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I have built an electronic load. At the time I did not understand the SOA curves. Now I find the IRFZ44 datasheet does not have the DC curve. Any idea what the max current might be at 30V and 40V so as not to cause damage? Otherwise, I may have to change it all to a 2N3055 (and make a mess).

The electronic load does work @ (badly smoothed) 40V DC 3A. (I have tested it only for few seconds at this power). However, I have tested it a lot at 15-25V and 1A. Seems to hold up well. I have a large heatsink with a fan on it. Case temp does not go over 40C.

(The IRFZ44N and IRF540 seem to be the only power mosfets readily available locally, so any pointers about the DC capability of either will do.)

Added on 2018 APR 13: Well, the mosfet died. So to answer my own question... The IRFZ44 will happily work at slightly more than 20V (maybe 23-24) at close to 2A (I tested numerous times at 1850mA). I tested today at 42.5V, and it died shortly after reaching 400mA (maybe upto 500mA, I'm not sure). It was not even warm, maybe only at 35C or so. So that's that. In hindsight, I should have added a couple of resisters upstream of the drain to lower the voltage across the mosfet. It should probably tolerate a lot if the Vds is limited to only about 10V.

This also means the SOA curves in the datasheet are probably not all correct. Extrapolating the points where the mosfet worked and did not work (the imaginary DC operation line) does not give me a line parallel to the other lines at difference pulse widths. So, they too may in reality be curving downwards much more at higher voltages (i.e. a curve instead of a straight line in the datasheet).

So there... my 30 cents worth of IRFZ44N...

Added on 2018 APR 15: Another mosfet died. This one failed with only 2.5-3.5V at about 2.4-2.5A. So that probably explains the missing dc line in the SOA curve. This mosfet will fail at above 2A at all voltages below 20V. The dc curve is a flat line at about 2A till 20V, and falls sharply after that to 400mA at 40V.

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    \$\begingroup\$ I see a DC characteristic curve for the IRFZ44N so I don't know why the internet should be preventing you from doing that. \$\endgroup\$
    – Andy aka
    Mar 13, 2018 at 17:11
  • \$\begingroup\$ Which figure? I couldn't find anything at 0Hz linear operation. \$\endgroup\$
    – Indraneel
    Mar 13, 2018 at 17:38
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    \$\begingroup\$ 25V*1A = 25W. 40V*3A = 120W. Big difference. And the absolute maximum power rating is based on a die temperature of 175ºC. Expecting reliability at any more than 30W (even with excellent cooling) is unrealistic. \$\endgroup\$
    – Bruce Abbott
    Mar 13, 2018 at 19:52

3 Answers 3

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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: -

enter image description here

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.

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    \$\begingroup\$ This is why Active load designs use current feedback \$\endgroup\$
    – Tony Stewart EE75
    Mar 13, 2018 at 19:01
  • \$\begingroup\$ @TonyStewart.EEsince'75 ampage feedback won’t protect a mosfet if it’s cold and you apply a high gate voltage to get enough current flowing. Within about a milli second, a hot spot can develop in the hexfet and a localized temperature of over 600 centigrade results and blows a hole in it before the feedback loop can respond. \$\endgroup\$
    – Andy aka
    Mar 13, 2018 at 19:37
  • \$\begingroup\$ I was thinking even if shorted and 100ns delays 100kHz BW or even 10kHz loop BW is not too hard with V to I control to prevent that with 0.3J capacity on this device.. \$\endgroup\$
    – Tony Stewart EE75
    Mar 13, 2018 at 19:48
  • \$\begingroup\$ The trouble is that it’s the centre 10 percent of the die that gets warm so the control loop closes down to keep running at the set point but all that current is flowing through the hot bit and that hot bit is about to boil off. \$\endgroup\$
    – Andy aka
    Mar 13, 2018 at 20:16
  • \$\begingroup\$ Thanks @Andyaka Now I know about Paolo Spirito and the recent Spirito Effects known in the last 10 yrs. and that sufficiently aggressive thermal clamping of the case can help prevent it with feedback faster than thermal time constant \$\endgroup\$
    – Tony Stewart EE75
    Mar 13, 2018 at 20:16
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It's on page 1 of spec

Continuous Drain Current (Absolute max)

ID=50A @25'C
. = 36A @125'C, Vgs at 10 V ,

For a linear electronic load, the critical factor is the Rth thermal resistance from case to ambient \$\Delta T = (Vin-Vout)\cdot I\cdot d\cdot R_{th} \$ 'C above ambient, for some pulse wuth duty cycle d ( 0~1)

Use a flat greased heatsink Rjc=0.5 and a CPU cooler < Rca=0.5
Rth=Rjc+Rca ( Rca case to ambient with a fan is as low or lower than chip case to junction)

So for 50W you can have 1A at a 50V drop or 50A with a 1V drop.

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  • \$\begingroup\$ The continuous current is when the mosfet is fully on. In the linear region, like in an electronic load where Vgs around 4V, the SOA curves are the guide. However, IRFZ44 has no curve for DC in the SOA curves. \$\endgroup\$
    – Indraneel
    Mar 13, 2018 at 17:44
  • \$\begingroup\$ DC is the SOA curve > 1 second. VGS must be greater than 4V to lower Ron. Usually 3x Vgs(th) \$\endgroup\$
    – Tony Stewart EE75
    Mar 13, 2018 at 17:53
  • \$\begingroup\$ Can't find SOA curve > 1 second. Can you please link the datasheet? [In an electronic load, Vgs has to be low to control the current, so mosfet is never fully on, so linear operation] \$\endgroup\$
    – Indraneel
    Mar 13, 2018 at 18:27
  • \$\begingroup\$ Did you not understand my answer with Temp rise? that is the steady state DC ( from Ohms' law for P=VI) and spec Vgs = Vds , Id = 250µA ONLY so Vgs >>Vgs(th).. sorry forget about SOA \$\endgroup\$
    – Tony Stewart EE75
    Mar 13, 2018 at 18:41
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Damage could be caused by overheat in your case (DC load).
You have to work with power dissipation figures. It is huge difference between 3A @ 40V and 3A @ 5V.

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