Khai D. T. Ngo, Chair
Tuesday, April 11, 2017
Fred C. Lee
A high-efficiency high-density LLC converter with a novel matrix transformer is proposed in this paper. The proposed matrix transformer integrates four elemental transformers into one magnetic core and utilized simple four-layer PCB as the windings. Flux cancellation is utilized to reduce core size and loss. The matrix transformer is arranged to further reduce the flux density in the top and bottom plates by half. Synchronous Rectifiers (SRs) and output capacitors are part of the secondary winding to minimize leakage and termination loss. Further improvement of the magnetic structure is proposed to reduce the core loss without sacrificing the power density. A detailed design procedure is provided. A 1MHz 380V/12V 800W LLC converter with GaN devices is built. The prototype can fit in the quarter-brick footprint and achieves a peak efficiency of 97.6% and a power density of 900W/inch3.
With fast power semiconductor devices based on GaN and SiC becoming more common, there is a need for improved driving circuits. Transformers with smaller inter-winding capacitance in the isolated gate drive power supply helps in reducing the conducted EMI emission from the power converter to auxiliary sources. This paper presents a transformer with a small volume, a low power loss and a small inter-capacitance in a gate drive power supply to fast switching devices, such as GaN HEMT and SiC MOSFET. The transformer core is embedded into PCB to increase the integration density. Two different transformer designs, the coplanar-winding PCB embedded transformer and the toroidal PCB embedded transformer, are presented and compared. The former has a 0.8 pF inter-capacitance and the latter has 85% efficiency with 73 W/in3 power density. Both designs are dedicated to a 2 W gate drive power supply for wide-band-gap device, which can operate at 200 °C ambient temperature.
Khai D. T. Ngo
We used gelcasting, a pressure-less processing technology for fabricating ceramic parts, to make NiCuZn ferrite cores, which are traditionally made by a process that requires high hydrostatic pressure. A commercial NiCuZn ferrite powder was mixed with water, dispersant, and organic monomers to form a slurry, and then cast into a mold of toroid shape followed by sintering at 900, 950, and 1000 °C respectively for two hours. The sintered core mass density was found to increase with sintering temperature. The magnetic properties of the cores, i.e. complex permeability and core-loss density, were measured. We found that the real part of the permeability increased with sintering temperature from about 44 at 900 °C to 77 at 1000 °C. The core-loss density data at 5 MHz showed that the cores sintered at 950 °C had the lowest core-loss density, about 50% lower than that of a commercial NiZn ferrite (4F1) core. Since gelcasting does not require pressure and is scalable and low cost, it has the potential to make magnetic cores with intricate shapes and sizes for desired coupling of magnetic fluxes to improve efficiency and power-density of power electronics converters.