The Voltage and Current Oscillation Analyzation at Non-ZVS Model of Phase-Shift Full Bridge Converter

DC/DC phase-shift full bridge (PSFB) converter is widely used as a result of high power density, high efficiency and simple control strategy. However, it is hard to achieve zerovoltage-switching (ZVS) performance when the load is light which causes large switching loss. It is noted that the lagging bridge arm of the PSFB converter is difficult to achieve ZVS cause the current of resonant inductor usually reaches zero point in advance, thus it can't extract the charge of the power switches parasitic capacitor before turning on the switches and then induces switching losses. When the current reaches zero point at 0 level of the inverter bridge output voltage, it is ought to keep at zero until the switches state changes. But in the actual circuit it begins to oscillate and the primary and secondary voltage of the transformer also oscillate. It is mandatory to analyze the reason of this phenomenon for a better performance of the converter. In this paper, the mathematical model of PSFB converter based on first harmonic approximation (FHA) is built and working models of the converter are analyzed in detail. The oscillation model is built to explain the phenomenon and reason, the simulation on PSIM is built to back up the theoretical analyzation. Finally, a 2-kW prototype is provided to verify the reliability of the analyzation.