Addressing design issues when working with UVC LEDs
As the performance of UVC LEDs increases, adoption of this relatively new technology is gaining momentum in life sciences and environmental monitoring instrumentation. As in all emerging technologies, the designer must be aware of some fundamental differences with respect to the incumbent solution and not assume “drop-in” replacement. This allows designers to realize the full benefits of UVC LEDs. With careful consideration, UVC LEDs can decrease footprint and power consumption – improving cost of ownership for the end user.
UVC LEDs in instrumentation
Interest in UVC LEDs for spectroscopy is increasing as they can address market trends around miniaturization, decreasing costs, and real-time measurements. Unlike deuterium or xenon flash lamps, LEDs emit a narrow spectrum where all the light output from the device is useful for measurement. Users can select the specific peak wavelength of interest based on their application requirements. In specific applications, standardized measurement methods have been developed with a mercury lamp emission line at 254 nm. For instance, water and air quality as measured to EPA standards requires an LED closely matched to the 254 nm peak wavelength. Table 1 illustrates some of the important organic compounds in life sciences research, pharmaceutical production, and environmental monitoring that can be identified with spectroscopy.
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Table 1 Common organic compounds with peak absorption wavelength
The other primary criterion for light source selection in instrumentation is light output at the peak wavelength. Because LEDs have a single peak, the light output is concentrated at a particular wavelength, unlike other UV lamps. Absorption spectroscopy applications generally require a low level of light output – 1 mW or less. However, in cases where the flow cell is isolated from the light source, higher output is required due to significant light attenuation before the signal reaches the cell. This can raise the required light output from the LED to well over 1 mW. In fluorescence spectroscopy, signal strength is directly proportional to light intensity. The excitation power depends on the trace concentration levels that need to be detected, so in these applications the light output required from a single LED can be greater than 2 mW. Figure 1 shows the irradiance comparison between common UV light sources in instrumentation. Although the input power is much less for the LED, the irradiance at the desired UVC wavelength is higher than the other sources making it a more efficient light source for the specific measurement.
Figure 1 This graph compares the irradiance of a UVC LED, xenon flash lamp, and deuterium lamp.
After wavelength and light output are selected, another important parameter is the viewing angle as it impacts the instrument optical train. Broadly, there are two options – narrow or wide angle. The former is achieved with a ball lens, the latter with a flat window. The narrow viewing angle allows for high intensity of light available over a small area. This package type is typically used when directly focusing the light into the instrument.
A flat window package has a wider radiation pattern that has a greater tolerance in alignment with fiber for remote coupling. It is particularly useful in applications where the flow cell must be isolated from the light source and electronics, like in monitoring high temperature chemical processes or in chromatography with highly volatile solvents. When practical, a narrow angle ball lens can keep components in the instrument to a minimum, while the flat window provides enhanced flexibility in design.
Optimizing the drive current allows the designer to balance the light output with the lifetime requirements of the application. Driving an LED below the manufacturers rated current will decrease the light output, but it will also increase the lifetime of the light source. In applications that require a high LED output power, some end users choose to operate LEDs at an elevated current above datasheet specifications. Increasing the drive current in this way allows for an increase in light output, but also poses certain risks in performance.
Overheating is a common issue that can negatively impact both the light output and lifetime of LEDs. Due to the instant on-off capability of LEDs, one can quickly turn on and off an LED in a periodic manner. Applications in fluorescence, which typically require higher light outputs, commonly use pulsed mode (duty cycle) operation to more safely increase LED current. Duty cycle is the percentage in one period in which an LED is turned on; where a period is the total time it takes to complete an on-and-off cycle. An LED operating at a 50% duty cycle, for example, would be turned on exactly half of the time and off half of the time. Figure 2 shows the normalized light output at various drive currents and duty cycles.
Figure 2 Here we see the impact of a varied duty cycle on normalized light output, while on time is held constant at 500 μs. Normalized power is relative light output power compared to the light output at maximum rated operating current of 100 mA with the appropriate heat sink.
Operating the LED at high currents impacts the LED junction temperature, which consequently impacts lifetime and light output. Optimizing the duty cycle can minimize the impact of the increased drive current on the junction temperature, thereby preserving the LED performance. Figure 3 illustrates the impact duty cycle can have on maintaining the junction temperature of the LED. By operating at a 5% duty cycle, one could achieve over three times the light output (as seen in Figure 2) with minimal impact on junction temperature.
Figure 3 This graph shows the impact of varied duty cycle on junction temperature, while on time is held constant at 500 μs.
Excess heat negatively impacts the light output and lifetime of an LED. Over the long-term, this heat decreases the useful life of the LED. Thermal management is extremely important when designing with UVC LEDs because of the greater proportion of energy converted to heat compared to longer wavelength LEDs. Proper thermal management keeps the junction temperature as low as is required for the given application and maintains the performance of the LED. In addition to passive and active cooling methods, the selected PCB can also result in better heat dissipation.
Figure 4 This figure shows the thermal pad temperature of FR4 and aluminum core PCBs without a heat sink (a) compared to the thermal pad temperature of an aluminum core PCB with and without a heat sink (b).
FR4 is one of the most commonly used PCB materials due to its relatively low cost, but it also has low thermal conductivity. In systems where the thermal load in the system is high, metal core PCBs, which have better thermal conductivity, are superior options. As heat dissipation needs increase, designers typically turn to increasing the PCB area and add heat sinks for superior thermal management. Designers can employ more active cooling techniques if further heat dissipation is required.
As the performance of UVC LEDs increases, designers are leveraging the benefits of design flexibility for spectroscopic instruments and disinfection reactors. The benefits of LEDs in these applications allows for more compact, efficient, and, quite often, more cost-effective designs. As this technology continues to develop, clever designers will discover more ways to leverage the benefits of UVC LEDs to address challenges in these markets.
References
"The Case for UVC LEDs in Spectroscopic Instrumentation," Crystal IS.
"Optan SMDs for Biofilm and Biofouling Control," Crystal IS.
OPTAN UVC LEDs datasheets and product documentation
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