Thermal Management Considerations to Meet LM-79 and the DOE CALiPER Testing Program

By Robert Sagebiel

Contributed By Convergence Promotions LLC


Thermal management parameters are not just dictated by LED datasheets, they are also mandated by the government. This article explains those parameters and how to meet them.

Whether you are building LED A19s, PAR38s, or any other lamp type in between – as well as LED fixtures – you need to be mindful of thermal management issues. However, if you want to do business in the USA, the Department of Energy (DOE) and the Environmental Protection Agency (EPA) have something to say as to what is acceptable for this market. The Design Lights Consortium (DLC) also uses LM-79 criteria, so keep this in mind when designing LED lighting products for the USA.

During phase one of evaluation, samples of proposed products will undergo performance testing at independent laboratories such as those participating in the S-DOE CALiPER program. To achieve a DOE CALiPER-quality LED solution, design engineers need a combination of an efficient LED array, a well-designed electronic driver, an effective heat sink, and an optimized optical solution. The proper combination of these key items directly determines the effectiveness, light output, and lifetime of these products. The US-DOE CALiPER testing has clearly indicated that first-rate components cannot be simply cobbled together to produce a “quality” product.

This article discusses how customers should focus on meeting LM-79 and the DOE CALiPER testing program in their designs.

Design requirements

The list of requirements that DOE and EPA have laid out for LED lighting products can only be characterized as daunting. The luminaire designer must keep in mind a myriad of requirements:
  • CCT (IESNA LM-79, LM-58, LM-16, CIE 15-2004)
  • The four ANSI defined Macadam ellipses: 2700 k, 3000 k, 3500 k and 4000 k
  • Lumen depreciation (IESNA LM-80): 70 percent of initial lumen to 35,000 hours
  • Color spatial uniformity (IESNA LM-79, LM-58, LM-16, and CIE 15-2004)
  • Color maintenance (ANSI C78.377A, IESNA LM-79, LM-58, and CIE 13.3-1995)
  • 80 CRI minimum (indoor)
  • Off-state power: zero
  • Warranty: three years
  • Thermal management: meet component supplier’s guidelines as well as key items for the LED driver/power supply/ballast
  • Power supply efficiency: 90 percent or more
  • FCC EMI/RFI: 47 CFR part 15/18 (consumer and non-consumer)
  • Transient protection (voltage spikes)
  • Frequency: 120 Hz or above
  • Noise: Class A, 24dB maximum
  • Power factor (ANSI C82.77): 0.90+ (commercial), 0.70+ (residential)
  • Temperature range: -20°C to maximum rating by power supply manufacturer
  • Safety (ANSI / UL 153, UL 1598)
The DOE CALiPER program tests for six items: total luminous flux, luminaire efficacy, correlated color temperature, color rendering index, steady state module/array temperature, and maximum power supply case/TMP temperature. Since four of the six tests are thermally related – total luminous flux, luminaire efficacy, steady state module/array temperature, and maximum power supply case/TMP temperature) – this article will focus mainly on how thermal design impacts LED performance and luminaire efficacy.

Maximizing efficiency

The first step toward thermal management is obviously to maximize the efficiency of your design. The following four key items help maximize luminaire efficacy:
  1. LED selection: This impacts total luminous flux, luminaire efficacy, correlated color temperature, and color rendering index.
  2. Thermal management: This impacts steady state module/array temperature, maximum Supply case/TMP temperature, and luminaire efficacy as well as lumen depreciation, color maintenance and CRI.
  3. Power supplies: This impacts steady state module/array temperature, maximum power supply case/TMP temperature, and luminaire efficacy as well as FCC, transient protection, noise, off-state power, and power factor.
  4. Optics: This impacts total luminous flux, and luminaire efficacy.
Two industry measurement references are used for fixture efficiency. The first is the coefficient of utilization (CU), which is an indication of a fixture's efficiency for delivering incandescent lamp lumens onto the horizontal surface to be illuminated. Highly efficient lighting fixtures have CU values above 0.9. The second is LER (fluorescent luminaire efficacy) which is the luminaire's light output divided by the input power. The formula is as follows:

LER = [luminaire efficiency (EFF) x total rated lamp lumens (TLL) x ballast factor (BF)] divided by [luminaire watts input]

Note that the effects of all components of the luminaire system are included in the LER. "LER gets to the core of what energy efficiency is all about – to get more energy service using less energy," said Francis Rubenstein, Staff Scientist, Lawrence Berkeley Laboratory.

