Enhancement of luminous flux of InGaAlP-based low-power SMD LEDs using substrates with different thermal resistances

Optimization of light-emitting diodes’ (LEDs’) design together with long-term reliability is directly correlated with their photometric, electric and thermal characteristics. For a given thermal layout of the LED system, the maximum luminous flux occurs at an optimal electrical input power and can be determined using a photo-electro-thermal (PET) theory. The purpose of this study is to extend the application of the luminous flux equation in PET theory for low-power (LP) LEDs.

LP surface-mounted device LEDs were mounted on substrates of different thermal resistances. Three LEDs were attached to substrates which were flame-retardant fiberglass epoxy (FR4) and two aluminum-based metal core printed circuit boards (MCPCBs) with thermal conductivities of about 1.0 W/m.K, 2.0 W/m.K and 5.0 W/m.K, respectively. The conjunction of thermal transient tester and thermal and radiometric characterization of LEDs system was used to measure the thermal and optical parameters of the LEDs at a certain range of input current and temperature.

The validation of the extended application of the luminous flux equation was confirmed via a good agreement between the practical and theoretical results. The outcomes show that the optimum luminous flux is 25.51, 31.91 and 37.01 lm for the LEDs on the FR4 and the two MCPCBs, respectively. Accordingly, the stipulated maximum electrical input power in the LED datasheet (0.185 W) is shifted to 0.6284, 0.6963 and 0.8838 W between the three substrates.

Using a large number of LP LEDs is preferred than high-power (HP) LEDs for the same system power to augment the heat transfer and provide a higher luminous flux. The PET theory equations have been applied to HP LEDs using heatsinks with various thermal resistances. In this work, the PET theory luminous flux equation was extended to be used for Indium Gallium Aluminum Phosphide LP LEDs attached to the substrates with dissimilar thermal resistances.