Electronic – Selecting a MOSFET for DC use


I have a general question regarding MOSFET Selection. I am trying to select a MOSFET for DC use. I am looking to replace a 5A 24V Relay with a N Type MOSFET.

The MOSFET would be driven from a micro so I would need a logic level gate. The micro is 5v Logic.

I am going to be mass producing these so cost is my main driver.

Most of the MOSFETs that I have come across do not have a DC area called out in the SOA curves. For instance the one I was potentially looking at was the IRLR3105PBF.

Datasheet here

Here are the params I looked at:

VDSS Max = 55V which is >> than my 24Vdc Bus so that is fine.

Power Calc –
5A*5A*0.37mOhm = .925W (High but I think a DPAK can handle that)

enter image description here

FIGURE 1 & 2 –
VGS @ 5V -> VDS = 0.3V @ 25C (but the graph 20uS Pulse I want this to be DC?)
VGS @ 5V -> VDS = 0.5V @ 175C (again I want this to be DC?)

SOA Curve

Looking at VDS – 0.5V (Worst case) it only shows 1V. 1V can go up to 20A way more than I need for a 10mSec pulse. (I am actually confused on this should I just assume that I will have VDS of 1V looking at this?)

But then comes my main question I want DC where do I look for that?

Is this just a bad choice? (I get the feeling it is because no where in the data sheet does it talk about DC) What should I look for when searching Digikey?

TLDR How should I select FETs for DC use?

Best Answer

If you need DC operation, you should really use a MOSFET that has a DC rating in its Safe Operating Area.

MOSFETs that don't have the DC curve may suffer from thermal runaway when used in DC applications and are intended or specified for switching applications only. Internal, local hotspots may occur and the MOSFETs may fail ("Spirito Effect").

The reason is a falling gate-to-source threshold voltage for a rising temperature, usually at low gate-to-source voltages. The details of this issue are usually not specified in the data sheets, so the only indicator is often the SOA diagram that has or doesn't have a DC curve. Fig. 3 in your MOSFET's data sheet looks like the point of thermal VGS crossover is a bit below 4 V. In my opinion, you are on the risky side when you use this particular MOSFET with a driver that can supply 5 V only. For a worst-case scenario, consider your supply to be on the low end (4.5 V), and allow some voltage drop for the driving stage. Sooner than you might like, you end up somewhere around 3.5 V.

Note that the absolute maximum ratings (25 or 18 A at 25 or 100 °C, respectively) are specified at a gate-to-source voltage of 10 V, when your MOSFET is fully on . They do not apply at lower gate-to-source voltages.

More about the background here: https://electronics.stackexchange.com/a/36625/930