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LED Basics: Technology Fundamentals for Novices
2011 DOE SSL Market Introduction Workshop Michael Poplawski
Pacific Northwest National [email protected]
Seattle, WAJuly 22, 2010
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Format• 20 minutes presentation• 10 minutes Q & AObjectives• Technology fundamentals• Do’s and don’ts• Beware oversimplification• Identify and prioritize
what you need to learn more about
Later Today• Lighting Performance• Standards (LM-79, LM-80)• Products (DOE Lighting
Facts, CALiPER)• Applications (GATEWAY)
LED Basics
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1) Value Proposition2) Physics3) Photons4) Heat5) Voltage, Current & Time6) White Light7) Directionality8) Lumen Maintenance9) Thermal Management
10) System11) Packages12) Power Control13) Dimming14) Efficiency15) Efficacy16) Lifetime17) Cost18) Learning Curve
2010: 18 Key Fundamentals
http://http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/poplawski_basics_philly2010.pdf
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1) Terminology2) System3) Physics4) Photons5) Voltage, Current & Time6) Spectral Power7) Directionality8) Heat9) Thermal Management
10) Thermal Ignorance11) Electric Loads12) Power13) Compatibility14) Lifetime15) Oversimplifications16) Interdependencies17) Marketing Hype18) Life Cycle
2011: Another Spin
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• Efficiency, Efficacy = Desired Output / Input– Efficiency = no unit change (%)– Efficacy = unit change (e.g. watts in, lumens out)– LED efficacy (lm/W) = luminous flux (IF, TJ) / IF x VF (IF)– System efficacy (lm/W) = LED efficacy (lm/W) x Electrical
Efficiency (%) x Optical Efficiency (%)
• Power– Power source– Power supply– LED control circuitry– LED driver
1) Terminology
IES RP-16-10: “Nomenclature and Definitions for Illuminating Engineering”
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• An LED lamp/luminaire is a system• Every component in the system not only adds function
(and cost), but often affects the performance of the other components, and can be a factor in determining overall performance and lifetime
2) System
PackagePower Control
Secondary Heat Sink
Secondary Optics DiffuserDimmer50/60 Hz
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• LEDs are polar semiconductor devices formed by the creation of a P-N junction
• Current only flows in one direction, under forward bias and the collapse of the depletion region
• Narrowband light is generated in the P-N junction as a result of current flow under forward bias
• The wavelength of the light generated depends on the band gap energy of the materials forming the P-N junction.
3) Physics
P ND
eple
tion+
+++ -
---
P N++++ -
---
P N
Dep
letio
n++++ -
---
+- -+
Zero bias Reverse bias Forward bias
~ No current ~ No current High current
----+
+++
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• LED photons are generated by electroluminescence (due to electric current), as opposed to incandescence (due to heat).
• Electroluminescence is the result of radiative recombination of electrons and holes in a semiconductor.
4) Photons
Source: LED Transformations
electron hole photon
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• Luminous flux is a non-linear function of LED current (IF)• LED current is a non-linear function of forward voltage
(VF)• Manufacturers typically define maximum rated LED drive
currents• LED response time is very fast
– Allows for PWM dimming– Improves potential for control
(e.g. occupancy sensing)– Can result in undesirable flicker
5) Voltage, Current, & Time
PWM Dimming
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• LEDs are narrowband light sources
• Many techniques for making white light
• Phosphors– Downconvert short
wavelength (higher energy) to longer wavelength (lower energy)
– Inefficiency (Stokes loss)– Performance degradation
over time/temperature
6) Spectral Power
Cool White
Warm White
Source: Cree data sheet
Source: Cree data sheet
Blue LED Yellow Phosphor
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• Omni-directional light– External reflector, lens– Size dependence
• Generated heat radiated away (IR)
• Directional light– Internal and/or external reflector, lens– Index of refraction change
• Generated heat transferred by conduction (primary) and convection
7) Directionality
Visible Light
Visible Light
Visible Light
Heat Heat Heat
Visible Light
Visible Light
Heat
HeatHeat
Visible Light
Visible Light
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• Recombination of electrons and holes does not always result in electroluminescence
• Heat is the result of non-radiative recombination of electrons and holes in a semiconductor.
