Research On Combined Two-Transistor Forward Converters

The goal of this paper is,by serial/parallel combination technique of Two-transistos Forward Converters(TTFCs),to keep the merits of the TTFC,and to overcome the shortages of it.The interleaving technique is adopted,while in combining TTFCs.The interleaving technique lowers the net ripple amplitude and raises the effective ripple frequency of the overall converter without increasing switching losses or device stresses.A novel parallel-serial type CTTFC(Combined Two-Transistor Forward Converter) with coupled inductor is present in this paper,and the performance of three parallel-serial type CTTFCs is studied and compared.The performance of CTTFC with coupled inductor is similar to the other two CTTFCs while coupling coefficient is less than one and the converter working CCM(Continuous Current Mode),and input-to-output voltage gain increase one times while coupling coefficient is one.Three parallel-parallel type CTTFCs is studied and compared in this paper.The performance of the CTTFC with coupled inductor is similar to that with sharing freewheeling diode while coupling coefficient is one,and similar to that with sharing output capcitor while coupling coefficient is less than one and the converter working CCM.To reduce voltage stress of transformer secondly diodes,a novel parallel-parallel/serial type CTTFC is presnt.It has many attractive characteristics,such as low current stress of power switches,small intput and output filter size,high reliability,and sharing output voltage.To reduce voltage stress of power switches,a series-parallel type CTTFC is studied in this paper.Voltage-sharing of input capacitors can be realized naturally in discontinuous conduction mode,andvoltage-sharing of input capacitors can also be realized naturally in continuous conduction mode.The voltage stress of power switches is only half of the input voltage,so power MOSFET can be used in high input voltage applications.A series-series type CTTFC with sharing freewheeling diode is studied.Its performance of voltage-sharing of input capacitors is similar to that of the above series-parallel type CTTFC.In order to increase the utilization of transformer core and reduce the number of transformers,the two-transisteor integrated-magnetics forward converter is studied in this paper.The phenomena about magnetic deviation due to asynchronism of duty cycle and other parameters is analyed,and the converter works normally although there is magnetic deviation.The working principle of circling current is analyed,and a improving circuit is present to decrease the circling current.To reduce voltage stress of power switch in the flyback-forward converter,a novel two-transister flyback-forward converter is present in this paper.By adjusting the turns ratio of forward and flyback transformer we can distribute the delivering power of forward and flyback transformer.