Steady-State Modeling Of The Series-parallel Resonant Converter And Its Designing Research

Series-parallel resonant converter (SPRC), which has a good soft-switchingperformance and capability of absorbing the parasitic parameters, is widely used in highvoltage DC power supply systems and distributed DC/DC applications. Continuous currentmode (CCM) and discontinuous current mode (DCM) are the two different operationmodes of SPRC. The fundamental harmonic approximation is a simple and effectivemethod to establish the steady-state model of SPRC operating in CCM, yet not suitable inDCM for the approximation error resulted from the serious sinusoidal distortion.In order to solve steady-state model of the SPRC operating in DCM, this paperproposes a piecewise description function, which represents the characteristic among theoutput voltage, the switching frequency and the load.In the just continuous current mode, the response of the resonant circuit is solved bythe state space method. The constraint relation in double pulses form and single pulse formis obtained separately. Based on the energy balance equation, a piecewise functionalfunction among the output voltage coefficient, the JCCM switching frequency and the loadquality factor is derived. In addition, the voltage gain characteristics of the SPRC is alsopresented. Via discontinuous coefficient, the piecewise function in just continuous currentmode is extended to the DCM, and it can accurately describe the steady-state behavior ofthe SPRC operating in DCM, providing a favorable way for designing the series-parallelresonant converter. Considering two applied situations, the design procedure based on thepiecewise function is given, and then, a digital simulation is established, verifying thecorrectness of the design procedure and the piecewise description function.