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Courses/LED & Lighting Design/LED Technology

lesson · 8min · Lesson 4 of 24

Thermal management: heatsinks and junction temperature

Course syllabusCourse overview
01LED Technology
  1. ReadHow LEDs produce light: p-n junction
  2. ReadEfficacy: lumens per watt explained
  3. ReadLED chip formats: COB, SMD, filament
  4. ReadThermal management: heatsinks and junction temperature
  5. quizLED technology quiz
02Driver Circuits
  1. ReadConstant current vs constant voltage drivers
  2. ReadDriver efficiency and power factor
  3. ReadDimming methods: PWM, 0-10V, DALI
  4. ReadDriver selection and compatibility
  5. quizDriver circuit quiz
03Emergency Lighting
  1. ReadBS 5266 categories: escape, standby, high-risk
  2. ReadSelf-contained vs central battery systems
  3. ReadMaintained vs non-maintained operation
  4. ReadTesting requirements and log books
  5. quizEmergency lighting quiz
04Lux Calculations
  1. ReadIlluminance, luminous flux, and efficacy
  2. ReadLumen method: average illuminance
  3. ReadRoom index, UF, and MF
  4. exerciseLux calculation worked example
05Lighting Control & Quality
  1. ReadColour temperature: warm, neutral, cool white
  2. ReadColour rendering index (Ra/CRI)
  3. ReadGlare: UGR and how to reduce it
  4. ReadSmart lighting systems: DALI 2 and IoT
  5. quizFinal assessment
Lesson · 8min
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In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
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ElectraCore lesson handout

Thermal management: heatsinks and junction temperature

Led Lighting · Lesson 4 · Intermediate

IntermediateReview: professional review pending

Purpose

Design the junction-to-ambient thermal path and avoid confusing case temperature with junction temperature.

Before you beginVoltage, current and power · Basic semiconductor and circuit concepts

Learning objectives

  • Trace electrical-to-light conversion
  • Use declared operating boundaries
  • Separate component and luminaire metrics
  • Check control and load compatibility

Most LED input energy becomes heat. That heat must flow from junction through package, board/interface, heatsink and ambient while staying inside declared limits under worst installation conditions.

LED JUNCTION LIGHT AND HEAT PATHLED JUNCTION LIGHT AND HEAT PATHDRIVERLEDlight · heatinput · regulation · junction · optics · control · environment

Core theory

Thermal resistances in series create temperature rise Q×Rθ. Use dissipated heat, not automatically rated electrical power, and include contact/interface and enclosure/insulation effects.

Tc is a declared case test point; Tj is internal junction temperature. Estimate Tj only with the exact thermal model and current operating condition.

Dust, insulation, recessed ceilings, orientation, ambient temperature, driver heat and blocked airflow can raise temperatures. Derating and lifetime projections require manufacturer data and validation.

Terms, symbols, and units
TermMeaningSymbolUnit
TjLED semiconductor junction temperatureNot applicableNot applicable
TcDeclared case temperature at a specified test pointNot applicableNot applicable
Thermal resistanceTemperature rise per heat flowRθK/W
Tj ≈ Ta + Q × Rθ,ja

Simplified steady-state model for declared complete junction-to-ambient resistance.

°C or K for rise; W and K/W
Worked exampleAmbient is 35°C, heat is 18 W and declared Rθ,ja is 3.2 K/W.

Assumptions: Supplied photometric/electrical values share the stated measurement boundary; Manufacturer limits remain governing.

  1. Rise: ΔT=18×3.2=57.6 K.
  2. Junction: Tj≈35+57.6=92.6°C.
  3. Check: Compare with declared limit/derating and verify actual mounting boundary.

Illustrative Tj is 92.6°C.

Reasonableness check: The result identifies whether it describes LED package, module, driver or complete luminaire and avoids claiming field performance from one laboratory number.

Common mistakes
  • Dividing lumens by LED-chip watts while calling it luminaire efficacy
  • Connecting constant-current LEDs to a constant-voltage source without current control
  • Assuming every dimmable driver works with every dimmer

Where this appears in practice

A reliable LED installation coordinates light source, optics, thermal path, driver, supply/protection, controls, environment and maintainability.

SafetyLED outputs can create glare/photobiological hazards and drivers can contain mains voltage and stored charge after switch-off. Isolate, prove dead, observe discharge instructions and never defeat thermal interfaces or enclosure barriers.
Local code checkConfirm current luminaire/module/controlgear safety and performance standards, BS 7671, exact driver/module/luminaire data, photobiological risk, thermal/fire enclosure requirements, EMC/flicker/harmonic obligations and control-protocol certification. A CE/UKCA or DALI-2 mark does not prove two arbitrary components are electrically, thermally or functionally compatible.

Knowledge check

Does a safe-to-touch heatsink prove the LED junction is cool enough?

No. Use the declared thermal path and test-point/model data.

Answer: No. Use the declared thermal path and test-point/model data.

Practical exercise

Build a worst-case thermal budget for open, recessed and insulation-covered installations.

Summary

  • Metric boundary changes the answer
  • LED current and junction temperature govern life/output
  • Driver and dimmer must match electrically and functionally

Sources and review

  • IEC 62031:2026: LED modules safety requirements: IEC; 2026; International
  • IEC 61347-2-13:2024: LED controlgear safety: IEC; 2024; International
  • DALI-2 certification overview: DALI Alliance; Current certification database/status must be checked; International

Editorial review date: 2026-08-22. Professional electrical review is pending.

Educational material for learning and preliminary checks. Verify current local requirements and exact equipment instructions. This lesson does not replace competent professional work.

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