half-H drivers, as they're often easy to search for.[1]
>
> There are also MOSFETs designed for logic-level drive, and driver ICs that
> boost the drive voltage up nice and high with a charge pump so you can use
> cheap N-channel MOSFETs for the high side as well as the low side, and
> drive your volume up on one part rather than having e.g. 2 N's and 2 P's
> for an H-bridge.

Right. I've been looking at low/high side drivers for motor controllers.
You want to use N channel mosfets for the motor because P channel mosfets
have higher rds and so burn more power. In a motor drawing a lot of
current, a high Rds can translate into a lot of power. So, N channel,
and then some kind of driver chip on the front end.

But this was not for a motor controller.

> The resistor to the gate is most likely a throwback to BJTs, _but_ it also
> does drop the edge rate a good bit and thus is good for EMI reduction at


Ah. I hadn't thought of that. This is a fairly static signal,
so  I probably don't need a resistor cause it'll change once
and stay that way for at least a few minutes.

> the expense of slightly more power burned by the MOSFET during switching.
>  It can let you reduce the switching speed of the FET as well, via RC time
> constant.
> GS resistor is not bad to have around while micro is booting.
>
> [1] No, these things are not all the same animal, but they're useful for
> very similar things.
>
> *
> Drew Van Zandt
> Artisan's Asylum Craft Lead, Electronics & Robotics
> Cam # US2010035593 (M:Liam Hopkins R: Bastian Rotgeld)
> Domain Coordinator, MA-003-D.  Masquerade aVST
> *
>
>
>
> On Thu, May 17, 2012 at 2:41 PM, Greg London <email at greglondon.com> wrote:
>
>> I'm looking for an N channel mosfet that can be turned on/off
>> with 5v/0v on the gate and can sink a hundred milliamps or two
>> of load.
>>
>> I found one example here:
>>
>>
>> http://search.digikey.com/us/en/products/SI2312BDS-T1-E3/SI2312BDS-T1-E3CT-ND/1656844
>>
>> Rds On (Max) @ Id, Vgs   is listed as
>> 31 mOhm @ 5A, 4.5V
>>
>> [question 1]
>> I'm I making this too hard?
>> I can't seem to find very many of these at all.
>> Most datasheets list the "on" gate voltage at around 10v.
>> Are there a bunch of mosfets I can use to switch 5v that
>> I'm just not seeing?
>>
>>
>> On a related n channel mosfet topic,
>>
>> [2] Can I simply hook a 5v TTL logic output to the gate of a mosfet?
>> A lot of schematics show a resistor from gate to source,
>> but usually say this is to drain the capacitance on the gate pin.
>> If I use the output pin of a 7408 AND gate and tie it directly
>> to the gate of the mosfet, won't the 7408 drain the capacitance
>> when it outputs zero volts? It seems like the resistor is for
>> when the thing driving the mosfet gate is either 5v or tristated, and
>> doesn't actually drive to 0 volts.
>>
>> [3] Some schematics also show a resistor between whatever is driving
>> the mosfet and the gate pin of the mosfet. If the gate to source
>> impedance on most mosfets is 6.28 giga-watts, I mean giga-ohms,
>> then I'm not sure why bother adding another resistor in there.
>> A resistor into the base of a NPN BJT transistor will limit the
>> current into the base, but that's because the base is just a diode
>> drop away from the emitter.
>>
>> Thanks for any help,
>> Greg
>>
>>
>>
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>


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