Techniques to Improve LED Drivers by Reducing Voltage Stress and Energy Storage

摘要

High-brightness light-emitting diodes (HB LEDs) provide many advantages over other existing electric light sources, including high efficacy, long lifetime and small form factor. However, the overall lifetime of off-line LED applications is limited by the low-quality electrolytic capacitors utilized for energy storage. In order to use long-life capacitors while limiting cost increase, the required energy-storage capacitance should be reduced, which can be achieved with several techniques addressed in this thesis. The constant input current approach can achieve a power factor (PF) of 0.9, which meets ENERGY STAR requirements, while reducing required energy storage by one-third compared to unity-PF case. When ripple is allowed on the LED current, the trapezoidal LED current approach minimizes energy storage with small control effort. A second stage can significantly reduce required capacitance by allowing large voltage variation on the capacitor, while bidirectional structure helps limit additional power loss. The small form factor of LEDs offers flexibility for diverse and sophisticated design. In order to take this advantage, LED drivers should have a small size or thickness. Series-input structure provides a possibility to apply low-voltage components in high-voltage circuits, while the common duty cycle approach achieves automatic input voltage sharing and LED current copying, which can significantly simplify system design. With reduced rated voltage, integration of semiconductor devices becomes much easier and converters are able to operate at high switching frequencies with small components, both of which lead to high-level monolithic integration. All of the principles and control approaches are verified in experiments, with the results provided in this thesis.