Ballast for independent control of multiple LED lamps

Light-emitting-diode (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 lighting 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 first-stage isolated current-fed power factor correction (PFC) pre-regulator that puts the storage capacitor to the secondary, thus allowing the use of lower-voltage storage capacitor in order to extend the overall system lifetime. 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 non-cascading structure to improve efficiency. Multiple non-cascading structures can be used for LED lamps for instant independent brightness control. Analysis, design example and prototype verification are given for the LED ballast.