Junction Temperature

Formula

Description

Junction temperature is the temperature at the semiconductor die inside a package, and it determines the device's reliability and lifespan. The thermal resistance Rθja (junction-to-ambient) describes how effectively heat flows from the die to the surrounding air. Lower thermal resistance means better heat dissipation. This single-resistance model is a simplification but is widely used for quick estimates. Most semiconductors have a maximum junction temperature of 125-175°C, and exceeding it causes degradation or failure. For every 10°C above rated temperature, device lifetime roughly halves.

Variables

• Tj — Junction temperature (°C)
• Ta — Ambient temperature (°C)
• P — Power dissipation (W)
• Rθja — Thermal resistance junction-to-ambient (°C/W)

Practical Notes

Rθja depends on the package type, PCB copper area, and airflow. A TO-220 package has Rθja of about 60°C/W without a heatsink and 1-5°C/W with a good heatsink. SOT-23 packages are typically 200-300°C/W. Always design for worst-case ambient temperature, not room temperature.