Maximum Power (Junction to Case)

Pmax = (Tj − Tc) / θjc

Quick answer

This calculator solves Pmax = (Tj − Tc) / θjc for Maximum Power. With Max Junction Temp 500 °C, Case Temperature 235 °C, θjc (Junction-to-Case) 200 °C/W, the result is 1.325 W. It solves in 4 directions, so you can enter the values you have and get the one you need.

Calculator

Maximum Power (W)
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Formula

Pmax = (Tj − Tc) / θjc

Description

This thermal formula determines the maximum power a device can dissipate based on the temperature difference between the junction and the case and the junction-to-case thermal resistance. When a heatsink maintains the case at a known temperature, this formula directly gives the allowable power dissipation. Lower θjc values allow higher power dissipation for a given temperature differential. Exposed-pad packages (QFN, PowerPAD) achieve much lower θjc than packages that rely solely on lead frame thermal conduction.

Variables

  • Pmax — Maximum allowable power dissipation (W)
  • Tj — Maximum junction temperature (°C)
  • Tc — Case temperature (°C)
  • θjc — Junction-to-case thermal resistance (°C/W)

Practical Notes

θjc is a fixed property of the package and die attach. Typical values: SOT-23 about 80-100°C/W, SOIC-8 about 30-60°C/W, exposed-pad QFN about 2-10°C/W, TO-220 about 1-3°C/W. For multi-die packages, θjc may be specified per die. Always verify case temperature with thermal measurements or simulation.

Symbols and units

Symbol Meaning Default unit Accepted units
Pmax Maximum Power W µW, mW, W, kW
Tj Max Junction Temp °C °C
Tc Case Temperature °C °C
Rjc θjc (Junction-to-Case) °C/W °C/W

Step by step

  1. 1. What you know: Max Junction Temp 500 °C, Case Temperature 235 °C, θjc (Junction-to-Case) 200 °C/W
  2. 2. The relation: Pmax = (Tj − Tc) / θjc
  3. 3. Substitute: Tj = 500 °C, Tc = 235 °C, Rjc = 200 °C/W
  4. 4. Result: Maximum Power = 1.325 W

Worked examples

Minimum range

Given: Max Junction Temp 25 °C, Case Temperature 11.8 °C, θjc (Junction-to-Case) 1 °C/W

Result: Maximum Power = 13.2 W

Low range

Given: Max Junction Temp 250 °C, Case Temperature 118 °C, θjc (Junction-to-Case) 10 °C/W

Result: Maximum Power = 13.2 W

Typical

Given: Max Junction Temp 500 °C, Case Temperature 235 °C, θjc (Junction-to-Case) 200 °C/W

Result: Maximum Power = 1.325 W

High range

Given: Max Junction Temp 850 °C, Case Temperature 400 °C, θjc (Junction-to-Case) 1000 °C/W

Result: Maximum Power = 0.45 W

Maximum range

Given: Max Junction Temp 8500 °C, Case Temperature 4000 °C, θjc (Junction-to-Case) 10000 °C/W

Result: Maximum Power = 0.45 W

Solving for each variable

Solve for Maximum Power

Max Junction Temp 25 °C, Case Temperature 11.8 °C, θjc (Junction-to-Case) 1 °C/W → 13.2 W

Solve for Max Junction Temp

Maximum Power 10 mW, Case Temperature 5.5 °C, θjc (Junction-to-Case) 0.1 °C/W → 5.501 °C

Solve for Case Temperature

Maximum Power 10 mW, Max Junction Temp 2.5 °C, θjc (Junction-to-Case) 0.1 °C/W → 2.499 °C

Solve for θjc (Junction-to-Case)

Maximum Power 10 mW, Max Junction Temp 2.5 °C, Case Temperature 5.5 °C → -300 °C/W

How Maximum Power changes with Max Junction Temp

Max Junction Temp (input) Maximum Power (result)
250 °C 75 mW
375 °C 0.7 W
500 °C 1.325 W
750 °C 2.575 W
1000 °C 3.825 W
2000 °C 8.825 W
3500 °C 16.33 W
5000 °C 23.83 W

The same result in other units

Unit Value
µW 1325000
mW 1325
W 1.325
kW 0.001325

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Frequently asked questions

How do you calculate Maximum Power (Junction to Case)?

Use Pmax = (Tj − Tc) / θjc. With Max Junction Temp 500 °C, Case Temperature 235 °C, θjc (Junction-to-Case) 200 °C/W, Maximum Power works out to 1.325 W.

What is the formula for Maximum Power (Junction to Case)?

The formula is Pmax = (Tj − Tc) / θjc. This thermal formula determines the maximum power a device can dissipate based on the temperature difference between the junction and the case and the junction-to-case thermal resistance.

How do you find Max Junction Temp when you know the other values?

Rearrange for Max Junction Temp: with Maximum Power 10 mW, Case Temperature 5.5 °C, θjc (Junction-to-Case) 0.1 °C/W, Max Junction Temp is 5.501 °C.

How do you find Case Temperature when you know the other values?

Rearrange for Case Temperature: with Maximum Power 10 mW, Max Junction Temp 2.5 °C, θjc (Junction-to-Case) 0.1 °C/W, Case Temperature is 2.499 °C.

What units does Maximum Power (Junction to Case) use?

Maximum Power is shown in µW, 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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