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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 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 [deg.C] = ΔTj [deg.C] + Ta [deg.C]
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.

Therefore the permissible power P[W] is expressed below.
P[W] = ( Tj [deg.C] - Ta [deg.C] ) / Rth [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.
・Concepts of Derating : (https://toshiba.semicon-storage.com/ap-en/design-support/reliability/appendix/03/1340594.html)
・“Thermal Design and Attachment of a Thermal Fin: Power MOSFET Application Notes (PDF:1,061KB)

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