LEDs play a variety of roles in modern products, from indicating status to assisting with operation to backlighting displays. As support technology, they may not be part of the essential value proposition of the product, but their operation is frequently critical to its function. Here’s what you need to know to successfully integrate LEDs in your next design.
Tip #1: Pay Attention to Thermal Management
LEDs have high wall-plug conversion efficiency. They don’t dissipate energy as heat through the bulb. They do, however, generate a significant amount of heat on the backside. Without thermal management, this heat can build up and reduce LED lifetime. One strategy is to use an LED printed circuit board (PCB), which has a metal substrate of high thermal conductivity, such as aluminum or copper. These materials allow for efficient heat dissipation, crucial for maintaining optimal LED performance.
LEDs are not unique – all electronic components dissipate some electrical power as heat. Placing an LED as close as possible to the panel may seem like a good idea but if it’s in the middle of a cluster of components, it can be damaged by their waste heat. A light pipe provides an easy solution. Light pipes are designed to capture LED output and transfer it to the point of use, whether that is millimeters away or meters away. A light pipe allows the LED to be placed in a less populous part of the board, helping to reduce heat and maximize lifetime.
Tip #2: Match Viewing Angle to the Application
LEDs are available with a variety of packages that modify the output distribution. Flat face LEDs produce very even output patterns suitable for broad viewing angles. LEDs with hemispherical lenses generate more narrowly focused output that is effective for front viewing (see Figure 1). Parabolic lenses result in highly forward directed output that provides good coupling into light pipes. Diffusers provide another degree of design freedom, homogenizing optical output, while transparent lenses create highly focused output.
Figure 1: Directivity radiation diagrams for (from left) a flat-face SMD LED (the Bivar SM0807), a through-hole LED with a hemispherical lens (the Bivar R20BLU-5-0080), and a domed through-hole LED (the Bivar 3BC-X).
Tip #3: Consider a Multicolor LED
Particularly for indication or functional illumination, lights frequently need to change colors. A panel indicator may need to shine green under one set of conditions and red under another. Placing two LEDs side-by-side increases footprint, cost, and energy consumption, as well as wiring. Multicolor LEDs provide an alternative. Consisting of multiple co-packaged LED chips, multicolor LEDs are single devices that can change output depending on how they are driven. The range of colors that a multicolor LED can produce depends upon device design and how it’s driven. Bicolor devices, for example, produce one color when forward biased and the other one reverse biased. RGB triads, on the other hand, can be programmed to generate any color of the rainbow.
Tip #4: Account for Unique LED Failure Modes
A number of factors can influence LED lifespan. As previously mentioned, operating the devices outside of rated temperature will reduce lifetime. LED output is proportional to drive current, but over driving can have negative effects ranging from shortened lifespan to catastrophic failure, which is why LEDs should always be wired in series with a breakdown resistor.
It’s important to note that outside of being significantly overdriven, LEDs don’t typically undergo catastrophic failure. Instead, they simply grow dimmer and dimmer over time until they are no longer bright enough to be effective. For most applications, an LED is considered to have reached end of life when its output has dropped by 30%. Check to make sure that this behavior is compatible with your application and that you have specified the optimal LED.
Tip #5: Take Advantage of Integrated Products
Using LEDs requires more than just the LED itself. They need to be wired with drivers and possibly controllers, as well as current-limiting resistors to protect against over driving. The cost, complexity, and opportunity for error only increase for applications that require multiple LEDs to provide sufficient output. Integrated assemblies are available that include the support electronics, or that combine multiple LEDs in a single unit that simply needs to be connected to power. These devices provide an easy way to speed assembly, reduce complexity, boost yield, and control costs.
Tip #6: Consider a Light Pipe
Light pipes provide a number of benefits for design, manufacturing, and operation of devices incorporating LEDs. They add flexibility to board layout, eliminate hotspots, prevent light bleed between adjacent LEDs, and add ingress protection, when necessary. They’re fast and easy to install and require no special tools. Talk with your vendor today about whether a light pipe is appropriate for your design.
The Bivar engineering team has decades of aggregate experience in integrating LEDs and light pipes into a host of products. Find out how the technology can improve your next design – contact us.