Aaron L. F. Stein

High-Q self-resonant structure for wireless power transfer

The range and efficiency of wireless power transfer systems are limited by the quality factor of the transmit and receive coils used. Multi-layer self-resonant structures have been proposed as a low-cost method for creating high-Q coils for high-frequency wireless power transfer. In these structures thin foil layers are separated by a dielectric material in order to form a capacitance that resonates with the inductance of the structure, while also forcing equal current sharing between conductors. In order to reduce winding loss, these structures are made with foil layers much thinner than a skin depth, which makes the layers of the structure extremely difficult to handle. In this paper, we present a modified self-resonant structure in which the layered conductors are made from standard PCB substrates with no vias. The PCB substrates provide an inexpensive way to handle thin conductive layers, and the modified self-resonant structure ensures that the poor dielectric properties of the PCB substrates do not impact the quality factor of the structure. The modified self-resonant structure makes it feasible to achieve advantages similar to litz wire, but at multi-MHz frequencies where effective litz wire is not commercially available. Experimental results show that the structure has a quality factor of 1177 at 7.08 MHz, despite only being 6.6 cm in diameter. The quality factor normalized by the diameter is more than 6.5x larger than other coils presented in the literature.

High-Q resonator with integrated capacitance for resonant power conversion

Resonant power conversion at MHz frequencies is useful for miniaturization of power electronics, but requires resonators or inductors with high efficiency. Because litz wire is not effective above several MHz, an alternative winding structure for inductors is required for operation at these frequencies. In this paper, we present resonators with a multi-layer foil winding which can be constructed using industry-standard polyimide laminates. The proximity effect losses are reduced due to current sharing between multiple conductor layers much thinner than a skin depth. The integrated capacitance of the resonator eliminates the need for a physical connection between the inductor and the capacitor required in a typical LC resonant tank. Low-loss NiZn ferrite cores were used to straighten field lines to reduce lateral current crowding. Prototypes of both parallel and series resonators achieved quality factors over 800 in the 7–9 MHz range, confirming the theoretical analysis of multi-layer foil windings.

Analysis of high efficiency multistage matching networks with volume constraint

Matching networks have useful applications in transforming voltages and impedances in resonant inverters and dc-dc converters. Stacking multiple stages of matching networks can, in some cases, increase the efficiency because each stage is responsible for smaller transformation, but it also reduces the available inductor volume for each stage which can increase the loss. We present optimization of matching networks with volume constraints to determine the optimum number of stages and other design choices for various transformation ratios, volumes and impedances. Scaling models of inductor performance with size are presented and their effect on the efficiency of single-stage and multistage matching networks is analyzed. The analytical results are verified by an experiment using 1- and 2-stage matching networks with a total volume constraint and a voltage transformation ratio of 4. Simple design rules for designing matching networks are presented for voltage transformation ratios lower than 20.

Figure of merit for resonant wireless power transfer

Improvements in performance of resonant coils are important for the range and efficiency of wireless power transfer (WPT); however, these improvements are difficult to measure using the conventional figure-of-merit (FoM), which is the product of quality factor and coupling factor. The conventional FoM does not account for important WPT system performance parameters such as coil size and range of power transfer; furthermore, it requires information which is not commonly reported. In this paper, we propose a new FoM that is the ratio of the loss fraction of a reference system and and the system-under-test. The reference system models two-loops of solid wire that are scaled to the same overall size as the system-under-test. The FoM is an indication of the performance of a WPT coil technology independent of a specific implementation, and can be calculated using commonly reported parameters: coil diameter, transmission range, and coil-to-coil efficiency. We computed the FoM for a few WPT systems in the literature, and found the highest FoM to be 3.4, achieved using a self-resonant structure.

Power density optimization of resonant tanks using standard capacitors

High-frequency power conversion is useful for miniaturization of power electronics, but requires low loss passive components to achieve high power densities without thermal issues. This paper investigates the lowest achievable ESR of a resonant tank using an air-core inductor with a single-layer foil winding and commercially available capacitors. A loss model is presented and online catalogs of multilayer ceramic capacitors are searched for components that can provide a low ESR when combined with an optimally designed inductor for various resonance frequencies. The resulting resonator has a measured sub-mΩ ESR and high efficiency with 250 V dc rating in a 1 cm3 volume. The resonant tank, when used in a resonant switched-capacitor converter, can theoretically handle up to 12 kW assuming a tolerable loss of 2 W, and the power converter itself will be limited by the power density of switches and interconnects rather than by passive components.

Can higher frequencies reduce magnetics size? An exploration of the impact of frequency on optimized flyback transformers

A well-discussed strategy to miniaturize magnetic components is to increase frequency. We consider the impact of frequency on the size of the transformer in a flyback converter. We have built a flyback transformer optimization routine that employs accurate and computationally efficient loss models to evaluate practical designs. An 18/11 pot core was picked and optimized over a range of frequencies. The best performing designs fell in the 500 kHz–2 MHz range suggesting that the biggest opportunities for miniaturization lie in this frequency range.

A step-by-step guide to extracting winding resistance from an impedance measurement

Impedance analyzer measurements can be helpful in assessing inductor and transformer winding resistance and predicting winding loss, but the measured ESR does not directly correspond to winding resistance. Neglecting the effects of core loss and winding capacitance can yield significant errors in the prediction. A step-by-step method to account for such effects and extract winding resistance from an impedance measurement is described. The proposed methodology is applicable to both inductors and multi-winding transformers. Several measurements are needed in this method; one is to determine the effects of core loss and the others yield the impedance from which winding resistance is extracted to form a resistance matrix. The winding resistance of a transformer was determined experimentally and the interactions between the winding resistance, effects of core loss, winding capacitance and inductance and their contributions to the measured impedance are demonstrated.