Research On ZVS Three-transistor Push-pull DC/DC Converters

The soft-switching technology can reduce device stress, switching loss, electromagneticinterference (EMI) and improve power density of the power electronic equipment. Thetraditional push-pull converter have advantages of simple power structure and driver circuit.But the voltage stress of its two power switches at primary side is twice of the source voltage,so the traditional push-pull converter is suitable for applying in low voltage and high currentinput field. In addition, the power switches in the traditional push-pull converter are hardswitching. Many researches are developed to improve the tradition push-pull converter andthere can be divided into five types:the1st type is the push-pull forward converter,the2ndtype is the push-pull converter with LC net,The3rd type is the active-clamped push-pullconverter,the4th type is four-transistor push-pull converter and the5th type is the proposedthree-transistor push-pull converter in this paper.Firstly, A novel three-transistor push-pull converter is proposed. An auxiliary transistoris inserted between the positive-pole of input power source and midpoint of two primarywindings in traditional push-pull converter. The driving logic of two main transistors is asinverse as those in traditional push-pull converter, and the driving logic of the auxiliarytransistor is as inverse as one driving logics&the other driving logic in traditional push-pullconverter. The proposed topology is simple. Its main transistors can achieve ZVS under wideload range and turn-off under half the current occurred in traditional push-pull converter. Theauxiliary transistor can realize ZVS under big load or with large leakage inductance. The ratedvoltage of the auxiliary transistor is half of that of the main transistors and is equal to theiuput voltage. The theory in each operation mode is discussed in detail. The main switchingwaveforms and variable equations are analysis. The soft-switching conditions are discusses.The PWM IC and driving circuit are presented. A prototype based the proposed topology wasbuilt. The experimental results verified the discussed theory correctly.Secondly,the other two coverters are proposed in this paper to sovle the limitation ofachieving ZVS for the auxiliary transistor. One is a soft-switching three-transistor push-pullDC/DC converter with LC net in parallel,and the other is ZVS three-transistor push-pullconverter with a saturable inductance in series.Both of them can realizezero-voltage-switching of the auxiliary transistor under a wide load range,with the help of asaturable inductance. Especially,the former can achieve ZVS of the auxiliary transistor underall load. The operating principles are analyzed in detail,and then the key problems ofachieving ZVS of three transistors are discussed.Two prototypes are built to verify the theoryanalysis and the feasibility of the proposed two converters.At last,the proposed three-transistor push-pull converter is compared with the basic ZVS phase-shifted full-bridge converter, as well as the other two proposed converters.For each ofthem,advantages and disadvantages are summarized to provide theoretical basis for practicalapplications.