[sorted] Tesla member but never used a MOSFET: clarity please?

Last visit to my preferred supplier I picked up a couple of IRL520N logic level mosfets. I've never used a mosfet before.

If I want to use one connected to an Arduino digital output does it need a current limiting resistor on the base oops I mean gate?

Hi,
JimboZA, I usually use a 470R, no matter what the design along with 10K gate to source.

http://irf.custhelp.com/app/answers/detail/a_id/215

Tom...... :slight_smile:

I have used them without. But it's a not a good idea. There is a brief surge of current when the MOSFET is switched which may damage the pin. Anything to keep the current below 20mA will work.

Grumpy_Mike occasionally publishes a simple current-limiting circuit for LEDs (two transistors + resistor). That circuit works great with a MOSFET (plus transistor and resistor). I highly recommend it as a first circuit.

TomGeorge:
JimboZA, I usually use a 470R, no matter what the design along with 10K gate to source.

Actually, it should not be a 10k gate to source, but Arduino output to ground. You do not want a potential divider - however minor - between the Arduino and the FET.

The difference between reply 1 and reply 3 being on which side of the gate resistor the resistor to source / ground goes?

Would one of you guys mind doing a quick sketch when you have a moment?

Hi,
At your service.

Tom..... :slight_smile:

Thanks Tom... so @Paul, you would prefer the vertical resistor (Tom's G-S one) to be on the left of the current limiter?

Tom's pic:

JimboZA:
Last visit to my preferred supplier I picked up a couple of IRL520N logic level mosfets. I've never used a mosfet before.

If I want to use one connected to an Arduino digital output does it need a current limiting resistor on the base oops I mean gate?

Its not an easy question to answer definitively. The Arduino outputs are rated at an
absoute max of 40mA, yet will be designed to handle a certain amount of capacitive load
(although how much isn't given in the datasheet). Capacitive loads will mean brief
spikes of current as the outputs switch which could exceed 40mA, but last a few
nanoseconds.

So a small MOSFET with a few hundred pF of gate capacitance might be quite OK
to drive directly. But what the maximum capacitive load is before a resistor should be
used is unclear. Most modern MOSFETs have more gate capacitance that you would
expect to see on a logic signal (measured in nanofarads) so a resistor is generally
advised. 150 ohms is a good value, since the lower the better for switching speed.

If you have a beefy MOSFET and want to switch it fast, you should always consider
a MOSFET driver chip which boosts the current output to much higher levels safely,
reducing switching losses and appearing as a standard logic load to the microcontroller.

I'm finding that the 10k pull down is mandatory: if I tickle the "G" the led (above the mosfet, between +ve and "D", "S" to ground) comes on but stays on when the 5V is removed, unless I have that pull down.

Is that normal for mosfets?

Absolutely - the gate is one plate of a capacitor, that's why they are called "field effect
transistors" as the electric field between gate and channel is responsible for the thickness
of the induced channel which allows current to flow between source and drain.

The charge on the gate mirrors the charge forming the conducting channel (charge carriers
overwhelm the natural p or n-type of the channel to the opposite sign).

Since the gate is isolated on a thin layer of quartz (silicon dioxide) it will hold its charge for
quite a while.

Ah ok thanks Mark... I was wondering about capacitance. I actually turned all the power off, and when I reapplied it (but with nothing tickling the gate) the LED was on again. I took a while for it to go off and I said to myself, self, this is a capacitor.

Right then, the mystery of MOSFETs is sorted, at least as much as I need to know.

Another thing worth mentioning is that power MOSFETs are very unlike logic circuit
MOSFETs. They use vertical current flow (the substrate is the drain), there is an
integral diode between source and drain that is part of the 3D structure of the device,
the gate voltage relative to the source is the important thing (gate to drain voltage
has little effect on behaviour). On resistances of 0.001 ohm or lower are possible!

