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LED Luminaire Design Guide
Lighting-class LEDs are now available that deliver the brightness, efficacy, lifetime, color temperatures, and white-point stability required for general illumination.
LED-based luminaires reduce total-cost-of-ownership ( TCO ) in these applications through maintenance avoidance ( since LEDs last much longer than traditional lamps ) and reduced energy costs.
There are over 20 billion light fixtures using incandescent, halogen, or fluorescent lamps worldwide. Many of these fixtures are being used for directional light applications but are based on lamps that put out light in all directions. The United States Department of Energy ( DOE ) states that recessed downlights are the most common installed luminaire type in new residential construction . In addition, the DOE reports that downlights using non-reflector lamps are typically only 50% efficient, meaning half the light produced by the lamp is wasted inside the fixture.
In contrast, lighting-class LEDs offer efficient, directional light that lasts at least 50,000 hours. Indoor luminaires designed to take advantage of all the benefits of lighting-class LEDs can:
Exceed the efficacy of any incandescent and halogen luminaire Match the performance of even the best CFL ( compact fluorescent ) recessed downlights Provide a lifetime five to fifty times longer than these lamps before requiring maintenance Reduce the environmental impact of light: no mercury, less power-plant pollution, and less landfill waste.
Luminaires Or Lamps
Designing LEDs into general illumination requires a choice between designing either a complete luminaire based on LEDs or an LED-based lamp meant to install into an existing fixture. Generally, a complete luminaire design will have better optical, thermal and electrical performance than the retrofit lamp, since the existing fixture does not constrain the design. It is up to the designer to decide whether the total system performance of a new luminaire or the convenience of a retrofit lamp is more important in the target application.
LED-based Luminaires For The Following Four Characteristics:
- Luminaire light output ( lumens )
- Luminaire efficacy ( lumens per watt )
- Correlated color temperaure ( degrees Kelvin )
- Color-rendering index
DOEs CPTP sets a good precedent for LED luminaire design by focusing on the usable light output of a luminaire ¡ª not just the light output of the light source.
1. Define Lighting Requirements
The design goals should be based either on an existing fixtures performance or on the applications lighting requirements.
2. Define design goals
- The designer should specify any other goals that will influence the design, such as special optical requirements or being able to withstand high temperatures.
- Specify design goals, which will be based on the applications lighting 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
Based on the design goals and estimated losses, the designer can calculate the number of LEDs needed to meet the design goals.
5. Consider all design possibilities and choose the best
- As with any design, there are many different ways to best achieve the design goals.
- LED lighting is still a new field, so assumptions that work for conventional lighting sources may not apply to LED lighting design.
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 design to further refine the luminaire design.
- Record observations and ideas for improvement.
Estimate Efficiencies of the Optical, Thermal & Electrical Systems
An LEDs luminous flux depends on a variety of factors, including drive current and junction temperature. To accurately calculate the necessary number of LEDs, the inefficiencies of the optical, thermal and electrical systems must be estimated first. Personal experience with previous prototypes, or the example numbers provided in this document, can serve as a guide to estimate these losses. This section walks through the process of estimating these system losses.
Optical System Efficiency
Optical system efficacy is estimated by examining light loss. There are two main sources of light loss to estimate:
1. Secondary Optics
Secondary optics are any optical system that is not part of the LED itself, such as a lens or diffuser placed over the LED. The losses associated with secondary optics vary depending on the particular element used. Typical optical efficiency through each secondary optical element is between 85% and 90%.
2. Light Loss Within the Fixture
Fixture light loss occurs when light rays from the light source strike the fixture housing before hitting the target. Some light is absorbed by the fixture housing, while some is reflected back into the fixture. The efficiency of the fixture is dictated by placement of the light source, the shape of the fixture housing, and materials used in the fixture housing.
|Light source||Light source efficacy||Coefficient of utilization||Fixture efficacy|
|CFL||65 lm/w||54%||35 lm/w|
|COB||80 lm/w||80%||68 lm/w|
|MCOB||100 lm/w||85%||85 lm/w|
|Figure 2 - Comparison of CFL & COB LED Coefficient of Utilization|
Calculate Number of LEDs Needed
Actual Lumens Needed
With all the system efficiencies estimated, the actual number of LED lumens required to achieve the design goals can be calculated. For this calculation, only the light efficiencies (optical and thermal) are used. The electrical efficiency affects only the total power consumed and fixture efficacy, not the amount of light coming out of the luminaire.
