Self-driven synchronous-rectification technique for the Asymmetrical Half-Bridge converter in led lighting applications

Due to their high reliability and luminous efficacy, HB-LEDs are being widely used in lighting applications and, therefore, their power supplies are required to have also high reliability and efficiency. A very common approach for achieving this in ac-dc applications is using a two-stage topology. The Power Factor Corrector Boost converter operating in Boundary Conduction Mode is a very common converter used as first stage. It is normally designed without electrolytic capacitors, improving reliability but also increasing the low-frequency ripple of the output voltage. The Asymmetrical Half-Bridge (AHB) is a perfect option for the second stage as it has very high efficiency, it operates at constant switching frequency and its output filter is small (i.e., it can be easily implemented without electrolytic capacitors). Moreover, the AHB is an excellent candidate for Self-Driven Synchronous Rectification (SD-SR) as its transformer does not have dead times. However, the standard configuration of the SD-SR must be modified in this case in order to deal with the transformer voltage variations due to the input voltage ripple and, more important, due to the LED dimming state. This modification is presented in this paper. The experimental results obtained with a 60-W topology show that efficiency of the AHB may be very high (94.5%).