TECH NOTES: Design solutions for solid-state lighting applications

Andreas Pohl of Avago Technologies describes some of the difficulties that designers may experience when designing LEDs into their lighting applications, focusing on thermal, optical and driver issues.

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This article deals with the difficulties that designers may experience when designing LEDs into their lighting applications. It focuses on the application of discrete 1-watt LEDs in a thermal design that can efficiently dissipate heat from the high power LED (MCPCB, thermal vias), and on lenses that are currently available on the market to address customer-specific design demands. The article also discusses solutions on how to use LEDs in an efficient and space-saving way with small-scale drivers, and parallel or serial circuits.

High-power LEDs for solid-state lighting

Figure 3
Avago Technologies has developed a high-power LED solution that is ideal for designers of architectural and solid-state lighting applications. This high-power LED package targets those applications where efficient heat dissipation, long lifetime and low package height are required.

Due to the industry-standard TO-220 package, designers can use the pad-layout of a power transistor to include this high-power LED into their design. The advantage of this package is its low profile design (3.3mm height) and wide 110-degree viewing angle.

This LED package, which is very thin and provides good light distribution, is extremely useful in lighting application designs such as channel lettering or backlighting. As a result, this LED package from Avago will enable designers to save a lot of design time as they will not need to design a LED-specific soldering pad layout that has to be set it up in their design software.

The advanced heat dissipation capabilities of Avago’s high-power Moonstone™ LED package incorporates a metal slug design that helps to dissipate heat more efficiently through the package (Rth J-P = 10-12K/W depending on the chip technology) which supports reliability and long lifetime performance.

Another factor that plays a key role in extending the life of this LED package is the use of a silicone encapsulation and enhanced phosphor process. Based on 6,000 hours actual light output degradation measurement that was conducted by Avago at room temperature (Tambient = 25°C, Tjunction = 43°C), the average light output was reduced by less than 30 percent in projection after 50,000 hours of continuous operation at 350mA.

Avago’s Moonstone LED package is compatible with standard manufacturing processes such as vacuum-nozzle pick-and-place assembly and reflow soldering and can also be assembled in non-ESD-safe conditions (build-in Zener dioder; ESD Level 16kV). It is also possible to easily hand-solder this LED package; this makes it easier to build-up a demo board to illustrate the advantages of using high power LED light sources, or when manual solder is necessary, to mount the LED onto the board. Moreover, Avago offers an advanced tight binning to eliminate color drifts within one bin which are likely to arise when using a wider white binning structure commonly used in the market.

Thermal management of high-power LED packages

Low thermal resistance helps to reduce the internal chip temperature and to achieve a long life performance. However, the metal slug alone is not sufficient to dissipate the electrical power that is converted to heat and light during operation. A good thermal design helps to significantly improve the lifetime of the lighting fixture by maintaining the junction temperature within a specific temperature range. As a result, a thermal design is needed to control the temperature of the high-power LED.

Figure 1

To be able to use Avago’s Moonstone LED with electrical performance above 1W, it makes sense to have a scalable thermal design. There are basically two approaches that can be used to dissipate the heat from the high-power LED to the ambient:

  • Printed Circuit Board (PCB) design using thermal vias and a heat sink
  • Metal-core PCB and heat sink

Thermal Vias: This design approach includes a double-sided FR4 PCB, a large copper pad design, as well as metalized holes through the PCB. The soldering pad must be the size that is specified in the datasheet (10.7 x 8.4 x 17 mm), to avoid any mis-alignment after soldering. However, the copper pad must be bigger than the solder mask (copper pad: ca. 20 x 14 mm) and be present on the upper and the lower side of the FR4 PCB.

The heat will than be dissipated from the dice to the metal slug while it is being soldered to the copper pad. Due to the fact that copper is a very good heat conductor, the heat is distributed evenly across the copper pad. The thermal vias conducts the heat from the upper to the lower side of the PCB, to enable efficient heat distribution. It is recommended to have more than 50 thermal vias with a diameter of 0.3mm to 0.5mm on the copper pad area connecting the upper and lower copper pad.

Figure 2
The FR4 PCB with the thermal vias should then be mounted to a heat sink using a thermal interface material like thermal grease or glue (or another thermally conductive material) and screw holes. There are different heat sinks available in the market. It is recommended that designers use an extruded aluminium heat sink in the shape of the copper pad, but it is also possible to use different shaped heat sinks as long as the heat sink is able to connect to the whole copper pad area.