The calculation of "actual lumens needed" for the example luminaire is shown below:
Actual Lumens Needed = Target Lumens / (Optical Efficiency x Thermal Efficiency)
Another decision to be made is what operating current to use for the LEDs. Operating current plays an important role in determining the efficacy and lifetime of the LED luminaire. Increasing the operating current will result in more light output from each LED, thus reducing the number of LEDs needed. However, increasing operating current also has several drawbacks. Depending on the application, these drawbacks may be acceptable trade-offs for the higher per-LED lumen output.
For the example luminaire, lifetime and efficacy are top priority design goals. The luminaire will run at the minimu operating current listed on the XLamp XR-E datasheet (350 mA) to maximize LED efficacy and lifetime.
1. Reduced efficacy
Higher operating current reduces the efficacy of current generation power LEDs. In general, the size of the power supply will increase as operating current increases, since it takes more power to generate the same number of lumens.
2. Reduced maximum ambient temperature OR Decreased lifetime
Higher current will increase the temperature difference between the LED junction and the LEDs thermal path. In practical terms, since the maximum junction temperature is already decided, this reduces the maximum ambient temperature for the luminaire.
If instead of lowered maximum ambient temperature, the maximum junction temperature is raised, the LED will degrade in light output faster over its operational life.
Number of LEDs
After deciding on operating current, the lumen output of each LED can be calculated. Since the thermal loss of the LED has already been taken into account through the actual-lumens-needed calculation, the numbers specified in LED-supplier documentation can be used directly without further interpretation.
For this calculation, it is important to use the minimum flux listed for your LED order code and not the typical number on the data sheet. Most LED companies sell to minimum flux ranges. By designing against this minimum number, you are ensuring that all luminaires made with that LED order code meet the target requirements.
Number of LEDs = Actual Lumens Needed / Lumens per LED.
Optical System Options
1. Bare LEDs & existing lamp reflector
As discussed earlier, the beam angle of the existing CFL fixture and the LEDs are very similar. So, one available option is to use no secondary optics. This option provides the lowest cost and lowest optical loss for the system. Using fewer components and less labor makes the luminaire easier and cheaper to assemble.
The drawback is the multiple-source shadow effect, explained on the next page. Also, if the light distribution of the LED is significantly different than the target luminairs distribution, then this option is not available.
2. LEDs with secondary optics & existing lamp reflector
Secondary optics are optical elements used in addition to the LED¡¯s primary optic to shape the LED light output. The general types of secondary optics are reflecting (where light is reflected off a surface) or refracting (where light is bent through a refractive material, usually glass or plastic). Secondary optics are available either by buying a standard, off-the-shelf part or by designing a custom optic through ray-trace simulation with an optical source model.
By using a secondary optic per LED, the beam angle of each LED can be customized to provide the exact light output pattern necessary. For instance, the beam angle of each LED can be narrowed to make the luminaire optimized for spot lighting instead of general lighting.
There are several drawbacks to this approach. First, the luminaire will have higher cost because of additional components and more complicated assembly. Second, since the optics are attached to each LED, there may still be multiple-source shadowing. Finally, the secondary optics will reduce the optical system efficacy.
3. Bare LEDs, existing lamp reflector & diffuser
Instead of using one optic per LED, a diffuser can be used over the entire LED array to spread the light. The benefits of this approach are a wider beam angle than is possible with the bare LEDs and eliminating the multiple-source shadow effect.
As with Option 2, the drawbacks are higher cost and reduced optical system efficacy. This is also not an option if the light distribution must be narrower than the bare LED, since diffusers can only spread light, not collect it.
Illuminance distribution, the multiple-source shadow effect, and aesthetics will usually drive the decisions on the optical system. Option 2 is the only option if the light output must be narrower than the bare LED. If not, Option 1 is better in terms of cost, efficacy and brightness. However, both Options 1 and 2 will exhibit the multiple-source shadow effect.
Also, users looking up at Options 1 and 2 will notice each individual LED. Users of Option 3 will see only a diffuse, uniform light source.
Thermal System Options
1. Existing fixture housing
The lowest-cost option is to reuse the fixture housing of an existing design as the housing and heat sink for the LED luminaire.
Obviously, this is not an option for new luminaire designs. Also, most existing housings are made of steel, which is a poor thermal conductor. Generally, a steel housing will be a bad choice for a heat sink.
2. Off-the-shelf heat sink
Another option is to buy an off-the-shelf heat sink. This heat sink will be a proven design and come with full specifications from the manufacturer.
However, it may not be optimized in performance, size or shape for the target application.
3. Custom heat sink
A custom solution provides the best opportunity to optimize the heat sink for the application but has several drawbacks.
