# How to use MOSFET thermal data for simulating in software

I have a MOSFET (irfb7434pbf) acting as load switch in a circuit.

The usual operation is well within the SOA region, but, on situations where the load would short circuit, the MOSFET is set to limit the current to a certain value. The ambient temperature is 20˚C and the MOSFET is allowed to cool down before the next short.

During the time the MOSFET is in current limitation I have a MCU integrating the current and the drop over the MOSFET to find out the MOSFETs current thermal status.
And yes, you guessed right, the MOSFET dies. Not always, not the first time around, but it does die.

Now, checking the MOSFET's data sheet the SOA looks like this:

I set the MOSFET to limit at 45A, which produces a voltage drop of 8.7V, which is outside of the SOA area.

And then it is when it gets tricky. In order to simulate the MOSFET's behaviour I run software, which uses the RthJC of the MOSFET for calculations.

At 10ms (which is the short duration), I have a ZthJC of about 0.33.

If I do the math in software, [email protected] gives me 391W, times ZthJC = 391*0.33 =129˚C. That is not a lot of margin but it seems that it would stay within the device's limitations for a carefully measured 10ms pulse.

Maybe I am not interpreting things right. Does it make sense that the MOSFET dies?

In the same datasheet there's a "maximum" power dissipation of 294W, which is lower than the power in one of my shorts. Is that the limit I am hitting?

• That's just really poor design. A mosfet should not be used to dissipate heat as a power resistor would. Any time you heat up a transistor, you're decreasing it's lifetime. Your ratings would change significantly depending upon how the fet is mounted as well as variations from part to part. Why can't you use a shorting power resistor instead of trying to create a circuit that's bound to fail, maybe not immediately, but well short of the expected lifetime of a part. Apr 4, 2014 at 21:48
• The event of entering current limitation is, in theory, extremely rare. This is a stress test, not part of the normal usage. Is what you propose to use a MOSFET to switch to a different path where a high power resistor can lower the current instead? Apr 4, 2014 at 22:07
• Something along those lines should work. Either shut off the power transistor to prevent a short by detecting a over-current condition or place a low value resistor in series at all times to help dissipate heat and lower current during a short. Apr 5, 2014 at 2:50

The IRFB7434 is a rather modern MOSFET, possibly employing trench gate structures. In linear operation, these modern MOSFETs can exhibit a positive temperature coefficient on a local scale, leading to current localization followed by local overheating and finally by failure. Further information on these effects can be found in the application note "Automotive MOSFETs in Linear Applications: Thermal Instability" by Infineon.

All SOA diagrams I have encountered so far in datasheets do not take this effect into account and are thus invalid for describing linear operation behavior. This localization effect also renders thermal models invalid if these are based on the thermal impedance graph, as a homogeneous power loss density within the silicon die is assumed.

It is important to note that the SOA data is not actually measured, it is calculated from models and statistical data. Apparently, the models used by most semiconductor manufacturers are somewhat outdated.

Long story short: You cannot use the SOA diagram of modern MOSFETs or the thermal impedance diagram to predict their behavior in linear operation. Maybe you can modify your circuit to prevent linear operation.

• From other experience I know this to be true.. any pointers for more information? Apr 5, 2014 at 2:16
• I would hope there's a trace of truth in those graphs. Are they completely off or is it just a matter of playing super safe? Also, does this runaway behaviour you describe apply at 10ms? Apr 5, 2014 at 10:56
• I extended my answer with a reference on the thermal runaway effects which lists some points that influence the stability. From my experience, a single 10ms pulse can be critical, especially at high drain-source voltages and for devices that are optimized for low on-resistance. In summary, I think that all SOA diagrams of devices that are not explicitly specified for linear operation are doubtful, and thus testing a particular MOSFET in the application or with a (quite powerful) curve tracer is essential. Apr 5, 2014 at 12:06

Old question, but I find important to note that SOA can be used only for rated(datasheet) cooling conditions. In order to use this particular SOA, the mosfet must be provided with a cooling system that dissipates 294W of power for indefinite time while keeping case temperature at 25 C!

OP did not mention cooling conditions (heatsink thermal resistance etc.), so it may be the reason of failure.