(A) compact flicker-free transformer-less LED driver with an enhanced power factor for omnidirectional multi-channel smart bulb applications

Abstract

Light emitting diode (LED) smart bulbs require efficient drivers to maximize its benefits. Efficient LED drivers should provide compact module form factor, low flicker, high power factor (PF), and high conversion efficiency. Depending on the presence of the switched mode power supply (SMPS), LED drivers can be classified into AC-DC and AC-direct topologies. AC-DC LED drivers can achieve low flicker and high PF with PF correction (PFC). However, the transformer for SMPS and the bulky components for PFC degrade omnidirectional performance of indoor LED bulbs due to its bulky module size. On the other hand, AC-direct LED drivers are suitable for implementing omnidirectional LED bulbs because a high PF can be achieved without requiring bulky PFC components. However, the nature of the LED driving scheme introduces high flicker with low frequency which harms the human body. This dissertation presents a flicker-free LED driver that adopts a new PF enhancement technique. The proposed LED driver divides the transmission path of the current to the LED string in two ways depending on the instantaneous input voltage in order to improve the PF relative to that of a conventional compact flicker-free LED driver that uses a fixed LED current regulation scheme with only one electrolytic capacitor. Since the proposed LED driver requires a minimum number of external components, a light bulb can be implemented with omnidirectional light emission due to its compact module. In order to prove the effectiveness and suggest design considerations of the proposed LED driver, mathematical analysis is conducted. Then, the design details of a multi-channel smart bulb system that adopts the proposed LED driver are described. The proposed LED driver IC is implemented in a 0.35-μm ultrahigh voltage CMOS process and occupies an area of 2.9 mm × 2.0 mm. A 10-W smart bulb prototype that adopts the proposed LED driver is flicker-free, and shows a PF of 0.715, and power conversion efficiency of 77.5% for an AC supply of 120-V.