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I use the Vishay SiA440DJ MOSFET in a PMSM inverter. For thermal simulation in Maltab/Simscape I need to provide thermal data. On pg. 1 of the datasheet the thermal resistance Junction to ambient = 80 K/W can be found, also junction to case = 6.5 K/W.

  1. Is it possible to get the thermal resistance case to ambient by RthJA-RthJC = (80-6.5)*K/W = 73.5 K/W ?

Simscape also needs a thermal time constant or a thermal mass in J/K. My approach is to use the Normalized Thermal Transient Impedance, Junction-to-Ambient chart on pg.6 of the datasheet. on the 0.5 duty cycle graph I look for the 63 % point and derive a time of 50 ms. For junction to case I found a time constant of 300 µs.

  1. Is that correct? To me it seems a little too fast to heat up a 1 sq. inch area of PCB with the MOSFET on it within 50 ms?!
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  • \$\begingroup\$ I don't think you're using the transient thermal impedance graph correctly. \$\endgroup\$
    – Hearth
    Commented Dec 9, 2023 at 15:09

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Vishay didn't link this to the FET but they offer a separate datasheet for the thermal characterstics here. i found all the information I need. I guess another approach would be to use a curve fitting technique and reproduce the curve by R and C elements.

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