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from the datasheet which one should I use? Thermal Junction to Case or Thermal Junction to Ambient?

Tj= Ta + Pd * Rth(j-c)

or

Tj= Ta + Pd * Rth(j-a)

when I use Rth(j-a), Tj become way too high, near TjMax.

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If you use Rth(j-c) you either have to know the maximum case temperature or you have to add the thermal resistance of an external heat sink (case to heat sink + heat sink to ambient) and the maximum Ta.

So, you would use Rth(j-a). If you are getting too high a Tj using Rth(j-a) at the maximum Ta then you require a different package or a heat sink.

Don’t forget to account for switching losses and the large increase in Rds(on) at high temperatures and ensure you have sufficient gate drive voltage to get guaranteed Rds(on).

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  • \$\begingroup\$ I already use Rds(on) at high temperature and Pd total (conduction losses, switching losses, dead time losses). Current at 30% mosfet can handle. \$\endgroup\$
    – urai5
    Jun 26, 2020 at 2:46

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