Thermal Resistance
Understanding Thermal Resistance
Thermal resistance is the key parameter for predicting device temperature and, consequently, reliability. Every degree of junction temperature reduction extends device lifetime. Thermal analysis using thermal resistance networks is standard practice for power amplifier and system design.
Thermal Resistance Network
- Rth_jc (junction-to-case): Determined by the device package. Manufacturer specified. 0.5-10 C/W typical.
- Rth_cs (case-to-sink): Determined by the thermal interface material. 0.1-1 C/W depending on TIM type and area.
- Rth_sa (sink-to-ambient): Determined by heatsink design. 0.2-5 C/W depending on size and airflow.
Temperature Calculation
T_j = T_ambient + P_dissipated x Rth_ja
Example:
GaN PA: P_diss = 50W, T_ambient = 50C
Rth_jc = 1.5 C/W, Rth_cs = 0.3, Rth_sa = 1.2
Rth_ja = 3.0 C/W
T_j = 50 + 50 x 3.0 = 200C
(Too hot for GaN max of 225C! Need better cooling.)
Frequently Asked Questions
What is thermal resistance?
Thermal resistance is the temperature rise per watt of dissipated power. Rth_ja = Rth_jc + Rth_cs + Rth_sa. Lower Rth = better cooling. For a PA dissipating P watts: T_junction = T_ambient + P x Rth_ja.
How do you reduce thermal resistance?
Better thermal interface material (lower Rth_cs), larger or better heatsink (lower Rth_sa), forced air or liquid cooling, copper heat spreaders, and selecting packages with lower Rth_jc. Each component in the thermal path must be optimized.
Why does junction temperature matter?
Device reliability decreases exponentially with junction temperature. A 10C increase roughly halves the MTBF for many semiconductor devices. Keeping T_j well below the maximum rated temperature is essential for reliability.