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- WWW. CREE.COM/XLAMP plications. Copyright © © 2011 document is is subject to to change without notice. Cree, the Cree logo and Copyright 2010 Cree, Cree, Inc. Inc. All All rights rightsreserved. reserved.The Theinformation informationininthis this document subject change without notice. XLamp are registered trademarks Cree, Inc. trademarks This document is provided Cree, the Cree logo and XLamp areofregistered of Cree, Inc. 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]. Other trademarks, product and company names are the property of their respective owners and do not imply specific product and/or vendor endorsement, sponsorship or association. Cree, Inc. 4600 Silicon Drive Durham, NC 27703 USA Tel: +1.919.313.5300 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