Metal-Core PCB: Using a metal-core PCB (MC-PCB) is probably the most convenient way of designing in the thermal management features. However, the MC-PCB approach is quite expensive if it is a customer-specific design, as the PCB consists of a solid 1.5-3 mm thick aluminum layer, depending on the dimensions of the design.

The thermal resistance junction-board for Avago’s Moonstone Star metal-core PCB is 18K/W. The lower side of the metal-core PCB is isolated, so it can be attached to a heat sink using a thermal interface material. The size and shape of the heat sink should match the size of the metal-core PCB and should be able to dissipate the heat efficiently. There are a variety of different heat sinks available in the market that are suitable for this purpose. Sometimes it’s also a good idea to mount the MC-PCB to a sufficiently thick metal plate to help dissipate the heat more efficiently.

Electrical design

LEDs are current-operated devices that must be driven at a specified current to achieve predictable luminosity and chromaticity levels. The luminosity can be controlled using pulse-width modulation (PWM) which will not affect the chromaticity. Avago’s Moonstone LED can be operated at 350mA, and has forward voltage up to 4.0V; these are the key values that are important for the electrical lighting design.

The most convenient way of driving a high-power LED is to use a high-power LED driver IC. There are several ICs available on the market, and most of them require an input voltage that is higher than the output voltage. Others have an integrated boost stage that converts the input voltage to the desired output level.

In the example below, we will focus on National Semiconductor’s constant current buck IC (LM3404HV). The output current can be adjusted by changing the sensing resistor; however, the input DC voltage needs to be higher than the required output voltage.

Figure 4

It is also possible to drive the high power LEDs using a constant voltage supply and a set of resistors to adjust the current. In this case, the resistors should be matched to the typical forward voltage of the Power LED that can be obtained from the datasheet so that the maximum forward current is not exceeded.

However, to achieve the required current, the resistor approach requires a customized resistor set for each forward voltage the Moonstone could have. Another disadvantage of this approach is the relatively high loss of power due to the electrical power that needs to be dissipated by the resistors. This will lead to a reduced efficiency of the luminaire.

Figure 5

Serial or parallel connection? It is recommended that multiple high-power LEDs be connected serially if the power supply supports the required voltage level. However, it is also possible to drive the high-powered LED in parallel if required.

Optical design

High-Power LEDs provide an advantage over conventional light sources because the light emission is already directional. Hence, it is not necessary to design a reflector to bring the light into a specific direction to get more light out of this light source. Avago’s Moonstone LED has a very wide viewing angle of 110-degrees which makes it an ideal light source for uniform illumination of areas close to the LED opening such as backlit letters on a wall.

On the other hand, the wide viewing angle of the package limits the distance between the LED and a surface that can be illuminated by a reasonable amount of light such as a desk light. Therefore, it is better to focus the 110-degree light beam to a narrower angle that provides a suitable amount of light at longer distances.

A narrower beam angle can be achieved by using a secondary reflector or secondary lenses with the high-power LED package. The optical efficiency of both approaches is between 80 and 90 percent -- depending on the connection and internal refraction between the reflector of the LED and the lens or reflector. Avago offers lenses for the most common viewing angles in signage and illumination applications, ranging from 6 to 30-degrees.

As the light spot might be considered irregular for some applications, like spot lights or reading lamps, Avago is working with several lens manufacturers in Europe to provide customers with lenses that create a more uniform light spot. These manufacturers have the ability to customize the shape and size of the lens, radiation pattern or include multiple LEDs into one lens to meet specific customer requirements.

For example, Avago worked closely with a lens supplier in the United Kingdom to help them fulfill the lighting project needs of one of their customers. The customer required a non-standard beam angle to achieve a certain radiation pattern. Though the application required lenses of different viewing angles, the project required lenses of the same height across the different angles.

Additionally, the lenses needed to be mounted on the LED using the normal flange design (plastic ring on Avago’s Moonstone package), and a much more mechanically robust interconnection. As a result, the lens supplier provided a feasible and timely customer specific optical solution for the Avago’s Moonstone power LED package.

[** Moonstone is a trademark of Avago Technologies.]

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