A soft self-commutating method using minimum control circuitry for multiple-string LED drivers

Light-emitting diodes (LEDs) are widely used in general lightings due to their several advantages including high efficiency, high reliability, long life, and environmental friendliness. Recently, various converter-free methods for non-isolated LED drivers with multiple LED strings connected in series have been introduced, enabling both a higher efficiency and power factor (PF) as well as lower total harmonic distortion (THD) [1-3]. In multiple-string LED drivers, the efficiency and PF are enhanced as the number of LED strings increases because of a low overhead voltage. However, as the operational voltage range decreases, it is difficult to find a proper commutation time using input voltage sensing approaches due to input voltage noise and LED voltage variation [4]. Other concerns are EMI and EMC noise caused by high di/dt and dv/dt in hard commutations. When the LED current is high, negative effects of hard commutation become worse and the required di/dt control circuits are more complicated [5]. To meet EMI and EMC regulations for lightings without adding on-board EMI filters, soft commutation is essential. In order to overcome these problems, we propose a soft self-commutating method using a Source-Coupled Pair (SCP) and reference voltages. The conventional control circuits required for an appropriate commutation time and soft commutation are no longer necessary. The fabricated 6-string LED driver IC is capable of achieving high efficiency (92.2%), high PF (0.996) and low THD (8.6%) under the 22W/110V AC condition.