DC operation for IRFZ44 mosfet, SOA curve, max current at 40V?
The IRFZ44N is a hexFET designed for switching applications so using it in linear applications runs a risk of destroying it and you being left scratching your head as to why it went pop. It might not even be very warm at all. For linear applications you should consider using MOSFETs designed for avoiding thermal runaway. Yes, a MOSFET will go into thermal runaway if the gate-source voltage is below the zero-temperature-coefficient threshold.

This isn't some theoretical annoyance that doesn't really happen. I can vouch for seeing it on a design I was asked to look at. Take a look at some IXYS MOSFETs that are designed to handle "linear" applications.

On semi document on thermal instability

Infineon document on thermal instability

Ditto from Fairchild

Nasa document explaining it


  Which figure? I couldn't find anything at 0Hz linear operation.


Figure 1 and figure 2 describe the DC operations despite this measurement being made with a 20 us pulse: -



It's made with a pulse to prevent self-heating and the remotest possibility of thermal runaway at lower gate voltages; note how a gate voltage of 4.5 volts (25 degC) produces a current of about 7 amps with 1 volt across drain to source; then note that as the silicon die warms (rapidly of course) the current increases to about 14 amps at 175 degC (figure 2).

This is the thermal runaway I refer to.