Deep UV light emitting diodes (UV-C LEDs) have numerous applications in UV curing, phototherapy, disinfection, water purification, fluorescent spectroscopy, bio-analysis and detection, sensors and monitors. Generally, the UV-C LEDs are based on AlGaN and present competitive new solid-state lighting ultraviolet sources. UV-C LEDs have numerous advantages over conventional mercury lamps. For instance, while the emission wavelength of low-pressure mercury lamps is fixed at 253.7 nm, the emission wavelength of DUV-LEDs can be tuned to various individual wavelengths across the UV spectrum. Consequently, they have earned a place in the ranks of most sought-after research areas. So far, research has shown that junction temperature (TJ) control is mandatory to improve the optical efficiency, lifetime and the wavelength accuracy of the UV-C LED light sources. Thermoelectric cooler devices have also been shown to allow a critical control of the junction temperature, based on direct measurement of the solder point temperature. Unfortunately, several fundamental shortfalls have inhibited their progress.
Research has shown that the optical performance of a UV-C LED device decreases, due to a variety of key factors such as quantum-confined Stark effect and electron leakage. In addition, excessive self-heating of the UV-C LED has also been reported to reduce the lifetime of the devices, the optical performance and yield spectral shift of the emission. Often, the range of the electrical LED current is specified by the manufacturer. If the maximal value of TJ is exceeded, the lifetime decreases substantially below the specified maximal life time value. Therefore, it would be beneficial to further assess the optical performance of the UV-LEDs devices.
In this view, a group of researchers from the University of Santiago de Chile, Professor Pablo Fredes, Professor Ulrich Raff, Professor Ernesto Gramsch, José Pascal (PhD candidate) and Javiera Cuenca, proposed to implement a PID control technique to control the voltage in the Thermoelectric Cooler Device (TEC) devices and therefore the desired range of junction temperatures. Their work is currently published in the research journal, Microelectronics Reliability.
In their study, a 285 nm LED source model: VPS171 by Nikkiso Co, LTD was used. To be precise, the researchers obtained the PID parameters with computational simulations based on physical models and experimental recordings of the solder temperature dynamics. More so, the maximal value of the junction temperature (TJ) provided by the manufacturer, which was specified for the samples used in their study was 100 °C.
The authors reported that the TEC made it possible to reduce the solder point temperature (TS), and therefore the TJ. In addition, the team observed that control of the TEC voltage allowed the temperature control of TJ. The consistency between the PID parameters obtained in the simulation and recorded temperature data, verified their junction temperature control model. Moreover, each UV-C LED device was seen to have a different thermal performance, depending on many factors, e.g. thermal conductivity of the heat dissipaters, the fan extractor velocity and operation ambient temperature.
In summary, the study by Professor Pablo Fredes and his colleagues demonstrated that the junction temperature of the UV-C LED devices could be controlled using a Thermoelectric Cooler Device applying an appropriated PID control strategy. Overall, in an interview with Advances in Engineering, Professor Pablo Fredes emphasized that every LED device can be modeled using the above developed control strategy, including a pre-cooling procedure and the voltage driving mode, owing to the fact that at lower temperature operations, optical performance was improved.
P. Fredes, U. Raff, E. Gramsch, J. Pascal, J. Cuenca. Junction temperature control of UV-C LEDs based on a thermoelectric cooler device. Microelectronics Reliability, volume 98 (2019) page 24–30.Go To Microelectronics Reliability