Light output stabilisation of LED based streetlighting luminaires by adaptive current control

Temperature dependence of LED operation is often not fully considered during the design of solid state lighting products. If temperature dependence is not carefully considered, solid-state lighting products are typically overdesigned to be too robust enough to fulfil the requirements under any possible environmental conditions. Temperature dependent nature of LEDs though, could even be a new benefit if properly considered. Overdesign means designing for the worst case that is the highest possible environmental temperature when LED efficiency/efficacy is low. With a control scheme resulting in constant emitted total luminous flux significant electrical power saving can be achieved since at lower temperatures, due to increasing efficiency/efficacy less electrical power, thus, lower forward current levels are sufficient. This paper describes different methods to specify the so called iso-flux control of LEDs' operating point, in which effect of temperature changes on light output characteristics is compensated by adjustment of the forward current. Parameters for an automated temperature compensation can be identified with the help of multi-domain LED models. This paper describes our LED multi-domain model based approach applied to the design of the light output control of an existing street-lighting luminaire. During the design of the control scheme real, archived meteorological temperature data set was considered. Based on our model we implemented the temperature compensated iso-flux control of a luminaire and the planned operation was validated by actual measurements. The verified luminaire model was further investigated with multi-domain models of aged LEDs obtained during an LM-80 standard compliant aging of a set of LEDs, characterizing LEDs up to 6000 + h of operating life time.