Power Derating (Temperature)

Formula

Description

Many electronic components have a power derating curve that specifies full rated power up to a knee temperature, then linearly decreasing power to zero at the maximum temperature. Below the knee temperature, the component can dissipate its full rated power. Above it, the allowable power decreases linearly because the reduced temperature headroom limits how much additional heat the component can generate internally without exceeding its maximum junction or case temperature. This applies to transistors, ICs, resistors, and many other components.

Variables

• Pmax — Maximum allowable power dissipation at temperature T (W)
• Prated — Full rated power at or below Tknee (W)
• Tmax — Maximum rated temperature at zero power (°C)
• T — Actual operating temperature (°C)
• Tknee — Temperature where derating begins (°C)

Practical Notes

Common knee/max temperatures: resistors 70°C/155°C, power transistors 25°C/150°C, ICs vary by package. Always check the specific derating curve in the datasheet. Below the knee temperature, this formula returns the full rated power. Military and automotive temperature ranges (-55°C to +125°C) require more aggressive derating than commercial (0°C to 70°C).