Time Domain Analysis of Three-Phase Single-Stage AC/DC Resonant Converter Using Numerical Calculation

Since LLC converter is capable of naturally creating the soft-switching condition to improve the overall efficiency, it is widely used in dc-dc applications. Using LLC resonant tank to realize soft switching for three-phase ac-dc converters would be a very promising solution to high efficiency and low cost ac-dc conversion, an LLC-based three-phase single-stage ac-dc converter was configured by taking advantage of three-phase voltage feature u(a) + u(b) + u(c) = 0 and incorporating three bidirectional switches. Because of time-varying feature of three-phase voltages, the input voltage of the converter to the LLC resonant tank is no longer a symmetrical square wave, but a time-varying and asymmetrical three-level waveform, which makes the analysis of the resonant circuit behavior much complicated. Some traditional analysis methods, such as fundamental harmonic approximation and its variants, are impossible to deal with. To overcome this analysis obstacle, this article proposes a numerical analysis method to analyze the time-varying three-level LLC resonant tank for this converter. According to the monotonous change of LLC input voltage in one fundamental cycle, it calculates and determines the time durations of three phases and the corresponding operation modes in each switching cycle. And this numerical analysis method is used to calculate the numerical relationship between the two control variables (switching frequency f(s) and duty cycle D-M) of this converter and the voltage gain and power level, which is a guideline for the design of the resonant tank parameters and control strategy of the LLC-based three-phase single-stage ac-dc converter. Finally, the time-varying waveforms of f(s) and D-M obtained by numerical calculation well match the PSIM simulation results, and a 3.3-kW three-phase 220Vac input and 48Vdc output prototype was built to verify the effectiveness of the proposed numerical analysis method and the feasibility of the control strategy.