2025 Theses Doctoral

Optimal Modulation and Air-core Inductor Design for High-Ripple Soft-Switching Inverters

Fahmy, Youssef

Three-phase converters play a critical role in the electrification of the energy used to power modern life. Improving the understanding and best operating practices for these converters provides benefits for electric vehicles and their chargers, distributed energy, and any sectors that are experiencing an increase in electricity demand and production. Techniques and novel components for improving the performance of bidirectional variable frequency soft-switching three-phase grid-tied inverters were explored. These techniques were then tested in hardware using oscilloscope and power analyzer measurements for performance verification.

Examination and improvement of variable frequency soft-switching techniques were conducted first for a capacitor neutral point connected three-phase converter. Losses were considered and zero voltage switching operating regions were explored. Simulations and experiments were conducted to test the regions and to increase the maximum processed power using these techniques. In excess of 20kW was processed using a single inverter. Two such inverters were then parallelized in two different ways in an effort to increase the power processed by a single system. A discretization and synchronization technique was developed for interleaving the current ripple of the two inverters when appropriate. The highest power and efficiency achieved were 50kW and over 99%, respectively, more than sufficient for high efficiency DC fast charging.

Next, a novel method was created for the generation of soft-switching frequencies for the more commonly used neutral-point isolated three-phase inverter. The method frames soft-switching as a convex optimization problem. This inverter, in contrast with the previous topology, was constrained to use a single switching frequency for all three phases. This constraint was exploited to simplify the devised quadratic program that was constructed to minimize the filter inductor current ripple, subject to zero-voltage switching constraints. Simplifications showed the problem has a closed form solution. The technique was shown to work across different space-vector modulation strategies that are used to generate the controlling duty cycles. As in the first topology, simulation and experimental results verified the performance of the novel soft-switching strategy up to 11kW. The new algorithm was able to operate in real time on the same microcontroller as was used for the neutral point connected topology.

During development, it was noted that the concurrence of high frequencies and high ripple currents necessary for the soft-switching techniques used in both topologies cause large, unpredictable losses in the magnetic cores of the filter inductors. To remedy this, the cored inductors were substituted by novel air-core inductors as drop-in replacements. Using the neutral point connected topology for experimental comparisons, these inductors were found to increase the system efficiency by more than 0.5% at rated power. The final inductor design has twice the volume but just two-thirds of the weight. An electromagnetic simulation study was conducted and experiments show that circuit operation was not disrupted by the change in inductor.

Finally, the increased volume required by the air-core inductor was compensated for by interweaving three of the inductors into a single novel power processing structure called the triaxial inductor. Two triaxial inductor designs were constructed by winding the three coils orthogonally to one another. One design remains as close as possible to the first air-core inductors while the other attempts to minimize the proximity losses. The triaxial inductors were characterized as magnetically independent while their volume was reduced by up to 3.3x compared to the first air-core inductor. A second magnetic field simulation was performed to better understand the more complicated rotating field. It was found that the majority of the efficiency savings from the individual air-core inductors were maintained when the triaxial inductor was used in the same neutral point connected circuit.