This option requires the designer to have access to thermal simulation software or access to a third party with thermal design expertise. Tooling and manufacturing fees may drive the per-unit cost of the custom heat sink higher than an off-the-shelf design.
Target luminaire cost, available heat sink development time, and target maximum ambient temperature will usually drive the decisions for the thermal system. In general, Option 2 is better for situations where low cost is more important than maximum ambient temperature. Option 3 is better when maximum ambient temperature is more important (e.g., outdoor lighting or indoor lighting in unconditioned spaces).
The example LED luminaire will use an off-the-shelf heat sink with a thermal resistance of 0.47¡ãC/W. With the heat sink thermal resistance value, the maximum ambient temperature can be calculated with the following formula:
Tj = Ta + ( Rth b-a x Ptotal ) + ( Pth j-sp x PLED )
Tj = LED junction temperature
Ta = Ambient temperature
Rth b-a = Heat sink thermal resistance
PLED = Single LED power consumption
= (Operating current) x (Typical Vf @ Operating current)
Ptotal = Total power consumption = (# LEDs) x PLED
Pth j-sp = LED package thermal resistance
Example luminaire values:
Tj MAX = 80°C
Rth b-a = 0.47°C/W
PLED = 0.35 A x 3.3 V = 1.155 W
Ptotal = 16 x 1.155 W = 18.48 W
Rth j-sp = 8°C/W
Ta MAX = Tj MAX - ( Rth b-a x Ptotal ) - ( Rth j-sp x PLED )
Ta MAX = 80°C - ( 0.47°C/W x 18.48 W ) - ( 8°C/W x 1.155 W )
Ta MAX = 80°C - 8.6856°C - 9.24°C
Ta MAX = 62°C
A maximum ambient temperature of 62°C for the example luminaire is acceptable for this indoor application. For an operating environment needing higher maximum ambient temperature, either the maximum junction temperature should be raised (which may impact lifetime) or the thermal system ( Rth b-a ) improved ( e.g., better heat sink ).
Electrical System Options
1. Off-the-shelf LED driver
An existing LED driver will provide the quickest design time, since it is already available and will come with a reference circuit design. All parts will be tested for EMI and safety regulations and will typically have the lowest per-unit cost in volume.
The drawbacks are that existing LED driver efficiencies are typically in the mid-80% range. Lifetime and operating temperatures may also be an issue, depending on the vendor and the application.
2. Next generation LED driver
As LED lighting is gaining in popularity, more semiconductor companies are turning their attention to optimizing LED driver designs. Another option is to partner with one of these companies on the next generation of LED drivers, which will have higher efficiencies and full regulatory approval.
However, waiting for the product development may delay the development of the LED luminaire. Also, smaller companies may not be able to work together with a driver company on an unreleased product.
3. Custom design
As with thermal design, a fully customized electrical system is an option. While it may be possible to get a higher efficacy than by using an off-the-shelf part, there are many potential drawbacks.
The burden of development and regulatory approval is now on the designer. Even after development, the per-unit cost may be higher than an existing solution. Also, keep in mind that driver companies will continue to develop more efficient and cheaper drivers during the LED luminaire development period.
Available development resources and target efficiency will usually drive the decisions for the electrical system. In todays high-power LED environment, improvements in the overall luminaire efficacy are driven more by the LEDs themselves and not the drivers. It may be advantageous to get a product out sooner rather than trying to wait until the electrical design is perfect.
Once the design decisions have been made, Table 8 (below) details the final steps to build and evaluate a prototype luminaire.
1. Board layout
♠ Complete the circuit board layout.
♠ Choose board material (FR4 vs. MCPCB) based on thermal and cost constraints.
♠ Keep in mind how the layout and positioning of parts will affect the light output and thermal flow of the luminaire.
2. Build a prototype
♠ Building one prototype (or several) is a valuable way to validate the design.
♠ Verify that the optical, thermal and electrical systems perform as they should.
♠ Test how easy the unit is to assemble.
3. Test prototype against design goals
♠ Test the prototype to make sure it achieves all the design goals.
♠ Testing can be done either internally or externally by a contracted luminaire-measuring company.
4. Finalize design & BOM
♠ Make final changes to the design (if any) based on the new information learned from analyzing the prototype.
♠ Document the final design and bill of materials.
5. Draw conclusions
♠ How could the existing design be improved if a different design choice was made?
♠ Are all of the original design goals still applicable, or are some less important than they seemed initially?
♠ Are there other applications that would benefit from LED light?
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