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junction temperature is the temperature at the LED die itself — the tiny chip that produces light. It is the single most important factor in LED lifespan and output. Every degree above the rated maximum costs you lumens and hours of life. Thermal resistance is how you predict and control junction temperature.
What Is Thermal Resistance?
Thermal resistance (°C/W) measures how much temperature rises per watt of heat flowing through a material or component. It works like electrical resistance — heat flows from hot to cold, and thermal resistance determines how much temperature “drops” along the way.
The thermal path from LED die to ambient air has three links in series:
| Link | Symbol | What It Is | Typical Value |
|---|---|---|---|
| Junction to solder point | R_j-sp | Inside the LED package. Set by the manufacturer. | 2–10 °C/W |
| Solder point to heatsink | R_sp-hs | Through the MCPCB and thermal interface (tape or compound) | 1–5 °C/W |
| Heatsink to ambient air | R_hs-a | Through the heatsink fins to the surrounding air | 1.4–26 °C/W (our range) |
Total thermal resistance: R_total = R_j-sp + R_sp-hs + R_hs-a
Why It Matters
junction temperature = ambient temperature + (power × R_total)
Example: A single white Rebel LED at 350mA dissipates ~1.12W. With a 25mm heatsink (R_hs-a = 13.7 °C/W) and assuming ~5 °C/W for the LED package and interface:
T_junction = 25°C + (1.12W × 18.7 °C/W) = 46°C
That’s well within safe limits. Now the same LED on a 19mm heatsink (21.1 °C/W):
T_junction = 25°C + (1.12W × 26.1 °C/W) = 54°C
Still safe, but warmer. In a 40°C ambient environment (enclosed fixture in summer), that becomes 69°C — still acceptable but getting closer to the point where output starts derating.
What Happens When junction temperature Is Too High
| junction temperature | Effect |
|---|---|
| 25°C (rated) | Full rated lumen output |
| 85°C | ~85–90% of rated output (typical 10–15% loss) |
| 120°C | ~70–75% of rated output, accelerated aging |
| 150°C (absolute max) | Risk of permanent damage |
The relationship is gradual — there is no cliff where the LED suddenly fails. But every 10°C increase roughly doubles the rate of lumen depreciation over time.
What You Can Control
You cannot change R_j-sp (that’s the LED manufacturer’s domain). You can control:
- R_sp-hs — Use thermal compound (lowest resistance) or quality thermal tape. Ensure the module sits flat against the heatsink with no air gaps.
- R_hs-a — Choose a heatsink with adequate °C/W rating for your power dissipation. See our Heatsink Selection Guide.
- Ambient temperature — Ventilate the enclosure. Mount away from other heat sources.
Practical Rule of Thumb
For most applications, keep the heatsink surface temperature below 55°C (comfortable to touch briefly). If the heatsink is too hot to hold, the junction temperature is likely above 85°C and the LED is significantly derated.
