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LED Lighting Design Process (Part 5)
  Form: Eupple    Date : 2009/10/13    『Back
5. Consider All Design Possibilities & Choose the Best

With the number of LEDs calculated, consider all the possibilities to accomplish the design goals. Since each LED is a small light source and has a much longer lifetime than traditional light sources, LEDs can be integrated into luminaires with new and unusual design elements. Designers can take full advantage of LED light’s directionality and wide variety of available secondary optics to create original designs.

At the same time, keep in mind that there are many different regulations that constrain the design choices. Providing a comprehensive list of worldwide standards applicable to LED luminaires is beyond the scope of this document, but Table 7 below gives examples of regulations that will apply in some portions of the world.

The rest of this section explains three design options for each system of our example LED luminaire: optical, thermal and electrical. For each system, guidelines for choosing the best option are provided.

Optical System Options

a. 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 luminaire’s distribution, then this option is not available.

b. 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’s 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.

c. 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

a. 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.

b. 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.

c. 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°C/W. With the heat sink thermal resistance value, the maximum ambient temperature can be calculated with the following formula:

Electrical System Options

a. 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.

b. 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.

c. 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 today’s 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.

Previous : LED Lighting Design Process (Part 6)
Next : LED Lighting Design Process (Part 4)
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