Integrated Automotive High-Power
LED-Lighting Systems
in 3D-MID Technology
Integrated Automotive High-Power
LED-Lighting Systems
in 3D-MID Technology
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof.ir. K.C.A.M. Luyben; voorzitter van het College voor Promoties,
in het openbaar te verdedigen op maandag 10 maart 2014 om 10.00 uur door
Werner THOMAS
Diplom-Ingenieur (FH), Ingolstadt University of Applied Sciences geboren te Kösching, Germany
Prof.dr. J. Pforr
Samenstelling promotiecommissie: Rector Magnificus, voorzitter
Prof.dr.eng. J.A. Ferreira, Technische Universiteit Delft, promotor
Prof.dr. J. Pforr, Ingolstadt University of Applied Sciences, Germany, promotor Prof.dr. J.A. Cobos, Universidad Politécnica de Madrid, Spain
Prof.dr. techn. N. Seliger, Rosenheim University of Applied Sciences, Germany Prof.dr. G.Q. Zhang, Technische Universiteit Delft
Prof.ir. L. van der Sluis, Technische Universiteit Delft Dr. J. Popoviü-Gerber, Technische Universiteit Delft
Bibliografische Information der Deutschen Nationalbibliothek Die Deutsche Nationalbibliothek verzeichnet diese Publikation in der
Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über http://dnb.d-nb.de abrufbar.
1. Aufl. - Göttingen : Cuvillier, 2014 Zugl.: (TU) Delft, Univ., Diss., 2014 978-3-95404-643-0
© CUVILLIER VERLAG, Göttingen 2014 Nonnenstieg 8, 37075 Göttingen
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1. Auflage, 2014
Gedruckt auf umweltfreundlichem, säurefreiem Papier aus nachhaltiger Forstwirtschaft. 978-3-95404-643-0
Acknowledgements
The research presented in this thesis has been performed at the Institute of Applied Research at Ingolstadt University of Applied Sciences. The work has been carried out in cooperation with the Electrical Power Processing (EPP) group at the Delft University of Technology. Over the past years, many people have contributed to the thesis either directly or indirectly. I would like to take this opportunity to thank those involved.
I am very grateful to my promotor, Professor Braham Ferreira, for giving me the opportunity to do my Ph.D. in his research group, for his support and his constructive comments to my work and to the thesis.
I would like to express my gratitude to Professor Johannes Pforr, for giving me the possibility to work in the field of power electronics and LED-lighting systems. Thank you for the tireless engagement in reviewing the publications, the endless discussions and for the guidance throughout my five years at the institute.
I would like to thank my daily supervisor Dr. Jelena Popović-Gerber for the support and discussions during writing the thesis as well as for being “my place to go” in Delft for all my questions independently of orginsational or technical nature.
Sincere thanks goes to the AUDI AG for supporting my research project. Especially, I would like to thank Stephan Berlitz, head of development lighting functions and innovations, for his continuous support and for giving me the possibility to carry out research in the field of Solid-State-Lighting.
I would like to thank my Ph.D. commission members: Professor J.A. Cobos, Professor N. Seliger, Professor L. van der Sluis and Professor G.Q. Zhang for the time and effort they spent reading my thesis, for their comments and suggestions.
Special thanks go to Ivan Josifovic for helping me through the organisational “paperwork” and to Martin van der Geest for translating the summary into Dutch.
I would like to thank Thomas Baier, my contact person at AUDI AG for the help and the great collaboration, especially in the first part of the project.
I am very grateful to my former colleagues at Ingolstadt University of Applied Sciences, especially Christian Augustin, Roland Cziezior, Thomas Hackner and Michael Stadler for the technical discussions and for making the time enjoyable. In particular, I would like to thank Sebastian Utz for “enduring” five years in the same office, all the discussions, countless night shifts in the lab or in the office and for being such a good friend.
I am deeply grateful to my parents and my sister for their love, for being there when ever needed and for their boundless support during my entire life. Thank you so much.
Most of all, I would like to thank my “better half” Anja for everything you did and meant to me in the last six years; for always being there, for enduring my unavailability, for bringing so much joy in my life and for always believing in me.
Table of content
List of symbols ... xiii
1. Introduction ... 1
1.1. Background ... 1
1.2. Applications of LED-lighting ... 2
1.2.1. Automotive lighting... 2
1.2.2. General lighting and consumer electronics ... 5
1.3. Requirements on three-dimensional LED-lighting systems ... 6
1.4. Problem description ... 8
1.4.1. Derived objectives ... 9
1.5. Thesis layout ... 9
2. Overview of three-dimensional LED-lighting systems ... 13
2.1. Introduction ... 13
2.2. LED-lighting systems: components and functions ... 13
2.2.1. Light-Emitting-Diodes (LEDs) ... 14
2.2.2. LED-driver ... 16
2.2.3. External thermal management components ... 19
2.2.4. Circuit carrier technology ... 20
2.3. Evolution towards three-dimensional LED-lighting ... 21
2.3.1. Printed Circuit Board (PCB)-based assemblies ... 21
2.3.2. Insulated Metal Substrate (IMS)-based assemblies ... 24
2.3.3. Flexible Printed Circuit Board (Flex-PCB)-based assemblies ... 26
2.3.4. 3D-Moulded Interconnect Device (3D-MID)-based assemblies ... 27
2.4. Requirements on future 3D LED-lighting systems ... 32
2.4.1. Conclusions on evolution towards three-dimensional LED-lighting ... 32
2.4.2. Requirements for future 3D LED-lighting systems ... 32
2.5. Summary ... 33
3. Enabling 3D-MID-based high-power LED-lighting systems ... 39
3.1. Introduction ... 39
3.2. 3D-MID-based high-power LED-lighting systems ... 39
3.2.1. Concept idea ... 40
3.2.2. Concept challenges ... 41
3.3. Making 3D-MID-based high-power LED-lighting possible ... 43
3.3.1. LED-driver topologies ... 44
3.3.3. Thermal management design ... 45
3.3.4. Interrelation between domains ... 45
3.3.5. Spatial configurations ... 46
3.4. Summary ... 47
4. Integration of LED-driver functions ... 51
4.1. Introduction ... 51
4.2. Survey of LED-drivers for application on 3D-MIDs ... 52
4.2.1. Series LED-structures ... 52
4.2.2. Multiple power converters and converter-cells ... 53
4.2.3. Parallel LED-structures ... 54
4.2.4. Summary ... 59
4.3. Development of inductive current balancing technique for high-power LEDs ... 60
4.3.1. Basic idea and operation principle... 60
4.3.2. Parallel input-structures ... 63
