CREE CLD-AP70

Figure 1
CLD-AP70 REV 0
APPLICATION NOTE
Cree/NXP Task Light Reference Design
Figure 1: NXP Cree task light
TABLE OF CONTENTS
INTRODUCTION
Introduction......................................................... 1
This application note demonstrates that an LED task
Design Approach/Objectives................................... 2
light can readily outperform its linear fluorescent equiv-
The 6-step methodology........................................ 2
alent and incorporate dimming for mood setting and for
1. Define lighting requirements............................ 2
additional energy savings. Cree, NXP Semiconductors
2. Define design goals........................................ 4
and Bright View Technologies collaborated to design an
3. Estimate efficiencies....................................... 4
under-cabinet SSL fixture that exceeds linear fluores-
4. Calculate the number of LEDs needed............... 5
cent performance while keeping cost low and meeting
5. Consider all design possibilities........................ 6
Energy Star performance specifications. This design is
6. Complete final steps......................................10
a simple approach to fluorescent lamp replacement and
Conclusions........................................................11
can easily be integrated into many linear lighting ap-
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NXP CREE TASK LIGHT REFERENCE DESIGN
DESIGN APPROACH/OBJECTIVES
In the Cree Application Note “LED Luminaire Design Guide,”1 Cree advocates a 6-step framework for creating LED luminaires. All Cree reference designs use this framework, and the design guide’s summary table is reproduced below.
Step
1. Define lighting requirements
Explanation
•
The design goals can be based either on an existing fixture or on the application’s
lighting requirements.
2. Define design goals
•
Specify design goals, which will be based on the application’s lighting requirements.
•
Specify any other goals that will influence the design, such as special optical or
environmental requirements.
3. Estimate efficiencies of the optical,
thermal & electrical systems
•
Design goals will place constraints on the optical, thermal and electrical systems.
•
Good estimations of efficiencies of each system can be made based on these constraints.
•
The combination of lighting goals and system effiiciencies will drive the number of
LEDs needed in the luminaire.
4. Calculate the number of LEDs
•
needed
5. Consider all design possibilities
and choose the best
Based on the design goals and estimated losses, the designer can calculate the
number of LEDs to meet the design goals.
•
With any design, there are many ways to achieve the goals.
•
LED lighting is a new field; assumptions that work for conventional lighting sources
may not apply.
6. Complete final steps
•
Complete circuit board layout.
•
Test design choices by building a prototype luminaire.
•
Make sure the design achieves all the design goals.
•
Use the prototype to further refine the luminaire design.
•
Record observations and ideas for improvement.
THE 6-STEP METHODOLOGY
The major goal for this project was to demonstrate a straightforward task light design using Cree XLamp LEDs that
meets or exceeds the performance of a comparison fluorescent fixture.
1.
DEFINE LIGHTING REQUIREMENTS
A desirable task light is low in power consumption and efficiently illuminates the area where it is installed. Listed in Table
1 are specific metrics that can quantify luminaire performance.
1
LED Luminaire Design Guide, Application Note AP15, www.cree.com/products/pdf/LED_Luminaire_Design_Guide.pdf
Copyright © 2011 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree, the Cree logo and XLamp are registered trademarks
of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at
www.cree.com. For warranty information, please contact Cree Sales at [email protected]
2
NXP CREE TASK LIGHT REFERENCE DESIGN
Importance
Critical
Important
Characteristics
Units
Luminous flux
Lumens (lm)
Luminance/illuminance
candela/m2 or lux
Electrical power
Watts (W)
Meet safety standards
Agency listing/mark
Price
$
Lifetime
Hours
Correlated Color Temperature (CCT)
Kelvin
Color Rendering Index (CRI)
100 point scale
Manufacturability
$
Ease of installation
Time = $
Comply w/Energy Star
Has label
Compatible w/controls
Yes/No
End-of-life disposition
Cost to recycle
Table 1: Design criteria
Table 2 summarizes general Energy Star® requirements to be met to be eligible to qualify for the Energy Star Program.2
Characteristic
Requirement
CCT
The luminaire must have one of the following designated CCTs and fall within the
7-step chromaticity quadrangles as defined in ANSI/NEMA/ANSLG C78.377-2008.
• 2700 K
• 3000 K
• 3500 K
• 4000 K
Color angular uniformity
The variation of chromaticity shall be within 0.004 from the weighted average point on
the CIE 1976 (u’, v’) diagram.
