Conception et réalisation thermique
Designer’s Complaint… LEDs are specified @ single test current @ 25°C Tj
A new trend in the data- sheet characterization of the LEDs - The LEDs are tested and binned at real world operating conditions
What is LED Junction Temperature
Measurement Point Application Brief AB33 http://www.philipslumileds.com/uploads/10/AB33-pdf
Heat Generation LEDs are not 100% efficient power consumed is not completely converted to light Approximately, 30% to 50 % (depending on the technology) is converted to light and the rest is converted to heat
Heat Flow LED thermal pad does not provide enough surface to dissipate the heat
Effects of Heat on LEDs Heat affects the LEDs in 5 different ways: - Light output
- Color shift
- Forward voltage shift
- LED lifetime
- Permanent damage
Effects of Heat on LEDs
Effects of Heat on LEDs Shifts dominant wavelength
Effects of Heat on LEDs
Effects of Heat on LEDs
Lumen Maintenance - (Bxx, Lyy) Notation used to describe the average lumen maintenance characteristic of the LEDs. Lumen maintenance for SSL devices is typically defined in terms of the percentage of initial light output remaining after a specific period of time. (Bxx, Lyy) Example – (B50, L70) at 50000hours: - On average, the light output of 50% (B50) of the LEDs within the system will drop to lower than 70% (L70) of their initial light output after 50000hours.
Effects of Heat on LEDs
Effects of Heat on LEDs
Basic cooling considerations Conduction: - The transfer of heat energy through a substance or from one substance to another due to temperature difference
Thermal Management It is critical to extract the heat away from the LED module and transfer it to ambient
Heat Sinks Efficiency of heat sinks depends mainly on: - Surface area
- The larger the surface area, the more heat dissipated
- Structure or shape
- Proper structure increases turbulent airflow which creates a more efficient heat sink
Heat Sinks - Material
- Use of materials with better thermal conductivity gives a more efficient heat sink
- Ex. cooper 401 W/m-K vs. aluminum 235 W/m-K
Thermal Resistance RTH
Thermal Resistance RTH
Thermal Conductivity (k) The measure of a material’s ability to conduct heat (W/mK)
Case Study
FLS has jointly developed with Qfinsoft, QLED, a thermal design and simulation software In parallel, FLS has launched a thermal design and simulation service to assist customers 4 FLS Engineers are assigned to carry out this service
What is QLED? FLS jointly developed QLED with Qfinsoft QLED is a thermal design and simulation software developed for modeling LUXEON LED lighting systems The accuracy of the LED models and their behavior were endorsed by Philips Lumileds
What is QLED? QLED is a virtual environment which allows customers to create different models. For example, models can range from: Multiple LEDs on a custom made board within an enclosed space or casing with active cooling
Benefits of Using QLED It minimizes the number of design cycles, reduces development costs, and decreases time to market
Benefits of Using QLED
Key Features Offers an easy to use library system for material selection Includes a powerful, yet easy to use design optimizer
QLED Capabilities Simulation modes include: - Steady state: DC current (constant ON)
- Transient: Pulse or strobe LEDs
- Parameterized Trials
- Optimization
Scenario A
Fortimo DLM 1100lm Thermal path basic solution Temperatures: - 1 = test point Tc
- 2 = heat sink @ module side
- 3 = ambient
Resistances: - R1 = LED DLM path 1-2
- R2 = heat sink path 2-3
Fortimo DLM 1100lm Thermal Resistances
Fortimo DLM 1100lm Thermal resistance of heat sink Example of standard heat sink: Needed 4.214 K/W (max) Heat sink: Aavid Thermalloy - Length @ 4.01 K/W = 35 mm
- Width= 76.2 mm, height= 38.1 mm, #fins= 8
Thermal Simulation – Open Frame
Tc = 90oC Exceed the max. Tc Thermal design must be modified
Larger heat sinks: - Tripled the heat sink height
Tc ≈ 73oC We still need to lower Tc to 65oC
Solutions? – larger heat sink Fins extended to touch the fixture Tc ≈ 59oC
Solutions? – vented fixture Vents on upper and lower sections of the fixture Tc ≈ 82oC Even with larger heat sinks, it may be difficult to reduce Tc
Scenario B
Nuventix – Open Frame Each setting has a thermal resistance depending on the performance setting
Nuventix – Open Frame At the standard setting and ambient temperature = 35oC, Tc ≈ 44.7oC Tc = P x Rth(hs-ambient) + Tambient Tc = 13 x 0.75 + 35 = 44.75oC
Nuventix – Closed Fixture Experimental testing SynJet to be modeled in QLED
Do'stlaringiz bilan baham: |