Conception et réalisation thermique Troyes, 23 février 2012

Conception et réalisation thermique

• Troyes, 23 février 2012

Designer’s Complaint…

• LEDs are specified @ single test current @ 25°C Tj

LED Datasheet Specifications

• 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

• Tj

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)

• Bxx: percentage of LEDs, on average
• Lyy: percentage of light output remaining

• 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

• Reduces operating life

Effects of Heat on LEDs

• May cause severe damage

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

• This can be done using the principles of conduction, convection and radiation

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

QLED – Thermal Simulation

• 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:

• A single LED on a heat sink

• to

• 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

• 2. Simple user interface

Key Features

• Provides very fast simulation results, with most simulations taking only minutes

• 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

• Passive cooling

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

Thermal Simulation – Closed Fixture

• Tc = 90oC

• Exceed the max. Tc

• Thermal design must be modified

Solutions? – larger heat sink

• 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

• Active Cooling

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