Color maintenance
The change of chromaticity over the first 6,000 hours of luminaire operation shall be
within 0.007 on the CIE 1976 (u’,v’) diagram.
CRI
Indoor luminaires shall have a minimum CRI of 80.
Off-state power
Luminaires shall not draw power in the off state.
Lumen maintenance requirement
L70 > 25,000 hours
Power factor (PF)
Total luminaire input power < 5 W: PF > 0.5
Total luminaire input power > 5 W: PF > 0.7
Warranty
3-year warranty
Operating frequency
> 120 Hz
Table 2: General Energy Star requirements
2
Energy Star Program Requirements Product Specification for Luminaires (Light Fixtures) - Eligibility Criteria - Version 1.0
http://www.energystar.gov/ia/partners/prod_development/new_specs/downloads/luminaires/
ES_Luminaires_V1_Final_Specification.pdf
Copyright © 2011 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree, the Cree logo and XLamp are registered trademarks
of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at
www.cree.com. For warranty information, please contact Cree Sales at [email protected]
3
NXP CREE TASK LIGHT REFERENCE DESIGN
The under-cabinet shelf-mounted lighting requirements3:
Characteristic
Requirement
Minimum light output
125 lumens per lineal foot
Zonal lumen density
•
•
Minimum luminaire efficacy
29 lm/W
Minimum of 60% of total lumens within 0-60° zone
Minimum of 12.5% of total lumens within 60-90° zone
Table 3: Under-cabinet shelf-mounted lighting requirements
2.
DEFINE DESIGN GOALS
The design goals for this project:
Characteristic
Unit
Minimum Goal
Target Goal
Luminaire light output
Lm
200
300
Illuminance/luminance profile
Lux
Same
Better
W
8
6
System power
Luminaire efficacy
Lm/W
40
50
Lifetime
Hours
25,000
50,000
K
3,500
2,700
80
85
CCT
CRI
Maximum ambient temperature
°C
49
The guiding principle for this design was to meet Energy Star guidelines and provide an off-the-shelf design that can be
used immediately or easily modified to meet specific requirements.
Since it is advantageous to be able to dim a task light, a main goal was to provide flicker-free dimming down to < 1%
light output. Another goal was to ensure the task light can be switched on at very low dimmer levels.
3.
ESTIMATE EFFICIENCIES OF THE OPTICAL, THERMAL & ELECTRICAL SYSTEMS
Figure 2 shows basic LED electrical data and optical output from Cree’s Product Characterization Tool (PCT).4 We chose
a configuration using XP-E LEDs, and another using MX-6 LEDs. Equally efficient configurations could be created using
XLamp ML-B, ML-E, MX-3 or XP-E HEW LEDs.
3
4
Ibid.
Available at http://pct.cree.com
Copyright © 2011 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree, the Cree logo and XLamp are registered trademarks
of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at
www.cree.com. For warranty information, please contact Cree Sales at [email protected]
4
NXP CREE TASK LIGHT REFERENCE DESIGN
Figure 2
Figure 2: Cree Product Characterization Tool data
4.
CALCULATE THE NUMBER OF LEDS NEEDED
An iterative process was used to determine the number of LEDs in the task lamp. The initial design used 6 LEDs at 1.25
in. pitch, but the light uniformity was not sufficient. Using five LEDs at 1.5 in. pitch, obscured by an precision engineered
diffuser and driven at 350 mA, proved to provide optimum illumination. Other design choices with larger numbers of
lower power LEDs would be equally plausible. For an extended discussion of LED pitch and various optical system tradeoffs, see Cree Application Note AP34, “Cree XLamp LEDS for Distributed Illumination Applications.”5
5
http://www.cree.com/products/pdf/XLamp_Distributed_Illumination_Apps.pdf
Copyright © 2011 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree, the Cree logo and XLamp are registered trademarks
of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at
www.cree.com. For warranty information, please contact Cree Sales at [email protected]
5
NXP CREE TASK LIGHT REFERENCE DESIGN
5.
CONSIDER ALL DESIGN POSSIBILITIES AND CHOOSE THE BEST
Figure 6
Designing a solid-state lighting (SSL) linear fixture that exceeds linear fluorescent performance, is cost competitive, and
exceeds Energy Star performance criteria requires a system-level design approach. All four system-level elements, LED
Figure 4
selection, secondary optics, driver selection/performance and thermal management, must be considered.
