Thermal management
for high power LEDs
M. Scheubeck / C. Rafael / H. Varga,
6.06.2014, Istanbul
Agenda
Thermal considerations for high power LEDs
1
Thermal characteristics of LEDs
2
PCB technologies and PCB layouts for OSLON SSL / Square
3
Estimation of junction temperature
4
Practical examples
High Power LED – Thermal Management | 6.06.2014| Page 2
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Energy conversion
Incandescent bulb
LED*
8%
30%
19%
Visible light
IR
Light
Heat
Light
Tj ~ 60 -120°C
* Varies depending on LED efficacy.
High Power LED – Thermal Management | 6.06.2014| Page 3
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
73%
Heat
Heat
70%
T ~ 3000 K
Light output
decreases with higher junction temperature
Data sheet LCW CQAR.EC
Effect in the design (3000K)
Luminous flux [lm]
400
300
250 lm
238 lm
200
100
TJ = 25°C
TJ = 85°C
0
LCW CQAR.EC: 250 lm @ 700 mA
High Power LED – Thermal Management | 6.06.2014| Page 4
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Chromaticity shift
shift to lower values with higher junction temperatures
Data Sheet LCW CQAR.EC
Effect in the design
Cy
Tj = 25˚C
Tj = 85˚C
Cx
High Power LED – Thermal Management | 6.06.2014| Page 5
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Dominant wavelength
increases with higher junction temperature
Data Sheet LA CPDP
Effect in the design
TJ= 85°C: Ddom = 4nm
~20% radiant power
loss at TJ=120°C
-20%
~60% loss caused by
LDom displacement
0.8 * 0.4 = 0.32  32%
lum. flux at 120°C
-60%
32%ΦV@120°C
High Power LED – Thermal Management | 6.06.2014| Page 6
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Forward voltage
drops down with higher junction temperatures
Data Sheet LCW CQ7P.PC
Effect in the design
LCW CQAR.EC
 IF = 700mA
 typ. VF = 3.05 V
TJ= 85°C: DVF = -0.11 V
typ. VF (700 mA; TJ = 85°C) = 2.94 V
High Power LED – Thermal Management | 6.06.2014| Page 7
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Lifetime
Θv(T,t)
Higher temperatures
accelerate the degradation of an
LED
For maximum lifetime Tj should be
as low as possible!
But:
Good LEDs still have a design life
of >50 000h at Tj=100°C
High Power LED – Thermal Management | 6.06.2014| Page 8
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Example of life time testing according to LM80 (necessary
for Energy Star)
OSLON SSL LUW CQ7P
High Power LED – Thermal Management | 6.06.2014| Page 9
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Agenda
Thermal considerations for high power LEDs
1
Thermal characteristics of LEDs
2
PCB technologies and PCB layouts for OSLON SSL / Square
3
Estimation of junction temperature
4
Practical examples
High Power LED – Thermal Management | 6.06.2014| Page 10
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Network of thermal resistances
Overview
TJunction
Rth JS
TSolderpoint
Rth SB
TBoard
Rth BC
TCase
Rth CA
TAmbient
High Power LED – Thermal Management | 6.06.2014| Page 11
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Thermal resistance
Small cross-sectional area
Large cross-sectional area
Q
Q
Rth 
thickness
area * thermal conductivity
High Power LED – Thermal Management | 6.06.2014| Page 12
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Larger area  lower resistance
Thermal conductivity
Thermal conductivity of typical materials within LED-Systems
Air
0.026
Aluminum
200
PC
0.2
Housing
PSA
0.2
Printed Circuit Board
Aluminum Alloy
120 - 180
FR4
0.3
PI
0.2
MCPCB
2.2
SAC
60
Thermal Adhesive
1
Resin
0.20
LED Package
0.01
Germanium
60
Premold
0.25
0.1
Al2O3
10 - 25
1
High Power LED – Thermal Management | 6.06.2014| Page 13
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Copper
385
10
AlN
170
Copper Alloy
280 - 350
100
1000
Thermal conductivity  [Wm-1K-1]
PCBs: Base materials
Isolated Metal Sheet (IMS)
Info
Pro:
Thermal simulation
- high thermal conductivity
- widely used
Con:
- high thermal expansion
The thermal performance is strongly
influenced by the used dielectric!
Dielectrics diversify in thickness from 35µm
to more than 150µm and in thermal
conductivity from 0,3W/mK to 10W/mK.
Typical thermal conductivity is about
1W/mK to 2W/mK
High Power LED – Thermal Management | 6.06.2014| Page 14
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Dielectric: 2W/mK
100μm
 Rth S-B = 6 K/W
Dielectric: 4W/mK 35 μm
 Rth S-B = 2,5 K/W
PCBs: Base materials
Ceramic PCB
Info
Pro:
- electrically isolating
- good thermal conductivity
- matched thermal expansion to
ceramic LED
Con:
- brittle
- expensive
Thermal simulation
Al2O3: Rth S-B = 3,6 K/W
No dielectric necessary! Circuit path is
directly attached to the ceramic.
