High Efficiency Ac / Dc Flyback Converter

The ever-present trend of reducing the size and weight of the portable data-processing equipment has created new challenges to the design of their power systems-AC/DC adapters.The flyback converter is the most well-used topology for the low power AC/DC adapters. In this dissertation, the flyback converter is processed as the main research object, and the application of synchronous rectification technique in flyback converter is focused. The main contents of the dissertation include:In chapter I, a varied-frequency flyback converter with diodes output rectifiers is introduced. The flyback converter with diodes output rectifiers has the simplest structure. Combined with varied frequency control, low switching loss of the primary switches can be achieved. With optimized parameters, ZVS even can be achieved for the primary switch under low input voltage. Evaluation on losses is presented and an experimental prototype is built up.In chapter II, the external-driven synchronous rectification method applied in a fixed frequency flyback converter is introduced. The external-driven method is attractive for its simplicity and low cost. However, the driven signal should be inverted and isolated before it is sent to drive the SR. Another limitation is the stand-by loss is large with this driven method and is difficult to be reduced. Theoretically, two methods can be adopted to reduce the standby power loss, i.e. frequency reduction and burst mode. But it will bring some new problems if those two methods are adopted. Detailed theoretical analysis and evaluation on losses are presented. An experimental prototype is built up.Current driven methods for SR applied in flyback converters are introduced in chapter III. In current driven methods of SR, the driving signal is fully achieved in the secondary side. Combined with varied frequency method, ZVS for the primary switch can be achieved. Furthermore, the stand-by loss can satisfy the design specification and no other problems occur. In this chapter all kinds of current driven methods are summarized. The merits and limits of these methods are analyzed. An optimum-selected driven method is singled out for detailed analysis. Parameters optimization and loss evaluation for the converter is presented. At last, an experimental prototype is built up to verify the theoretical analysis.In the final part of the dissertation, the mentioned three flyback topologies are summarized and evaluated and an optimum topology is singled out. Furthermore, possibilities to improve the performance of the optimum-selected current driven method for SR are analyzed.