basics of heat transfer: led fundamentals
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LED Fundamentals
Basics of Heat T fTransfer
08-16-2011
Cooling of LEDsHeat Path through a systemg y
Heat Sink Housing AmbientPCB
Heat tracks out to the ambient
Component Heat Sink Housing AmbientPCB
LED Fundamentals | Basics of Heat Transfer | Page 2
Basics of heat transferHeat transfer occurs when two surfaces have different temperatures, thus causing heat energy eat t a s e occu s e t o su aces a e d e e t te pe atu es, t us caus g eat e e gyto transfer from the hotter surface to the colder surface.
In the simplest of terms, the discipline of heat transfer is concerned with two things:H t Fl Heat Flowrepresents the movement of thermal energy from one place to another
Temperaturerepresents the amount of thermal energy availablep gy
[J]HeatTotal amount of energy transferred
Q
[W] = [Js-1]Heat Flow (Heat Transfer Rate)Heat Transfer per unit time
dQQdt
[Wm-2]Heat FluxHeat Transfer Rate per unit area
2d Qqdt dA
LED Fundamentals | Basics of Heat Transfer | Page 3
Modes of Heat Transfer
Heat Transfer
Conduction Convection Radiation
v
e-m
m
e-
Energy transfer through direct molecular collisions (phonon transfer) and by conduction band electrons
Heat transfer by liquid motion. The fluid motion may be caused by density differences (free convection) or external mechanical
Heat Transfer through electro-magnetic radiation
forces (forced convection).
Within solids the heat transfer is done by heat conduction. In gases (air) the heat transfer is dominated by convection and radiation.
LED Fundamentals | Basics of Heat Transfer | Page 4
heat transfer is dominated by convection and radiation.
Heat Transfer in a thermal system
Conduction
Convection
Radiation
Heat Transfer from the die to the surrounding environment by conduction, convection and radiation.
LED Fundamentals | Basics of Heat Transfer | Page 5
Fundamental law of heat conductionFourier Law
When there exists a temperature gradient within a body, heat energy will flow from the region of high temperature to the region of low temperature.
TT L
TTAQ 21 Q dTA dx
The heat transfer rate (dQ/dt) is directly proportional to ( ) y p pthe area A and to the temperature difference along the path of heat flow.
Proportionality Ratio: thermal conductivity i t i l t
LED Fundamentals | Basics of Heat Transfer | Page 6
is a material property
Thermal ConductivityThermal Conductivity of typical materials within LED-Systemsy yp y
Air0.026
Aluminum Alloy120 180
Aluminum200
PC0 2H i 120 - 1800.2Housing
SAC CopperFR4
MCPCB2.2
PSA0.2
SAC60
Copper385
FR40.3
PI0.2
Thermal Adhesive1
Printed Circuit Board
Premold
Resin0.20
Al O
Germanium60
Copper Alloy
AlN170
LED Package
0.01 0.1 1 10 100 1000Th l C d ti it [W 1K 1]
Premold0.25
Al2O318 - 25
Copper Alloy280 - 350
LED Package
LED Fundamentals | Basics of Heat Transfer | Page 7
Thermal Conductivity [Wm-1K-1]
Electrical Analogy for 1D Heat ConductionBy comparing the steady state heat flow equation with Ohm’s Law for current flow through a resistor,
Fourier Law Ohm‘s Law
y co pa g t e steady state eat o equat o t O s a o cu e t o t oug a es sto ,we see that they have similar forms:
)( 21 TTL
AQ
)(1
21 VVR
I
Definition of Thermal Resistance Rth
Valid for steady state one dimensional heat transfer The thermal resistance isn‘t a real physical value. It is
only a tool in the field of thermal engineering.TTLRth
)( 21 only a tool in the field of thermal engineering. In reality heat flow and temperature distribution are 3-
dimensional problems
QAth
LED Fundamentals | Basics of Heat Transfer | Page 8
Heat Generation of LEDs
optPelPheatPLEDQ
elLEDopt PP FFel IVP
elLEDopt
elLEDheat PP 1
LED = optical efficiency of the LED
VF(T), LED(T), Popt(T) Pheat(T)
Rule of Thumb for white LEDsTemperature Dependencies
Approximation: 100 lm ~ 320 mW
LED Fundamentals | Basics of Heat Transfer | Page 9
General Resistor Networks1-dimensional heat conduction
Serial Configuration Parallel Configuration
Rth,1
QRth,1
Rth,2
QRth,2
Rth,3
Rth,3
i
i,thser,th RR i i,thpar,th R
1R
1
LED Fundamentals | Basics of Heat Transfer | Page 10
Simplified Thermal Resistor Networkfor a LED System
TJunction
Rth JS Component
y
TSolder Point
Rth SB SubstrateTechnology
Aluminium PlateTBoard
Rth BA CoolingSystem
Technology
Tambient
System
Thermal Resistor NetworkThermal System Configuration Thermal Resistor Network
LED Fundamentals | Basics of Heat Transfer | Page 11
Disclaimer
All information contained in this document has been checked with the greatest care. OSRAM Opto Semiconductors GmbH can however, not be made liable for any damage that occurs in connection with the use of these contents.
OSRAM Opto Semiconductor GmbH makes no representations and warranties as to a possible interference with third parties' intellectual property rights in view of products
i i ti f f OSRAM O t S i d t G bH' t i i foriginating from one of OSRAM Opto Semiconductor GmbH's partners, or in view of products being a combination of an OSRAM Opto Semiconductor GmbH's product and a product of one of OSRAM Opto Semiconductor GmbH's partners. Furthermore, OSRAM Opto Semiconductors GmbH cannot be made liable for any damage that occurs in p y gconnection with the use of a product of one of OSRAM Opto Semiconductor GmbH's partners, or with the use of a combination of an OSRAM Opto Semiconductor GmbH's product and a product of one of OSRAM Opto Semiconductor GmbH's partners.
LED Fundamentals | Basics of Heat Transfer | Page 12
DisclaimerThis document constitutes neither an offer to sell nor a solicitation to buy or subscribefor securities. Any such offer will be made solely on the basis of the SecuritiesProspectus yet to be approved by the German Financial Supervisory Authority(BaFin) and published thereafter. The information legally required to be provided toinvestors will be contained only in the Securities Prospectus.The information contained herein is not for distribution, directly or indirectly, in orinto the United States of America (including its territories and possessions of anyS f h U i d S f A i h Di i f C l bi ) d bState of the United States of America or the District of Columbia) and must not bedistributed to U.S. persons (as defined in Regulation S under the U.S. Securities Actof 1933, as amended ("Securities Act")) or publications with a general circulation inthe United States of America This doc ment is not an offer of sec rities for sale in thethe United States of America. This document is not an offer of securities for sale in theUnited States of America. The securities have not been and will not be registered underthe Securities Act and may not be offered or sold in the United States of Americaabsent registration or an exemption from registration under the Securities Act Theabsent registration or an exemption from registration under the Securities Act. TheIssuer does not intend to register any portion of the offering in the United States ofAmerica or to conduct a public offering of the securities in the United States of America.This document is not an offer of securities for sale in the United Kingdom Canada
LED Fundamentals | Basics of Heat Transfer | Page 13
This document is not an offer of securities for sale in the United Kingdom, Canada,Japan or Australia.
Thank you for your attention.Thank you for your attention.
LED Fundamentals | Basics of Heat Transfer | Page 14
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