Conception et réalisation thermique Troyes, 23 février 2012

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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

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:

  • 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

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

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