LED Selection
A linear light fixture is expected to provide uniform light transmission, color temperature and, in
the case of a dimmable LED-based fixture, maintain color temperature while dimming. This design
shows that both the Cree XLamp XP-E and MX-6
Figure 3: XP-E LED
Figure 4: ML-E LED
LEDs exceed the targets for this application. The
XP-E LED can be driven at a much higher current
than the MX-6 LED, producing more light within the
fixture, if requirements dictate.
LED placement within the fixture is also critical.
Linear fluorescent lamps have uniform light transmission and are omnidirectional. This gives a nice
lighting profile when looking directly at the fixture,
but also comes with a performance penalty in the
Figure 5: MX-6 LED
Figure 6: XP-E HEW LED
constrained space of undercabinet design. Since SSL luminaires are point sources, careful consideration of LED placement within the fixture is vital.
Secondary Optics
The diffuser in the comparison linear fluorescent fixture traps as much as 50% of the light produced by the fluorescent
tube. This has a huge negative impact on total efficacy of the fixture. Working with Bright View Technologies6, a linear
diffuser was designed and optimized for this application. This allowed for almost 85% of the light generated by the LEDs
to be transmitted through the diffuser and onto the work surface.
Luminous Flux
CCT
CRI
SSL without diffuser
307 lm
4051
85
SSL with diffuser
261 lm
4033
85
Table 4: Performance of LED task light vs. linear fluorescent
6
www.brightviewtechnologies.com
Copyright © 2011 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree, the Cree logo and XLamp are registered trademarks
of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at
www.cree.com. For warranty information, please contact Cree Sales at [email protected]
6
NXP CREE TASK LIGHT REFERENCE DESIGN
To eliminate shadowing and produce light uniformity, the LED-to-diffuser distance had to be tailored so the point-source
LEDs are essentially invisible. The original design placed the LEDs 0.75 in. behind the diffuser. Though Bright View
worked to optimize the diffuser, the distance between the LEDs and from the LEDs to the diffuser produced a suboptimal
result, with the individual LEDs clearly visible and corresponding shadowing apparent. A second design placed the LEDs
1.75 in. behind the diffuser. The end result was a task light that has little to no shadowing from end-to-end of the fixture
and hides the LEDs so the point sources are eliminated.
Driver Topology and Performance
It is highly likely that a user will come in contact with the fixture, so a safe topology that isolates the user from the AC
mains is required. One of the great features of the SSL21017 is its ability to be configured for both buck (non-isolated)
and flyback (isolated) configurations. We selected the flyback topology for this design, providing the maximum safety
Figure 7
available in an SSL fixture.
Driver efficiency is critical, and this design achieves 80% efficiency at full load, as shown in Figure 7 and Figure 8. Figure
9 shows the power factor. The curves in these figures are for three drivers tested under identical conditions. Differences
in the curves are due to variances in the driver performance based on manufacturing/component spreads in the driver
assemblies.
EFFICIENCY (%)
85.00%
83.00%
81.00%
#1
79.00%
#2
#3
77.00%
75.00%
80
100
120
140
INPUT VOLTAGE (V)
Figure 7: SSL2101 driver efficiency without triac dimmer in the circuit
7
http://www.nxp.com/#/pip/pip=[pip=SSL2101]|pp=[t=pip,i=SSL2101]
Copyright © 2011 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree, the Cree logo and XLamp are registered trademarks
of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at
www.cree.com. For warranty information, please contact Cree Sales at [email protected]
7
Figure 8
NXP CREE TASK LIGHT REFERENCE DESIGN
EFFICIENCY (%)
85.00%
83.00%
81.00%
#1
79.00%
#2
Figure 9
77.00%
#3
75.00%
80
90
100
110
120
130
140
INPUT VOLTAGE (V)
Figure 8: SSL2101 driver efficiency with triac dimmer in the circuit at maximum lumen output
0.990
Figure 10
pf
0.960
0.930
#1
0.900
#2
0.870
#3
0.840
80
100
120
140
INPUT VOLTAGE (V)
Figure 9: Power factor without triac dimmer in the circuit
Using the SSL2101 enabled a sought-after feature, triac dimming, to be provided. One of the main design goals was to be
able to dim the task light down to < 1% light output without flickering. Since the NXP SSL drivers are specifically designed with
this in mind, the goal was achieved with a straightforward design.
Figure 10: SSL2101
Another goal was to ensure a user can switch on the task light at very low dimmer levels. The low voltage startup of
the SSL2101 made this achievable.
