Matthias Wendt

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.

LEDs in Real Lighting Applications: from Niche Markets to General Lighting

Due to the enhancements in the field of solid state lighting, being in price per lumens or lumens per watt applications for LEDs in lighting products migrate from niche to general lighting. A number of shortcomings today are discussed as well as concepts for improvements

Alternative sustain driver concepts for plasma display panels

Plasma display panels (PDP) are used in large flat TV sets today. Although plasma displays show high performances their use is still restricted to high-end professional applications because of high cost. A large part of the cost is caused by the electronic which covers the whole backside of the panel. Almost three quarter of the electronic is related to sustain drivers. These drivers provide the panel with high frequency AC voltages of about +/-175 V. Because of the energy stored in the electrode capacitance and the high repetition frequency, sustain drivers in PDP consist of transistor H-bridges and energy recovery circuits. These energy recovery circuits are based on resonant topologies, which are formed by the parasitic capacitances of the panel and discrete chokes. Ideally all energy, repetitively stored in the capacitance of the panel, can be recovered. However, the required MOSFETs, the chokes and the PCB connections cause voltage drops, which damp the oscillation, leading to hard-switching operation in the sustain driver units applied today. The paper thus describes the topology and the fundamental operation of standard sustain drivers and it proposes two alternative solutions. Both solutions allow real zero voltage switching. The first alternative sustain driver concept only uses another control scheme, which can be easily installed in the present hardware. The second new concept uses split auxiliary DC voltages for the resonant commutation. The two DC voltages are adapted to perform a complete resonant commutation of the panel voltage with the lowest current. This solution needs a small buck and a small boost converter to be designed for continuous power flow. Since up to 8 sustain driver units are operating in parallel at the same DC link voltage, the required buck and boost converter can be used for all sustain driver units. Prototypes of the standard and the alternative sustain drivers have been built and successfully tested.