4.3.3. Series input-structures ... 66
4.3.4. Comparison of equal power and equal current LED operation ... 69
4.3.5. Experimental verification ... 72
4.4. Compensation of increased LED-tolerances and of LED failures... 75
4.4.1. Basic idea ... 76
4.4.2. Analysis of current balancing behaviour ... 77
4.4.3. Operation with LED failures ... 82
4.4.4. Experimental verification ... 84
4.5. Integration of external PWM dimming ... 87
4.5.1. Basic idea and operation principle... 87
4.5.2. Investigation of dimming related colour shift ... 88
4.5.3. Converter design for modulated dimming ... 90
4.5.4. Experimental verification ... 95
4.6. Overview of developed LED-drivers for 3D-MID application ... 97
4.7. Summary ... 98
5. Integration of spatial and electrical functions... 105
5.1. Introduction ... 105
5.2. Comparison of 3D-MID and PCB construction ... 106
5.2.1. Substrate technology ... 107
5.2.2. Circuit artwork assembly ... 109
5.2.3. Via interconnection technology ... 112
Table of content xi
5.3. Contacting challenges of components ... 117
5.3.1. Current carrying capacity of 3D-MID circuit tracks ... 117
5.3.2. Integration potential of passive components with 3D-MIDs ... 123
5.3.3. Circuit trace parasitics ... 125
5.4. Spatial- and electrical performance of 3D-MID-based power converters ... 133
5.4.1. Case study introduction ... 133
5.4.2. Spatial performance ... 136
5.4.3. Electrical performance ... 142
5.4.4. Estimation of power limits of 3D-MID-based power electronics ... 150
5.5. 3D-routing possibilities and concepts ... 156
5.5.1. Increasing the circuit- and the component-density ... 157
5.5.2. Integration of additional functions ... 158
5.5.3. Summary on 3D-routing ... 159
5.6. Summary ... 161
6. Integration of thermal management functions ... 167
6.1. Introduction ... 167
6.2. Identification of dominant heat transfer modes in LED-lighting systems ... 167
6.2.1. LED-driver components ... 168
6.2.2. High-power LEDs ... 169
6.2.3. 3D-MID challenges ... 169
6.3. Converter level thermal management ... 170
6.3.1. Perpendicular heat transport and heat spreading ... 170
6.3.2. Layout and geometry optimisation ... 177
6.3.3. Integration of extra thermal pathways ... 180
6.3.4. Substrate material modification ... 180
6.3.5. External cooling structures: Heat sinks ... 181
6.3.6. Constraints for 3D-MID-based LED-lighting applications ... 182
6.4. Thermal management with the 3D-MID circuit carrier – a case study ... 183
6.4.1. Design of circuit carrier based thermal management ... 185
6.4.2. Case study implementation ... 185
6.4.3. Case study results ... 191
6.5. Integrated Reflector Heat Sink ... 194
6.5.1. Basic idea ... 195
6.5.2. IRHS construction ... 197
6.5.3. IRHS design ... 198
6.5.4. Implementation – a case study ... 200
6.6. Summary ... 205
7. Case study: 3D-MID-based high-power LED-lighting system ... 211
7.1. Introduction ... 211
7.2. System specification ... 211
7.3. System design ... 212
7.3.1. LED-driver topology selection ... 213
7.3.2. Spatial design and routing ... 215
7.3.3. Thermal management design ... 217
7.4. Realisation of the case study prototype ... 221
7.5. Spatial evaluation ... 224 7.6. Electrical evaluation ... 225 7.6.1. Current-balancing ... 225 7.6.2. LED failures ... 226 7.6.3. Modulated dimming ... 227 7.6.4. LED-driver efficiency ... 228 7.7. Thermal evaluation ... 229 7.7.1. LED-driver ... 229 7.7.2. LED-section ... 231 7.8. Summary ... 231
8. Conclusions and recommendations ... 235
8.1. Summary ... 235
8.2. Conclusions ... 236
8.2.1. Present practice and evolution of (3D) LED-lighting system construction ... 236
8.2.2. 3D-MID technology application to enhance the 3D-design of high-power LED-lighting systems with LED-driver ... 237
8.2.3. Development of adapted LED-driver topologies for 3D-MID realisation ... 238
8.2.4. Influence of 3D-MID usage on electrical and spatial realisation of power electronics238 8.2.5. Thermal management of 3D-MID-based LED-lighting systems ... 239
8.2.6. Thesis contributions... 240
8.3. Recommendations for further research... 241
A. Appendix: Influence of inductor tolerances ... 245
B. Appendix: Loss analysis and -comparison ... 251
C. Appendix: Thermal-modelling and -simulation ... 265
SUMMARY ... 273
SAMENVATTING ... 277
ZUSAMMENFASSUNG ... 281
List of symbols
Latin Letters
A Area [m²]
A Active Source
A Attenuation
Aactive Surface area that contributes to radiation [m²]
Aback Back surface [m²]
Abot Bottom surface [m²]
Achip Chip area [m²]
Afront Front surface [m²]
Aplate Plate area [m²]
Aside Side surface [m²]
Atop Top surface [m²]
B Magnetic flux density [T]
B Width [m]
Bpk Peak magnetic flux density [T]
C Capacitance [F]
Cdg Drain gate capacitance [F]
Cds Drain source capacitance [F]
Cgs Gate source capacitance [F]
Cin Input capacitance [F]
Ciss MOSFET input source capacitance [F]
Cout Output capacitance [F]
CP Heat capacity of the fluid [J/K]
Crss MOSFET output source capacitance [F]
Cx Branch capacitance [F]
ci Inner diameter of vias [m]
cvia_3D-MID Diameter of 3D-MID vias [m]
D Duty cycle [%]
D Diode
Dr Driver
dtrace Distance among circuit traces [m³]
dvia Distance between vias [m]
E Energy [J]
FC Frequency response of C-filter FCL Frequency response of C-L-filter
fe Effective frequency [Hz]
fo Repetition frequency [Hz]
fs Switching frequency [Hz]
fmod Modulation frequency [Hz]
hc Convection based heat transfer coefficient [W/m²K]
hinductance Inductor height [m]
hIRHS Height of Integrated Reflector Heat Sink [m]
hhor Horizontal heat transfer coefficient [W/m²K]
hvert Vertical heat transfer coefficient [W/m²K]
Iavg Average Current [A]
ID Diode Current [A]
Idr Gate driver current [A]
IL Current through inductance [A]
ILED LED current [A]
ILp_x Current through primary winding of transformer x [A] ILs_x Current through secondary winding of transformer x [A]
In Input current [A]
IRMS Root mean square current [A]
J Current Density [A/m²]
K0 Core volume [m³]
K1 AC loss constant
Kf Frequency constant Kb Flux density constant
L Inductance [H]
L Length [m]
LA Parasitic anode inductance [H]
Ldx Parasitic drain inductance [H]
lCu Copper length [m]
lIRHS Length of Integrated Reflector Heat Sink [m]
LK Parasitic cathode inductance [H]
Lin Input inductance [H]
Lloop Parasitic loop inductance [H]
Lo Output inductance [H]
Ls Leakage inductance [H]
Lsx Parasitic source inductance [H]
Lx Branch inductance [H]
M Mutual inductance [H]
M Modulation signal
N Number of windings
nbranch Number of branches
ncell Number of (converter-) cells
nLED Number of LEDs
P Power [W]
Pc Core losses [W]
Pconduction Conduction losses [W]
