Using Adaptive Off-time Synchronous Rectification to improve efficiency in low output voltage converters

The design of efficient, isolated low output voltage converters is a major concern due to their widespread use. One of the preferred methods used to maximize their efficiency is synchronous rectification, i.e. the replacement of the secondary side diodes by MOSFETs to decrease conduction losses. However, depending on the topology being used, synchronous rectification might not provide the required efficiency improvement or even be easily implemented. This paper presents a novel synchronous rectification system that can be applied to converters with symmetrically-driven transformers and to converters from the Flyback family. In both cases, the proposed system adaptively generates a control signal that turns on a MOSFET synchronous rectifier, placed in paralell with a secondary side diode, during the conduction interval of the diode. The proposed system uses only information from the secondary side, thus avoiding breaking the isolation barrier; it can be built using a few low cost analog components, it is reliable and simple, and can be easily implemented in an integrated circuit. Up to a 3 % improvement is demonstrated in a 3.3-5 V, 120 W Push-Pull converter and up to a 2.5 % improvement is obtained in a 5 V, 50 W Flyback converter, both of them designed for telecom applications.