LED Junction Temperature & Thermal Design Simulator

Parameters

Forward current I_F

Forward voltage V_F

Luminous efficacy

lm/W

Datasheet value at 25°C (white LEDs are typically 100-170 lm/W)

Ambient temperature T_a

°C

R_thJC (junction → case)

K/W

Internal thermal resistance of the LED package (from datasheet)

R_thCS (case → heatsink)

K/W

TIM (grease or pad) plus MCPCB combined

R_thHA (heatsink → ambient)

K/W

External thermal resistance set by the heatsink and fan

Target lifetime

Results

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Total power (W)

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Heat dissipation (W)

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Total R_th (K/W)

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Junction temperature (°C)

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Predicted lifetime (hr)

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L70 lifetime (hr)

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LED thermal path — die → package → heatsink

Shows the die, MCPCB and heatsink together with the thermal-resistance network (R_thJC + R_thCS + R_thHA). Colour represents temperature (blue → green → orange → red); the die turns redder as T_j increases.

Lifetime vs junction temperature (Arrhenius)

Thermal-resistance breakdown (R_thJC + R_thCS + R_thHA)

Theory & Key Formulas

$$T_j = T_a + P_{thermal} \cdot R_{th,total},\quad L_{70} = 0.7 \cdot L_{ref} \cdot 2^{(T_{ref}-T_j)/10}$$

T_j: junction temperature (°C). T_a: ambient temperature. P_thermal = P_total − P_optical: heat generated at the junction (W). R_th,total = R_thJC + R_thCS + R_thHA: series thermal resistance (K/W). L_ref: reference lifetime (50,000 hr at 85°C). The Arrhenius-style rule doubles the life for every 10°C drop in T_j.

LED junction temperature, thermal resistance and life design

🙋

I always heard LEDs "never burn out", but car headlamps and street lamps obviously get replaced. So they do have a lifetime?

🎓

Good observation. Unlike an incandescent bulb, an LED does not go dark when the filament breaks — it just gets gradually dimmer over thousands of hours. That gradual dimming is "lumen maintenance", and the industry calls the time to fall to 70% of the initial flux L70. Think of an LED as ending its life by going dim, not by burning out.

🙋

So L70 is the "time until dim". What controls how slowly it dims? The lifetime number jumps around dramatically when I move the sliders.

🎓

The single biggest driver is the junction temperature T_j — the temperature of the semiconductor die itself. As a rule of thumb, every 10°C drop in T_j roughly doubles the life and every 10°C rise halves it. This is the Arrhenius law, and the tool models it as L = L_ref · 2^((85 − T_j)/10). So if a better heatsink can knock T_j down by 10°C, the design lifetime shifts by a full factor of two.

🙋

OK, so how do you actually compute T_j? You cannot stick a thermometer on the die.

🎓

The trick is that "ΔT = heat × thermal resistance" is exactly the same equation as Ohm's law (V = IR). The part of the power that is not converted to light (the heat P_thermal) flows from the die out to ambient through three series resistances: R_thJC (die → case), R_thCS (case → heatsink) and R_thHA (heatsink → ambient). Add them and you get T_j = T_a + P_thermal × R_th_total. With the defaults I = 350 mA, V = 3.2 V, efficacy 130 lm/W and R_th = 5 + 0.3 + 3, that gives P_total = 1.12 W, P_thermal ≈ 0.91 W and T_j = 25 + 0.91 × 8.3 ≈ 32.5°C — exactly what the tool prints.

🙋

Same form as a resistor network! So if I grow the heatsink to drop R_thHA, T_j falls and the life goes up?

🎓

Exactly. Try moving R_thHA from 3 to 1 K/W: T_j drops sharply and the predicted lifetime jumps by an order of magnitude. Two caveats, though: R_thJC is fixed by the LED package, so you cannot reduce it from outside, and ambient T_a in an automotive headlamp bay can reach 60-85°C, where no heatsink alone will save you. In those cases the usual trick is to derate the current I_F to about 70% so that P_thermal itself is smaller.

🙋

"R_thJC cannot be reduced" is a useful catch. What about luminous efficacy — higher efficacy means less heat and lower T_j, right?

🎓

In principle yes. The tool models P_optical = P_total × (η/683), so a larger η shifts more of the input into light and less into heat. The latest white LEDs exceed 200 lm/W, which has cut the thermal load noticeably compared with five years ago. The tool also drops the efficacy by about 0.5%/°C as T_j rises, which can create a vicious circle: poor heatsinking → efficacy droop → more heat → shorter life. In thermally dense applications such as Cree XLamp or Lumileds Luxeon high-power LEDs, horticultural LEDs for plant factories, outdoor lighting and Micro-LED displays, managing T_j is the deciding factor for product quality.

Frequently Asked Questions

Junction temperature T_j is the ambient temperature T_a plus the product of heat dissipation P_thermal and total thermal resistance R_th_total: T_j = T_a + P_thermal · R_th_total. P_thermal is the part of the input power that is not converted to light (P_total − P_optical). R_th_total is the series sum R_th-JC (junction to case) + R_th-CS (case to heatsink) + R_th-HA (heatsink to ambient). Enter the forward current, forward voltage, efficacy and each thermal resistance and the tool returns T_j and the predicted lifetime instantly.