Logic MOSFETs are completely symmetrical, source and drain are interchangable,
and the substrate has to be kept at one of the supply rails to remain isolated. On
resistances in MOSFETs in a typical processor are measured in k-ohms (but
gate capacitances in fF (femto farads)...)

Chips with integrated DMOS MOSFETs are a hybrid between the two - current flow
is horizontal since the drains have to be isolated from the substrate, and thus
the on-resistance is significantly higher than with discrete power MOSFETs (down to
about 0.2 ohms)

One other thing just struck me, and that's due to the word "gate" as distinct from "base" in bjt-speak.....

Are mosfets thus switches always?- whereas a bjt is only a switch when it's saturated and otherwise is an amplifier?

power MOSFETs originally were analog amplifiers, but the switching use
has come to dominate as all power electronics today is switchmode, more
or less. Modern power MOSFETs are optimised for switching, not for low
distortion.

I think the gate/base difference was purely to have a different name and prevent
confusing circuits designed for FETs being used for BJTs and vice versa. Although
SCRs also have gates, so its not particularly consistent.

The base was named after the early transistor design which had a sliver of silicon with
two dopant areas diffused in from opposites sides, nearly meeting in the middle, the
sliver was the base in a mechanical sense...

Incidentally IGBT's have gate/emitter/collector....

JimboZA:
Thanks Tom... so @Paul, you would prefer the vertical resistor (Tom's G-S one) to be on the left of the current limiter?

Yes, because as first stated, you do not want a potential divider reducing in any way the voltage applied to the gate.

It may be a small point, but - why not do it properly?


The term "gate" is more appropriate for FETs as that is what it is - it controls; opens and closes the path from source to drain, like a tap or valve.

The action of a transistor is quite different - it amplifies the current you feed into the base; this is the beta (ß) parameter. The ß of a FET is almost infinite as virtually no current is drawn by the gate (except of course, to charge it up); it has instead a voltage/current transfer factor or "transconductance" like a (thermionic) valve (translation for Yanks: vacuum tube).

I did a page about driving stuff with MOSFETs: Gammon Forum : Electronics : Microprocessors : Driving motors, lights, etc. from an Arduino output pin.

It shows current sources and sinks, plus calculations for heat dissipation etc.

In other words the transfer parameter for bipolars is the unitless Ic/Ib, while the Fet's is Id/Vg, which has the unit of mho or sieman - the reciprocal of ohms.

JimboZA:
Last visit to my preferred supplier I picked up a couple of IRL520N logic level mosfets. I've never used a mosfet before.

If I want to use one connected to an Arduino digital output does it need a current limiting resistor on the base oops I mean gate?

Most everyone will tell you you need a series resistor to absorb the "current spike caused by charging and discharging the gate capacitance (and fighting the Miller effect)".

However, that is completely wrong. The mosfet drivers in the AVR chip are basically voltage controlled resistors (i.e. mosfets!). The way to damage them is to overheat them. But the current spike caused by driving a mosfet gate is SO short in duration that you will never even begin to warm the output drivers, let alone damage them.

Also, a series resistor will slow down the switching time of the mosfet you are controlling your load with and keep IT in the linear region longer, resulting in a higher power dissipation in the mosfet.

What resistor to use? 0 ohms.

However, that is completely wrong.

If you work in electronics for some time you are maybe aware of something called "good engineering practice".

If Atmel claims the max i/o current is 20mA, then the good practice is to limit the current to be less than 20mA under any conditions.

The input capacitance of the fet is 450pF, so you may (can?, able to?) calculate easily what is the max charge/discharge current for a certain speed of a falling/rising signal edge :wink:

pito:
The input capacitance of the fet is 450pF, so you may (can?, able to?) calculate easily what is the max charge/discharge current for a certain speed of a falling/rising signal edge :wink:

And further on that, it entirely depends on what you are doing with the FET.

If you are switching it on or off every ten seconds or so, it may well not be critical. If on the other hand you are performing PWM with it, than it absolutely may be drawing these charge/ discharge currents for a significant proportion of the time.