Electronics > Beginners
MOSFETs - Safe Operating Area
joeyjoejoe:
--- Quote from: Hero999 on September 27, 2018, 08:43:34 am ---If it's under 30V or so, a BJT might be better.
--- End quote ---
Yup under 30V for sure - "true" current would be around 4A.
MiDi:
--- Quote from: BravoV on September 27, 2018, 11:53:24 am ---An example from IXYS linear fet, IXTN46N50L datasheet, at the SOA chart, sometimes they called "Extended" FBSOA, watch carefully the highlighted areas I made, most common switching fets (or almost all) didn't make this kind of details.
Also watch the bold DC line as they deliberately made it.
--- End quote ---
Bit confused both SOAs are for 150°C Tj (junction/die), what has the Tc (case) to do with the SOA?
Edit: Oh, they are doing it on others the same way :o
aheid:
--- Quote from: MiDi on September 27, 2018, 10:11:28 pm ---Bit confused both SOAs are for 150°C Tj (junction/die), what has the Tc (case) to do with the SOA?
--- End quote ---
If Tj is the same for two different Tc's then, assuming the systems are in equilibrium, the lower Tc system must have a better cooling solution and hence is able to dissipate more power as heat.
Alternatively, the absolute temperature of Tj depends on Tc. Higher Tc means less current to reach the same Tj.
MiDi:
--- Quote from: aheid on September 27, 2018, 10:50:52 pm ---
--- Quote from: MiDi on September 27, 2018, 10:11:28 pm ---Bit confused both SOAs are for 150°C Tj (junction/die), what has the Tc (case) to do with the SOA?
--- End quote ---
If Tj is the same for two different Tc's then, assuming the systems are in equilibrium, the lower Tc system must have a better cooling solution and hence is able to dissipate more power as heat.
Alternatively, the absolute temperature of Tj depends on Tc. Higher Tc means less current to reach the same Tj.
--- End quote ---
:palm: too late here, I should go :=\
It is the Rthjc which defines how much power at given dTjc...
KE5FX:
Relying on Tj is problematic for power FETs that are constructed by paralleling multiple dies internally. In the region below the point where the transfer curves cross over, a given drain current will be achieved at a much lower Vgs as temperature goes up. The dies are never 100% identical, so thermal runaway becomes a possibility as the hotter dies start to carry more drain current.
To paraphrase Dr. Bronner, "Derate! Derate! OK!"
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