LED degradation in the package resin, phosphor and bonding layers is a chemical process that follows the Arrhenius law. As an empirical rule, this tool predicts the lifetime as L_pred = L_ref · 2^((T_ref − T_j)/10) using a reference lifetime of 50,000 hr at T_ref = 85°C. Dropping T_j from 85°C to 75°C roughly doubles the life; pushing it to 95°C halves it. L70 (70% lumen maintenance) is taken as 70% of that predicted lifetime.

L70 is the time to fall to 70% of the initial luminous flux and is the standard lifetime metric in the lighting industry. LEDs do not burn out — they simply dim — so ENERGY STAR, LM-80 and TM-21 define L70 as the performance lifetime. Residential LED lamps are typically rated for 25,000-40,000 hr L70, while outdoor street lamps often require 50,000+ hr. Use the tool's predicted lifetime and L70 alongside T_j and compare them against your design target.

Four basic levers: (1) increase fin area by optimising the fin count, height and thickness, (2) move from natural convection to forced convection (a fan), (3) anodize or black-paint the surface to add radiative cooling, and (4) add a heat pipe or vapour chamber to spread heat quickly. For aluminium extruded fins, 3-5 K/W is typical under natural convection and 0.5-1 K/W with a fan. Dropping R_thHA from 3 to 1.5 K/W in this tool lowers T_j significantly and extends life exponentially.

Real-world applications

Automotive headlamps and tail lamps: The engine-bay ambient sits at 60-85°C and rises another 10°C or more when the car is idling without airflow. Cree XLamp and Lumileds Luxeon LEDs (1-10 W class) are used in large arrays, so designers must verify with this kind of tool that T_j stays below the cut-off (often 125°C) under the worst signal-stop condition. Following the datasheet derating curve, currents I_F are usually held to 70-80% of the absolute maximum.

Outdoor lighting, street lamps and sports lighting: An L70 of 50,000 hr (about 11 years at 12 hr per night) is a common requirement, written into ENERGY STAR and DLC specifications. By holding R_thHA to 1-2 K/W with an aluminium extruded fin under natural convection — and keeping T_j around 70°C — high-maintenance outdoor luminaires can deliver the rated life that owners expect.

Horticultural / plant-factory LEDs (high PPFD): Closed-environment strawberry and lettuce farms run red, blue and far-red LEDs for nearly 24 hours a day to hit PPFDs of 300-600 μmol/m²/s. Insufficient heatsinking causes flux to drop within a few thousand hours, which directly hits yield. Planning L70 and R_thHA together with this tool helps keep the light output steady through the entire growth cycle.

Micro-LED, UV-LED and laser-based lighting: Micro-LED displays with high pixel density and UV-C LEDs for medical or water sterilisation (around 265 nm) have extreme heat density. Even with R_thJC of 3-5 K/W, the die can locally exceed 150°C. Advanced techniques such as direct die attach on a sub-mount, copper-base MCPCBs and vapour chambers are used to push R_thCS below 0.1 K/W — and this tool is a quick way to estimate the gain.

Common misconceptions and pitfalls

The first pitfall is assuming that "if the case temperature T_c is low, the junction temperature T_j is also low". The T_c on the datasheet is the temperature at the bottom of the LED package — the actual die temperature is T_c plus P_thermal × R_thJC. This tool reports T_c and T_j separately, but for high-power LEDs where R_thJC is 5-15 K/W, a measured T_c of 60°C can correspond to a T_j of 80-100°C. Always evaluate life and efficacy against T_j; datasheet lifetime tables are quoted at T_j unless stated otherwise.

The second pitfall is "using the 25°C efficacy from the datasheet as if it were constant". This tool drops the efficacy by about 0.5%/°C as T_j rises, but real parts often droop more steeply. Red LEDs are far more temperature-sensitive than blue, with some products losing over 20% of their flux at 85°C. In RGB mixed-colour lighting, the temperature drop in the red channel skews the colour temperature higher, which is a frequent field complaint. For colour-critical applications, always consult the datasheet's "Junction temperature vs Luminous flux" curve.

The third pitfall is "thicker thermal grease cools better". The opposite is true: a TIM (grease or thermal pad) should be applied as thin and uniform as possible to minimise R_thCS. The thermal conductivity of grease is roughly 1/100 of aluminium, so a 0.3 mm layer can be three times more thermally resistive than a 0.1 mm layer. When mounting an MCPCB, torque the screws evenly to the recommended value and apply just enough grease that you can barely see it. Try raising R_thCS in this tool from 0.3 to 1.0 K/W — you will see a clear rise in T_j.

LED Junction Temperature & Thermal Design Simulator

LED junction temperature, thermal resistance and life design

Frequently Asked Questions

Real-world applications

Common misconceptions and pitfalls

How to Use

Worked Example

Practical Notes