8) Heat
Source: LED Transformations
electron hole heat
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• The key to LED lifetime• Manufacturers typically define maximum rated LED
junction temperature (TJ)– Operation at lower junction temperatures results in higher lumen
output and/or longer lifetime– Light output, lifetime vs. junction temperature relationships
(shape of curves) can vary for different LED colors, types, and/or manufacturers
• TJ must be managed in system design & installation– LED power– Thermal path between the LED junction and ambient– Ambient temperature
9) Thermal Management
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10) Thermal Ignorance
• Do not expect to install your LED replacement lamp anywhere and see equal performance
• Some manufacturers specify maximum ambient temperatures for installation
• Exceeding maximum rated temperatures can result in lower lifetime, and/or lower luminous flux
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• Resistive loads– For AC input, only care about Vrms– Time independency: Irms = (1/R) x Vrms
• Incandescent sources effectively only care about Vrms– Constant R at steady state – R is a function of filament temperature
• Complex loads– Contain devices which store energy (e.g. capacitors, inductors)
and/or devices with non-linear current-voltage relationships– Time dependency: dv/dt, di/dt, on/off switching
• LEDs are non-linear devices– Different current-voltage relationships in different regions of
operation– Small change in voltage can equal large change in current– (Average) current must (typically) be controlled
11) Electric Loads
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12) Power
Vrms = 120V?
Vrms = 120V?
Vrms = 120V?
Vrms = 120V?
Same Different
Black Box
(average) light output
Black Box
Black Box
Black Box
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• The LED driver controls the emission of luminous flux– Average value– Modulation (flicker)
• LED driver compatibility with any element (power and/or control) in its electrical path can affect how well it delivers upon its designed performance– Transformers, dimmers, etc.– Some manufacturers do compatibility testing and generate
compatible product lists• LEDs ARE capable of high performance dimming
– Dimming control, power control compatibility is key– Systems with separate power and control signals (e.g. 0-10V)
minimize compatibility issues– MANY variables affect compatibility with systems with coincident
power and control signals (e.g. forward, reverse phase)
13) Compatibility
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• Lifetime is a statistical metric– How is it calculated?– Statistical significance
• Mean? Median? Byy?• B50 = median = traditional source “rated life”
• Warranty is conditional metric– What are the terms?
• System lifetime is what matters– Lumen Maintenance (L70)– Color Maintenance– Catastrophic Failure
• Maintenance is still a necessary part of the equation• Long life introduces new application-specific considerations
– Cleaning– Increased transient event exposure (e.g. surge)
14) Lifetime
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• Performance– LED efficacy is not a constant – LED driver efficiency is not a constant
• Lifetime– Lifetime ≠ lumen maintenance– LED lumen, color maintenance are not constants
• System– System efficacy ≠ LED efficacy– System lifetime ≠ LED lifetime
15) Oversimplifications
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16) Interdependencies
When you are buying or specifying LED lamps/luminaires, make sure you know what parameters the manufacturer prioritized
Number of LEDsCost
Flux
Efficacy
Current (IJ)
Lumen Maintenance
Color shift
Temp (TJ)
DriverRequirements
ThermalManagement
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• Do know your value propositions– Energy, maintenance savings– Light spaces differently (small, color without filters, fast, control)
• Do your own payback calculation(s)• Don’t be green-washed
– 100,000 hour no-maintenance lifetime– Legacy form-factor, non-legacy light distribution– Efficacy vs. light quality
• Don’t succumb to scare tactics– Flicker– Blue Light– Electromagnetic radiation– Hazards may be real, risks are limited
17) Marketing hype
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• Existing studies– Use energy dominates (> 95%) total energy (use + manufacturing) for all light sources– Incandescent sources consume the most total energy, and have the worst environmental
impact, by far– SSL is comparable to CFL today– Continuing improvements in SSL efficacy will improve its life cycle performance further
• DOE is re-booting its own LCA study, starting next month, with a new global team including researchers from PNNL, Navigant, and academia.
18) Life cycle
http://www.osram-os.com/osram_os/EN/About_Us/We_shape_the_future_of_light/Our_obligation/LED_life-cycle_assessment/index.htmlhttp://randd.defra.gov.uk/Default.aspx?Menu=Menu&Module=More&Location=None&Completed=0&ProjectID=16519#RelatedDocuments
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• Disruptive technology• Innovation ongoing in all
system areas • Get Smart. Ask
questions. Validate information.
Using LEDs, Fact Sheets• ssl.energy.gov/ssl_basics.html• ssl.energy.gov/factsheets.html • Ongoing updatesTechnical Reports• Program Plan:
http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/ssl_mypp2011_web.pdf
• Manufacturing Roadmap: http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/ssl_manuf-roadmap_july2010.pdf
• Energy Savings: http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/ssl_energy-savings-report_10-30.pdf
Conferences• R&D, Manufacturing, Market Intro:
ssl.energy.gov/conferences.html
Evolving learning curve
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