Structure-Aware Compact Thermal Models of Power LEDs

This paper based on the example of white power LEDs illustrates the methodology for the generation of device compact thermal models, whose element values can be assigned physical meaning. The diode thermal behaviour was studied both with the forced water cooling and with the natural convection air cooling. Moreover, owing to the fact that the investigated devices had an electrically isolated thermal pad, the measurements were carried out with the thermal pad properly soldered and with the pad left unconnected, what facilitated the identification of particular sections in the heat flow path. All the measurements of device heating or cooling curves were taken according to the JEDEC standards. The determination of the optical power allowed the computation of the real heating power, which was used then as the input quantity for thermal computations and analyses presented in this paper. Based on the measurement results, thermal structure functions and time constant spectra were computed using the Network Identification by Deconvolution method. The compact thermal models of the investigated LEDs were derived based on the time constant spectra. Owing to the proposed methodology, it was possible to attribute physical meaning to model element values. The accuracy of generated compact models was validated by comparing the simulated heating curves with the measured ones. Although the compact models for the investigated cases consisted only of four RC stages, they provided excellent simulation accuracy with errors below 4% of the maximum temperature rise value.