Maximizing lumen output of LEDs for automotive exterior signal lighting applications

Light emitting diodes (LEDs) operated at relatively low currents when first used for automotive exterior signal lighting. These devices were generally the 3 mm or 5 mm LEDs that were lighted with an input current of no more than 30 milliamperers per LED. Generally these LEDs were used in center high mounted stop lamps or side marker lamps. In most cases the center high mounted stop lamps required 30 to 70 of these low powered devices depending on LED type, location on the vehicle and customer requirements. Today LEDs are being used for not only the center high mounted stop lamp, but literally all exterior automotive signal lamps. These LEDs are exceedingly brighter than the 3 mm or the 5 mm devices were. These brighter devices require the use of fewer LEDs per lamp function, but at the expense of requiring higher drive currents thus generating more heat. It is this heat generation that creates an issue that must be addressed by the lamp designer. The heat being generated by the latest generation of LED devices must be removed to ensure that the device maintains it capabilities to efficiently generate light. To enable the LED to operate efficiently certain factors of the LED must remain unchanged as it is operated over the life of the lamp in an automotive application. Two factors that must experience little or no change are the lumen output of the LED and the emitted color. The heat issue must be analyzed and eliminated or controlled when utilizing the newest "high power," high current devices in an automotive exterior signal lamp. The scope of this paper will identify the parameters that must be controlled to minimize the heat generated by the chip such that the light output of these latest "high power" LEDs is maximized and maintained.