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Author Topic: Basic questions about MOSFETS (what do those terms mean?)  (Read 1161 times)
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I'm looking to use a MOSFET to switch on/off an array of LEDs that can draw up to about 1.6A (16 LEDs at 100mA each). I've used basic transistors before and have had good times with the ole TIP120, but I'd like to find a SMD package for a PCB I'm working on.

I'm struggling with some of the terms that are used in MOSFETs, and how to pick just the right parameters that I need. In particular, if I want to find a MOSFET that is controllable with 5V logic, what field should I be looking at in Digikey? Vgs(th) max? What does that mean?

I *think* I've sourced a suitable MOSFET, but I'm wondering if there is something that I missed: http://www.digikey.com/product-detail/en/DMN3404L-7/DMN3404LDICT-ND/2052805
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The key characteristic you want is the gate voltage required for full rated current flow (lowest Ron value). This a a spec you won't find listed by itself but rather shown in a graph table plotting source/drain current Vs source/gate voltage applied and is usually in the datasheet for the device. The description field that is most useful on that digikey page is:
Quote
Logic Level Gate

That makes is a so called logic level mosfet, which is what you want to interface with an arduino.

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Is a series resistor required (or good practice) on the gate?
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From the datasheet:

Static Drain-Source On-Resistance RDS (ON): 42 mΩ max @ VGS = 4.5V, ID = 4.8A

So that mosfet is suitable for driving from an Arduino (because the Rds(on) is specified at 5V or less), but you shouldn't switch more than 4.8A with it (because the effective Rds(on) may increase dramatically above that current).

The other important parameters are Vds(max) which must be higher than the voltage you want to switch, and Pd(max) which (in conjunction with Rds(on)) also limits the continuous current you can switch.

Is a series resistor required (or good practice) on the gate?

It's good practice, especially if you are using PWM. 100 ohms is a suitable value.
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Is a series resistor required (or good practice) on the gate?

It's commonly used, not because the mosfet requires it but because the current flow when switching from high to low or low to high discharge and charge the rather high gate capacitance typical of power MOSFETs can tax the driving circuit in your case the arduino output pin. Once the mosfet is fully turned on or off no gate current is being consumed.  A 200 ohm series resistor should protect the output pin well enough.

Lefty
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I not sure about all the new IC parts, but logic-level MOSFETs will usually have an "L" in the
p/n, eg DMN3404L, assuming L means logic-level here.

Also, IRF540 vs IRL540, on and on. All those long letters at the end of the p/n refer to
different package types.

http://www.digikey.com/product-detail/en/IRF540STRLPBF/IRF540STRLPBFTR-ND/856559
http://www.digikey.com/product-detail/en/IRL540SPBF/IRL540SPBF-ND/812105
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@dc42: that is a great example of some terms that I don't fully understand. Can you explain a little more about what R(ds) is about? V(ds) seems pretty obvious; the maximum amount of voltage that can be passed between source and drain.

I notice that the schematic symbol for N-channel MOSFETs has an arrow pointing inward, to the gate. But I thought N-channel MOSFETs were beefy NPN transistors, which have the arrow pointing outward, away from the 'gate'?

Would the attached snippet of my schematic be an appopriate use of an N-channel MOSFET?


* mosfet-schematic.png (77.16 KB, 800x579 - viewed 26 times.)
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A MOSFET is not a "beefy" NPN, it's simply a MOSFET.  MOSFETs and NPN are completely different topologies.  You can have not-so-beefy MOSFETs (capable of mere mA of thoughput) and high-power NPNs (capable of hundreds of amps.)

R(ds) is the resistance between drain and source, and is a function of gate voltage and the inherent properties of the substrate.  Lower is better -- it acts more like a switch the closer to 0 ohms you get, while higher resistance burns off excess heat.
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Rds is the turn-on resistance between drain and source. The lower the better, because current*current*resistance (I^2 * R) is the power dissipated in the transistor.
As drawn, you really need a P-channel MOSFET.
Or, put the N-channel below all the LEDs so are controlling the connection to ground.
Parallel LEDs like that are generally frowned up. If you buy them all from 1 batch and each group is the same color you could be okay for a while (I have 3 in parallel like this making up the segments of a 7-segment display,  been 2 years and still working ok, turned on about 8 hours a week).
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One very common confusion that arises is that people use the Vthr (threshold voltage) thinking its the
turn-on voltage at the gate.  It isn't, its the voltage below which the device is fully off.  The fully-on
gate voltage is usually 2 to 3 times the threshold voltage.

When you use a MOSFET as a switch the Vthr contains no useful information, the Rds(on) at particular Vgs
values is what's important.

Second mistake people make is assume a "5A" MOSFET can take 5A happily - it usually can't, it'll need heavy
heatsinking (liquid cooling even) to handle that current as it is usually the thermal limit that determines the
Ids (continuous) - the thermal limit is usually quoted with the device case held at 25C by infinite heatsink!

Again just work out the power dissipation and voltage drop from the Rds(on) value, ignore the current
rating, but ensure the power dissipation is in spec for your setup (with or without heatsinking).
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Is a series resistor required (or good practice) on the gate?

It's commonly used, not because the mosfet requires it but because the current flow when switching from high to low or low to high discharge and charge the rather high gate capacitance typical of power MOSFETs can tax the driving circuit in your case the arduino output pin. Once the mosfet is fully turned on or off no gate current is being consumed.  A 200 ohm series resistor should protect the output pin well enough.

Lefty


You can't stop there, you have to tell him the rest of the ugly truth.  smiley  He doesn't want to arbitrarily pick a very high value resistor to put between the pin and the gate.  The turn on time would be severely impacted making for much more heat dissipation than would have been expected based upon datasheet calculations.  IOW, turning the switch on and off slower generates more heat.
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Is a series resistor required (or good practice) on the gate?

It's commonly used, not because the mosfet requires it but because the current flow when switching from high to low or low to high discharge and charge the rather high gate capacitance typical of power MOSFETs can tax the driving circuit in your case the arduino output pin. Once the mosfet is fully turned on or off no gate current is being consumed.  A 200 ohm series resistor should protect the output pin well enough.

Lefty


You can't stop there, you have to tell him the rest of the ugly truth.  smiley  He doesn't want to arbitrarily pick a very high value resistor to put between the pin and the gate.  The turn on time would be severely impacted making for much more heat dissipation than would have been expected based upon datasheet calculations.  IOW, turning the switch on and off slower generates more heat.

That is the truth. And frankly the few times I've used power mosfet in my arduino projects I've used no series resistor at all with no problems, even though I know there is a risk there.

Lefty
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I usually don't worry much about it either; I usually don't use a resistor when tinkering.  If I'm soldering, then that's different.  I have used a gate driver in exactly one project to "toggle" an IGBT, which I've also only used once.  Poor little PIC couldn't begin to turn it on in a reasonable time.  It'd get hot really easy.  The gate driver sure fixed that.
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