Maximum Power (Junction to Case)

Formula

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.