Advances in High Frequency Inductor Design for Power Converter

Research output: Book/ReportPh.D. thesis

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Abstract

The Power Sources Manufacturers Association roadmap identifies high-efficiency and high-power-density end-use of electricity as one of the key challenges to increase the economic growth rate with a minimal environmental impact. The challenge in realizing smaller, faster, and in turn greener power supplies is the integration of magnetic materials that can operate efficiently at high frequency. The most successful work will involve magnetic material development, optimization of magnetic component designs, and consideration of fabrication constraints. This project primarily aims for a high-efficiency and high-power-density power management (DC/DC converter) from the USB-C port to the battery within the next generation of fast-charging smartphones or other portable electronic devices. This thesis hypothesizes that employing superparamagnetic nanoparticles material, which has high permeability and negligible hysteresis loss, in high-frequency inductor design can enable high-efficiency and high-power-density converter. The focus of this thesis will be on the magnetic material characterization and the inductor design. The first step is to establish a baseline prototype with state-of-the-art magnetic material. Multi-phase Zero Voltage Switching (ZVS) Buck converter is the selected topology to achieve high power density. A novel utilization of 4-leg magnetic core is proposed as the integrated inductor. From the constructed inductor model and semiconductor losses calculation, the design trade-off is analyzed. The optimal switching frequency is found to be around 2 MHz in this case, which is when the core material performance factor peaks. As proof of concept, a highly compact prototype, featuring stacked PCB solution is built. The input voltage ranges from 6 V to 12 V and the converter can deliver up to 10 A at 4 V output. The converter reaches a nominal efficiency of 88% and has a power density of 301 W/in3. The second step is to build a platform for magnetic core characterization, in particular the core loss. Mutual inductance compensation method is employed in the measurement circuit to reduce sensitivity to phase errors. An addition to the circuit is proposed in this work to make easier DC bias generation. This platform is used to study the core losses behavior at high frequency with superimposed DC bias in ferrite. The tested MnZn ferrite has a nominal relative permeability of 1500 and 800, which was tested at frequency from 500 kHz to 3 MHz. The measurement results are explained thoroughly with three controlling parameters: excitation frequency, AC flux density amplitude, and DC bias. Furthermore, an improved Steinmetz Premagnetization Graph and Artificial Neural Network are used to create core loss prediction model. The measurement data and built model can be accessed online for use by another magnetics designer. The third step is to analyze the switching transition in multi-phase half-bridge circuit with coupled inductors. It was found that the coupled inductors can achieve or maintain ZVS in the cases where non-coupled inductors cannot. Moreover, the coupled inductors can reduce the required deadtime to achieve ZVS. This benefit comes from the increased amplitude and frequency of resonant voltage in the presence of coupled inductors. The impact is quantified, analyzed, and verified in this thesis. The final step is to design a prototype with superparamagnetic nanoparticle material. Multi-phase ZVS Buck converter is still the selected topology to keep a fair comparison. The new material must be characterized in terms of permeability level and core loss performance. The core loss measurement platform is again used for this. Considering the fabrication constraint, two coupled inductor structures are analyzed.
Due to the very low permeability level, 3D FEM is used to obtain accurate inductance and coupling factor. The core loss, winding loss, and thermal models are incorporated into the inductor design. After optimization, a new prototype is built featuring 84.9% nominal efficiency and 1049 W/in3 power density. There is almost three-fold improvement in the power density, thanks to the extra-flat structure of the inductors. The converter can deliver up to 6 A at 4 V output.
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages244
Publication statusPublished - 2023

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