Synchronous High Step-Down Ratio Non-Isolated LED Constant Current Driver Based on Improved Watkins-Johnson Topology

In the field of non-isolated light-emitting diode (LED) driver, the omnipresent buck-based LED constant current driver shows poor efficiency in occasions of high step-down ratio due to the marginal operating duty cycle. In order to overcome such drawback and to provide an alternative topological scheme for an LED driver, a Watkins-Johnson topology-based solution is proposed in this paper, by improving the original circuit structure into a dual low-side power switch version and introducing a feedback network for regulating the output current to a constant level. Theoretical analysis, including accurate model construction, steady-state analysis, dynamic analysis, and design consideration, is conducted. Also, an experimental prototype is realized to drive all kinds of LED arrays for acquiring electrical parameters and evaluating performance. The innovative points include invoking the low-side-ize approach to realize the power circuit transformation, and furthermore, theWatkins-Johnson topology is first applied to the constant current power supply. The proposed LED driver holds salient features of low-terminal current ripple, ease of driving, high-voltage conversion ratio tolerance, and inherent energy recovery functionality. The application scope covers low-voltage dc input or automotive lighting use.