How to calculate the junction temperature when the power is dissipated in a semiconductor device

When the power is dissipated in the semiconductor device, the junction temperature rises.
The temperature rise ΔTj is shown as follows:
ΔTj  [deg.C]= Rth [deg.C /W] × PLOSS [W]
ΔTj: Junction temperature rise
Rth: Thermal resistance
PLOSS: Power dissipation in the semiconductor device

Junction temperature is shown as follows:
Tj = ΔTj + Ta
Tj: Junction temperature
Ta: Ambient temperature
*In a bipolar transistor, Tj is used as the symbol of junction temperature.
In a MOSFET, Tch is used as the symbol of channel temperature instead of Tj.

For calculation of junction temperature, please refer to “Thermal Design and Attachment of a Thermal Fin: Power MOSFET Application Notes”

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