LED Junction Temperature
Estimate LED junction temperature from drive power, thermal resistance, and board temperature.
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The engineering
An LED's junction temperature drives everything you care about — lumen output, dominant wavelength shift, and lifetime — yet you can't stick a probe on the die. Instead you back it out: measure or specify a reference temperature at the case or solder pad, multiply the dissipated heat by the thermal resistance from junction to that point, and add the rise. This is the standard bench method from Cree/LED-maker app notes.
The gotcha is the optical term. Only the heat that stays in the package raises Tj, so a high-efficiency LED radiating 40% of its input as photons dissipates only 60% as heat — set η_opt accordingly. If you leave it at 0 you get the conservative worst case, which is usually what you want for a thermal margin check. Keep Tj comfortably under the datasheet max (often 125–150 °C) or lumen maintenance falls off a cliff.
Where this math comes from
The junction-temperature bookkeeping LED engineers use is inherited directly from semiconductor packaging, where θ_jc (junction-to-case thermal resistance) has rated transistors and diodes since the 1960s. As power LEDs crossed the watt-per-emitter line in the early 2000s, thermal management became the limiting factor rather than the diode physics, and manufacturers began publishing θ_jc alongside the optical specs.
Cree's XLamp application notes formalized the practical recipe for solid-state lighting designers: measure the solder-point temperature, apply the rated thermal resistance, and account for the fraction of input power that leaves as light. That workflow — copied across the industry — is exactly what this card computes.
- 1960Semiconductor industryθ_jc adopted as the standard junction-to-case thermal resistance metric for packaged devices.
- 1962Nick Holonyak Jr.Demonstrates the first practical visible LED, seeding solid-state lighting.
- 2003CreeLaunches XLamp power LEDs and publishes thermal app notes defining the Tj = Tsp + P·θ method.
- 2012JEDEC JESD51-14Standardizes transient dual-interface measurement of junction-to-case thermal resistance.
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