Fannon Lim

Nonlinear and Parametric Vibrations in Ultrasonic Cutting Systems

Previous research has shown that multimodal interactions typical of autoparametric systems can be analytically modelled in simple structures. In this paper the vibration behaviour of two complex ultrasonic cutting systems is experimentally characterised. The energy exchange from the operating mode into a modal frequency close to half of the tuned frequency, and excitation of combination resonances consisting of two modal frequencies whose sum is close to the tuned frequency, are detected. The measured energy transfers from the primary responses to the internally excited modes are shown to be qualitatively similar to the theory and typical of autoparametric systems.

Effects of Modal Interactions on Vibration Performance in Ultrasonic Cutting

The effects of modal interactions and nonlinear response characteristics can hamper implementation of high power ultrasonic technologies, due to the resulting modal coupling, increased stress, audible noise levels and poor control of the operating response. These different adverse responses are illustrated by characterising the vibration behaviour of single-blade and multi-blade ultrasonic cutting systems. This paper proposes design strategies to eliminate the effects of modal interactions, by focusing on reducing the number of vibration modes, and to reduce the effects of nonlinear responses, by serial coupling of tuned components with appropriate cubic softening and hardening response characteristics.

Implications of Phosphor Coating on the Thermal Characteristics of Phosphor-Converted White LEDs

The phosphor layer in phosphor-converted white light-emitting diodes (pcLEDs) affects their optical and thermal performances. This paper reports the effects of phosphor thickness and particle concentration on the optical efficiency and temperature rise on conformal phosphor-coated LED package. It is observed that a thicker phosphor layer and a higher phosphor particle concentration will increase the amount of backscattering and back reflection of light from the phosphor layer. These light extraction losses not only reduce the optical efficiency of the light output but also cause heat accumulation in the phosphor layer, leading to higher LED junction temperature. At 2700-K correlated color temperature (CCT), the temperature rise is observed to increase by as much as 2.6 times as compared with its blue LED package. However, the self-heating effect can be reduced through its die-bonding configuration. Structure function-based thermal evaluation shows heat accumulation in the phosphor layer and that flip-chip bonding can dissipate heat generated in the GaN LED and phosphor layer effectively. Evidence in this study demonstrates that optical efficiency and thermal resistance of pcLEDs are dependent on the CCT ratings.

Achieving accurate electro-optical-thermal measurements of high-power LEDs

High-power Light Emitting Diode (LED) generates significant amount of heat fluxes that can affect the temperature-dependent properties of the device. This self-heating effect can upset the measurement setup and produce inaccurate readings, leading to misinterpretation of results such as electrical and thermal resistances. Optical, electrical and thermal performances of high-power LED packages were analysed under different temperature feedback controls. The results of these experiments demonstrate the importance of the temperature control module in the measurement setup affecting the devices properties such as the series resistance Rs and the thermal resistance Rth. In the electrical current-voltage measurements, the temperature control module cannot control the self-heating effect effectively, resulting in a lower Rs compared to when the measurements are made manually. In transient thermal measurements, it was found that lower Rth values are obtained when the controller operates in closed-loop adaptive temperature control compared to when it operates in open-loop adaptive temperature control. This paper recommends the manual electrical and open-loop thermal measurement methods for accurate parametric LED analyses.

Effect of packaging architecture on the optical and thermal performances of high-power light emitting diodes

The phosphor and die bonding configuration affect the optical efficiency and thermal performance in phosphor-coated white LEDs. In this paper, light emission studies reveal that the chromaticity shift and light extraction losses depend on the uniformity of phosphor particles deposited over the LED surface. A non-uniform and sparse phosphor layer affects the correlated color temperature (CCT) and the spectral Y-B ratio due to the disproportionate contribution of light emission between the LED device and the phosphor layer. Furthermore, the Y-B ratio was observed to reduce with temperature due to higher Stokes energy and light extraction losses in the phosphor layer. As a result, the Y-B ratio exhibits an inverse relationship with the packages thermal resistance as a function of temperature. On the other hand, the thermal performance of a LED package is dependent on the die-bonding configurations (conventional and flip-chip). Due to the improved heat dissipation capabilities in flip-chip bonding, the temperature rise and thermal resistance of the package was observed to reduce with temperature. By alleviating the heat accumulation in the package, more stable colorimetric properties such as CCT and Y-B ratio can be achieved.

Effects of humidity on the electro-optical-thermal characteristics of high-power LEDs

LEDs are subjected to environments with high moisture in many applications. In this paper, the experiments reveal photometric and colorimetric degradation at high humidity. Corresponding spectral power analysis and parameter extraction indicate that the flip-chip bonded LED samples show accelerated chip failure compared to the conventionally bonded samples. The chip-related failure induces greater heat accumulation, which correlates with the increase in heating power observed in the package. However, the temperature rise and thermal resistance for the flip-chip bonded LEDs do not increase substantially as compared to the conventionally bonded LEDs. This is because the junction temperature can be reduced with a flip-chip die-bonding configuration where the heat generated in the LED chip is dissipated effectively onto the AlN substrate, thereby reducing the increase in temperature rise and thermal resistance. The experimental results are supported by evaluation of the derivative structure functions. In addition, as the thermal resistance of the LED package varies with different humidity levels, there is a need to specify the conditions of humidity in data sheets as LED manufacturers routinely specify a universal thermal resistance value under a fixed operating condition.