Pconv Power dissipated by convective heat transfer [W]
PLED LED power [W]
Pload Power of load [W]
Ploss Power loss [W]
Prad Power dissipated by radiative heat transfer [W]
Psw Switching losses [W]
PV Magnetic core loss [W]
Q Heat dissipated by a source [W]
Q MOSFET
q Heat flux [W/m²]
Qgd MOSFET gate charge [C]
R Resistance [Ω]
Rbx Balancing resistance [Ω]
Rac Ac resistance [Ω]
List of symbols xv
Rdc Dc resistance [Ω]
Rel Resistance of circuit trace [Ω]
Rdson On resistance of MOSFET [Ω]
Rg Gate resistance [Ω]
Rgate_dr Internal driver gate resistance [Ω]
Rloop Loop resistance [Ω]
Rm Magnetic resistance [H-1]
Rmax Maximum resistance [Ω]
Rm_air-gap Magnetic resistance of air gap [H-1]
Rm_ferrite Magnetic resistance of ferrite [H-1]
Roper Temperature corrected resistance [Ω]
Rth_Cu Thermal resistance of copper (-layer) [K/W]
Rth_Cu_spread Thermal spreading resistance of copper [K/W]
Rth_interface Thermal resistance of interface material [K/W]
Rth_IRHS Thermal resistance of Integrated Reflector Heat Sink [K/W]
Rth_IRHS_ambient Thermal resistance of Integrated Reflector Heat Sink to ambient [K/W]
Rth_j_c Junction to case thermal resistance [K/W]
Rth_LED Thermal resistance of LED package [K/W]
Rth_perp Perpendicular thermal resistance [K/W]
Rth_sub Thermal resistance of substrate [K/W]
Rth_sub_ambient Thermal resistance of substrate to ambient [K/W]
Rconv Convection resistance [K/W]
Rsp Thermal spreading resistance [K/W]
Rtotal Total resistance [Ω]
Rtot Total thermal resistance [K/W]
sx Edge length [m]
Tambient Ambient temperature [K]
TC Time response C-filter [s]
TCL Time response C-L-filter [s]
tCu Copper thickness [m]
tif Current fall-time [s]
Tj Junction-temperature [K]
toff Off-time [s]
ton On-time [s]
Tmax Maximum temperature [K]
Ts Switching period [s]
tsub Substrate thickness [m]
tvr Voltage rise-time [s]
V Voltage [V]
VCx Voltage of capacitor [V]
VD Diode forward voltage [V]
Ve Core volume [m³]
Vg Gate voltage [V]
Vin Input voltage [V]
VL Voltage across inductance [V]
VLED LED forward voltage [V]
Vo Output voltage [V]
Vout Output voltage [V]
Vplt MOSFET plateau voltage [V]
VRx Voltage of resistor [V]
Vs Voltage across power switch [V]
Vth Threshold voltage of MOSFET [V]
Vx Volume [m³]
wCu Copper width [m]
wIRHS Width of Integrated Reflector Heat Sink [m]
wwindow Width of magnetic core’s winding window [m]
Greek Letters
α Angle [°]
α Aspect ratio
α20 Linear temperature coefficient [1/K]
β Expansion coefficient of the fluid [1/K]
δ Skin depth [m]
ε Permittivity [F/m]
ε Surface emission coefficient
εr Relative permittivity
εr Relative surface emission coefficient
ζrelative Relative volume utilisation factor
η Electrical efficiency [%]
ηLED Electrical LED efficiency [%]
Θ Magnetomotive Force [AT]
Δ I Peak inductor current [A]
Δ ILED_max Maximum branch current deviation [%]
Δ Irel Relative current deviation [%]
Δ Irel_simp Simplified relative current deviation [%]
Δ L Inductance deviation [%]
Δ Lloop Change of parasitic loop inductance [H]
Δ PLED_max Maximum branch power deviation [%]
Δ Prel Relative power deviation [%]
ΔT Temperature increase [K]
ΔTLED Temperature increase of LED chip [K]
ΔT22 Time interval [s]
Δton Rise time [s]
Δtoff Fall time [s]
ΔTtrace Temperature increase of circuit trace [K]
Δ Vds Overvoltage at MOSFET [V]
Δ VLED LED branch voltage deviation [V]
Δ vo Output voltage deviation [V]
Δx Colour shift in x-direction chromaticity diagram CIE 1931 Δy Colour shift in y-direction chromaticity diagram CIE 1931
Φ Magnetic Flux [Wb]
λ Thermal conductivity [W/mK]
λCu Thermal conductivity of copper [W/mK]
λIRHS Thermal conductivity of Integrated Reflector Heat Sink [W/mK]
λsub Thermal conductivity of substrate [W/mK]
μ Absolute viscosity of the fluid [Ns/m²]
μ0 Magnetic permeability of vacuum [H/m]
μr Relative magnetic permeability
List of symbols xvii
ξ Damping ratio
ρ Fluid density [kg/m³]
σ Electrical conductivity [S/m]
σ Stefan-Boltzmann constant [J/K]
tan(δ) Loss tangent
ψcomponent Component volume [m³]
ψtotal_assembly Total assembly volume [m³]
ψunused Total unused volume in the assembly [m³]
ω Angular frequency [rad-1]
ωd Angular frequency diode current [rad-1]
Acronyms
2D Two-dimensional
3D Three-dimensional
3D-MID 3D Moulded Interconnect Device ac Alternating current
CCC Current Carrying Capacity CCD Charge Coupled Device CCM Continuous Conduction Mode CTE Coefficient of Thermal Expansion
dc Direct current
DCM Discontinuous Conduction Mode DRL Daytime Running Light
EMC Electromagnetic Compatibility EMI Electromagnetic Interference FEM Finite Element Modelling IMS Insulated Metal Substrate IRHS Integrated Reflector Heat Sink LCD Liquid Cristal Display
LCP Liquid Crystal Polymer LDS Laser Direct Structuring LED Light Emitting Diode
MOSFET Metal Oxide Field Effect Transistor
PA Polyamide
PC Phosphor coated
PC-ABS Polycarbonate/Acrylonitrile Butadiene Styrene PCB Printed Circuit Board
PEEC Partial Elements Equivalent Circuit PEN Polyethylene Naphthalate
PET Polyethylene Terephthalate
PI Polyimide
PMMA Polymethyl Methacrylate
PPA Polyphtalamide
PWM Pulse Width Modulated
RMS Root Mean Square
SEPIC Single Ended Primary Inductor Converter SMT Surface Mount Technology
SSL Solid State Lighting
1.Introduction
1.1. Background
Rising luminous fluxes of high-power LEDs as well as the growing energy consumption of lighting – 19 percent of the global energy production in 2006 [WT06] – have contributed to the widespread use of LEDs in modern lighting systems [St08]. Today, single LED-packages reach a light output that is competitive or even higher than those of incandescent- and compact-fluorescent-lamps, as illustrated in Figure 1-1. LED-arrays even surpass these values and achieve luminous fluxes of up to 9,000lm [Br11].
Figure 1-1: Total luminous flux of different light-sources derived from [WT06] with updated values for high-power LEDs [Os07], [Os08]
Increasing luminous efficacies, i.e. the emitted luminous flux per watt electrical power consumption, are another important reason for the growth of LED-lighting [Wh057].
The technological progress of LED-lighting can be seen from Figure 1-2, where a comparison of the luminous efficacy of different selected light sources is given. Currently, high-power LEDs achieve a maximum luminous efficacy of up to 157 lm/W, with a mean value of about 75 lm/W considering different power-classes [SSL11]. This is a factor 5 to 10 higher performance when compared to conventional incandescent- or halogen- lamps. Commercially available high-power LEDs can also compete with energy-saving- and fluorescent-lamps in terms of their luminous efficacy.
