# Sizing a mosfet

#### jgauthier

Mar 22, 2013
53
Greetings,

A project I have embarked on consists of a micro controller that is "always on". Based on a change, a pin will go low, and that will tell it to turn another pin high. That high pin will then turn on the rest of the circuit. The circuit load could be as high as 2A. I am trying to determine the best device to power this circuit with. Currently, I have an IRF610 mosfet. I know I could also probably accomplish this with a relay. I would like to keep the circuit small, and I believe the TO220 package is about as big as I'd like to get. Am I approaching this the right way, and have I selected an appropriately powered mosfet?

I have not yet wired up the circuit, I just did research and bought the part.

I really appreciate any input!

#### duke37

Jan 9, 2011
5,364
The IRF610 is a high voltage low current power mosfet. I have not looked at the on resistance which needs to be low if it is to be used without a heat sink.

What voltage will the fet be switching and what will be the driving voltage?

#### jgauthier

Mar 22, 2013
53
The switching voltage will be 5 volts, and additionally the driving voltage will also be no more than 5 volts.

#### john monks

Mar 9, 2012
693
The IRFZ40 is rated at 50 volts, 51 amps, 125 watts, 0.028 ohms, and is an N-channel TO-220.

#### duke37

Jan 9, 2011
5,364
The IRFZ40 threshold voltage could be up to 4V. This means that 5V drive would not be adequate.
It may be better to use an npn transistor.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Or find a suitable logic level mosfet.

For 2A with a max voltage of 5V, there are a plethrora of tiny devices.

One example is an IRF7220. 14V at up to 8.8A continuous, 88A pulsed! And it has a Vgs(th) of 0.6V.

The best thing is to learn to use Digikey's parametric search. Select a range if useful parameters and then sort by price. Shop around if needs be to see if they're available at your normal distributor.

Let's do this with Digikey...

1) Go here
2) Type "Mosfet" in the search box and click GO
3) Select the appropriate component type. Typically this is the one with the most hits In this case, under "discrete semiconductor products", I would click on "FETs - single"
4) Now for the parameters, select ranges of useful things, one at a time. The mosfet you mentioned is an N channel device, so select all the N channel types and click "apply filters"
5) another thing is the current -- 2A, so perhaps select Id ranging from 4A to 20A and click "Apply filters" again
6) V low Vgs(th) is important, so select all those between (say) a couple of hundred millivolts and 2.5V (we want to be able to turn the device hard on, so we need to exceed the threshold by a significant amount)
7) Now we're down to a little over 3000 results, so do an advanced sort on price, ascending for a quantity of 1 (this is the blue triangle on the left, under the heading for the price column)

The first thing I notice is a lot of SOT023 (small surface mount) mosfets capable of switching over 5A! (and remember they don't make heatsinks for SOT-23!)

Maybe you want through-hole? If so, go back to the parameters and select a mounting type of "through hole" and again click on "apply filters" (remember to repeat the sort on price)

If you want a particular package (say TO-220) you can scan through the list, or you can also subset it by package.

There's lots of options there for under \$1

I would recommend selecting the ones which have their headline specification Rds at a Vgs of 5V since that avoids the additional step of looking up the datasheet to double-check Id and Rds at Vgs of 5V.

Other suppliers have parametric searches as well, but I find that digikey's works better than others I've found.

#### jgauthier

Mar 22, 2013
53
I started with an npn transistor, (2n7000) but it could not drive the load.

Through some other research, I also found this one which I think will work.

http://www.st.com/web/catalog/sense_power/FM100/CL824/SC1164/PF188057

At the moment, as a work around, I am using a 7n2000 triggered from a 5v source, to trigger the IR610 on a 12v source, and this seems to work. But I want to switch to a single device in the future, and will look at the STU95N2LH5 to see if it will suffice.

Thanks for the pointers!

E
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