Commonly used ceramics:
Al2O3, ~20W/mK,
AlN,
~180W/mK,
High Power LED – Thermal Management | 6.06.2014| Page 15
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
AlN: Rth S-B = 0,9 K/W
PCBs: Base materials
FR4
Info
Pro:
Thermal simulation
- electrically isolating
- widely-used
- cost-effective
-electrical & thermal vias
Con:
- very bad thermal conductivity
Rth S-B = ~ 42K/W
High Power LED – Thermal Management | 6.06.2014| Page 16
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
PCBs: Base materials
FR4 – thermal improvements by layout (0,8mm FR4)
35µm copper layer
Rth S-B = 42 K/W
35µm copper layer
Rth S-B = 37 K/W
High Power LED – Thermal Management | 6.06.2014| Page 17
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
70µm copper layer
Rth S-B = 31 K/W
PCBs: Base materials
FR4 – thermal optimization
effective thermal conductivity
(0,8mm FR4 board, 25µm Cu plating in vias)
 Thermal Vias
thermal conductivity [W/mK]
140,0
The effective vertical
thermal conductivity is
influenced by density and
diameter of the thermal
vias!
via diameter
0,1mm
0,2mm
0,3mm
0,4mm
0,5mm
0,7mm
1,0mm
120,0
100,0
80,0
60,0
40,0
20,0
-
High Power LED – Thermal Management | 6.06.2014| Page 18
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
0,2
0,4
0,6
via pitch [mm]
0,8
1,0
FR4 – thermal improvements
Variants with thermal vias (ø300µm, pitch 700µm, 25µm plating, 0,8mm FR4)
35µm
35µm
35µm
Rth S-B = 29 K/W
Rth S-B = 42 K/W
35µm
70µm
Rth S-B = 25 K/W
High Power LED – Thermal Management | 6.06.2014| Page 19
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Rth S-B = 26 K/W
70µm
Rth S-B = 17 K/W
Rth S-B = 8 K/W
FR4 – thermal improvements
Variants with capped thermal vias (ø300µm, pitch 700µm 25µm plating)
70µm
35µm
Rth S-B = 17 K/W
Rth S-B = 25 K/W
Capped vias:
70µm
Rth S-B = 8 K/W
70µm
35µm
filled with epoxy
(complex and expensive)
Rth S-B = 14 K/W
High Power LED – Thermal Management | 6.06.2014| Page 20
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Rth S-B = 8 K/W
PCB: Base Materials
Overview
Base Material
FR4
Aluminium IMS
Copper IMS
Pro
Widely-used
Con
Low thermal conductivity
Rth- Range
20 .. 50 K/W
Cost effective
Thermal improvement adds costs
5 .. 20 K/W
Widely-used
High thermal expansion
2 .. 15 K/W
High thermal conductivity
Costs
Very high thermal conductivity
High costs
1 .. 10 K/W
Thermal expansion
Aluminium oxide
ceramic (Al2O3)
Electrical isolating, thermal expansion High costs
matched to ceramic LEDs
Brittl
3 .. 5 K/W
Aluminium nitride
ceramic (AlN)
High thermal conductivity, electrical Very high costs
isolation, thermal expansion matched Brittl
to ceramic LEDs
Flexible
Bending could be dangerous to
solder joint
1 .. 2 K/W
Flexboard
(similar to FR4)
20 .. 30 K/W
Referenzlayout: OSLON SSL
High Power LED – Thermal Management | 6.06.2014| Page 21
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
PCB base materials: Comparison in price
5,00
4,50
Price index
4,00
3,50
3,00
2,50
2,00
1,50
1,00
0,50
0,00
FR4
IMS
0,9W/mK
IMS
1,3W/mK
IMS 3W/mK
PCB material
High Power LED – Thermal Management | 6.06.2014| Page 22
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Al2O3
AlN
Heatsink
Rth =16,3 K/W
Rth =19,5 K/W
Rth =15 K/W
Rth =17,3 K/W
High Power LED – Thermal Management | 6.06.2014| Page 23
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Agenda
Thermal considerations for high power LEDs
1
Thermal characteristics of LEDs
2
PCB technologies and PCB layouts for OSLON SSL / Square
3
Estimation of junction temperature
4
Practical examples
High Power LED – Thermal Management | 6.06.2014| Page 24
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Definition of Rth
~30%
Important for “high power” LEDs:
Consideration of radiated power
Light e
Pel
Q
Real thermal resistance
RthP1P2  real 
T1  T2
Pel   e
RthJS-real OSLON SSL ~ 10 K/W
High Power LED – Thermal Management | 6.06.2014| Page 25
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
~70%
Electrical thermal resistance
RthP1P2 el 
T1  T2
Pel
RthJS-el OSLON SSL ~ 6..7,5 K/W
Estimating the junction temperature
T junction
 Q  Rth _ real  Ts
Tjunction  Pel  Rth _ el  Ts
High Power LED – Thermal Management | 6.06.2014| Page 26
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Q  Pel   e
Measuring Ts of an OSLON LED
location for Ts measurement
High Power LED – Thermal Management | 6.06.2014| Page 27
