Theoretical considerations for the design of self-diagnostic circuit for LED based street lightings Sándor Glisics 1 , Zoltán Kovács 2 , Gábor Marosy 3 , András Poppe 4 1 glisics@eet.bme.hu, 2 kovacsz@eet.bme.hu, 3 marosy@eet.bme.hu 4 poppe@eet.bme.hu Budapest University of Technology & Economics, Dept. of Electron Devices, PO Box, H-1521 Budapest, Hungary ABSTRACT: Nowadays street lights employing Solid State Lightning devices are increasingly gaining ground. This paper deals with measurement methods of self-diagnostic electronic circuits for street lamps. Diagnostics is based on thermal transient measurement of LEDs. The measurement is achieved via the change in the forward voltage of the p-n junction. This paper shows and compares three different measurement schemes. One of the schemes is demonstrated with a concrete circuit realization. Shunted LED method was selected for demonstration. 1 Introduction Nowadays street lights employing Solid State Lightning devices are increasingly gaining ground. These light emitting high power diodes are mounted into the lamp in groups of multiple units [1][2][3]. As these are relatively high power LEDs, producing heat during their operation, an appropriate cooling must be installed by the manufacturer. The cooling of light emitting diodes is usually implemented using the metal housing of the lamp to conduct the heat. As the lifetime of an LED is highly dependant on its operating temperature, it is therefore an important task to ensure that the temperature stays below a critical limit. The elevated temperature also decreases the emitted light intensity [2][3]. Using the temperature dependence of the p-n junction conclusions can be drawn regarding the heat transfer paths. Using this quantity it is possible to determine qualitative quantities of the heat conduction [8]. 2 Thermal transient measurement of LEDs During a thermal transient measurement of LEDs the measurement of the network of heat resistances and heat capacitances from the semiconductor to the outside world is targeted. These values and the time dependence of them originating from ageing can be determined using heating and cooling curves. Therefore, based on the cooling and heating transients it can be decided whether the thermal resistance of any of the LED's heat transfer path changed drastically due to transformation, based on which the degradation of thermal heat conductors (e.g. de- lamination) might be induced. Forward Voltage Range U LED Input dynamic range U Time Heating curve Figure 1: LED heating curve The heating curve on Figure 1 can be obtained by suddenly increasing (stepping) the current flowing through the LED. As a result of the abrupt current change, the on-voltage of the LED abruptly increases as well at the beginning. The following, second phase is the so- called heating curve. In case of a cooling curve, the high current flow of the LED is abruptly decreased. Then the cooling curve can be measured, which is conducted similarly as for the heating curve, but with the process running in the opposite direction [2][3][7]. LEDs used in our design have the following parameters: typical forward voltage is 3,7V, which has -2mV/°C typical temperature coefficient and 9°C/W thermal resistance junction to case [9]. 3 Measurement principle During the measurement, a current change which can be considered as a unit step function is forced on serially connected LEDs, while the time variance of the on- voltage is measured. According to measurements, a function with a rise time of few hundred microseconds can be considered as an unit step, as it is 2 or 3 magnitudes faster than the dominant time constants [3][8]. Considering the measurement arrangement, it seems obvious to measure the on-voltage of the LEDs directly via an A/D converter, as shown on Figure 1. 115 2012 13th Biennial Baltic Electronics Conference (BEC2012) Tallinn, Estonia, October 3-5, 2012 978-1-4673-2774-9/12/$31.00 ©2012 IEEE