Copyright © 2011 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree, the Cree logo and XLamp are registered trademarks
of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at
www.cree.com. For warranty information, please contact Cree Sales at [email protected]
8
NXP CREE TASK LIGHT REFERENCE DESIGN
Thermal Management
Thermal management is critical in SSL luminaires. Ensuring the recommended maximum LED junction temperature is
not exceeded is critical to long life and color point stability of the LEDs. We designed an aluminum housing, increasing
the housing cost by 30% over a sheet metal version. This increased cost was offset by improved thermal performance
without the necessity of an additional heat sink. An aluminum housing allows the LED assembly to be produced with
standard FR-4 PCB material.
Thermal images of the LED board show that the system
well–below
Figureis 11
part the
1 recommended maximum operating temperatures of the components in the design.
Figure 11 shows a thermal view of the LED area of the LED task light. The temperature/time chart plots the maximum
temperature in the LED area versus time
Figure 11 – part 2
Figure 11: Thermal view of LED task light
Copyright © 2011 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree, the Cree logo and XLamp are registered trademarks
of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at
www.cree.com. For warranty information, please contact Cree Sales at [email protected]
9
NXP CREE TASK LIGHT REFERENCE DESIGN
Task Light Design
The LED task light utilizes five Cree XP-E or MX-6 LEDs driven by the NXP SSL2101 driver. The LEDs are a great fit in
linear applications and feature an electrically isolated thermal path, enabling heat removal through low-cost PCBs, and
excellent color point stability, all with industry-leading lumen maintenance.
The design utilizes NXP’s SSL210x triac dimmable driver family to achieve excellent deep dimming, isolated flyback
design, ease of mechanical design and excellent efficiency, approximately 79%, all with a PF > 0.9. This, in conjunction
with Cree’s superb lighting class XP-E or MX-6 LEDs, offers lighting designers a new standard in LED lighting.
The output is set at 6 W and delivers approximately 307 lumens without the diffuser. With the Bright View diffuser, the
task light produces approximately 261 lumens. The equivalent fluorescent light had a lumen output through the diffuser
of 201 lumens for 8 W of power.
6.
COMPLETE FINAL STEPS
Figure 12
Performance Summary
Table 5 compares the performance the LED task light to a linear fluorescent. The LED task light produces 25% more
lumens for 25% less power (6 W vs. 8 W) and has a significantly better CRI than the fluorescent. These tremendous
results are achieved using state of the art Cree XP-E or MX-6 LEDs coupled with NXP’s triac dimmable SSL2101 driver,
capable of dimming the LED fixture down to < 1% of light output completely flicker free.
Luminous Flux
CCT
CRI
LED task light
261 lm
4033
85
Fluorescent task light
201 lm
4068
62
Table 5: Performance summary of LED task light vs. linear fluorescent
The LED task light boasts triac dimming capabilities that the comparison fluorescent fixture lacked. The dimming is smooth and flicker free down to < 1%
of light output and the unit can be switched on at very low light output levels.
The unit is compatible with a wide range of off-the-shelf triac dimmers from
popular manufacturers including Lutron, Leviton and Cooper.
The basic design of this 6-W LED task light can be adapted quite easily for any
type of 5-12 W LED fixture.
Figure 12: Various triac dimmers
Copyright © 2011 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree, the Cree logo and XLamp are registered trademarks
of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at
www.cree.com. For warranty information, please contact Cree Sales at [email protected]
10
NXP CREE TASK LIGHT REFERENCE DESIGN
CONCLUSIONS
This LED task light readily outperforms a fluorescent equivalent. With good design practice we have shown how thermal
performance can be managed for a long lifetime design. The completely flicker-free deep triac dimming brings another
dimension to the fixture compared to fluorescent equivalents.
Using industry-leading Cree lighting class LEDs and NXP’s driver, the flexible design can be applied as is or modified to
different mechanical formats for multiple applications, such as an LED downlight or a wall-washing fixture. If more light
is required, the power level of the design can be easily modified up to 15 W using the SSL2101 driver and up to 25 W
using the SSL2102. This flexibility, coupled with brighter or more Cree LEDs, can accommodate most applications.
Copyright © 2011 Cree, Inc. All rights reserved. The information in this document is subject to change without notice. Cree, the Cree logo and XLamp are registered trademarks
of Cree, Inc. This document is provided for informational purposes only and is not a warranty or a specification. For product specifications, please see the data sheets available at
www.cree.com. For warranty information, please contact Cree Sales at [email protected]
11