Georg Sauerländer

Power driver topologies and control schemes for LEDs

This paper deals with power electronic drivers for LED strings. Due to the enormous progress recently achieved in the technology of light emitting diodes (LEDs) it can be expected that LEDs lighting will replace incandescent and halogen bulbs in general illumination in the near future. A LED light source typically consists of a series connection of single LED cells. It shows a similar behaviour like a zener diode. For efficiency reasons LED strings can not be supplied via series resistors but need switched mode power drivers with current control. Different standard DC-DC converter topologies are discussed which can be adapted to feed a constant current into a LED load. For future LED driver developments it has to be considered that LEDs can also efficiently be supplied by pulsating currents. This simplifies the converter and control design and reduces the number of components. Hence, different converter topologies are studied which are able to stabilise the average value of a pulsating output current. This also includes topologies with galvanic isolation. Resonant operating LED drivers seem to be specially suited for this task. Hence, a series resonant galvanic isolating LED driver is studied in detail. Under certain conditions this converter does not need a current sensor to stabilise the average current in the LED load. Finally, the features of different pulsating current waves are investigated concerning their peak, RMS and high frequency content.

Driver Electronics for LEDs

Large-volume applications of high-brightness LEDs are well established for signaling and signage. They are expected to replace conventional lamps in automotive applications in the near future and incandescent, halogen and fluorescent lamps at least in some general illumination applications within a few years. Innovative driver circuits optimized with respect to functionality, efficiency, cost, size and reliability are an enabler for the successful introduction of new LED based lighting products. This paper consists of two parts. In the first part, the influence of the LED current waveform, which depends on the driver topology and control, on the luminous flux and hence, the luminous efficiency is investigated experimentally for different LEDs, i.e. a red, a green and a blue 1 W LED. In the second part, an LED driver with PCB integrated capacitive layers and a transformer made of ferrite polymer compound having an extreme high power density is presented. This driver uses highly integrated passive components. This new concept is named emPIC (embedded passives integrated circuit). All passive components will be integrated in the printed circuit board (PCB) using structured layers of different materials. Experimental results taken from the driver demonstrate that the LED current ripple stays within the margins defined in the first part of this paper