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Measuring Ts of an OSLON LED
OSLON with glued thermocouple (TC)
High Power LED – Thermal Management | 6.06.2014| Page 28
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Measuring Ts of an OSLON LED
Ts measurement with IR camera
TC measurement: 28,7°C
IR camera measurement: 28,9°C
(at cross, (e = 0,9))
In the flat
region(green)
the temperature
of the silicone is
equivalent to Ts
Flir i60
High Power LED – Thermal Management | 6.06.2014| Page 29
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Accesories
 As thermocouple we use Omega 5TC-TT-KI-36– 0,13mm wire diameter
http://www.omega.com/pptst/5TC.html
 As adhesive we use Arctic Silver Thermal Adhesive:
http://www.arcticsilver.com/arctic_silver_thermal_adhesive.htm
High Power LED – Thermal Management | 6.06.2014| Page 30
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Agenda
Thermal considerations for high power LEDs
1
Thermal characteristics of LEDs
2
PCB technologies and PCB layouts for OSLON SSL / Square
3
Estimation of junction temperature
4
Practical examples
High Power LED – Thermal Management | 6.06.2014| Page 31
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Temperature compensation for Brilliant Mix
Visual Impression of CCT
t = 0 min
Visible
Visible
Visible
Color
Color
Color
shift
shift
shift
Visual Impression of CCT
t = 30 min
Brilliant Mix WITHOUT
Brilliant Mix WITH
Phosphor only
Temperature Compensation
Temperature Compensation
Warm White LED
Compared to a single phosphor warm white LED the Brillitant Mix without
temperature compensation has a significant and visible change in CCT!
High Power LED – Thermal Management | 6.06.2014| Page 32
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Brilliant Mix basics
Mixing of EQ White and amber LEDs enables warm white light sources.
MW
0,510
MU
MS
MT
MQ
MR
0,460
MN
MM
Cy
MJ
MK
MH
MI
0,410
MF
M9
2700K
MG
3000K
3500K
ME
MD
0,360
E. g. Target Color
Point at 2700 K
Warm-white
MP
ML
MC
MB
EQ White Groups
(operating temperatures)
MV
Amber
LED
4000K
MA
M8
Cx
High Power LED – Thermal Management | 6.06.2014| Page 33
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
0,650
0,600
0,550
0,500
0,450
0,400
0,350
0,300
0,310
Main reasons for color shifts (Lum. flux ratio)
Different LED technology and behavior
InGaN / UX3
High Power LED – Thermal Management | 6.06.2014| Page 34
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
InGaAlP
Brilliant Mix simulation and design in SPICE
Without Temp. Comp.
With Temp. Comp.
High Power LED – Thermal Management | 6.06.2014| Page 35
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Temperature in Volt  °C
Brilliant Mix simulation and design in SPICE
CIE x
w/o Temp. Comp.
CIE y
CIE x
With Temp. Comp.
CIE y
High Power LED – Thermal Management | 6.06.2014| Page 36
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
Temperature in Volt  °C
CCT shift (Monte Carlo Analysis)
Tolerances
Resistors: ± 5 %
With temperature compensation
Without temperature compensation
High Power LED – Thermal Management | 6.06.2014| Page 37
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
NTC:
± 3%
Φv LED:
± 5%
Color shift over time
Due to the temperature compensation circuitry, the CCT shift versus
temperature is significantly lower and in the same range like for white
LEDs!
CCT in K
CCT Variation of Bulb Retrofit
3000
2950
2900
2850
2800
2750
2700
2650
2600
2550
2500
2450
2400
2350
2300
2250
2200
2150
2100
2050
2000
Brilliant Mix with Temperature Compensation
Brilliant Mix without Temperature Compensation
LCW CQDP.EC 6T
0
200
400
600
800
1000
Time in s
High Power LED – Thermal Management | 6.06.2014| Page 38
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
1200
1400
1600
1800
2000
A) Temperature compensation for Brilliant Mix
Luminous flux versus time
The drop in luminous flux of a temperature compensated brilliant mix
setup is similar to a white LED
Flux Variation of Bulb Retrofit
100%
95%
Relative Luminous Flux in %
90%
85%
80%
75%
70%
65%
Brilliant Mix with Temperature Compensation
60%
Brilliant Mix without Temperature Compensation
55%
LCW CQDP.EC 6T
50%
0
200
400
600
800
1000
Time in s
High Power LED – Thermal Management | 6.06.2014| Page 39
OS SSL AE | Manfred Scheubeck / Christine Rafael / Horst Varga
1200
1400
1600
1800
2000