Conception et réalisation thermique

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Conception et réalisation thermiqueConception et réalisation thermiqueTroyes, 23 février 2012

Thierry SuzanneIngénieur d’application

© Future Electronics Inc. Confidential and Proprietary Making LED Lighting Solutions Simple TM

Designer’s Complaint…Designer’s Complaint…

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

• My application is different!

• What is the real light output for my application?


LED Datasheet SpecificationsLED Datasheet Specifications

• A new trend in the datasheet characterization of the LEDs– The LEDs are tested and binned at real world

operating conditions

Hot Binning @ 85oC


What is LED Junction TemperatureWhat is LED Junction Temperature

LED Junction Temperature (Tj)

• Temperature directly on the LED chip/die

• What does tested and binned at 25oC or 85oC at a specific drive current of for example 700mA mean?

– The LED was driven at 700mA and light output measurements were made while the junction temperature at the LED was maintained at 85oC


Measurement PointMeasurement Point

• Application Brief AB33 http://www.philipslumileds.com/uploads/10/AB33-pdf


Heat GenerationHeat 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

Radiometric Power (power converted to light)


Heat FlowHeat Flow

• LED thermal pad does not provide enough surface to dissipate the heat

No heat in the light beam

• We add board, thermally conductive material and heat sink to transfer the heat from the LED junction to the air surrounding the LED

Thermal Pad

• Heat generated by the LED is dissipated via the thermal pad underneath the LED

• No heat in LED’s main light beam


Effects of Heat on LEDsEffects of Heat on LEDs

• Heat affects the LEDs in 5 different ways:

– Light output– Color shift– Forward voltage shift– LED lifetime– Permanent damage



0%

50%

100%

150%

200%

-40 -20 0 20 40 60 80 100 120

Junction Temperature TJ [°C]

Rel

ativ

e L

igh

t O

utp

ut

(LO

P)

AmberRed GreenBlue

White

CyanRoyal Blue

Reduces Light Output

100% light output at 25oC

70C

90%

• AlInGaP: Red, Red-Orange, Amber

• InGaN: Royal-Blue, Blue, Green, Cyan, White

More sensitive to heat



Shifts dominant wavelength

Color K (nm/ ºC)

Amber

.09

Red

.03

Blue

.04

Green

.04

Cyan .04

*



Tj

0

50

100

150

200

250

300

350

400

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Forward Voltage (V)

Forw

ard

Cu

rren

t (m

A)

Red, Reddish Orange,

Amber(AlInGaP)

Royal Blue, Blue, Cyan,

Green, White (InGaN)

Vf -2.0 to -4.0mV/°C

LED Driver: Vout= 43-48V

@ 25oC Vf=3.0V

15 LEDs:

→ 15 x 3.0 = 45V

OK!!

@ 87oC Vf=2.85V

15 LEDs:

→ 15 x 2.85 = 42.75V

Not OK!!

Vf



(B50, L70)

What is (B50, L70)?


Lumen Maintenance - (Bxx, Lumen Maintenance - (Bxx, Lyy)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.



Reduces operating life

~50.0%

~155k

~165k

~175k

(B50, L70)



May cause severe damage

Thermal management is critical


Basic cooling considerationsBasic cooling considerations

• Conduction:– The transfer of heat energy

through a substance or from one substance to another due to temperature difference

Radiation

ConductionConvection• Convection:

– The process in which hot air rises and cool air delves down. Hot air will cool down as it flows through the cooler air mass until it reaches equilibrium.

• Radiation:– The transfer of heat via

electromagnetic waves through space


Thermal ManagementThermal 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 SinksHeat 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 SinksHeat 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

Laminar Flow Turbulent Flow


Thermal Resistance RThermal Resistance RTHTH

• Thermal resistance describes how much that material resists the flow of heat through it

— Units: oC/W or oK/W

• It changes with the material type, thickness, surface area, and power (number of LEDs)

• We want this number to be as low as possible to make sure heat flows easily from one point to another


Thermal Resistance RThermal Resistance RTHTH

LED thermal resistance: RTH

junction to slug

Board thermal resistance: RTH

board

Thermal interface material thermal resistance: RTH thermal interface

Heat sink thermal resistance: RTH heatsink

RRTHTH = R TH board+ R TH thermal interface + R TH heat sink+R TH junction to slug

+

+

+


Thermal Conductivity (k)Thermal Conductivity (k)

Thermal Conductivity Units are in W/mK.

• The measure of a material’s ability to conduct heat (W/mK)




Case Study Case Study


Scenarios:

Scenario A: Passive Cooling

• Open Frame

• Closed Fixture

Scenario B: Active Cooling

• Open Frame

• Closed Fixture


QLED – QLED – Thermal SimulationThermal 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?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?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 sinkto

Multiple LEDs on a custom made board within an enclosed space or casing with active cooling


Benefits of Using QLEDBenefits of Using QLED

1. It minimizes the number of design cycles, reduces development costs, and decreases time to market

Concept Prototype Testing Product

QLEDConcept ProductPrototype Testing


2. Simple user interface

Benefits of Using QLEDBenefits of Using QLED

Main Toolbar

Simulation Manager

3D Toolbar

Message WindowComponent Toolbar

3D Model View


Key FeaturesKey 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 CapabilitiesQLED Capabilities

• Simulation modes include:

– Steady state: DC current (constant ON)

– Transient: Pulse or strobe LEDs

– Parameterized Trials

– Optimization




Scenario AScenario A

Passive cooling


Fortimo DLM 1100lm Fortimo DLM 1100lm Thermal path basic solutionThermal 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

13

2

3

2


Fortimo DLM 1100lm Fortimo DLM 1100lm Thermal ResistancesThermal Resistances

Rth

c-hs

Rth

hs-am

b

Rth

hs-am

bTc

Ths

Tamb

0.2 K/WSide view

Top view

1

3

2

3

3

12

12


Fortimo DLM 1100lm Fortimo DLM 1100lm Thermal Thermal resistance of heat sinkresistance 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 FrameThermal Simulation – Open Frame

• Ambient = 35oC• Tc ≈ 62oC• Matches the theoretical calculations

LEDs junction temp.

Tc


Thermal Simulation – Closed Thermal Simulation – Closed FixtureFixture

• Tc = 90oC• Exceed the max.

Tc• Thermal design

must be modified

• Fully enclosed can (air tight)• No vents for air to go in and

out• Steel Fixture

Tc


Solutions? – larger heat sinkSolutions? – larger heat sink

• Larger heat sinks:– Tripled the heat

sink height

• Tc ≈ 73oC• We still need to

lower Tc to 65oC Tc


Solutions? – larger heat sinkSolutions? – larger heat sink

• Fins extended to touch the fixture

• Tc ≈ 59oCTc


Solutions? – vented fixtureSolutions? – 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

Tc




Scenario BScenario B

Active Cooling


Nuventix – Open FrameNuventix – Open Frame

• Each setting has a thermal resistance depending on the performance setting


Nuventix – Open FrameNuventix – 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 FixtureNuventix – Closed Fixture

• Experimental testing

• SynJet to be modeled in QLED

Conception et réalisation thermique

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