Furthermore, the general research goal for white high-power LEDs is set to reach 200 lm/W [SSL11] (single-chip LEDs), which is even higher than the efficacy of High-Intensity-Discharge lamps. 0 500 1000 1500 2000 2500 3000 3500 High-power LEDs (single package) (different types) Xenon lamp (low- to high-beam) Compact fluorescent (9-25W) Standard incandescent (40-100W) Luminous flux [lm] Light-Emitting-Diode (single package) (0.01-12W) Xenon lamp (low- to high-beam) Standard incandescent lamp (40-100W) Compact fluorescent (9-25W)
Figure 1-2: Luminous efficacy of selected light-sources derived from [AL03]with updated values for high-power LEDs [Os08], [Os07], [Ph07a], [Cr08b], [SSL11]
Another advantage of LEDs is their long lifetime which can reach a peak value of over 100,000 operating hours [Ph06] at optimal environmental conditions. Due to this, a multitude of lighting applications can be designed without considering maintainability. When combining this feature with the small geometrical dimensions of the LED-chips – a typical chip has an area of ܣ݄ܿ݅=1-2mm² – very compact or thin systems get possible.
These benefits as well as a wide range of available colours make LEDs and especially high-power LEDs the technology of choice for a multitude of applications. Besides, a large variety of customised lighting functions can be fulfilled by Solid-State-Lighting.
1.2. Applications of LED-lighting
LED-lighting applications comprise the automotive-sector, general-lighting and consumer electronics. These will be characterised in the following.
1.2.1. Automotive lighting
In the last years, LED exterior lighting has started to become a prominent innovation in automotive lighting. The beginnings have been already made in the 1990’s with the introduction of the ’third stop-light’ in LED-technology, where the fast turn-on behaviour of LEDs was used to decrease the reaction time of the following drivers [Ve06].
The use of complete LED taillights has been a further step to advance automotive exterior lighting. With the introduction of white light generated by LEDs and rising luminous fluxes, LED-based automotive front-lighting emerged and is already used in insular series applications today.
Besides to increased efficacies and lifetime, other technological benefits have contributed to the success of LED-lighting in automotive applications:
0 50 100 150 200 250
Efficacy [lm/W]
Standard incandescent Tungsten halogen
Halogen infrared reflecting Mercury vapor
Compact fluorescent (5-26W)
Compact fluorescent (27-55W) Linear fluorescent
Metal halide Compact metal halide
LED (1-3W) LED (5W)
LED (12-20W)
Introduction 3
x The small footprint and height of LEDs allows new degrees of freedom in placing light elements and allows improved as well as complex three-dimensional lamp designs.
x LED-lighting systems often comprise a multitude of single LEDs, which can be individually arranged or electrically driven to enable new lighting functions highly exceeding the possibilities of conventional single and central light sources.
Figure 1-3 shows several examples of modern automotive LED-lighting systems which already benefit from the flexibility in lamp design obtained by the LED-technology. It is common to these solutions that the LEDs are spatially arranged in three-dimensions (3D) to create a more individual design of the day- and night-appearance of the automobile-front and -rear when compared to conventional halogen- or xenon-lamps. These systems will be called
3D LED-lighting systems throughout this thesis.
Figure 1-3: Trends in automotive exterior lighting: LED-lighting used as recognition feature to stand out from the competition and to differentiate model specific design
The arrangement of the LEDs is particularly used to underline the exterior shape of the automobile. The LED-design therefore provides a diversification in between the model-range of a car manufacturer. Furthermore, it is a recognition feature to stand out from the competitors.
Solutions with (advanced) 3D-designs, however, require a complex assembly to mount and to electrically contact the LEDs in space that also comes at the cost of a large component count (Figure 1-4). Besides, conventional cooling solutions have to be adapted according to the desired 3D-shape. Unfortunately, these aspects limit the design versatility that can be achieved in 3D LED-lighting systems and increase system costs when using state of the art assemblies. Therefore, the majority of contemporary automotive lamp designs are still
(Source: Audi AG)
focused on conventional shapes with single and central light source, as known for the past decades.
Figure 1-4: Full-LED headlamp with limited 3D-design
State of the art solutions for creating three-dimensional LED-lighting systems and their limitations will be discussed in detail in Chapter 2.
Environmental conditions and requirements
Automotive LED-lighting systems often consist of a multitude of LEDs that can have different power levels and colour values to perform lighting functions, like in stop-lights, day-time-running lights or even as low- or high-beam (Figure 1-5). The coloured and white LEDs are often located as clearly visible single light sources.
Figure 1-5: Full LED-headlamp (left) and LED tail-lamp (right)
As the conventional 14V automotive electrical power net has a variable input voltage with typical values of Vin =8V-17V, a stable LED-brightness level has to be achieved over the entire input voltage range. Commonly, switched mode power converters are used to maintain and control the LED-brightness. When a large number of LEDs is used, a uniform brightness distribution is additionally required in order to maintain the required light output distribution as well as for optical reasons. In addition, brightness control is a key requirement in automotive lighting to provide basic lighting functions, e.g. day-time-running-lights are operated at night as position light which requires dimmed LED-operation. Finally, a high
LEDs (behind lenses) Design element Optical system Thermal management (backside) Circuit carrier (backside) (Source: Cadillac) Low Beam: 14 LED-Chips Daytime-Running-Light: 24 LEDs Indicator LEDs High Beam: 2 x 4 LED-Chips
Tail- and stop light: 41 LEDs
Introduction 5
availability of the lighting system is required due to its security-related function. All these attributes define essential lighting functions, which require LED-drivers that are optimised towards automotive requirements.
3D LED-lighting systems further demand on solutions that allow the mechanical- and the electrical-connection of LEDs and LED-driver components in space to achieve the required design and functionality. As constructed space is limited in automobiles, the desired 3D-shapes have to be realised without consuming excess space.
Harsh environmental conditions within the vehicle, with vibrations and shocks, amplify the requirements on the LED-lighting system. Furthermore, the LEDs and LED-driver components are excited to wide temperature ranges of -40ιC to +80°C (up to +120°C in special cases) which requires an effective thermal management to keep component temperatures below critical values (Chapter 2). The heat dissipated by high-power LEDs increases temperature levels above environmental temperatures in the lamps. The electrical system should therefore operate at reduced losses when integrated in the lighting system.
1.2.2. General lighting and consumer electronics
Next to the automotive environment, LED-lighting is increasingly used in general lighting and in consumer electronics. In the latter case, LEDs have been used as signal or control lights for decades, comparably to automotive (stop-) lighting. With rising luminous fluxes, however, also consumer articles emerged that contain high-power LEDs, e.g. in video projectors or in backlights of LCD-displays [Lu09].
The use of Solid-State-Lighting in general lighting has started to grow in the last few years. General lighting is considered in this thesis, as indoor- and outdoor-lighting, e.g. in street-lamps with LEDs that exhibit high luminous fluxes of 2,000-10,000 lumens [Ca09], [PT096], [Ow096]. The resolution of the European Commission for phasing out incandescent lamps and of lamps with a non-tolerable luminous efficacy [Eu08] further accelerated the demand on high-power LEDs as an alternative light-source in general lighting.
