Power quality issues in single-stage AC-DC HBLED drivers at low power levels: Problems and solutions

In this paper the design of efficient single-stage High Brightness Light Emitting Diodes (HBLEDs) ac-dc drivers for low power applications is addressed using variable frequency hysteretic control. An adaptive hysteresis window modulated by the input voltage and the output LED current reference is used. The double modulation strategy helps to avoid the distortion near the zero crossings of the input current and to improve the dimming performance of the HBLEDs. This results in a low value of Total Harmonic Distortion (THD) under low current/power conditions and therefore in a very good dimming performance in single-stage ac-dc HBLEDs drivers with PFC. The operation principle of the proposed technique is presented and numerical simulations are shown to demonstrate its functionality. A laboratory prototype is designed and tested to verify its feasibility obtaining a significant improvement in terms of power quality. Under universal input line voltage operation, good efficiency and low THD can be achieved at low power levels.