To estimate LED Lamp Efficacy (LLpE), a “rule of thumb” formula to utilize is:

LLpE = ([LED lumens/watt)] X [LED junction temperature derating %]) X [power supply efficiency %] x [optical efficiency %]

Using a 120 lm/W LED, a 90 percent derating value, an 82 percent power supply efficiency, and an 88 percent optical efficiency and solving the equation:

LLE = ([120] X [90%]) X [82%] X [88%]

LLE = 77.93 lm/W

This result is on par with the current top efficacy of fluorescent fixtures at 70 percent or more efficiency when using 100 lm/W T8 FL lamps.

LED output degrades as the junction temperature increases. The LED junction temperature is impacted by the ambient temperature, current through the LED, forward voltage drop (and collectively the electrical, quantum, extraction and white conversion loses, which can waste 75 to 80 percent of the power within the LED as heat), LED junction to LED housing (case temperature), LED housing to circuit board, circuit board to heat sink, and air convection thermal transfers. Even the highest lumen output LEDs will be tremendously derated by poor thermal design.¹ It is critical to use the most efficient thermal transfer systems available, such as a Bergquist MCPCB, to mount your LEDs to, along with external heat sinks that are ambient air cooled. If the physical mounting space for the LED heat sink is constrained or limited air flow is an issue, the SynJet® by Nuventix should be considered for generating additional air flow cooling.

When using a power LED, the relationship between its junction temperature and its thermal resistance (resistance to the conductivity of heat, measured as °C/W) is very important. If the LED’s power consumption is constant, a smaller thermal resistance makes for a smaller rise in temperature at the junction. This allows the LED to be operated within a higher ambient temperature environment.

A typical thermal model

Figure 1: A typical thermal model for an LED package.


In Figure 1, the LED’s package thermal resistances are modeled as resistors, the local ambient temperature is modeled as a voltage source, and the LED power dissipation is modeled as a current source.² The thermal resistance between two points is defined as the ratio of the difference in temperature to the power dissipated; the unit is °C/W.

The engineering design challenge here involves the selection of an appropriate sized heat sink for the given heat source. Working in units of thermal resistance helps simplify the design calculation. As an example, when thermal resistances occur in series, they are additive, so when heat flows through two components, each with a thermal resistance of 1°C/W, the total resistance is 2°C/W.

The LED package design governs its thermal resistance, from the LED junction to the thermal contact at the bottom of package, and it is referred to as the thermal resistance between the junction and the solder point (Rjc). Different components in the heat conduction path can be modeled as different thermal resistances. By this “thermic Ohm’s Law”, the following equation results:

TJ = TA + (RJA × PLED), and Rja = Rjc + Rcb + Rbhs + Rconvection

If the thermal impedance value is reduced and a lower ambient temperature exists, the junction temperature will be lower. To maximize the working ambient temperature range for a given power dissipation, the total thermal resistance from LED die junction to ambient must be minimized.² The right choice of thermal interface materials such as thermal grease (e.g. ITW Chemtronics) or thermal pads (e.g. Bergquist) can improve the heat transfer at these mechanical interfaces.

As an example, using a Cree® XM-L white LED, the maximum forward current is determined by the thermal resistance between the LED junction and the ambient temperature. Given an existing thermal resistance of 2.5°C/W between the junction and the solder point, it is crucial for the LED retrofit lamp or LED luminaire to be designed in a manner that minimizes the thermal resistance from the solder point to ambient in order to optimize lamp life and lumen output. Minimizing the number of thermal path interfaces is a key part of good thermal design. Each interface adds more thermal resistance to the total system. An LED package with an internal heat sink that does not require external electrical isolation will have a lower total system thermal resistance since it does not need the extra layer of electrical isolation which can interrupt the thermal path.

The thermal impact is highlighted the derating chart shown in Figure 2, which plots maximum current versus ambient temperature and the relative luminous flux chart³, shown in Figure 3. For example, the LED lumen output at 55°C ambient would be reduced by about 30 percent if the total system thermal resistance Rj-a is 15°C/W instead of 4°C/W.

 

Current versus temperature

Figure 2: Maximum allowable current versus temperature.


Luminous flux versus current

Figure 3: Luminous flux versus current.


