Study On Forward Power Converter With Passive Lossless Snubber

Forward converter has been widely used for low-to-medium power conversion applications due to its topology simplicity, low cost, high reliability. But one inherent limitation of forward converter is that transformer must be reset during the power transistor off period, or lead to transformer core saturated. In recent years, researches about magnetic reset technique are very many, and several magnetic reset methods have been developed in order to overcome this inherent limitation. These methods include tertiary reset winding, R-C-D clamp, resonant reset, active clamp reset, and so on.Hower,These solutions have many disadvantages which block comprehensive applications of forward converter.For PWM converters, snubber technology can be broadly classified into two groups:passive snubber and active snubber. Passive snubber can achieve many functions which active snubber achieves. For example, it can reduce switching losses, and achieve zero-current turn on and zero-voltage turn off, and reduce voltage stress and current stress across power transistors, and weaken EMI noises, and so on. Moreover passive snubber also have many advantages which active snubber does not achieve. For instance, their topologies are simple, and they do not require an extra switch or additional control circuitry. Consequently, they are less expensive, have higher reliability, and can achieve higher performance/price ratios than active methods. Passive snubber is a kind of strongly attractive technology, and has received more and more people's attention.The main objective of this paper is to apply passive snubber to forward converters, and to enhance power density, reduce costs and improve efficiency. Firstly, classification, respective features and applications areas of switching power supply topologies are introduced. Various soft switching technologies which domestic and overseas power electronic fields are fond of are analyzed and discussed, and their many drawbacks are pointed out. Subsequently, the basic operating principles, advantages and disadvantages of various magnetic reset techniques are introduced. Moreover, synchronous rectifier technology which forward converters widely employ is introduced. In chapter 3, the basic principle and classical circuits of passive snubber technology are introduced, and principles and rules of composition of passive snubber networks are analyzed and generalized, and passive snubber is compared with active snubber. Two operating modes of single-ended forward converter with passive snubber are analyzed in details in chapter 4, and the simulation results are given to verify the analysis results. In chapter 5, to overcome the drawbacks that forward converter can not transfer energy during the power transistor off period and output power is small, a new passive forward converter with current-doubler rectifier is put forward. The converter can operate under two states including forward mode and flyback mode, so output power raises. Moreover, the converter can reset transformer excitation flux reliably ,and achieve soft switching, and efficiency is high. Another important feature is its topology simplicity and low cost, and does not include lossy components, so it has definite meaning of theory and practical value.