Noncascading structure for electronic ballast design for multiple LED lamps with independent brightness control

LED light sources, which are more compact, capable to change color in real time, less dissipative, and more durable are finding more applications than conventional light bulbs in domestic, commercial, and industrial environments. However, requirements such as high-power factor, long lifetime, accurate current control, and high-efficiency pose challenges to the design of LED ballast circuits. This paper proposes an LED ballast with a dual noncascading structure. The first-stage noncascading structure is an isolated current-fed power factor correction (PFC) preregulator. In the proposed design, the short-lifetime high-voltage storage capacitor at the primary is replaced by a long-lifetime low-voltage capacitor at the secondary, thus extending the overall system lifetime. The PFC is programmed by the conventional averaged current-mode control for high-power-factor applications. Furthermore, the high-voltage stress on the main switch, which is typical in current-fed converters, is reduced substantially by appropriately exploiting the transformer leakage inductance. The design uses two secondary transformer windings and an LED current driver to form a second noncascading structure to improve efficiency. Multiple noncascading structures can be used for LED lamps for instant independent brightness control. Analysis, design example, and prototype verification are given for the LED ballast.