The MOSFET (metal-oxide-semiconductor field-effect transistor) can be used to obtain some pretty fast switching times, but it does have its limits. We found those limits.

In a recent project, we had to determine the highest possible frequencies a certain MOSFET could effectively switch a circuit on and off. Several MOSFETs were melted during the process. While destroying components is to be expected in the development stages of a project, we experienced something unusual: The MOSFETs were reaching temperatures of above 700 degrees Fahrenheit! Hot enough to boil solder! Of course, this can easily be explained by the fact that we are switching .8 amps at 48V, which is not a normal thing for us; that high power is the cause of the exceptional temperatures.

But… what actually causes the MOSFETs to overheat in the first place?

The only time a MOSFET experiences heat is during the transition period. When it is fully switched one way or the other, it provides minimal resistance, generating very little heat. We were pushing the MOSFET to 250 kHz and above, which was not providing enough time for the MOSFET to actually switch. It spent all of its time in the transition phase, constantly generating heat.

The MOSFET handled a 135 kHz switching frequency with no issues at all, and that is good enough for us.

Featured image from DigiKey