Thermal Design and Testing: Key Factors for Reliable SSL

Unlike incandescent lamps, which radiate heat, the heat generated by LEDs must be dissipated away from these semiconductor devices by conduction. Without proper thermal management, LED light output will decrease, the dominant and peak wavelengths will increase, and the color temperature will change. Addressing these thermal challenges is critical to ensuring the efficacy and life expectancy of solid-state lighting (SSL) luminaires.

Temperature Effects
If proper attention is paid to thermal design, LED lifetimes can far exceed 50,000 hours (see Figure 1). However, in addition to shortening operating life, increasing temperatures can degrade LED performance. Relative luminous flux (light output) decreases with increasing temperature (see Figure 2). For some LEDs, output can be reduced by 80% or more when case temperatures reach 130°C, depending on the color temperature rating.

Figure 1 As current increases, lower junction temperature is required to achieve longer operating life

Figure 2 As the thermal pad (housing) temperature increases, the relative light output decreases

The critical temperature of an LED, the junction temperature, is measured deep inside the package, away from the surface of the chip. Because junction temperature is not easy to measure, it is usually read from the package (or case) temperature and then the actual junction temperature is calculated. For some LEDs, the maximum junction temperature can be as high as 150°C, while the normal operating case temperature is between -40 and 135°C. Each LED manufacturer determines the maximum junction temperature of its LEDs, and this value is increasing for new devices.

Measurement and Management
In the LED Lighting Reliability Report published by the U.S. Department of Energy, the authors noted that the lifetime of LED lighting fixtures is not equivalent to the estimated LED lifetime. Therefore, they emphasized the need to understand how temperature is measured and its impact on the expected lifetime of the system. Centrally located temperature sensors are not sufficient to monitor, detect and protect multi-LED designs.

The National Electrical Manufacturers Association standard "Recommended Standard for Solid-State Lighting Accessory Interfaces for Lighting Fixtures" "NEMA LSD 45-2009" provides detailed recommendations for LED temperature test points. With properly placed temperature sensors, the temperature can be measured, monitored and controlled by adding circuits. The added circuits will increase the complexity of SSL design, but will also improve the performance, reliability and life of lighting fixtures.

Although thermal management is critical to the electronics of SSL, thermal design is one of the most important factors affecting the aesthetics of the luminaire. The challenge facing lighting fixture designers is to make the aluminum housing aesthetically pleasing. Thermal management and heat sinks combine to ensure high performance and long-term reliability, making SSL an ideal technology for energy-efficient lighting.

Thermal testing is an important factor in SSL luminaire design. Several questions related to thermal performance need to be answered satisfactorily, including:

● Is the thermal design adequate to ensure the efficacy of the light source?

● Is a heat sink required, or does the lighting fixture structure ensure adequate heat dissipation?

● Does the thermal design take into account the worst-case scenario?

Answering these questions requires a detailed thermal analysis and may require multiple iterations to optimize the final design. This can be very time consuming, especially when outsourced testing services are utilized. [page]

A Systems Approach
Given the complexity of accurately measuring temperature, coupled with the desire to reduce time to market, a test tool specifically designed to accurately record temperature can simplify the task of verifying that a design is correct.

An example of this approach is the tool provided as part of TE Connectivity’s NEVALO system. The use of this tool demonstrates how the key elements required for thermal analysis can be integrated to meet the needs of the user.

The thermal assessment tool (see Figure 3) consists of three parts:
1. An LED lighting module (LLM) configured for thermal testing. This module is almost identical to the production LLM, except that it includes the thermal test circuit (thermistor).

2. Interconnects for connection to thermal test equipment.

3. Graphical user interface (GUI) test software running on a PC.

Figure 3 A four-LED lighting module (a) and its thermal test board counterpart (b). The special plug on the right side of the test board can be connected to a computer's USB port.

With such a tool, designers can verify the thermal performance of each LLM in an actual lighting fixture. Therefore, the tool helps to monitor the LED junction temperature under different operating and environmental conditions. In addition, the tool can also provide the following assistance:

● Analyze LLM design and correlate the design with LED junction temperature;

● Optimize LLM reliability test;

● Optimize the size of the heat sink required to keep the LED at a safe operating temperature.

The tool will record and log the LLM temperature in a chart recorder-like format on a GUI displayed on a computer screen. Temperature readings can be in degrees Celsius or Fahrenheit with an accuracy of ±0.1°C. Although the tool provides quite detailed information, it also provides a simple "pass"/"fail" indication for acceptable thermal design. A stable temperature in the green zone indicates an acceptable thermal design, but temperatures too close to the bottom of the green zone indicate that a heat sink is unlikely to be needed.

Figure 4. Computer GUI display of thermal test tool data provides documentation and record of LLM temperatures.

Using the data obtained from the thermal assessment tool, the entire SSL system can be tested and the results recorded to verify whether the system performance meets the LED design specifications well, while also determining the safety factor of the luminaire design.

As lighting manufacturers transition from traditional (incandescent) designs to solid-state lighting designs, thermal design and thermal performance add a new, and perhaps unfamiliar, layer of design complexity. Rigorous evaluation of thermal performance is essential to ensuring the efficacy and life expectancy of SSL lighting fixtures.