Concept Study of an LLC Converter with Magnetically Resonant Inductor

Due to their high efficiency, LLC-type converters have gained wide application fields, e.g., as DC-DC converter stage in automotive chargers or in power supplies in industrial applications. The amazingly high efficiencies of 97 to 98%, in some cases up to 99% are achieved by zero-voltage switching techniques that allows the use of Superjunction MOSFETs with small channel resistances but relatively large parasitic capacitances. However, if the converter is to be operated in wider voltage gain range, operation at the lower end of this range can result in a huge switching frequency which would degrade the good efficiency. The idea presented in this paper is to make use of inductors with a resonance of the magnetic field. As a result, the switching frequency would have to be varied only in a smaller range. The idea is particularly aimed at future designs with switching frequencies approaching the MHz range as it will occur with GaN transistors very soon. In this frequency range, the wavelength within the magnetic material is only in the dimension of some millimeters so that the core geometry could be designed for certain resonance effects that would reduce the peak switching frequency considerably. However, this favourable effect could be impaired by other side effects such as increased losses, so the situation should be clarified by detailed experiments in the near future.