Like in automotive applications, in general lighting or in consumer electronics LEDs are not only used for reasons of saving energy and for lifetime considerations. Moreover, there are some applications that benefit from LEDs’ small footprints and the ability to spatially arrange individual light sources. Figure 1-6 (a) shows an LED street lamp with a three-dimensional design as one possible example. In this lamp, the LEDs are used to create a completely new design which allows a diversification among other street lamps, which is comparable to the approaches in the automotive segment. Further, a three-dimensional LED-arrangement can also be used to obtain an improved brightness distribution on the street (Figure 1-6 (b)).
In contrast to the automotive sector, in consumer electronics or in general lighting, there is no general trend towards three-dimensional shaping of lighting systems, due to the wide span of applications, which neither require enhanced shaping possibilities nor need to save construction space.
Figure 1-6: Three-dimensional shaped LED streetlamps
The absence of a simple solution for forming 3D LED-lamps and the high assembly complexity – with a large number of components (Figure 1-6 (b), (c)) – further contributes to the low number of applications, which benefit from a three-dimensional alignment of LEDs up to now.
Environmental conditions and requirements
General lighting solutions often comprise a multitude of high-power LEDs to meet the requirements on high luminous fluxes, as shown in Figure 1-6. Also consumer electronics, like the background illumination of flat-screen TVs, contains a large number of LEDs. A power electronic system is therefore required to maintain required LED-brightness-distribution and -control, e.g. for dynamic background illumination [WKM09]. Further, in 230 VAC mains application systems, galvanic isolation is required to decouple high input voltages from the LEDs. This is especially necessary when a compact lamp-design without extra LED-housing is desired. Hence, a power converter is required to transform high ac voltages into appropriate dc voltages for the LEDs.
Different environmental conditions, e.g. ambient temperatures, have to be considered in domestic applications, depending on indoor- or outdoor usage. In the vast majority of applications, the environmental impacts are significantly lower than in automotive LED-lighting. It is therefore assumed that most of the foregoing environmental conditions are also covered by the demands of automotive exterior lighting.
Thus, the automotive environment, with its conventional 14V automotive electrical power net, will be the considered environment in this thesis. However, special applications suitable for the mains will be commented throughout the work when applicable.
1.3. Requirements on three-dimensional LED-lighting systems
Progresses in the LED technology have led to a variety of applications in automotive- and general-lighting. Especially the field of automotive exterior LED-lighting uses the small footprint of LEDs and their characteristic as point light sources as key features to combine lighting tasks with design (Section 1.2).
(Source: Schréder GmbH)
LED power supply
Additional wiring LED Optical system Printed circuit board Housing Environmental protection Additional wiring Closing clips
(a)
(b)
(c)
(Source: Siteco GmbH)Introduction 7
Although automotive LED-lighting is at the leading edge regarding three-dimensional lighting solutions, the current construction of 3D LED-lighting systems is not optimised towards complex design requirements. Contemporary assemblies require a large number of construction parts to perform 3D-contacting and -mechanical fixation of LEDs and their related power electronic LED-drivers.
Thus, the plurality of automotive LED-lighting systems is still focused on conventional designs of front- and tail-lights, as known for decades. The same limitation has been observed for the majority of LED-based general-lighting systems.
To improve 3D LED-lighting systems, the following target functions can be identified and should be addressed:
x Spatial und mechanical functions:
o The lighting systems must be able to follow complex three-dimensional shapes to further improve the degrees of freedom in the lamp design. For this reason, the LEDs and the power electronics, for their electrical drive, require a 3D-structure which fixes them.
o Due to the requirement of reduced constructed space (automotive), the system should also be able to achieve the required 3D-shape at a minimum of excess volume. Hence, solutions which allow a reduced construction height are desirable.
o The systems should be built at a reduced number of components to reduce efforts in their spatial fixation. Furthermore, they must be robust against application specific environmental conditions, e.g. vibrations or maximum temperatures.
x Electrical functions:
o In 3D LED-lighting systems, the LEDs and the power electronics must be electrically contacted in three dimensions and the appropriate signals have to be delivered to the spatially distributed LEDs.
o The electrical drive has to ensure that a homogeneous brightness distribution is achieved among the LEDs, as they are often clearly visible as single light-sources. Here, LED specific requirements concerning temperature- and electrical-behaviour (Chapter 2 and 4) have to be observed.
o As input-voltage levels can vary, the LED drive has the function to keep stable LED brightness levels over input voltage variations. Furthermore, the power electronics should provide a galvanic isolation when high (input-) voltages appear to separate them from the remaining system.
o (Automotive) illumination requires high system availability. Therefore, the LED-driver should be able to maintain a high operational availability even at LED failures.
x Thermal functions:
o An effective cooling of the used high-power LEDs and the power electronic components, especially for high power levels, is required and has to take care of the demand on high degrees of freedom in 3D-design.
1.4. Problem description
The high versatility in placing individual and compact light sources is a key feature provided by LED-lighting technology. It allows an enhanced design flexibility which can skilfully be used to improve the appearance, but moreover to enhance functions of modern lighting systems, as introduced in Section 1.2. So far, the LED-technology’s potential for improving three-dimensional designs is, however, insufficiently used in the vast majority of LED-lighting applications.
The physical realisation of contemporary LED-lighting systems requires a large number of components and is identified as the main hurdle for limiting the distribution of 3D LEDlighting systems. These systems comprise LEDdrivers, 3Dmounting and -contacting components as well as thermal management structures that have to be mounted and arranged in three-dimensions leading to high efforts and costs (Section 1.2).
A new approach is therefore needed for the realisation of 3D LED-lighting systems to decrease the number of components and to enhance the design versatility. This directly addresses the components which are necessary to fulfil spatial-, electrical- and thermal-functions, defined in Section 1.3.
Determining a new concept for the realisation of 3D LED-lighting systems requires the analysis of the current practice and evolution of 3D LED-lighting assemblies to identify the main technological boundaries as well as to derive requirements on future assemblies.
The main objective in this concept, and hence in this thesis, is to decrease the number of components of contemporary automotive LED-lighting systems and to enhance the design versatility in three-dimensions by integrating the functions provided by individual parts into one or more multifunctional components. This concept requires the investigation of the integration potential for the LED-driver, for the electrical- and spatial-contacting and for the thermal-management in the ‘3D multifunctional-component(s)’.
As a wide range of applications, with different spatial arrangements and power levels, exist for LED-lighting it is further required to provide techniques that derive the limitations and possibilities for the concept’s electrical-, spatial- and thermal-design. These parameters can be used to determine the feasibility of prospective applications and to derive adapted designs.
Introduction 9
1.4.1. Derived objectives
Analysing the foregoing problem description, the main objective of this thesis is to:
decrease the component number required in high-power 3D LED-lighting systems with power electronic LED-driver to reduce the complexity in the assembly and to increase the degrees of freedom in the design.
To achieve this aim the following objectives have to be determined:
x Identification of the main reasons that limit enhanced designs of contemporary high-power LED-lighting systems by analysing the present practice of construction and the evolution towards 3D LED-lighting systems
x Determination of an approach to use the technology of 3D-Moulded Interconnect Devices (3D-MID) for enhancing the 3D-design whilst decreasing the construction complexity of high-power LED-lighting systems with LED-driver by increasing the level of function integration
x Development of optimised LED-drivers with integrated lighting functions for a simplified 3D-MID realisation
x Determination of merits and limitations to mount and contact the LED-driver and the LEDs on the 3D-MID as well as to analyse influences of the 3D-MID technology on the spatial- and electrical- realisation of power-electronics
x Examination of the potential to integrate thermal management functions into the 3D-MID for low complexity systems and to derive a solution to enhance the power level of 3D-MID-based LED-lighting systems whilst maintaining high degrees of freedom
1.5. Thesis layout
Figure 1-7 visualises the layout of the thesis.
