“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 ti = 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:
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.
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?”