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].
R_th(j-a) = (T_j(max) – 25 [deg.C]) / P_C
T_j(max): Junction temperature in Absolute Maximum Ratings
P_C: Collector power dissipation in Absolute Maximum Ratings

Junction temperature is calculated by using the above thermal resistance.
ΔT_j [deg.C]＝ R_th(j-a) [deg.C/W] × P_LOSS [W]
T_j = ΔT_j + T_a
ΔT_j: Junction temperature rise
R_th(j-a): Thermal resistance, junction to ambient
P_LOSS: Power dissipation in semiconductor device
T_j: Junction temperature
T_a: 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 P_C values @ T_c = 25 [deg.C].

For the meanings of symbols, please see below.

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

• Thermal Design and Attachment of a Thermal Fin: Power MOSFET Application Notes (PDF:1,061KB)