Electronic – Understanding MOSFET characteristics

component-selectionmosfet

I am trying to select an n-channel MOSFET to switch a 12 V power rail. The use case here is to switch on/off a couple of LEDs which are wired in series. The LEDs (datasheet) can draw a maximum of 700 mA. I want to control the MOSFET via a microcontroller (3.3 V logic level).
I am now struggling to choose a suitable MOSFET, mostly because I do not understand some of their characteristics.

What I know so far is:

  • MOSFET needs an SMD package for my application
  • Vcgs(th) should be smaller than my 3.3 V logic level
  • FET type should be n-channel

What I don’t know:

  • What does Continuous Drain Current (Id) mean? What is important here?

  • What does Drain to Source Resistance mean? What is important here?

  • How important is the Maximum Power Dissipation?

For more information on the circuit, you can look at my original question here.


An example circuit.

Enter image description here

This MOSFET may be suitable for the use case.

Best Answer

What does Continuous Drain Current (Id) mean? What is important here?

It means continuous current (as apposed to pulses of current) between the Drain and Source. Generally it is specified in the 'maximum' or 'absolute maximum' ratings section, which means it's the absolute maximum current the FET can pass continuously (at the specified case temperature) without getting so hot that it burns out. You need to keep the actual Drain current well below this for reliability and efficiency.

What does Drain to Source Resistance mean? What is important here?

It's the ratio of voltage drop across the Drain-Source junction to current passing through it, when the FET is fully turned on. A Drain current should be specified for this because at some higher current the resistance will increase dramatically as the FET 'saturates'. Temperature should also be specified because the 'ON' resistance also increases as the FET gets hotter. Several Gate voltages may also be specified because at lower Gate voltage the FET will not turn on as hard (ie. it will have higher 'ON' resistance) and it will reach saturation at lower current.

RDSon is the most important characteristic you need to calculate voltage drop, power loss, and temperature rise.

How important is the Maximum Power Dissipation?

Not very important, because it is not a realistic number. What is actually important is how hot the FET gets (ie. its junction temperature), which is dependent on heat sink performance and ambient temperature as well as power dissipation. In practice these numbers are poorly defined, so temperature cannot be accurately predicted and you cannot run the FET at its 'maximum' power dissipation rating without severe risk of overheating it - which makes the figure almost useless.

For reliability and safety the FET should be operated well below its maximum junction temperature, and for efficiency you want it to dissipate negligible power compared to the load. Therefore in practice the actual power dissipation generally needs to be much lower than the 'optimistic' datasheet spec.

What I know so far is:... Vgs(th) should be smaller than my 3,3V logic level

Not just smaller, much smaller. 'Threshold' voltage is the Gate voltage at which the FET just begins to turn on. It is typically specified at a Drain current of only 0.25 mA, and varies over a wide range between individual units. If the threshold voltage is 3.0 V the FET will not turn on fully with 3.3 V. The minimum voltage you should use is the one specified in the RDSon spec.