Since the invention of the incandescent lightbulb in the 19th century, lighting devices have become brighter, smaller, more efficient, and more specialized. Light-emitting diodes (LEDs), which entered the market in the late 1960s, are among the most critical developments since Edison's original bulb.
LED technology offered a more efficient alternative to the incandescent lightbulb, but the manufacturing cost made early versions prohibitively expensive for many designs. As the adoption of LED technology grew, semiconductor manufacturing became more streamlined, eventually making LEDs cheaper and more accessible. Semiconductor manufacturing ― much of which was optimized for LED and transistor development ― focused on creating smaller, more efficient devices. Chip-on-Board LEDs (COB LEDs) are one of the results of this compact, high-efficiency semiconductor manufacturing.
Difference Between COB and SMD LEDs
COB LEDs feature many LED chips that are fused and integrated into a printed circuit board. The board utilizes a single circuit to power the LED chips simultaneously. Surface-mounted-diode (SMD) LEDs use many independent circuits to power a single diode.
The main advantage of using a COB LED is its diode density. Diode density is a measurement of how many diodes can fit into a single area. COB LEDs generally have nine or more diodes arranged in a matrix configuration, all of which utilize the same circuitry for power. While this setup limits the LED's color-changing ability, it also allows for extremely high-density lighting and increased efficiencies over SMD options. Some refer to COB LEDs as LED modules since they are technically a module collection of many diodes on a single board package.
Managing COB LED Temperature
If you incorporate a COB LED into your design, you must understand the LED board's cooling requirements. LED modules are extremely efficient and advantageous for lighting applications ― such as roadway and parking lighting ― but they can create significant design challenges when it comes to managing their thermal load. COB LEDs are high-power components that therefore generate high temperatures. For example, the XHP70.2 LED designed by Cree can utilize up to 28.8 Watts of electricity at 6V, which creates a significant amount of light and plenty of unwanted heat.
With no thermal management efforts, the XHP70.2 LED junction's temperature can easily exceed 250 degrees Celsius in a matter of seconds. This will cause the COB LED to degrade and malfunction and can cause adjacent device failures. Consult the COB LED's datasheet to find its Maximum Junction Temperature, the maximum temperature allowed at the junction point before failure. The lower the operating junction temperature, the better.
Regulating COB LED Heat
Cooling a COB LED is similar to cooling of any other type, but a COB LED has a high thermal as a result of its small footprint and high power. This combination presents a significant challenge for the electrical or mechanical engineer, as the high-heat equipment relies on passive or active cooling methods, or a combination of the two. Given the COB LED's high thermal output ability, engineers use active cooling methods such as:
· Liquid cooling
· Synthetic jet cooling
In mass-produced devices that utilize high-power LEDs, passive thermal management devices may be built into the device, such as cooling fins integrated into the head of a streetlight post. Passive cooling doesn't require extra energy to cool the COB LEDs, but it's a complicated endeavor. A passive cooling design must be meticulously engineered to ensure minimum junction temperatures are operationalized.
Cree provides a series of whitepapers that outline specific thermal management techniques for their XLamp LEDs and illustrate how to optimize thermal performance of the PCB that host an XLamp LED module. Engineers can take advantage of a variety of thermal management methods to cool COB LEDs, but precision is essential. With correct thermal management, COB LEDs will remain an efficient, safe, and desirable option for lighting applications.