Figure 1-7: Thesis layout Chapter 1 Introduction Chapter 2 Overview of 3D LED-lighting systems Chapter 3 Enabling 3D-MID based high-power LED-lighting
systems
Chapter 7 Case Study: 3D-MID based high-power
LED-lighting system Chapter 8 Conclusions and Recommendations
Chapter 5 Integration of spatial and
electrical functions Chapter 6 Integration of Thermal management functions Chapter 4 Integration of LED-driver functions
Bibliography
[AL03] ALG: Advanced Lighting Guidelines. New Buildings Institute Inc, White Salmon, Washington, D.C, 2003.
[Br11] Bridgelux: Bridgelux RS Array Series: Product Data Sheet DS25, 2011.
[Ca09] Cardenas, A.: Operating the City Green Lighting Building the Harmonious Society - DMX Solar LED Street Light Specification-, 2009.
[Cr08b] Cree XLamp MC-E Datasheet, online available at: http://www.cree.com/products/xlamp.asp, 2008.
[Eu08] Phasing out incandescent bulbs in the EU, Technical briefing, 2008.
[Lu09] Lucas, J.: Samsung Begins Producing Ultra-slim, Energy-efficient LCD Panels with Edge-lit LED Backlighting. In Samsung Press Release online available at:
http://www.businesswire.com/news/home/20090326005854/en, 2009. [Os07] Osram Semiconductors: Osram Golden Dragon Datasheet ZW W5SG, 2007. [Os08] Osram Semiconductors: Osram Ostar Datasheet LE UW E3B, 2008.
[Ow096] Owen, B.: Big Apple goes green with LED pilot projects. In LEDs Magazine, 2009. [Ph06] Philips Lumileds: Custom Luxeon Design Guide, Application Brief AB 12, 2006. [Ph07a] Philips Lumileds: Luxeon K2 Datasheet DS51, 2007.
[PT096] Photonics Industry; Technology Development Agency: Taiwan develops LED Cluster in Southern Taiwan Science Park. In LEDs Magazine, 2009; pp. 45–46.
[SSL11] Solid-state lighting research and development. Multi-year program plan. U.S.
Department of Energy, Office of Energy Efficiency and Renewable Energy, [Washington, D.C.], 2011.
[St08] Steel, R.: High-Brightness LED Market Overview & Forecast: Proc. Strategies in Light 2008, 2008.
[Ve06] Verband der Automobilindustrie: Leuchtdioden für mehr Sicherheit und Langlebigkeit. In Auto Jahresbericht, 2006, 1; pp. 145-147.
[Wh057] Whitaker, T.: LEDs in the mainstream: technical hurdles and standardization issues. In LEDs Magazine, 2005; p. 44.
[WKM09] Wonbok, L.; Kimish, P.; Massoud, P.: White LED Backlight Control for Motion Blur Reduction and Power Minimization in Large LCD TVs. In Journal of the Society for Information Display, 2009; pp. 1–18.
[WT06] Waide, P.; Tanishima, S.: Light’s labour’s lost. Policies for energy-efficient lighting; in support of the G8 plan of action. OECD, Paris, 2006.
2.Overview of three-dimensional LED-lighting
systems
2.1. Introduction
Chapter 1 has shown that enabling LED-lighting systems with an enhanced three-dimensional shape for improved design and functionality is the core topic of this thesis. The main limitations of contemporary assemblies – namely the limited degrees of freedom and the multitude of components needed – have been indicated.
In this chapter an overview of the essential components found in state of the art LED-lighting systems is given with a focus on the required power electronic system (Section 2.2). This includes the components’ technological requirements as well as functions that they have to fulfil.
The technical evolution towards three-dimensional LED-lighting is outlined in Section 2.3 and gives an overview of how contemporary LED-lighting assemblies are able to create
3D-structures.
The technological hurdles but also potentials, identified in Section 2.3, are used in Section 2.4 to derive requirements which should be fulfilled for making enhanced three-dimensional design and versatility in future LED-lighting systems possible.
The chapter will be summarised in Section 2.5.
2.2. LED-lighting systems: components and functions
Modern LED-lighting systems comprise a variety of components that are required to fulfil lighting functions, as already introduced in Chapter 1. Figure 2-1 shows an automotive LED-lighting system used in front LED-lighting, visualised in an exploded view. In this system, all light-functions are realised with LED-technology.
It can be seen from the figure that a multitude of LEDs, circuit carriers for their electrical interconnection, external thermal management and optical components as well as several design elements are required to build high-power LED-lighting systems. The housing and the LED-driver complete the system, but are not shown in this view.
In contrast to the vast majority of (power) electronic systems, LED-lighting devices usually contain the LED-driver as well as the load – the LEDs – in the same enclosure and they are often also attached on the same circuit carrier. The electrical and thermal behaviour of the LEDs, e.g. regarding power losses, has therefore to be considered in the design of the power electronic system. Moreover, the planned design – including the number and size of the LEDs – defines the lamps’ power level as well as the component positions in three-dimensional space.
Figure 2-1: Example: exploded view of automotive front LED-lighting system (Source: AUDI AG)
It can also be deduced from Figure 2-1 that a variety of functional interdependencies appear in LED-lighting systems. For example, the electrical system containing the LED-driver, the LEDs and the circuit carrier is mechanically attached to thermal management components. A part of this structure is further connected to the optical system, and so forth. This leads to a complex combination of single components that are often individually constructed; this challenges the assembly of the final three-dimensional LED-lighting system.
In the following, relevant components of LED-lighting systems with LED-driver will be explained to give an overview of their main functions and requirements. The overview includes the LEDs, the LED-driver, the thermal management components as well as the circuit carrier technology. These components build the “electronic system” in the LED-lamp. The housing and the optical components will not be discussed further here, as their implementation is out of the scope of this thesis.
Detailed investigations on each domain of the electronic system will be individually performed in Chapters 4-6 to enhance the degrees of freedom of future 3D LED-lighting systems.
2.2.1. Light-Emitting-Diodes (LEDs)
LEDs play a central role in the realisation and assembly of LED-lighting systems. As Light-Emitting-Diodes and their high-power derivatives show a highly different optical, thermal and electrical behaviour compared to incandescent lights a short summary of state of the art LED characteristics and requirements concerning the power electronic system will be given in the following. LEDs Design element Optical system Thermal management component Circuit carrier
Overview of three-dimensional LED-lighting systems 15
The thesis’ focus lies on white high-power LEDs, as these build the cornerstone for using LEDs in general lighting, e.g. in automotive- or domestic-applications. White light can be created by three different methods using the LED technology [Sc03]:
x Multi-colour LEDs can be used to obtain white light by mixing the emission spectra of the individual LED (-chips).
x Wavelength converters use ultraviolet or blue LEDs attached with several layers of different phosphors. As a result of this combination, white light is excited. For this reason, the LEDs are also-called Phosphor-Coated (PC-) LEDs.
x Semiconductor converters use a primary light source, e.g. a blue LED chip, and an additional active semiconductor region that absorbs a fraction of this optical power and re-emits photons with a longer wavelength. As a composition white light is emitted.