Junction temperature has a second significant impact on LED lifetime (ANSI L70 rating). As the LED junction temperature increases, the L70 operating life decreases as seen in Figure 4.4 Heat sinks play a very valuable role in keeping high-power LEDs cool. It is important to know that not any piece of metal will do a good job. While many lighting fixtures use sheet steel in their fabrication, steel is not nearly as effective as aluminum or copper as a heat sink. While any heat sink may be better than none, an effective heat sink design can reduce overall product weight, increase heat dissipation, and can become an aesthetic part of the product’s design.

 

Operating life versus junction temperature

Figure 4: Operating life versus junction temperature.


The next biggest impact on LED lighting design is the power supply efficiency. In multiple rounds of DOE CALiPER testing, this has ranged from a low of 51 percent to a high of 95 percent. As mentioned above, the current benchmark is a T8 (100 lm/W) FL fixture at 70 percent or more total efficacy. A fixture using 120 lm/W LEDs with a 90 percent value for thermal, lumen, and optical efficiency would still only reach 73 percent efficiency. If your power supply is working at 51 percent efficiency, the lamp or fixture system total would then be reduced to 41 percent efficiency. While many off-the-shelf LED driver options are available, it is important to match the LED circuit load to the power supply as closely as possible. Tweaking the LED array to get a best match with an existing LED driver power supply can be very time consuming and may result in too few lumens to work with.

An optimal solution is to design your power supply to meet your fixture’s requirements to get your efficiency as high as possible. This task is not to be taken lightly and is not for the novice. However, design help is available from many of the IC suppliers that make the components used within power supplies. The downside is that you will have to undergo UL testing of your custom power supply versus buying one that is already UL recognized from a ballast/driver manufacturer.

The last key item on an LED lighting system is the optical design. The best optical TIR lenses and reflectors for LEDs may approach 92 percent efficacy. Lens material plays a critical role in this area. While acrylic materials have the best optical efficacies, their nominal upper operating temperature is 68°C. Polycarbonate materials offer 88 percent optical efficacy and are very good up to 90°C, however if used in an outdoor/high UV environment, they need to be UV stabilized. Matching up the right lens or reflector is critical in maximizing lumen output. Just as with thermal interfaces, optical interfaces create loses. The proper mounting of the lens and LEDs is critical to a good optical interface. Keeping the number of interfaces to a minimum is also crucial. Photons do not like to go through multiple optical materials and those losses quickly multiply with each additional interface. Optical manufacturers (e.g. Carclo Technical Plastics) produce lenses and reflectors that are matched to a specific LED’s optical output. Do not swap the lens from one LED with another model or brand. Optical output will be greatly decreased.

Component manufacturers have developed considerable infrastructure (R&D, manufacturing, purchasing, quality assurance, sales and marketing, shipping, customer service, etc.) that allows them to provide the best products for their target markets. This allows them to be very efficient in developing and processing their products. This incredible storehouse of technical knowledge can also produce a “narrow field view” side effect.

Where can you turn for help in dealing with so many variables? You can call a dozen component manufacturers and hope for the best, or you can call your local electronics distributor that has an LED lighting program in place to help you attain maximum lamp or fixture efficiency with a minimum of headaches. One of the key reasons that it is good to work with an electronics components distributor is to have faster time-to-market solutions. Distributors offer a one-stop solution for the numerous electronic technologies needed to develop an LED lamp or luminaire solution that meets your requirements. Other reasons include the fact that distributors provide global engineering and manufacturing support; product design/development expertise; and access to emerging technologies in LED light sources, optics, LED drivers, thermal management, and controls. Distributors also offer a number of centralized and/or local field applications engineers who are a quick phone call or e-mail away from helping you develop an efficient lamp or luminaire solution. You can reduce your design time and in turn speed up your time to market when all of your key LED lighting components are available off-the-shelf from Digi-Key today.

Thermal management is a critical issue that can make our break your LED lighting product design. If you want to do business in the USA, the Department of Energy (DOE) and the Environmental Protection Agency (EPA) Energy Star program will impact the products that are acceptable for this market. Many public utilities will be offering rebates on LED lighting products, which will be crucial for speeding up adoption rates, but they must meet either Energy Star or Design Lights Consortium (DLC) requirements. These requirements are all based on LM-79 test criteria.

References:
  1. http://en.wikipedia.org/wiki/Thermal_management_of_high_power_LED
  2. http://www.cree.com/products/pdf/XLampXM-L.pdf
  3. http://www.cree.com/products/pdf/XLampXR-E_lumen_maintenance.pdf
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