The Study On Combining Technique Of Two Transistor Forward Converter

Two Transistor Forward Converter(TTFC) has many attractive characteristics, such as low switch voltage stress, inherent anti-break-through capability, high reliability. It becomes one of the most widely used topology in the industrial applications, especially in the medium power capacity and low output voltage area, such as telecommunication energy systems, welding machines and computer power supply. There are, however, many shortages in a TTFC. Firstly, the operation duty cycle of the TTFC must be less than 50%. As a result, voltage stress of the secondary rectifier is high, which limits the usage of TTFC in high output voltage applications, especially when the reverse recovery of the freewheeling diode is taken into account. Secondly, the low duty cycle leads to the high voltage and current ripple, and enlarges the filter size. Finally, the transformer core can only operate in the first quadrant, decrease the utilization of the transformer core.The goal of this paper is, by proper combination technique of two TTFCs (Combined Two-Transistor-Forward Converter, CTTFC), to keep the attractive characteristics of the TTFC, and overcome the shortage of it. In combination, the interleaving technique is adopted. Interleaving technique can increase the effective frequency of the converter, and decrease the current ripple. Two TTFCs can consist four CTTFCs: parallel-parallel type CTTFC, serial-parallel type CTTFC, parallel-serial type CTTFC and serial-serial type CTTFC. The interleaving technique is used in all these CTTFCs.There are three methods to. parallel connect two TTFCs: parallel connecting them at the output capacitor, parallel connecting them at the freewheeling diode, and parallel them at the output capacitor with coupled inductor. In this paper, the steady and transient performance of these three CTTFCs is studied and compared In detail. Additionally, a novel phase-shifted controlled CTTFC with current doubler is presented in this paper, the core of this CTTFC can operate in the first and third quadrant symmetrically. As a result, the utilization of transformer core is increased.To reduce voltage stress of power switches, serial-parallel type CTTFC is presented in this paper. In this CTTFC, the voltage stress of the power switches is only half of the input voltage, so power MOSFET can be used in high input voltage applications. In this paper, the principle of this CTTFC is analyzed, and the methods of input voltage balance control are presented.The characteristics of the parallel-serial and serial-serial type CTTFCs are discussed in this paper, and comparison between these two CTTFCs and the other two types CTTFCs mentioned above has been done. To increase the capacity of the CTTFC and reduce the voltage stress of rectifier further, compositeABSTRACTCTTFCs are presented in this paper, which is fit for the high power, high output voltage applications. To improve the switching condition of the main switch, a Zero Voltage Transition (ZVT) circuit for the CTTFC is proposed. In this ZVT circuit, only one resonant loop is used, and both TTFC in the CTTFC can be turned art and off in zero voltage conditions, the auxiliary switch can be turned on and off in zero current conditions.A 8kW power system operation power supply module is developed by the author. In this module, the parallel-parallel type CTTFC in which two TTFCs are parallel connected at the freewheeling diode is adopted. To cooperate with this module, a 3.3kW stepless voltage regulator is also developed by the author. Both of them are measured by the authority organization of our country.