Describe
With the improvement of UVC LED performance, the adoption of this relatively new technology is gaining momentum in life science and environmental monitoring instruments. As with all emerging technologies, designers must be aware of some fundamental differences related to existing solutions, rather than assuming "plug-in" replacements. This enables designers to realize the full advantages of UVC LEDs. After careful consideration, UVC LEDs can reduce footprint and power consumption-increasing the cost of ownership for the end user.
UVC LED in the instrument
The interest of UVC LEDs for spectroscopy is increasing because they can address market trends around miniaturization, cost reduction, and real-time measurement. Unlike deuterium or xenon flash lamps, the spectrum emitted by the LED is very narrow, and all the light output of the device can be used for measurement. The user can select the specific peak wavelength of interest according to the application requirements. In certain applications, a standardized measurement method has been developed, and the emission line of the mercury lamp is 254nm. For example, water and air quality measured according to EPA standards require LEDs to closely match the peak wavelength of 254 nm. Table 1 shows some important organic compounds in life science research, drug production and environmental monitoring, which can be identified by spectroscopy.
Table 1 Common organic compounds with peak absorption wavelength
Another main standard instrument for light source selection is the light output of the peak wavelength. Because the LED has a single peak, unlike other UV lamps, the light output is concentrated at a specific wavelength. Absorption spectroscopy applications usually require low levels of light output-1 mW or less. However, in the case where the flow cell is isolated from the light source, a higher output is required due to the significant light attenuation before the signal reaches the battery. This can increase the light output required by the LED to more than 1 mW. In fluorescence spectroscopy, signal intensity is directly proportional to light intensity. The excitation power depends on the trace concentration level that needs to be detected, so in these applications, the light output required by a single LED may be greater than 2 mW. Figure 1 shows the irradiance comparison between common UV light sources in the instrument. Although the input power of the LED is much smaller, the required UVC wavelength irradiance is higher than other light sources, making it a more effective light source for specific measurements.
Figure 1 This chart compares the irradiance of UVC LED, xenon flash lamp and deuterium lamp.
After selecting the wavelength and light output, another important parameter is the viewing angle, because it will affect the optical system of the instrument. Broadly speaking, there are two options-narrow angle or wide angle. The former is achieved with a spherical lens, and the latter has a flat window. The narrow viewing angle allows high-intensity light to be obtained in a small area. This type of package is usually used when focusing light directly into the instrument.
The plane window package has a wider radiation pattern, and has a larger tolerance for alignment with the optical fiber, and can be used for remote coupling. It is particularly suitable for applications where the flow cell must be isolated from the light source and electronic equipment, such as monitoring high temperature chemical processes or high volatile solvent chromatography. In practical applications, the narrow-angle spherical lens can keep the components in the instrument to a minimum, while the flat window can improve design flexibility.
Optimize the drive current, so that the designer can balance the light output and application life requirements. Driving the LED below the manufacturer's rated current will reduce the light output, but will also increase the life of the light source. In applications that require high LED output power, some end users choose to run LEDs at higher currents than data sheet specifications. Increasing the drive current in this way can increase the light output, but it also brings certain performance risks.
Overheating is a common problem that will negatively affect the light output and life of the LED. Due to the instantaneous switching capability of the LED, people can quickly turn on and off the LED periodically. Applications in fluorescence generally require higher light output, and pulse mode (duty cycle) operation is usually used to more safely increase the LED current. Duty cycle refers to the percentage of a period of time that the LED is turned on; the period is the total time required to complete a switching cycle. For example, an LED operating at a 50% duty cycle will turn on exactly half the time and half the time. Figure 2 shows the standardized light output at various drive currents and duty cycles.
Figure 2 Here, we see the effect of varying duty cycle on normalized light output, while the on-time remains constant at 500μs. The standardized power is the relative optical output power, compared with the optical output of the maximum rated operating current of 100 mA, using an appropriate heat sink.
Operating the LED under high current will affect the LED junction temperature, which will affect the LED junction temperature and affect the life and light output. Optimizing the duty cycle can minimize the impact of increased drive current on junction temperature, thereby maintaining LED performance. Figure 3 illustrates the effect of influencing the duty cycle on maintaining the junction temperature of the LED. By working with a 5% duty cycle, more than three times the light output can be achieved (as shown in Figure 2), with minimal impact on junction temperature.
Figure 3 This graph shows the effect of varying duty cycle on junction temperature while the on-time remains constant at 500μs.
Overheating will have a negative impact on the light output and life of the LED. In the long run, this heat will reduce the lifespan of the LED. When designing with UVC LEDs, thermal management is very important because the proportion of energy converted into heat is greater than that of long-wavelength LEDs. Proper thermal management can keep the junction temperature at the lowest temperature required for a given application and maintain the performance of the LED. In addition to passive and active cooling methods, the selected PCB can also bring better heat dissipation.
Figure 4 This graph shows the thermal pad temperature (a) of FR4 and aluminum core PCB without heat sink compared to the thermal pad temperature (b) of aluminum core PCB with and without heat sink.
FR4 is one of the most commonly used PCB materials because of its relatively low cost, but it also has low thermal conductivity. In a system with a higher thermal load in the system, a metal core PCB with better thermal conductivity is a better choice. As the demand for heat dissipation increases, designers usually turn to increasing PCB area and adding heat sinks to achieve excellent thermal management. If further heat dissipation is required, designers can use more active cooling techniques. As the performance of UVC LEDs improves, designers are taking advantage of the design flexibility of spectroscopic instruments and disinfection reactors. The advantages of LEDs in these applications allow for more compact, efficient, and often more cost-effective designs. With the continuous development of this technology, smart designers will find more ways to use the advantages of UVC LED to meet the challenges of these markets.
