Power Derating (Temperature)

Pmax = Prated × (Tmax−T) / (Tmax−Tknee)

Quick answer

This calculator solves Pmax = Prated × (Tmax − T) / (Tmax − Tknee) for Derated Power. With Rated Power 22 W, Max Temperature 50 °C, Operating Temperature 110 °C, Derating Start Temp 235 °C, the result is 22 W.

Calculator

Result
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Formula

Pmax = Prated × (Tmax − T) / (Tmax − Tknee)

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).

Symbols and units

Symbol Meaning Default unit Accepted units
Pmax Derated Power W µW, mW, W, kW
Prated Rated Power W µW, mW, W, kW
Tmax Max Temperature °C °C
T Operating Temperature °C °C
Tknee Derating Start Temp °C °C

Step by step

  1. 1. What you know: Rated Power 22 W, Max Temperature 50 °C, Operating Temperature 110 °C, Derating Start Temp 235 °C
  2. 2. The relation: Pmax = Prated × (Tmax − T) / (Tmax − Tknee)
  3. 3. Substitute: Prated = 22 W, Tmax = 50 °C, T = 110 °C, Tknee = 235 °C
  4. 4. Result: Derated Power = 22 W

Worked examples

Minimum range

Given: Rated Power 22 mW, Max Temperature 2.5 °C, Operating Temperature 5.5 °C, Derating Start Temp 11.8 °C

Result: Derated Power = 22 mW

Low range

Given: Rated Power 0.22 W, Max Temperature 25 °C, Operating Temperature 55 °C, Derating Start Temp 118 °C

Result: Derated Power = 0.22 W

Typical

Given: Rated Power 22 W, Max Temperature 50 °C, Operating Temperature 110 °C, Derating Start Temp 235 °C

Result: Derated Power = 22 W

High range

Given: Rated Power 2.2 kW, Max Temperature 85 °C, Operating Temperature 187 °C, Derating Start Temp 400 °C

Result: Derated Power = 2.2 kW

Maximum range

Given: Rated Power 22 kW, Max Temperature 850 °C, Operating Temperature 1870 °C, Derating Start Temp 4000 °C

Result: Derated Power = 22 kW

How Derated Power changes with Rated Power

Rated Power (input) Derated Power (result)
4.84 W 4.84 W
12.1 W 12.1 W
24.2 W 24.2 W
36.3 W 36.3 W
48.4 W 48.4 W
72.6 W 72.6 W
96.8 W 96.8 W
0.1936 kW 0.1936 kW
0.3388 kW 0.3388 kW
0.484 kW 0.484 kW

The same result in other units

Unit Value
mW 22000
W 22
kW 0.022

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Frequently asked questions

How do you calculate Power Derating (Temperature)?

Use Pmax = Prated × (Tmax − T) / (Tmax − Tknee). With Rated Power 22 W, Max Temperature 50 °C, Operating Temperature 110 °C, Derating Start Temp 235 °C, Derated Power works out to 22 W.

What is the formula for Power Derating (Temperature)?

The formula is Pmax = Prated × (Tmax − T) / (Tmax − Tknee). 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.

What units does Power Derating (Temperature) use?

Derated Power is shown in mW, W, kW. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

Related Concepts

All Thermal formulas →
Junction Temperature Full Thermal Chain Required Heatsink Maximum Power (Junction to Case) Thermal Time Constant Forced Convection Heat Transfer Power Derating (Temperature) Natural Convection Thermal Resistance

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