by Steve Bowling,
While LEDs offer mainstream lighting applications benefits such as long life, durability and high efficiency, the lifetime of an LED product may be significantly shortened without proper thermal management safeguards in your design.
It’s hard to miss the news these days that high-brightness LEDs are finding their way into mainstream lighting applications. Suppliers are making daily announcements about the next model of LED that is more efficient than the last. The high efficiency of LEDs, along with their durability and long lifetimes, make the technology seem like a “no-brainer” for general-illumination products. However, LED technology introduces many new challenges to the designer of a lighting-fixture product. An electronic driver circuit is a must for most applications, and optics may be needed to get the necessary illumination performance. Furthermore, a proper thermal design is required to ensure that the LED temperature remains within limits. Without proper thermal protection and control, the lifetime of an LED product can be shortened from many years to just months.
LEDs Versus Incandescent Light Sources
Place your hand near a typical incandescent light source. You will likely feel some amount of heat. Much of the energy put into the incandescent bulb is released as heat through radiation. LED light sources are much different than the ordinary light bulb because they radiate a minimal amount of heat energy. This is a major advantage in some lighting applications because heat is not produced in the illuminated area. However, wasted energy is absorbed into the LED itself and must be conducted away from the LED through heat-sinking materials. Then, convection cooling must be used to transfer heat into the surrounding air.
The operating lifetime of an LED is directly tied to the operating current and, more importantly, the junction temperature during operation. Unlike an ordinary light bulb, the LED does not instantly fail. Instead, the light output of the LED gradually decreases with time. LED manufacturers characterize the lifetime of their products using lumen maintenance. The failure point of an LED is generally agreed to be the time when the light output, measured in lumens, has decreased to 70 percent of the original output. This operating lifetime, measured in hours, is known as L70.
LEDs Present Thermal Management Challenges
LED manufacturers and fixture designers have a huge thermal design challenge because of the high power densities that exist at the LED dice level. The LED packaging must take the high power density at the LED junction and spread it over a larger area. In effect, the manufacturer must provide the most efficient thermal conduction path possible, while balancing other design factors such as physical dimensions, optical performance and cost.
The LED has thermal limits similar to a silicon device. Most can survive junction temperatures as high as 150°C. However, the operating temperature must be kept much less than the maximum TJ specified in the datasheet; to achieve L70 times that make LEDs practical for use in lighting fixtures. For example, a typical high-power LED might have a L70 of 50,000 hours when operated at a TJ of 85°C.
The ability of the LED package or other heat-conducting medium to transfer heat is expressed in terms of thermal resistance, or q. The thermal resistance between two physical points is specified so that the effectiveness of a thermal path can be evaluated. The thermal resistance of a path between two points is specified in degrees Celsius per Watt (°C/W). Given the operating power, a temperature difference between two points can be calculated if the thermal resistance is known.
To find the actual junction temperature from a temperature measured at the solder point, the LED operating power and the thermal resistance of the LED package must be known. The LED power can be calculated from the LED forward voltage, VF, and the operating current, I. The thermal resistance from the LED junction to the solder point of the package, ØTS, can be found from the LED datasheet. The LED junction temperature, TJ, can then be calculated as follows:
TJ = VF • I • Ø + TS, where TS is the temperature measured at the solder point.
Clearly, a lighting-fixture designer must perform these measurements and calculations to ensure that the mechanical design provides proper cooling in the anticipated operating environment. However, there are some cases where electronic thermal protection is a good idea to ensure the long-term reliability of the product.
With LED fixtures, the heat sink must be an integral part of the fixture enclosure and must be exposed to the elements for convection cooling. In outdoor, industrial and automotive applications, air pathways can become clogged with dust or dirt. Additionally, lighting fixtures may get installed without proper airflow. For example, a ceiling installation may not allow proper cooling because of high temperatures above the ceiling or inadequate clearances for convective cooling.
Incorporate Temperature Sensors In Your Design
One method that can be used for electronic thermal protection is a logic-output temperature sensor. This temperature sensor has a fixed temperature threshold. An open-drain output allows the temperature sensor to be interfaced to a driver circuit in a variety of ways. Many solid state lighting driver (SSLD) ICs have an “enable” input that can be used for ON/OFF control or for dimming applications. The output of the temperature can be connected to this input and used to shut down the electronic driver when necessary, as shown in Figure 1.
Figure 1. A logic output temperature sensor can shut down an SSLD at a specific threshold or adjust the reference input to lower the operating temperature.
The open-drain output can also be connected to the analog reference circuitry associated with the driver. When the operating temperature becomes too high, the logic temperature sensor adjusts the current-regulation reference, reducing the LED drive current and operating temperature. This application of the logic temperature sensor allows the light fixture to continue operation, but at a reduced power level.
LED fixtures break the “light bulb and fixture” design model, as the LEDs could likely last for the fixture’s entire lifetime — there is no light bulb to be periodically replaced. However, it is possible to make modules or “light engines” that integrate LEDs and the electronic driver. A light-fixture designer may wish to build a module that can be installed into a variety of applications. In this scenario, the amount of cooling available for the light engine would be unknown.
Figure 2. The output of an analog temperature sensor is measured with a small MCU to provide adaptive thermal control.
A temperature sensor with an analog voltage output can be used to provide variable temperature protection. The sensor provides a calibrated, linear output that can be used to directly control the current output of an LED driver circuit. Another approach is shown in Figure 2. This approach uses the analog temperature sensor as an intelligent temperature controller. The output of the temperature sensor is connected to an analog-to-digital converter (ADC) input of a small microcontroller (MCU). The MCU generates a pulse-width modulation (PWM) signal that is connected to the dimming input of the SSLD. Using this method, the MCU can detect temperature trends and adjust the amount of light output, to lower the temperature as required. This type of circuit provides an adaptive solution that would provide thermal protection and preserve LED operating lifetime for any type of application or environment.
Steve Bowling is a staff engineer at Microchip Technology Inc. For more information, contact Microchip Technology, 2355 West Chandler Blvd., Chandler, AZ 85224; (480) 792-7200; www.microchip.com
For Further Reading
2. “ASSIST recommends: LED Life For General Lighting”, a publication of the Alliance for Solid State Illumination Systems and Technologies, Lighting Research Center, Rensselaer Polytechnic Institute.
3. “Cree Xlamp LED Thermal Management,” Cree Inc.