Research On Single-core Parallel Two-transistor Forward Converter

Two-transistor forward conversion topology has been widely used in the fieldsrequiring higher reliability with low voltage stress and anti-break-through, however,some inherent shortcomings still limited the development of two-transistor forwardconverter, such as the duty cycle can’t be more than50%for the transformerresetting, the ripple of the output voltage and output current is high, the utilizationof the transformer core is low because of just operating in the first quadrant. Thispaper selects two-transistor forward conversion topology as the basic topology, andovercomes some typical shortcomings by proper soft-switching and combinationtechnique while retaining its attractive characteristics.The development and current research status of two-transistor forwardconversion topology are analyzed from two aspects of soft-switching technique andcombination technique, and the advantages, disadvangtages and application fields ofthe current existing two-transistor forward conversion topologies are compared, onthis basis, a single-core parallel two-transistor conversion topology with LCDlossless snubber network and tapped filter inductor is selected. The ripple of theoutput current can be decreased so that the volume of the output filter conductor bedecreased by the combination style of interleaving parallel; the two parallelconverters use a transformer with two primary windings, and the transformer ofsingle-core structure can realize symmetry magneting to improve utilization ratio ofthe transformer core and power density of the converter; the power switches canachieve zero-voltage turn-off to reduce switching loss and improve switchingenvironment because of LCD lossless snubber network without increasingcomplexity of control and extra loss; a tapped filter inducter is selected to solve theproblem of primary circulating current loss caused by reverse recovery of thediodes.The principle of LCD lossless snubber network which can realize softswitching of the power switches and the mechanism and suppression measure ofprimary circulating current are analyzed, and a detailed theoretical analysis focusingon the single-core parallel two-transistor conversion topology has been done, andthe effect on maximum switching current stress and output voltage characteristic ofdifferent turn ratio is analyzed, and the small signal model of presented converter inCCM mode with voltage control loop is established depending on the state-spaceaverage method. Based on the theoretical analysis of the topology, the parametersare calculated and designed, and the effect and selection principle of LCD losslesssnubber network and tapped filter inductor are especially analyzed. A1.6kW experimental platform is built on the basic of theoretical analysis andPSpice simulation. Experimental results show that the researched single-coreinterleaved two-transistor forward converter can realize zero-voltage turn-off of theswitches and symmetry magnetization of the transformer core, and the structure oftapped filter inductor can suppress the primary circulating current obviously, so theconverter can have promotion both in power density and conversion efficiency.