How much power is it permissible for the semiconductor device to dissipate?

Permissible power dissipation is limited at the junction temperature.
It must be used within a range that does not exceed the junction temperature at the absolute maximum rating.

When the power is dissipated in the semiconductor device, the junction temperature rises.
The temperature rise ΔT_j is shown as follows:
ΔT_j[deg.C]＝ R_th [deg.C/W] × P_LOSS [W]
ΔT_j: Junction temperature rise
R_th: Thermal resistance
P_LOSS: Power dissipation in the semiconductor device
Junction temperature is shown as follows: T_j [deg.C] = ΔT_j [deg.C] + T_a [deg.C]
T_j: Junction temperature
T_a: Ambient temperature
＊In a bipolar transistor, T_j is used as the symbol of junction temperature. In a MOSFET, T_ch is used as the symbol of channel temperature instead of T_j.

Therefore the permissible power P[W] is expressed below.
P[W] = ( T_j [deg.C] - T_a [deg.C] ) / R_th [deg.C/W]

Although it is possible to use it at the maximum junction temperature Tj (max) or less, due consideration must be given to factors affecting reliability such as deterioration and lifespan. Degradation, such as that of performance, accelerates as junction temperature rises. In order to maintain stable performance for a long period of time on your equipment, please design with derating to the maximum junction temperature Tj (max).

Please refer to the information below.

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