The wavelength-conversion is the most common and widely distributed approach to create white light. This is mainly determined by cost reasons, the simplified drive of single LED-chips and the comparably stable colour values of phosphor-coated LEDs [Sc03], [BSS06]. Hence, general illumination applications and automotive (front) lighting use these types of LEDs and will therefore be focused on in this work.
LED power level
Light-Emitting Diodes are used in a variety of application fields and cover a wide range of light output levels and a variety of colours, as already shown in Chapter 1. Next to the light output, also the power level can be used as criteria to diversify LEDs. Besides it is an important figure to determine the electrical design of the LED-driver as well as for the implementation of an effective thermal management. The latter is linked to the electrical LED-efficiency which shows typical values of 15-20 percent for high power LEDs [QLH09]. The remaining power is dissipated as heat and has to be effectively cooled in the LED-lighting system.
Figure 2-2 gives an overview of common LED power levels with related drive currents, luminous fluxes and typical LED-packages of each power-class. The overview divides the LEDs into three classes that will be referred to subsequently in the thesis:
x Low-power LEDs with a power consumption of below PLED=0.3W
x High-power LEDs comprise power levels of PLED=0.5-3W as a typical indication of size
x Ultra high-power LEDs are mainly realised by combining several high-power LED-chips in a single package, leading to power levels PLED>3W and luminous fluxes of 1000 lumens out of one LED package and higher [Os08].
Figure 2-2: Overview of different LED power levels with selected electrical and photometric properties
Thermal characteristics
Next to the choice of the power class, the design and application of LED-lighting has to focus on LEDs’ thermal characteristics. Although LEDs are inherently robust and long-lasting components [Ph06], one big issue is the influence of the ambient temperature on the LEDs’ light emission and lifetime.
The emission intensity of LEDs decreases with increasing temperature [Sc03]. Furthermore, a gradual reduction in light output during lifetime appears. This is called light output degradation [Ph07b]. Next to the insidious decrease in luminous flux, complete LED failures are possible. LEDs can fail with open circuit, e.g. due to broken bond wires, or with short circuit caused by ’threading dislocations’ or by the ’degraded passivation’ [Ba97], [Wu09], [Ar08]. A comprehensive list with details to the LED failure modes is given in [Ar08].
Further, the dominant wavelength and the overall emission spectrum of LEDs is affected by the LED-junction temperature which causes a shift of colour values [Sc03], [BSS06] which could be perceived by the human eye, e.g. at white Multi-colour LEDs.
Considering rough environmental conditions, e.g. defined by automotive applications with a large temperature range from -40°C to +80°C, temperature cycles, mechanical vibrations and shocks, the ideal lifetime of LEDs will be further decreased and cannot be completely ignored in automotive LED-lighting. Especially the thermal management and the LED-driver have to be designed to ensure high system availability.
2.2.2. LED-driver
Common LED-lighting systems contain a multitude of LEDs that have to be driven in a manner that requirements on light output, light distribution and design are fulfilled. A so-called LED-driver is therefore essential to supply the LEDs with the required drive currents and that takes care of their electrical characteristics (Chapter 4).
Low-power High-power Ultra-high -power
Nominal drive current ILED[mA] 30-50 350-1000 >1000
Typical power consumption PLED [W] <0.3 0.5-3 >12
Luminous Flux ΦV[lm] ≤ 15 ≤ 350 >600
Overview of three-dimensional LED-lighting systems 17
Apart from that the LED-driver must be able to provide the following essential lighting functions:
x Creation of a uniform brightness distribution among a multitude of high-power LEDs x Control of LED brightness with stable colour values
x Compensation of single LED failures
The simplest application of an LED-driver can be seen in a constant voltage source connected to a series resistor which defines the current flowing through a string of LEDs. This system, however, suffers from low efficiency due to losses in the current limiting resistor and makes the system unsuitable for high-power LED applications.
In addition, systems with a variable input voltage, e.g. the automotive electrical power net, require more sophisticated solutions that allow driving the LEDs with a stable brightness level over input voltage changes. Therefore, most drivers for high-power LEDs contain a switched mode dc-dc or ac-dc power converter to drive the LEDs. Especially for high voltage applications, where the LEDs are mounted on the surface and are therefore easy to access, a galvanic isolation should also be provided by the LED-driver to decouple the high input voltages from the LEDs.
Power converter topologies used in LED-drivers
In respective papers, a large number of power converters has already been proposed for driving high-power LEDs. This contains non-isolated topologies which are mainly operated directly from (low-voltage) dc networks. Furthermore, isolated topologies have been used to bring solid-state-lighting into applications operating directly from the 230 VAC mains supply. Figure 2-3 gives a brief overview of published power converters for driving high-power LEDs with an excerpt of respective references. In the field of non-isolated converters, a variety of basic converter topologies have been suggested to act as LED-drivers. This contains boost- [XW08], buck- [Sa08], [Ya09], buck-boost- [TP09b], CUK- [Br08b] and SEPIC- [ZGZ08] converters.
Figure 2-3: Overview of non-resonant power converters used as LED-driver with related references
Isolated topologies like the flyback [Ri05], [Mi09] and forward [Fa06] converter have also been suggested to drive LEDs for mains applications. Likewise, isolated derivatives of SEPIC and CUK converters have been proposed in [Hu08].
LED driver topologies
non-isolated Buck [XW08] Boost [Sa08], [Ya09] Buck-Boost [TP09b] CUK [Bro08] SEPIC [ZGZ08] isolated Flyback [Ri05], [Mi09] Forward [Fa06] CUK SEPIC [Hu08] BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
Beyond non-resonant LED-drivers, resonant power converters can also be considered for driving LEDs and could be an option to obtain improved EMI behaviour. Resonant converters will not be covered in this work due to already high efficiencies of hard-switching LED-drivers, increased part number of many resonant converters and the pure mass of publications concerning resonant-switching.
Brightness distribution networks
The number of connected LEDs not only determines the power converter topology, but also influences the requirement on so-called brightness distribution networks. When high output voltages are unwanted due to security issues, e.g. above 54V in the conventional 14V automotive electrical power net, the number of LEDs that can be driven in a simple series connection gets limited.
As a consequence, it is often required that a single LED-string is split into various series connections which still have to be operated with a uniform brightness distribution. Different solutions have been suggested to obtain this operation mode and external active or passive networks are commonly used to unify LED-brightness levels [PZ08], [MM05], [BZ04], [DZ07]. These networks are therefore also an important component in the LED-driver.
A discussion of LED-driver topologies with brightness distribution networks will be performed detailed in Chapter 4.
Brightness control networks
Finally, brightness control (dimming) has also to be performed by the LED-driver. Two different methods are most commonly used in state of the art systems. The simplest solution is obtained by regulating the dc current flowing through the LEDs, caused by the direct relationship between LED current and luminous flux. An alternative method is Pulse-Width-Modulated dimming (PWM) which is used in the vast majority of LED-lighting. State of the art LED-drivers achieve PWM-control by connecting an additional external dimming network to the LEDs [NZ04]. The ratio of on- and off-time of an extra dimming switch in the network defines the level of brightness.
