Application of integrated magnetics in resonant converters

The physical size of inductors in resonant converters was examined in terms of inductor to transformer size ratio. Following the general discussion of the issue, the inductor size in a series resonant DC-DC converter is examined in detail. It is shown that the inductor to transformer size ratio is a function of the peak voltage of the inductor to the RMS voltage of the transformer and is independent of the inductance. The physical size of the inductor could be much larger than the transformer when the converter operates near resonance. Away from resonance, the inductor size becomes close to, and even smaller, than the transformer. Leakage inductance of the transformer can be used as the resonant inductance but if the leakage is associated with the secondary windings of the transformer, it will increase the peak reverse voltage of the rectifier diodes when a center-tapped rectifier topology is used. To solve this problem, an integrated transformer-inductor element with high primary leakage inductance and low secondary leakage is proposed and tested. The experimental unit was built for a power level of 1 kVA and switching frequency range 30-60 kHz.