PAGE 135 OF 150 · POWER SUPPLIES & BATTERIES
Thermal Design Basics
Cooler chips live longer and stay brighter.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
The thermal path
Heat travels from the chip junction to the case, to the heat sink and into the air. Each step has a thermal resistance in degrees C per watt, and they add up.
Formula
Junction temperature = ambient + power × (Rjc + Rcs + Rsa).
For 2.1 W, with Rjc 6, Rcs 1 and Rsa 10 degrees C per watt, junction temperature is 25 + 2.1 × 17 ≈ 61 degrees C.
- Keep the junction well under the datasheet maximum.
- Allow for hotter rooms and enclosures.
- Airflow lowers the heat sink to air resistance.
REMEMBER
Heat paths add up, so a weak link spoils the design.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
Explore LED clocks, stopwatches and electronics at cosycom.com.