Compact High-Efficiency DC/DC Converter using Planar Magnetics

High-efficiency, ultra-compact power converters are in high demand in many different applications, including renewable energy, electric vehicles, data centers, and medical equipment.
One of the primary reasons for this trend is the introduction of wide-bandgap (WBG) devices, such as silicon carbide (SiC) and gallium nitride (GaN), which have superior properties to commonly used devices. However, as the size of the converter decreases, so does the effective cooling surface area. In order to increase the cooling surface of converters, planar magnetic components have become increasingly popular in recent years. In addition to the increased cooling surface provided by planar magnetics, the converters benefit from the ability to use printed-circuit-boards (PCB) as copper windings. The use of PCBs as the copper windings improves production accuracy as well as design freedom.


This thesis, titled ”Compact High-Efficiency DC/DC Converter using Planar Magnetics”, is primarily focusing on the development of highly efficient compact planar magnetics for use in isolated DC-DC converters. Various calculation methods have been developed and used in different stages of the planar magnetics design process. The ability to precisely determine the copper winding parasitic components is critical when optimizing magnetic components. A precise simulation method was therefore created to determine these parasitic components. Both the calculation and simulation methods have been validated through measurements on various constructed planar transformers and inductors.


The background and motivation for the thesis is presented in Chapter 1. This chapter also discusses the state of the art in planar transformers used in isolated dc-dc converters, as well
as the method demonstrated for determining the parasitic components. The project objectives are then defined, followed by an overview of the thesis.


The design and analysis of two isolated full-bridge with WBG devices for an isolated hardswitched dc-dc converter is described in Chapter 2. It was demonstrated how the parasitic capacitance of the transformer influences the capacitive loss of the semiconductors. During the PCB design process, simulation software is utilized to analyze and predict the PCBs power loss.


The design process for a high turn-ratio planar transformer is shown in Chapter 3. In the optimization process, a novel capacitive transformer model is used to accurately determine
the parasitic capacitance. In addition, a simulation for the transformer’s various parasitic components is presented, with a low error of approximately 6 % compared to the measured
value. The 5 kW planar transformer has a turn-ratio of 28:4, an efficiency of 99.5 %, and a volume of 45.7 cm3.


Chapter 4 compares planar inductor designs for a large dc-current application. A design method for a planar inductor with an air-gapped ferrite core is compared to a powdered core
material design method. A precise simulation of the winding resistance of planar inductors is presented and compared to the measurements. The planar inductor designed for a 50 A
application has been shown to have an efficiency of more than 99.8 % and a volume of only 20.3 cm3. The main reason for this high efficiency is the implementation of a novel planar inductor stack-up, shown to be ideal for these high-current applications.


In Chapter 5, an analysis of the completed high-power dc-dc converter with WBG devices and planar magnetics is shown. This chapter contains all of the experimental results related
to the operation of the converter. The 5 kW isolated dc-dc converter has a power density of 17.2 kW/L and a peak efficiency of 97.7 %.


Finally, in Chapter 6, the thesis is summarized with a conclusion and recommendations for future research is suggested.