# What parameters are to be considered when choosing a MOSFET?

I have a circuit that uses a MOSFET to heat a nickel titanium wire (by switching of course). But I don't understand why does it use IRF740 specifically.

What parameters do we look into when choosing a MOSFET? Appreciate the help!

For reference: the image of the circuit

• Edi, just right click on the image in the linked article and paste the URL into this question using the image button on the editor toolbar. That way all the information required is in one place and we don't all have to follow links to understand your question. – Transistor Aug 1 at 13:05
• @Transistor thanks! Fixed it now. – Edi Aug 1 at 13:10
• Short answer: Rds_on*I^2 should be low and gate voltage should be below your MCU's output. – Navin Aug 2 at 6:45

## 3 Answers

All the parameters are important in one situation or another (that's why they're on the datasheet!). In this case you have a (presumably) slow-switching circuit that is either on or off.

The first ones you should look at are the drive voltage to switch it on (in the case of the IRF740, 10V is required, so it's unsuitable for direct logic drive). Look at voltage rating. Higher voltage rated MOSFETs will tend to be more expensive and/or have higher on resistance for the current. The IRF740 has a 400V rating. If you only have a 12V supply so that 25V or 30V rating is plenty, then you're leaving money (or silicon) on the table by using an unnecessarily high voltage rated part that probably runs hotter than some alternative.

Then look at the Rds(on) and thermal properties to estimate how much power it will dissipate when turned on (and therefore if a heatsink is required, and if it is required, what the thermal resistance to ambient needs to be).

In the case of the IRF740 it has a Rds(on) with 10V drive of 0.55 ohm, which increases to maybe 50% more at high junction temperature. So call it 0.83 ohm. At 5A it will dissipate 20.6W, requiring a large heat sink or heat sink and fan.

If you use a 25V MOSFET such as a PSMN0R9 you can drive it with 4.5V and it will have 1m$$\\Omega\$$ Rds(on) with 10V drive or 1.25$$\\Omega\$$ with 4.5V drive (with a similar temperature effect). Assuming 1.9m$$\\Omega\$$ that's power dissipation at 5A of less than 50mW and it won't get warm even without a heat sink (so the Rds(on) will actually be quite a bit lower again).

There are other effects when you try to switch the MOSFET quickly (gate charge) and so on, but the above-mentioned are probably the parameters that you should look at initially in your particular application.

And, of course there are practical considerations such as package, cost and availability and part status (active vs. obsolescent) if you're going into production.

As always, it depends. You always have to make compromises as an electrical engineer, there's no ideal components in the real world. You should prioritize things you care about.

I'll give you three examples.

If you are dealing with high power & a lot of switching such as the mosfets in a buck/boost converter
You care about:

• Fast switching so you don't waste energy as the transistor turns on or off
• Low Rds(on) so you don't waste energy when the transistor is on
• Low input, output and input to output capacitances, since these cause ringing (decaying high frequency oscillation in this case) and miller plateau

You might care about:

• The size/geometry of the transistor, maybe you want to attach a heatsink
• The maximum drain to source voltage, it's not clever to give an input that immediately breaks your transistors
• The threshold voltage, maybe you want to drive the gates directly from a 5 V microcontroller, though you should have a gate driver if you are making a professional buck converter

You don't care about:

• Delays, if the transistor turns on now or 1 microsecond later doesn't really matter, just be aware of it and you can make everything turn on and turn off when it should.

If you are dealing with high power & barely any switching such as a mosfet in a polarity protection for some circuit
You care about:

• Low Rds(on) so you don't waste energy when the transistor is on
• The threshold voltage so it turns on and works given your input voltage
• The maximum gate to source voltage allowed so your transistor don't break
• The maximum drain to source voltage allowed so your transistor don't break

You might care about:

• The size/geometry of the transistor, maybe you want to attach a heatsink
• Input, output and input to output capacitances, you should make sure that the gate doesn't ring

You don't care about:

• Switching speed of the transistor, if you want your protected circuit to ramp up slowly then you should do that with a RC circuit
• Delays, if the transistor turns on now or 1 second later doesn't matter

If you are dealing with low power & a lot of switching such as the mosfets in a 0-3.3 V to 0-5 V logic level converter
You care about:

• Delays
• Switching speed
• Input, output and input to output capacitances, you want them to be very low and not ring
• threshold voltage, so a 3.3 V signal can interact with the mosfet

You might care about:

• Delay variations, if you have several pins being level converted then maybe it is important that they all come around the same time
• Maximum drain to source voltage and gate to source voltage, most, if not all mosfets can work with 5 V, but at least look at these values

You don't care about:

• Rds(on), it could be 50 ohms, it doesn't need to be super low like 0.001 ohm, it will still do its job perfectly fine
• Heat dissipation, your transistors shouldn't heat up at all, so if they are very small then that's perfectly fine, no need for a heat sink.

There are more parameters, I know, but I mentioned three examples which might help you understand that it depends and it's up to you to find out which parameters you should care about.

Remember that you also have BJT's and IGBT's and many other types of active components. In my third example about the logic level converter it would make more sense to use a BJT than a mosfet because a BJT is usually faster than a mosfet and has a knee voltage of 0.7 V.

Keep in mind that what I just mentioned is when you use the mosfet in a binary way like a switch, either ON or OFF. You can also use a mosfet as an amplifier instead of an op-amp and then you should look for other parameters. Or you could use a BJT.

While not relevant to the example given in the text, very relevant to the question (by title as is):

SOA.

There are various physical layouts of MOSFET chips, of which some can, some cannot deal well with being operated continously in the linear range of their characteristic at significant power. This is important when linear amplifiers or linear power supplies are to employ the MOSFET in question.