01LED Thermal Design
Why is my LED losing lumen output and shifting color faster than the datasheet lifetime suggested?
Junction-temperature-driven LED thermal design validated against measured Tj and power-cycling fatigue data.
02The problem
An LED luminaire's flux, color point and lifetime are all governed by junction temperature, but junction temperature is rarely measured directly during design — it's usually inferred from a datasheet thermal resistance value applied to an assumed case temperature, which compounds error at every step.
The result is a lumen-maintenance and warranty specification built on an estimate rather than a measurement, and a design that may run hotter — or have less margin — than the specification assumes.
Why it's hard
- Junction temperature cannot be measured directly with a thermocouple; it has to be inferred from the LED's own electrical characteristics using transient measurement
- Datasheet thermal resistance values are measured under standardized conditions that rarely match the actual board, housing and mounting of a specific luminaire design
- Optical and thermal behavior are coupled — junction temperature affects flux and color point, and the housing design affects junction temperature — so the two need to be evaluated together, not separately
- Lumen-maintenance (L70/L90) and warranty specifications are long-duration extrapolations that depend heavily on the accuracy of the underlying junction temperature and fatigue data
- Switching duty cycle (on/off frequency) drives thermal-mechanical fatigue at the die-attach and board interconnect that a steady-state thermal analysis does not capture
03How we solve it
Predict — simulation
- Design the luminaire housing, board spreading and convection path in FLOEFD's CAD-embedded environment, including LED optical-thermal coupling
Measure — hardware
- Measure the LED's junction temperature and thermal resistance directly with Simcenter Micred T3Ster, separating die, die-attach and board contributions
- Validate thermal fatigue life under real switching duty with Simcenter Micred Power Tester power cycling
Correlate — close the loop
- Use the measured junction temperature and Rth to validate the FLOEFD thermal model of the housing design, rather than relying on datasheet Rth alone
- Base the lumen-maintenance projection and warranty specification on measured thermal and fatigue data
05Standards in scope
- LM-80 / TM-21 for LED lumen maintenance
- JEDEC JESD51 package characterization
07Questions engineers ask
Base your lumen-maintenance spec on measured junction temperature.
Tell us your luminaire's power and duty cycle, and we'll show you how measurement changes the thermal design margin.