Understanding the Relationship between UVC LED Lifetime and Power

“What is your product lifetime?” is a common question from engineers who are evaluating UVC LEDs for disinfection. This question is difficult to answer without adding many conditions, caveats and footnotes—many that often make the answer irrelevant to the engineer.

In UVC LEDs, lifetime is not a fixed specification.

Lifetime is a variable that engineers control after considering how the UVC LED will be used. UVC LED, by its nature, is a constant current device. As the forward current applied to the diode is changed, the UVC output of the diodes increases or decreases in a proportional manner.

In addition, the UVC output of an LED degrades overtime. The rate of degradation varies with both the UVC LED and the operating conditions. Principal among these are temperature (degradation is slower at lower temperatures), forward current (slower at lower drive currents), and materials properties. By balancing these factors, designers can build systems that prioritize output over lifetime or vice versa.

This article provides the basic definitions of lifetime in the case of UVC LEDs and the variables that impact lifetime in disinfection applications.

What is LED lifetime?

Let us start with the formal definition of lifetime: LED lifetime is defined by the L and B values. It is important to note that with any lifetime specification, we’re talking about behaviors of populations of LEDs, not discrete devices.

The L value, commonly known as the intensity or maintenance factor, refers to the percentage of the initial power output at time t_i = 0 which remains at time t = x.

For instance, for a 20mW LED driven at 350mA, a lifetime of 3,000 hours for L50 value would mean that after 3,000 hours of continuous on-time (at specific operating conditions), the LED output would be 50 percent of the initial rated value, and thus emits 10mW of power.

The B value, commonly referred to as the reliability value, is defined as the minimum percentage of an LED population which will work within the L-value specification.

For instance, an LED at 1,000 hours, you may have two different lifetime specifications for the same population of LEDs:

• L85B50: at least 50 percent of the LEDs used will have over 85 percent of the initial output after 1,000 hours
• L70B10: at least 90 percent of the LEDs used will have over 70 percent of the initial output after 1,000 hours

Changing the B value rating only affects the defined lifetime, not the LED. When evaluating LEDs based on B Values, the main question is: what are the reliability requirements in your application? For instance, having an array of 20 LEDs will be less impacted by one LED performing out of specs that an array of two LEDs.

Finally, lifetime needs to be considered for the system, not just the LEDs. This includes the LED driver, and other components.

How to design UVC LED-based devices around lifetime

When approaching disinfection applications, three main questions need to be answered.

1. How much power is required at end-of-life? This is typically answered looking at the target pathogen and/or disinfection level, application (surface, water, air), and sub-questions related to the application. For water: Ultraviolet transmittence (UVT), flow rate, geometry of reactor, and reactor materials. For surface: area size, environment, distance between light source and disinfection target, and Health & Safety
   
   
2. What does end-of-life mean for the application? Lifetime is relative—it’s the amount of power at a point in time. Therefore it is essential to understand what this point in time is. The usage is equally important as the lifetime refers to the total continuous on-time, and pulsing/duty cycle therefore increases the number of hours rating linearly. This information coupled with the amount of power required allows design engineers to optimize operation to ensure desired disinfection rates for the full lifecycle of the product.
   
   
3. Which UVC LEDs are being used? Information on operating specifications (maximum current, thermal resistivity, viewing angle, etc.), lifetime ratings (L, B values, with initial output and wavelength (including tolerances around binning)) is the final piece for designing around end of life requirements.

Conclusion

Lifetime is not a specification of the LED—like wavelength or thermal resistance—but a variable affected by operating conditions. Lifetime can be tuned by operation (current, pulse), system (temperature), and reliability (all LED lifetimes are provided based on the predictability of a tested population). Therefore, a number of hours with an L value is irrelevant and needs to be associated with a B value, operating conditions, solder temperature, and power requirements.

Rather than asking, "What is the lifetime of your product?" the more precise question to ask is, “How much power at a certain wavelength is required at a certain point in time in your application?”