Operating behavior and design of the half-cycle discontinuous-conduction-mode series-resonant-converter with small DC link capacitors

The series-resonant-converter operated in half-cycle discontinuous-conduction-mode is a realization option for bidirectional isolated DC/DC converter modules, which are key components of modern solid state transformer (SST) concepts, among other applications. In SSTs the DC/DC converters are interfacing the input and output inverter stages via DC links. For reasons of costs as well as possible application-specific restrictions on weight and volume it is beneficial to use as small DC link capacitors as possible. However, when the resonant capacitance is higher than about 10% of the DC link capacitances, the deviation of the resonant current waveform from the sinusoidal shape becomes significant and the half-cycle duration is altered from values calculated using common approaches. Here, a comprehensive discussion of this case is given and design guidelines for the choice of the resonant capacitor are derived. The importance of the effect is illustrated by applying these to a complete design example.