A discussion of brightness control solutions and their performances, as well as the development of a novel dimming approach will be introduced in detail in Chapter 4.
LED-driver components
It can be summarized that state of the art LED-drivers typically comprise individual brightness distribution and control networks that have to be combined with the power converter to perform essential lighting functions, shown in Figure 2-4.
Overview of three-dimensional LED-lighting systems 19
Figure 2-4: Components of LED-driver in state of the art systems
2.2.3. External thermal management components
Section 2.2.1 already showed that LEDs are temperature sensitive devices. Their junction temperatures influence the light emission spectrum in terms of wavelengths as well as amplitude. Further, low junction temperatures increase LED-lifetime and light output over time. An effective thermal management implementation is, therefore, required for the LEDs, as well as for the power electronic components in the LED-driver.
Chapter 6 will focus on the discussion of thermal management solutions and their application for three-dimensional LED-lighting systems, separately. However, a short overview on state of the art solutions will be shown here.
Different active and passive heat removal components exist for the cooling of power electronic equipment in general. Their application is dependent on the power class of the entire system, required space, environmental conditions and consequently on the loss density appearing in the system. Figure 2-5 gives an overview of cooling concepts and their thermal efficiency according to [To11].
Figure 2-5: Achievable power dissipation by heat fluxes with various thermal management solutions [To11]
The optimisation of cooling concepts is still a subject of research to find improved designs and boundaries of cooling [Cl05]. Especially the cooling of high-power LEDs is strongly
Power converter Brightness distribution network Brightness control network BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
focused in research and industry as typical LED-efficiencies of 15-20 percent [QLH09] are dramatically lower than those of modern power converters with efficiencies exceeding 80 percent. An overview of thermal management solutions for LEDs is given in [To11].
However, not only external components (Figure 2-5) like heat sinks or cold-plates are part of the thermal management but also the entire pathway from the component to the heat exchange structures. For this reason, also the circuit carrier plays an essential role in the thermal management implementation.
2.2.4. Circuit carrier technology
Figure 2-1 has already indicated that the circuit carrier technology plays a central role in the design and assembly of three-dimensional LED-lighting systems. The circuit carrier’s functions will be defined in accordance with the definition of packaging functions of power converters, introduced by [PF053]. The resulting packaging functions of the circuit carrier are:
x Electrical (integrity) function:
o Providing electrical interconnection between the LED-driver components and LEDs and creating electrical isolation
x Mechanical (integrity) function:
o Creating mechanical stability and fixation of the electronic components in the LED-lighting system
o Enabling environmental protection for the system and fixation of the optical system
x Thermal (integrity) function:
o Fulfilling thermal management functions to transport the heat from the components to an external thermal management component, e.g. heat-sink, or to directly dissipate the heat by itself
It can be derived from the foregoing functions, that the circuit carrier is the linking device between the electrical domain, with electron flow paths, the thermal management, defining heat paths, and the mechanical mounting of the entire system.
The evolution of LED-lighting towards three-dimensional designs is, therefore, strongly linked to the choice and availability of circuit carrier technologies. The next section will be used to describe how state of the art circuit carrier technologies contribute towards the creation of three-dimensional LED-lighting systems and technological shortcomings as well as limitations of contemporary realisations will be summarised.
Overview of three-dimensional LED-lighting systems 21
2.3. Evolution towards three-dimensional LED-lighting
Section 2.2 identified the essential power electronic components and functions they have to fulfil in lighting systems. In this section, the evolution towards three-dimensional LED-lighting systems will be discussed to determine the current practice of realising 3D-structures and to deduce requirements on the realisation of future three-dimensional LED-lighting systems.
A classification of the geometrical degrees of freedom of electronic systems, as defined by [RC08], will be used in the progress of this section to rate the design versatility of state of the art LED-lighting assemblies. Figure 2-6 shows this classification with principle drawings used for illustration.
Figure 2-6: Classification of geometrical degrees of freedom in LED-lighting systems based on [RC08]
The overview shows that circuit carriers can be classified in four different dimensions, ranging from simple two-dimensional (2D) structures to free-form surfaces which equates to a completely three-dimensional (3D) system design. Intermediate stages are defined as 2.5D and Nx2D. In the prior, parallel shifted planes exist where components are attached, whereas in the latter also angled planes carry electronic components, e.g. LEDs to obtain improved light distributions.
The evolution towards three-dimensional LED-lighting will be presented according to the available circuit carrier technology. Each circuit carrier technology will be characterised in a short manner; the resulting assembly as well as the achievable dimensions will be described and solutions found in the market will be given.
2.3.1. Printed Circuit Board (PCB)-based assemblies
The change from incandescent lighting to LED-lighting in general illumination required a change towards a different circuit carrier technology. Previously, lead frames have been used
Dimensions 2D 2.5D Nx2D 3D
Drawing
Attribute planar component side
planar component side, 3D-snaps, ribs on opposite side
different angled planar component
sides
Standard forms (cylinder, cube,…) Planar component side,
3D-snaps, ribs
Free-form surfaces different plan parallel
component sides
for contacting of incandescent lamps and could have been found especially in automotive applications. This was possible, as usually no driver electronics has been required to drive the lamp. A direct connection of the lamps to the automotive 14V electrical power has been sufficient.
With LEDs, the number of light sources and LED-driver components increased significantly, which lead to the widespread use of Printed Circuit Boards (PCB) for contacting the LED-lighting systems.
Up to now, the PCB is one of the standard circuit carriers used for conventional LED-lighting systems.
Technology description
The PCB technology offers a standardised and inexpensive solution for contacting electrical components with high reliability and optimised assembly processes due to its wide distribution. It covers almost any electrical application, including power electronic systems [Po05]. Hence, standardised and fully automated processing of Surface Mount Devices (SMT) or of leaded components is available, including high-speed pick & place, as well as soldering and in-line electrical testing.
Additionally, a wide range of copper layer thicknesses, e.g. 35μm, 70μm and 105μm, is available with standard PCB technology. A large span of current levels can therefore be carried by the PCB’s copper tracks, which is sufficient for the majority of LED-lighting applications.
Moreover, the large number of available copper layers on PCBs – up to 48 [Ci12] – make the PCB to a very universal circuit carrier technique that allows solving challenging contacting demands, e.g. complex LED-driver circuits.
Low thermal conductivities of PCB substrates, e.g. FR-4 with λ=0.25W/mK, however, challenge the LED-cooling by means of only the circuit carrier.
PCB-based assemblies
PCBs are inherently dimensional devices which initially led to LED-lamps with two-dimensional shape only, shown in Figure 2-7. In this configuration, the PCBs mainly provide contacting and mounting functions for the LEDs as well as for the LED-driver components. Depending on the power class, the PCB is complemented by an external heat sink with active or passive cooling to dissipate the heat generated by the LEDs and the LED-driver components. Conventional systems use mechanical mounting elements, e.g. screws, to attach the PCB to the heat sink structures, whereas thermal interface materials are used to provide low thermal resistances between both components. The optical system, e.g. a reflector, is mounted in a similar way. Figure 2-7 shows typical examples of PCB-based LED-lamps used in different lighting applications.
