Average Modeling and Simulation of Series-Parallel Resonant Converters by SPICE Compatible Behavioral Dependent Sources

A new methodology for developing average models of resonant converters is presented and verified against cycle by cycle simulation. The proposed model ing approach applies the concept of Rac(t) which represent the instantaneous effective load of the resonant network. Unlike the treatment given in previous studies, the value of Rac is evaluated here dynamically as a function of the temporal low-frequency-average of other relevant variables . Once def ined, the model can be used as is to run steady state (DC), large signal (transient) and small signal (AC) simulations on any modern circuit simulators. The proposed methodology was used to develop a behavioral model of a seriesparallel resonant converter. Excellent agreement was found between simulation by the proposed model and cyc le by cyc le s imulat ion . models derived by the proposed high level presentation, is their ability to emulate the DC, large signal and small signal responses of the corresponding switch mode or resonant system. Once derived, the models can be run as-is on practically any modern circuit simulation package to obtain open or closed loop responses in the time and/or frequency domain. The fundamental ideas of the proposed approach are exemplified by developing the behavioral model of a seriesparallel resonant converter and verifying the validity of the model against cycle by cycle simulation. Following the same line of reasoning, other resonant topologies can be captured by analogous behavioral representation. II. MODEL DERIVATION Following Steigerwald [5], the basic operation of a resonant converter, such as a series-parallel converter (Fig. 1), can be represented by a damped resonant network (Fig. 2). In this representation the virtual AC resistor (Rac) expresses the effect of the dissipative nature of the load (Rout, Figs. 1, 2) on the resonant circuit. The value of (Rac), under steady state conditions, can be obtained by equating the AC power dissipated by it to the DC power delivered to the load (Rout). This yields: