Different Magnetic Flux Density Cases of Series Resonant Converter Operating in Discontinuous Conduction Mode Suitable for Medium-to-High Voltage Applications

The pulse frequency modulation with a constant on-time can be adopted for the series resonant converter operating in discontinuous conduction mode to regulate the output voltage. It is found for the first time that there are two different operation cases with different magnetic flux density (MFD) values. The boundary condition for the two cases is found out and the exact MFD expression of each case is derived. The MFD has smallest value under lowest switching frequency and increases with the increase of switching frequency for both cases, indicating the high-power transformer can be designed at highest switching frequency. A transformer turns ratio design guideline can be easily obtained to avoid the higher maximum MFD case. The theoretical analysis is verified by the simulation and experimental results.