Improving the design of the asymmetrical half-bridge converter without electrolytic capacitor for low-output-voltage ac-dc LED drivers

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 its 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 also 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 significant dead times. Another important issue regarding the AHB is that its closed loop controller cannot be very fast and it cannot easily cancel the previously-mentioned low-frequency ripple. In this paper, a feed-forward technique, specifically designed to overcome this problem, is presented. The experimental results obtained with a 60-W topology show that efficiency of the AHB may be very high (94.5%) while the inherent control problems related to the AHB can be overcome by the proposed feed-forward technique.