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# How to calculate the junction temperature of a semiconductor device whose datasheets do not include a “thermal resistance” value

At first, the value of “thermal resistance Rth(j-a)” is calculated by using the datasheet data below. The ambient temperature shall be 25 [deg.C].
Rth(j-a) = (Tj(max) – 25 [deg.C]) / PC
Tj(max): Junction temperature in Absolute Maximum Ratings
PC: Collector power dissipation in Absolute Maximum Ratings

Junction temperature is calculated by using the above thermal resistance.
ΔTj [deg.C]＝ Rth(j-a) [deg.C/W] × PLOSS [W]
Tj = ΔTj + Ta
ΔTj: Junction temperature rise
Rth(j-a): Thermal resistance, junction to ambient
PLOSS: Power dissipation in semiconductor device
Tj: Junction temperature
Ta: Ambient temperature

For a semiconductor device package used with a heat sink, in order to ensure more accurate calculations, thermal resistance of junction-to-case values should be used instead of thermal resistance of junction-to-ambient values.
In this example, it is calculable by the same formula as above by using PC values @ Tc = 25 [deg.C].

For the meanings of symbols, please see below.

 Bipolar Transistor Junction temperature : Tj Thermal resistance : Rth(j-a), Rth(j-c) Collector power dissipation : PC MOSFET Channel temperature : Tch Thermal resistance : Rth(ch-a), Rth(ch-c) Drain power dissipation : PD

For calculation of junction temperature, please refer to “Thermal Design and Attachment of a Thermal Fin: Power MOSFET Application Notes (PDF:1,061KB)

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