Asymmetric Pulse Frequency Modulation With Constant On-Time for Series Resonant Converter in High-Voltage High-Power Applications

The series resonant converter (SRC), controlled by the traditional pulse frequency modulation (PFM) with constant on-time, can operate in discontinuous conduction mode (DCM) and is applicable for high-voltage high-power applications with the requirement of a wide output voltage range. However, in the traditional PFM with constant on-time, the resonant capacitor voltage will be higher than the input voltage during the zero current stage, leading to a higher maximum magnetic flux density (MMFD) case. To avoid this, a novel asymmetric pulse frequency modulation (APFM) with constant on-time is proposed for SRC operating in DCM, where the MMFD of transformer core varies linearly with the operating frequency and output voltage among the whole output voltage range. The high-power transformer can be designed according to highest operating frequency and the transformer turns ratio can be designed to be small. Furthermore, the proposed APFM leads to smaller peak current for all switches and fully zero-current-switching can be achieved. The output power and voltage can be still regulated, meeting the high-voltage high-power applications. For the proposed APFM, there are four different driver combinations with exact the same effects and advantages. The theoretical analysis has been validated by the established simulation model and experimental platform.